3rd Edition
Biology
for the 2015 syllabus
for CSEC® Examinations
Biology for CSEC® Examinations is part of a well-established series
Also available in the CSEC Science series:
3rd Edition
Physics
Also available in the CSEC Science series:
Biology
3rd Edition for the 2015 syllabus
Alec Farley & Clarence Trotz
for CSEC® Examinations
Also available in the CSEC Science series:
3rd Edition
Chemistry
Physics
Chemistry
for CSEC® Examinations
Mike Taylor & Tania Chung
Biology
for CSEC® Examinations
Linda Atwaroo-Ali
3rd Edition
3rd Edition
08/12/2014 10:42
Also available in the CSEC Science series:
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Physics for CSEC® Examinations is an
independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Also available in the CSEC Science series:
3rd Edition
Chemistry
Also available in the CSEC Science series:
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Chemistry for CSEC® Examinations is
an independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Chemistry
for CSEC® Examinations
Mike Taylor & Tania Chung
I S B N 978-0-230-43882-8
9
780230 438828
08/12/2014 10:44
08/12/2014 10:42
3rd Edition
for the 2015 syllabus
Biology
for CSEC® Examinations
Linda Atwaroo-Ali
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Biology for CSEC® Examinations is an
independent publication and has not been
authorized, sponsored, or otherwise
approved by CXC.
Find us on Facebook
/macmillancaribbean
Tania Chung (MSc. EdLead, BSc. Hons, DipEd-Distinction) has been
actively involved in science education for many years. She has taught in
Jamaica at Calabar High School as well as in Barbados and the Cayman
Islands. Tania continues to work in the field of improving the teaching and
learning of science.
Also available in the CSEC Science series:
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Physics for CSEC® Examinations is an
independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
3rd Edition
Chemistry
CSEC Physics cover.indd 1
Find us on Facebook
/macmillancaribbean
Find us on Twitter
@MacCaribbean
www.macmillan-caribbean.com
for the 2015 syllabus
for CSEC® Examinations
3rd Edition for the 2015 syllabus
for CSEC® Examinations
Alec Farley & Clarence Trotz
Also available in the CSEC Science series:
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been actively involved in
education and teaching for many years. He has considerable experience
of teacher training, has examined science at ‘O’ and ‘A’ levels all over the
world, and has taught chemistry at School and University levels for
over forty years.
Tania Chung (MSc. EdLead, BSc. Hons, DipEd-Distinction) has been
actively involved in science education for many years. She has taught in
Jamaica at Calabar High School as well as in Barbados and the Cayman
Islands. Tania continues to work in the field of improving the teaching and
learning of science.
Find us on Facebook
/macmillancaribbean
780230 438842
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
3rd Edition
for the 2015 syllabus
Physics
Chemistry for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Chemistry for CSEC® Examinations
features comprehensive, systematic coverage of the latest CSEC syllabus
(2015). Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up to date
scientific information.
I S B N 978-0-230-43884-2
9
Linda Atwaroo-Ali is Head of Science at St Joseph’s Convent
in Trinidad and Tobago.
780230 438842
Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been actively involved in
education and teaching for many years. He has considerable experience
of teacher training, has examined science at ‘O’ and ‘A’ levels all over the
world, and has taught chemistry at School and University levels for
over forty years.
Find us on Twitter
@MacCaribbean
www.macmillan-caribbean.com
Find us on Twitter
@MacCaribbean
www.macmillan-caribbean.com
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Chemistry for CSEC® Examinations is
an independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Chemistry
for CSEC® Examinations
Mike Taylor & Tania Chung
I S B N 978-0-230-43882-8
9
780230 438828
CSEC Chemistry cover.indd 1
08/12/2014 10:44
08/12/2014 10:42
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Biology for CSEC® Examinations is an
independent publication and has not been
authorized, sponsored, or otherwise
approved by CXC.
I S B N 978-0-230-43883-5
9
780230 438835
08/12/2014 10:42
CSEC Biology cover.indd 1
08/12/2014 10:49
for CSEC® Examinations
Mike Taylor & Tania Chung
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Chemistry for CSEC® Examinations is
an independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
3rd Edition
Find us on Facebook
/macmillancaribbean
Find us on Twitter
@MacCaribbean
www.macmillan-caribbean.com
I S B N 978-0-230-43882-8
9
780230 438828
CSEC Chemistry cover.indd 1
09/12/2014 08:48
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Biology for CSEC® Examinations is an
independent publication and has not been
authorized, sponsored, or otherwise
approved by CXC.
08/12/2014 10:53
Biology
for CSEC® Examinations
Linda Atwaroo-Ali
With
interactive
digital
resources
Linda Atwaroo-Ali
3rd Edition
CSEC Biology cover.indd 1
3rd Edition for the 2015 syllabus
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Alec Farley BA (Physics), MS (Physics), Diploma in Advanced Studies
in Science Education, is a former Chief Examiner in CXC Integrated
Science. He has served as Education Officer, Science and Mathematics
within the Ministry of Education, Guyana and has been the head of both
Mathematics and Physics departments throughout the region.
Clarence Trotz MA (Cantab), Cert Ed. was a member of the panel which
formulated the first CSEC physics syllabus, and went on to become Chief
Examiner. He has also served in the Ministry of Education of Guyana as
Co-ordinator of Science and Mathematics Education.
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
I S B N 978-0-230-43884-2
9
Also available in the CSEC Science series:
for CSEC® Examinations
Physics for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Physics for CSEC® Examinations features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Linda Atwaroo-Ali
Chemistry
Mike Taylor
Tania Chung
www.macmillan-caribbean.com
Biology for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Physics
features
Rejuvenated in a third edition, Biology for CSEC® Examinations
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
for CSEC® Examinations
3rd Edition
Find us on Twitter
@MacCaribbean
Find us on Twitter
@MacCaribbean
3rd Edition for the 2015 syllabus
Alec Farley & Clarence Trotz
Key features of the CSEC Science series:
for the 2015 syllabus
for the 2015 syllabus
for CSEC® Examinations
for CSEC® Examinations
Linda Atwaroo-Ali
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
Find us on Facebook
/macmillancaribbean
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
780230 438842
Linda Atwaroo-Ali is Head of Science at St Joseph’s Convent
in Trinidad and Tobago.
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Physics for CSEC® Examinations is an
independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
CSEC Physics cover.indd 1
I S B N 978-0-230-43884-2
9
CSEC Physics cover.indd 1
Alec Farley BA (Physics), MS (Physics), Diploma in Advanced Studies
in Science Education, is a former Chief Examiner in CXC Integrated
Science. He has served as Education Officer, Science and Mathematics
within the Ministry of Education, Guyana and has been the head of both
Mathematics and Physics departments throughout the region.
Clarence Trotz MA (Cantab), Cert Ed. was a member of the panel which
formulated the first CSEC physics syllabus, and went on to become Chief
Examiner. He has also served in the Ministry of Education of Guyana as
Co-ordinator of Science and Mathematics Education.
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
3rd Edition
Find us on Twitter
@MacCaribbean
Key features of the CSEC Science series:
Alec Farley • Clarence Trotz
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
3rd Edition
3rd Edition
Physics With Biology
interactive
digital
resources
Biology
3rd Edition
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Also available in the CSEC Science series:
3rd Edition
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
Alec Farley • Clarence Trotz
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Physics for CSEC® Examinations is an
independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
08/12/2014 10:49
3rd Edition for the 2015 syllabus
3rd Edition
08/12/2014 10:53
Also available in the CSEC Science series:
for CSEC® Examinations
Mike Taylor
Tania Chung
780230 438835
Alec Farley & Clarence Trotz
Physics for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Physics for CSEC® Examinations features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Alec Farley • Clarence Trotz
I S B N 978-0-230-43883-5
9
CSEC Biology cover.indd 1
for CSEC® Examinations
Chemistry for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Chemistry for CSEC® Examinations
Physics
features comprehensive, systematic coverage of the latest CSEC syllabus
(2015). Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up to date
scientific information.
Chemistry for CSEC® Examinations
Find us on Twitter
@MacCaribbean
Find us on Twitter
@MacCaribbean
Physics for CSEC® Examinations
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
3rd Edition for the 2015 syllabus
Alec Farley & Clarence Trotz
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
CSEC Chemistry cover.indd 1
3rd Edition for the 2015 syllabus
Biology for CSEC® Examinations
Linda Atwaroo-Ali is Head of Science at St Joseph’s Convent
in Trinidad and Tobago.
for the 2015 syllabus
for CSEC® Examinations
for CSEC® Examinations
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been actively involved in
education and teaching for many years. He has considerable experience
of teacher training, has examined science at ‘O’ and ‘A’ levels all over the
world, and has taught chemistry at School and University levels for
over forty years.
Tania Chung (MSc. EdLead, BSc. Hons, DipEd-Distinction) has been
actively involved in science education for many years. She has taught in
Jamaica at Calabar High School as well as in Barbados and the Cayman
Islands. Tania continues to work in the field of improving the teaching and
learning of science.
780230 438842
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
3rd Edition
for the 2015 syllabus
Physics
Chemistry for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Chemistry for CSEC® Examinations
features comprehensive, systematic coverage of the latest CSEC syllabus
(2015). Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up to date
scientific information.
I S B N 978-0-230-43884-2
9
3rd Edition
Clarence Trotz MA (Cantab), Cert Ed. was a member of the panel which
formulated the first CSEC physics syllabus, and went on to become Chief
Examiner. He has also served in the Ministry of Education of Guyana as
Co-ordinator of Science and Mathematics Education.
Find us on Twitter
@MacCaribbean
Linda Atwaroo-Ali
3rd Edition
Key features of the CSEC Science series:
3rd Edition for the 2015 syllabus
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
CSEC Physics cover.indd 1
780230 438828
Alec Farley BA (Physics), MS (Physics), Diploma in Advanced Studies
in Science Education, is a former Chief Examiner in CXC Integrated
Science. He has served as Education Officer, Science and Mathematics
within the Ministry of Education, Guyana and has been the head of both
Mathematics and Physics departments throughout the region.
3rd Edition for the 2015 syllabus
Mike Taylor & Tania Chung
for CSEC® Examinations
Physics for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Physics for CSEC® Examinations features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Alec Farley BA (Physics), MS (Physics), Diploma in Advanced Studies
in Science Education, is a former Chief Examiner in CXC Integrated
Science. He has served as Education Officer, Science and Mathematics
within the Ministry of Education, Guyana and has been the head of both
Mathematics and Physics departments throughout the region.
Clarence Trotz MA (Cantab), Cert Ed. was a member of the panel which
formulated the first CSEC physics syllabus, and went on to become Chief
Examiner. He has also served in the Ministry of Education of Guyana as
Co-ordinator of Science and Mathematics Education.
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
I S B N 978-0-230-43882-8
9
for CSEC® Examinations
for CSEC® Examinations
Key features of the CSEC Science series:
3rd Edition
Mike Taylor
Tania Chung
www.macmillan-caribbean.com
CSEC Chemistry cover.indd 1
for the 2015 syllabus
Chemistry
Biology for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Physics
Rejuvenated in a third edition, Biology for CSEC® Examinations
features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
3rd Edition
Find us on Twitter
@MacCaribbean
08/12/2014 10:49
Mike Taylor
Tania Chung
780230 438835
Alec Farley • Clarence Trotz
I S B N 978-0-230-43883-5
9
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Chemistry for CSEC® Examinations is
an independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Chemistry for CSEC® Examinations
Linda Atwaroo-Ali
Alec Farley • Clarence Trotz
Find us on Facebook
/macmillancaribbean
3rd Edition for the 2015 syllabus
Alec Farley & Clarence Trotz
www.macmillan-caribbean.com
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Biology for CSEC® Examinations is an
independent publication and has not been
authorized, sponsored, or otherwise
approved by CXC.
Find us on Twitter
@MacCaribbean
www.macmillan-caribbean.com
CSEC Biology cover.indd 1
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Tania Chung (MSc. EdLead, BSc. Hons, DipEd-Distinction) has been
actively involved in science education for many years. She has taught in
Jamaica at Calabar High School as well as in Barbados and the Cayman
Islands. Tania continues to work in the field of improving the teaching and
learning of science.
3rd Edition
for CSEC® Examinations
for CSEC® Examinations
Chemistry for CSEC® Examinations
3rd Edition for the 2015 syllabus
www.macmillan-caribbean.com
Find us on Facebook
/macmillancaribbean
for the 2015 syllabus
for the 2015 syllabus
for CSEC® Examinations
for CSEC® Examinations
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Linda Atwaroo-Ali is Head of Science at St Joseph’s Convent
in Trinidad and Tobago.
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
780230 438842
3rd Edition
3rd Edition
Physics
Biology for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Physics
Rejuvenated in a third edition, Biology for CSEC® Examinations
features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
I S B N 978-0-230-43884-2
9
Physics for CSEC® Examinations
CSEC® is a registered trade mark of the
Caribbean Examinations Council (CXC).
Physics for CSEC® Examinations is an
independent publication and has not
been authorized, sponsored, or otherwise
approved by CXC.
Biology for CSEC® Examinations
Also available in the CSEC Science series:
Physics for CSEC® Examinations
Find us on Twitter
@MacCaribbean
Biology for CSEC® Examinations
Find us on Facebook
/macmillancaribbean
www.macmillan-caribbean.com
Chemistry for CSEC® Examinations
3rd Edition for the 2015 syllabus
Series Editor: Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been
actively involved in education and teaching for many years. He has
considerable experience of teacher training, has examined science at
‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
CSEC Physics cover.indd 1
Dr. Mike Taylor (MA., PhD., CChem., FRSC.) has been actively involved in
education and teaching for many years. He has considerable experience
of teacher training, has examined science at ‘O’ and ‘A’ levels all over the
world, and has taught chemistry at School and University levels for
over forty years.
3rd Edition
Biology
3rd Edition for the 2015 syllabus
for CSEC® Examinations
Physics for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Physics for CSEC® Examinations features
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Key features of the CSEC Science series:
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Alec Farley BA (Physics), MS (Physics), Diploma in Advanced Studies
in Science Education, is a former Chief Examiner in CXC Integrated
Science. He has served as Education Officer, Science and Mathematics
within the Ministry of Education, Guyana and has been the head of both
Mathematics and Physics departments throughout the region.
Clarence Trotz MA (Cantab), Cert Ed. was a member of the panel which
formulated the first CSEC physics syllabus, and went on to become Chief
Examiner. He has also served in the Ministry of Education of Guyana as
Co-ordinator of Science and Mathematics Education.
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
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• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
for the 2015 syllabus
for CSEC® Examinations
Chemistry for CSEC® Examinations
Also available in the CSEC Science series:
for CSEC® Examinations
Chemistry for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Chemistry for CSEC® Examinations
Physics
features comprehensive, systematic coverage of the latest CSEC syllabus
(2015). Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up to date
scientific information.
Key features of the CSEC Science series:
Key features of the CSEC Science series:
Physics for CSEC® Examinations
Physics for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
features
Rejuvenated in a third edition, Physics for CSEC® Examinations
Chemistry
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Physics for CSEC® Examinations
3rd Edition for the 2015 syllabus
3rd Edition
for the 2015 syllabus
Chemistry
for CSEC® Examinations
Physics for CSEC® Examinations
of books aimed at students preparing for their CSEC Science studies.
Physics
features
Rejuvenated in a third edition, Biology for CSEC® Examinations
comprehensive, systematic coverage of the latest CSEC syllabus (2015).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Biology for CSEC® Examinations
3rd Edition for the 2015 syllabus
I S B N 978-0-230-43883-5
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09/12/2014 08:53
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Log on to the Macmillan Caribbean website
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include interactive questions, audio-based activities,
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CSEC Sci IFC Biol sticker.indd 1
09/12/2014 13:59
Biology
for CSEC ® Examinations 3rd Edition
Linda Atwaroo-Ali
Series Editor: Dr Mike Taylor
CSEC® is a registered trade mark of the Caribbean Examination Council (CXC).
BIOLOGY FOR CSEC® EXAMINATIONS THIRD EDITION is an independent publication
and has not been authorised, sponsored, or otherwise approved by CXC.
Macmillan Education
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A division of Macmillan Publishers Limited
Companies and representatives throughout the world.
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ISBN 9780-230-47948-7 AER
Text © Linda Atwaroo-Ali 2014
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Copyright, Design and Patents Act 1988.
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First edition published 2003
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Note to Teachers
Photocopies may be made, for classroom use, of pages 332–367 without the prior written permission of
Macmillan Publishers Limited. However, please note that the copyright law, which does not normally
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9780230438835_CSEC Biology 3e.indb 2
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Contents
Series Preface
vii
About this Book
viii
Section A: Living Organisms in the
Environment
1
2
3
4
5
Chapter summary
Answers To ITQs
Examination-style questions
The Variety of Living Organisms
Characteristics of life
The major groups of organisms
Classification of organisms on the basis of visible
characteristics
The binomial system
Chapter summary
Answers To ITQs
Examination-style questions
9
10
12
13
13
Ecology and the Impact of Abiotic Factors on
Living Organisms
Ecology
Environmental factors
Ecosystem, habitat, population, community
Distribution of species
Chapter summary
Answers To ITQs
Examination-style questions
15
7
15
15
16
17
21
22
24
Feeding Relationships between Organisms
Producers and consumers
Herbivores, carnivores and omnivores
Predators and prey
Food webs
Decomposers and detritivores
Special relationships
Chapter summary
Answers To ITQs
Examination-style questions
Ecosystem, Habitat, Population, Community
Trapping the Sun’s energy
Pyramids of energy
Pyramids of numbers
Pyramids of biomass
Bioaccumulation
Chapter summary
Answers To ITQs
Examination-style questions
The Cycling of Nutrients
Biogeochemical cycles
The carbon cycle
The nitrogen cycle
Acid rain
2
3
3
6
Population Growth, Natural Resources and
their Limits
Growth of natural populations
Resources and their limits
Chapter summary
Answers To ITQs
Examination-style questions
The Effects of Human Activity on the Environment
Humans and the environment
Endangered and vulnerable organisms
Other effects of human activity
Impact of human activities on marine and wetland
environments
Impact of increase in greenhouse gases
Conservation and restoration of the environment
Chapter summary
Answers To ITQs
Examination-style questions
24
24
25
26
27 Section B: Life Processes and Disease
28
8
Cells
29
Why we need microscopes
30
Plant and animal cells
30
Unicellular microbes
31
Cell specialisation in multicellular organisms
33
Movement of substances into and out of cells
33
Chapter summary
37
Answers To ITQs
37
Examination-style questions
38
9
Photosynthesis
39
Plants are the food supply for animals
40
Photosynthesis
40
Products of photosynthesis
40
Limiting factors in photosynthesis
42
Etiolation
42
Chapter summary
43
Answers To ITQs
46
Examination-style questions
49
50
50
51
52
52
55
60
60
62
63
63
64
66
70
72
72
74
74
75
78
78
79
80
81
83
87
88
89
91
91
92
96
96
97
98
98
99
iii
10 Feeding and Digestion
Diet
A balanced diet
Malnutrition
Holozoic nutrition
Digestion and absorption along the alimentary canal
Assimilation
Functions of the liver
Chapter summary
Answers To ITQs
Examination-style questions
11 Respiration
Aerobic respiration
Anaerobic respiration
Chapter summary
Answers To ITQs
Examination-style questions
100 16 Excretion, Osmoregulation and Homeostasis
101
Metabolism
106
Excretory products in animals
107
Excretory products in plants
108
The human excretory system
111
Osmoregulation
116
Homeostasis
117
Chapter summary
117
Answers To ITQs
118
Examination-style questions
119
17 Movement
122
The importance of movement in animals
122
Movement in plants
125
The skeleton of humans
127
Chapter summary
128
Answers To ITQs
128
Examination-style questions
181
181
182
183
184
189
189
192
193
194
196
196
197
199
205
205
206
12 Gaseous Exchange
130 18 Irritability, Sensitivity and Coordination
Importance of gaseous exchange in humans
130
Irritability
Mechanism of gaseous exchange in humans
131
Stimulus
Importance and mechanism of gaseous exchange
The sense organs of humans
in plants
135
The nervous system
Characteristics common to gaseous exchange surfaces 135
The endocrine system
The effects of smoking
137
Drugs and the effects of drug abuse
Chapter summary
139
Chapter summary
Answers To ITQs
140
Answers To ITQs
Examination-style questions
140
Examination-style questions
208
209
209
210
211
217
219
222
223
224
13 Transport and Defence in Animals
142 19 The Eye, the Ear and the Skin
The need for a transport system
142
The eye
The circulatory system of humans
143
How we see
Blood
149
Sight defects and their corrections
Hypertension
152
The ear
The role of blood in defending the body against disease 153
How we hear
Chapter summary
156
Balance
Answers To ITQs
157
The skin
Examination-style questions
159
Temperature regulation in humans
Temperature regulation in birds
14 Transport in Plants
160
Skin care
The importance of transport in plants
160
Chapter summary
Transport systems of plants
161
Answers To ITQs
Movement of water through a plant
163
Examination-style questions
Transpiration
166
Adaptations in plants to conserve water
167 20 Reproduction in Animals
Uptake and movement of mineral salts
168
Reproduction
Transport of manufactured food
168
Reproduction in humans
Chapter summary
170
The male reproductive system
Answers To ITQs
170
The female reproductive system
Examination-style questions
172
Hormones of the gonads
Fertilisation
15 Storage in Plants and Animals
173
Development of the embryo, fetus and placenta
Why do organisms store food?
173
Birth
Food storage in plants
173
The role of contraception
Food storage in animals
178
HIV/AIDS and other STDs
Chapter summary
179
Chapter summary
Answers To ITQs
179
Answers To ITQs
Examination-style questions
180
Examination-style questions
225
226
227
230
232
233
234
235
237
239
239
240
240
242
iv
244
244
246
246
247
248
250
250
252
253
254
256
256
258
21 Reproduction in Plants
Life cycle of a plant
Structure of a flower
Pollination
Fertilisation and development of seed
Dispersal
Chapter summary
Answers To ITQs
Examination-style questions
22 Disease and Humans
Health and disease
Pathogenic diseases and vectors
Social and economic implications of disease
Chapter summary
Answers To ITQs
Examination-style questions
Section C: Continuity and Variation
23 Mitosis
Chromosome number
The cell cycle
Importance of maintaining species chromosome
number
The process of mitosis
Mitosis and asexual reproduction
Chapter summary
Answers To ITQs
Examination-style questions
259 24 Meiosis
259
The importance of meiosis
261
The process of meiosis
262
Significance of meiosis
263
Chapter summary
264
Answers To ITQs
267
Examination-style questions
267
25 Heredity and Genetics
269
Genes
271
Examples of genetic effects
271
Pedigree charts
273
Chapter summary
275
Answers To ITQs
275
Examination-style questions
276
26 Variation and Evolution
276
Genetic variation
Importance of genetic variation
DNA testing and forensic science
Natural selection
278
Artificial selection
278
Mutation
279
Genetic engineering
Chapter summary
281
Answers To ITQs
282
Examination-style questions
282
287
287 Section D: School-Based Assessment
288
27 School-Based Assessment
Practical work in Biology
School-Based Assessment contents
Index
290
290
291
293
294
295
295
296
297
302
304
306
306
309
310
310
312
313
313
317
319
321
323
324
325
328
328
332
368
v
vi
Series Preface, 3rd edition
Macmillan’s textbooks for the Caribbean Secondary Education Certificate (CSEC)
Science subjects have been written by teachers with many years’ experience of
preparing students for success in their examinations. These revised third editions
have been written to align with the new CXC syllabuses (to be first examined in
2015). Additional practical activities have been included to reflect the new emphasis
on practical work, and new features (such as group work and discussion activities)
will help teachers to cater to a variety of different learning styles within the
classroom.
These books are specially designed to stimulate learning, whatever the reader’s
needs. Students starting a topic from scratch may need to be led through the
explanation one step at a time, while those with prior knowledge of a topic may
need to clarify a detail, or reinforce their understanding. Others may simply need to
check that they understand the material.
Each CSEC science syllabus specifies the areas to be used for the School-Based
Assessment (SBA). Each book in the series has a section designed to help students
with their SBA, by offering advice on how to approach the task, presenting
examples of good SBA work or suggesting suitable material to use within it.
Teachers are free to photocopy these pages.
The CSEC Science series covers everything a student needs pass their CSEC
examination, as well as providing a firm foundation for more advanced study at
CAPE level.
Dr Mike Taylor
Series Editor
vii
About this Book
This book isn’t just words on a page.
This book contains a range of different features to introduce, teach and highlight key information
throughout the course. These pages explain how to use them. The larger column contains the main text
and diagrams; you can read straight down it without interruption.
The smaller column contains other useful facts, so make sure you use it to check your understanding.
You should remember to spend time studying the figures and diagrams as well as the text.
ITQ Where you see this icon, you
This icon shows how you can
make links between this
concept and other topics in Biology.
It is important to remember that
you are not just learning facts in
isolation but to think about how
they relate to your world and your
experiences.
will find an In-Text Question (ITQ). These
are spread throughout each chapter and will help
you to check your progress. If you can’t answer the
ITQ, you should refresh your knowledge by rereading the relevant paragraphs in the main text.
Answers to the ITQs are found at the end of each
chapter.
This symbol means that you can find
additional practice for this topic on the
Macmillan CSEC Science digital resources. These
stand-alone components will help you to learn
and revise key areas of the course. For more
information please visit: http://www.macmillancaribbean.com/pages.aspx/educationalbooks/
secondary/science/interactive_science_csec/.
Life Processes and Disease
Anaerobic respiration in humans
Anaerobic respiration in bacte
Human cells respire normally aerobically. However, during strenuous exercise,
muscle cells need much more energy for the extra work that they are doing.
The breathing rate and heart rate increase in an attempt to get more oxygen
to these cells. Sweating occurs to help lose some of the extra energy as heat.
With increased respiration, a lot of heat is produced which is lost from the skin
(chapter 19). After a period of sustained exercise, the oxygen supply becomes
inadequate, even with panting for air and the increased heart rate. The muscle
cells then respire anaerobically.
Energy is still produced when cells respire anaerobically, although it is a
much smaller amount for each molecule of glucose. This means that they can
continue to do work (contract and relax).
CHAPTER 19
ITQ9
Sometimes bacteria can be found
in canned foods or tins, despite the
fact that the cans and tins are sealed
so that no air can enter. How is this
possible?
inoculation
JVVSLK[V V *HUKHZ[HY[LYJ\S[\YLV
LN3HJ[VIHJPSS\ZI\SNHYP
lactic acid + energy
glucose
in muscle cells
lactic acid ❯
fatigue ❯
ITQ7
Humans respire mostly aerobically.
When do humans respire anaerobically?
oxygen debt ❯
fermentation
PUJ\IH[LKPUSHYNL]H[Z V *MVYHIV
SHJ[VZLJVU]LY[LK[VSHJ[PJHJPKWYVK\JPU
Lactic acid is a waste product of this reaction. It builds up in the muscles and
causes them to ache (figure 11.6). This is often called fatigue. After exercise,
the body has to get rid of the lactic acid as quickly as possible. This is done
by using oxygen to change it back to a chemical like glucose so that it can be
broken down completely in aerobic respiration. When anaerobic respiration
occurs in muscles it is in addition to aerobic respiration and not in place of it.
A person continues to ‘breathe hard’ or pant for some time after exercise as
oxygen is needed to get rid of the lactic acid. The oxygen required to get rid of
the lactic acid is called the oxygen debt (figure 11.7).
JLSSK\YPUN
HUHLYVIPJYLZWPYH[PVU
ITQ8
What is alcoholic fermentation and
what are two of its uses?
cool, add fruits, etc.
package and distribute
H[ V *[OLIHJ[LYPHYLTHPUHSP]LI
MLYTLU[H[PVUVJJ\YZH[[OPZ[LT
\ZLKMVYJVU[YHJ[PVUL[J
storeH[ V *
LULYN`ZTHSSLYHTV\U[
Figure 11.9 The manufacture of yoghurt depends on the a
bacteria.
V_`NLU
LNT\ZJSLJLSSZ
K\YPUNWYVSVUNLK
Z[YLU\V\ZL_LYJPZL
alcoholic fermentation ❯
sugar
fermentation
• sugar from barley seeds
• cane sugar or molasses
ethanol + carbon dioxide
fermentation
fermentation
beer
rum
•ÅV\YHUK`LHZ[KV\NOHM[LYRULHKPUN¶
ÅV\YOHZZ[HYJO^OPJOPZIYVRLUKV^U
to maltose
• yeast uses the maltose as a source of
sugar and fermentation occurs
after some
time
• dough rises as bubbles of CO2
get caught in the dough
•IHRPUNRPSSZ[OL`LHZ[HUK
evaporates the ethanol
Figure 11.8
Uses of fermentation.
SHJ[PJHJPK
Chapter summary
ZLYPLZVMYLHJ[PVUZ
SLHKPUN[VIYLHRKV^U[V
*6/6
• All cells respire to release energy to carry out the p
• Respiration takes place in the mitochondria of cells
• Food is oxidised during respiration, and carbon diox
waste products:
C6H12O6 + 6O2 energy + 6H2O + 6CO2
• Energy is stored in phosphate bonds in ATP (adenos
• There are many advantages to storing energy as sm
• There are two types of respiration: aerobic and ana
• Aerobic respiration uses oxygen and releases a lot o
• Anaerobic respiration releases a small amount of en
• Humans usually respire aerobically but their muscle
during prolonged exercise.
• Lactic acid is produced during anaerobic respiration
oxygen debt which has to be repaid.
• Anaerobic respiration in yeast produces ethanol wh
and carbon dioxide which is used in making bread.
• Anaerobic respiration in bacteria is used in the mak
Figure 11. 7 The oxygen debt is the oxygen needed to break down the lactic
acid formed during exercise.
Anaerobic respiration in yeast
During anaerobic respiration in yeast, ethanol and carbon
dioxide are produced as waste products. Ethanol is an alcohol
and the process is known as alcoholic fermentation.
Yeast is very important in the making of alcohol and bread
(figure 11.8). The ethanol can be produced in many ways to
make a wide range of alcoholic drinks, including beer and wine,
which are enjoyed by humans.
The production of carbon dioxide is used in bread-making
to make dough rise. The carbon dioxide produced by the yeast
as it respires accumulates inside the dough in small pockets.
The dough is seen to get bigger or rise as the gas expands with
warmth. Ethanol is also produced but in small quantities – it
evaporates when the bread is baking in the oven.
126
The first time an important new word
appears in the text, it is highlighted at the
side. A definition or in-depth explanation
is given in the main text.
viii
milkJVU[HPUZSHJ[VZL
pasteurisation
OLH[[YLH[TLU[ V *[VRPSSKPZLHZLJH
anaerobic respiration
Figure 11.6 The build-up of lactic
acid in muscle cells after strenuous
exercise can be painful.
Some bacteria also respire anaerobically. Like an
acid as a waste product. We make use of this in t
and cheese (figure 11.9).
Sign-off Proofs
As you can see on p126, the smaller column
can contain key details. It is good practice to
spend time reading this column as well as the
main text so that you don’t miss any important
information.
Sign-off Proo
Each image has a
caption and a figure
number to help with
cross-referencing.
Summaries of the key
facts from each chapter
will help you check
your understanding.
g
A list of objectives
at the beginning of
each chapter tells you
what topics you will
be covering. They will
help you to plan and
measure your learning.
By the end of this
chapter, you should
be able to:
understand the terms ‘ecology’, ‘ecosystem’ and ‘environment’
distinguish between abiotic and biotic factors
distinguish between habitat and niche
distinguish between community and population
distinguish between population and species
relate the distribution of species to abiotic factors
describe the components of soil
understand the advantages and disadvantages of the use of natural and
chemical fertilisers
Tables and definitions
are printed in coloured
boxes for easy
recognition.
Living Organisms in the Environment
Answers to ITQs
ITQ1
ria
mal cells, they make lactic
he manufacture of yoghurt
This is the style of
question you may
come across in your
exam. Your teacher
will suggest how you
can use them, but they
will measure what you
have learnt and help
to identify any gaps in
your knowledge so you
can revisit the relevant
sections of the book.
IHJ[LYPHHKKLK
[OV\YZ¶
UH[\YHS`VNO\Y[
UVTVYL
YH[\YL
aerobic respiration of Lactobacillus
cesses of life.
e and water are produced as
ne triphosphate).
all packets of ATP.
obic.
energy.
rgy without the use of oxygen.
cells can respire anaerobically
Biotic factors
The temperature of water.
Feeding relationships, e.g. between the lizard
and insects that are its prey.
The amount of light available to the organisms.
Behaviour of scad when attached by dolphin.
You may have noted other examples from the pictures.
ITQ2 A home aquarium is a limited ecosystem; it doesn’t contain the
diversity of species that would be found in the naute. A backyard pond is more
likely to be a complete ecosystem with all the diversity necessary to sustain
itself.
ITQ3 (i) A habitat is the place where an organism lives. A niche is the role an
organism plays in an ecosystem. (ii) population is a group of organisms of one
11 • Respiration
PUNVYNHUPZTZ
Abiotic factors
The School-Based
Assessment pages
contain activities which
enable you to explore
the theoretical concepts
in the chapter. They will
test your investigative
and problem-solving
skills and show realworld applications
of the facts you are
learning.
Examination-style questions
1
(i)
Explain, using examples, the meaning of the terms:
(a) abiotic factor;
(b) biotic factor.
(ii) Define:
(a) environment;
(b) habitat;
(c) population;
(d) community.
(iii) Describe, using examples, how abiotic factors of the environment affect the
distribution of species.
(iv) (a) Amoebae live in fresh and salt water habitats. Describe a major problem of
amoebae living in fresh water.
27 • School-Based Assessment
1.1 To observe visible characteristics of animals and plants
Chapter 1 The Variety of Living Organisms
Syllabus skills: O/R/R
Procedure: animals
1. Visit a backyard garden, a nearby cocoa estate, a nature centre, foothills of forest (anywhere a range of
organisms can be seen).
2. Copy the table below into your lab book and observe five animals (include three insects). Describe what
each animal was seen doing e.g. sucking nectar from a flower, sitting on the bark of a plant. Make a
n animals and creates an
h is used in the alcohol industry
g of yoghurt and cheese.
s
127
ix
x
Section A:
Living Organisms in
the Environment
1
By the end of this
chapter, you should
be able to:
The Variety of Living
Organisms
understand why there exists a range of living organisms on Earth
list and define the characteristics of life
describe the major groups of organisms
understand how a classification system is used to group all living organisms
observe and classify living organisms according to visible similarities and
differences
range of living organisms
characterisics
of life
growth
respiration
irritability
movement
nutrition
excretion
reproduction
Prokaryota
Protoctista
Fungi
Plantae
Animalia
classified according
to common features
Kingdom
Phylum
Class
Order
Family
Genus
Species
species – can
interbreed
with each other
breeds
varieties
races
The planet Earth, the third planet from the Sun, has all the conditions
necessary to support life as we know it. Our planet is positioned at such
a distance from the Sun that living organisms can survive in the range of
temperatures on its surface (although it is a fairly wide range). The presence of
water in all its forms (solid, liquid and gas) and the combination of gases which
make up the atmosphere (including nitrogen, oxygen and carbon dioxide) are
all conditions that are essential to life on Earth.
A huge variety of living forms exist on the planet Earth. They can inhabit
most of the Earth’s surface, land, air and water. They show an enormous range
in size and complexity – from the microscopic, which cannot be seen by the
naked eye and are as simple as one cell, to giant whales which must live in
water since they are too heavy to support themselves and move on land.
2
1 • The Variety of Living Organisms
Characteristics of life
characteristics of life ❯
ITQ1
List three characteristics of the planet
Earth that enable it to sustain life.
Biology is the study of life and how living things stay alive. All living
organisms, microscopic to gigantic, possess certain characteristics. These are the
characteristics of life that distinguish living things from non-living things.
There are seven of these characteristics.
1 Growth – Living organisms increase in mass, size and numbers.
2 Respiration – The energy released during respiration is needed to carry out
all life processes.
3 Irritability – Living organisms can respond to changes in their internal
environment and the world around them. These responses usually increase
their chances of survival.
4 Movement – Most living organisms can move. Plants show growth
movements such as growing towards the light. Most animals can move
from place to place to find food or a mate.
5 Nutrition – All living organisms need food which is used as a source of
energy. Plants make their food during photosynthesis. Animals get their
food by eating plants or other animals.
6 Excretion – All living things make waste products during metabolism.
These must be removed from the body.
7 Reproduction – This is the production of new organisms.
Living organisms are able to carry out all these processes on Earth. Most
organisms are adapted to live on land or in water, more or less close to sea
level. Some survive in ‘extreme’ places such as:
• in hot sulfur springs where chemical conditions are toxic to most living things;
• in extreme cold, such as at the North and South Pole;
• in deep parts of the ocean where no light can reach, such as the Marianas
Trench;
• in the upper atmosphere;
• in extremely hot deserts, such as the Gobi desert;
• inside other living organisms.
Wherever they live, as long as they are able to carry out the processes of life
living organisms survive and produce offspring. Most places on Earth can
support life.
The major groups of organisms
ITQ2
Animals and plants are able to carry
out certain processes which distinguish
them from non-living things. Describe
briefly how a plant (i) feeds (ii) moves.
All organisms used to be classified or placed in two kingdoms or main groups
– animals and plants, depending on whether they get their food from other
organisms or make their own food. However, living things are more diverse than
this and a classification system of five kingdoms is now used. These kingdoms
are the Prokaryotes, Protoctists, Fungi, Plants and Animals (figure1.1).
Living organisms
Prokaryotes
(chromosomes
not enclosed
in a nucleus)
Eukaryotes
(chromosomes enclosed
in a nucleus)
Protoctists
unicellular
Figure 1.1
Fungi
Viruses
Plantae
Animalia
multicellular
Living organisms are placed in five major kingdoms (shown in red).
3
Living Organisms in the Environment
virus ❯
The kingdoms have scientific names that are
slightly different from their common names.
Prokaryota
Protoctista
Fungi
Plantae
Animalia
Viruses do not fit into this classification. They are the smallest organisms,
though it is difficult to think of them as living because they can only ‘live’
inside another living cell. They also do not have a true cellular structure like
other organisms (figure 1.2).
Viruses that attack humans
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ITQ3
What are the five major groups of lifeforms or organisms?
Viruses that attack bacteria are called bacteriophages or simply phages
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ITQ4
Bacteria are described as being
microscopic and unicellular organisms.
What do these terms mean?
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Figure 1.2 The structure of some viruses.
Billions of viruses ‘exist’ around us and it is only when they enter the cells
of an organisms that they show some of the characteristics of life. There they
can reproduce and grow in numbers.
Viruses have a great impact on life on Earth because they can live inside
every type of living organism, from bacteria to plants and animals. It is believed
that they have changed the course of human history because of diseases like
smallpox, measles and now AIDS.
Figure 1.3 Escherichia coli is a rodshaped bacterium which is part of the
normal gut ‘flora’ of humans and other
vertebrates.
Figure 1.4 Anabaena is a bacterium where
the cells stick together in long chains.
4
Prokaryotes
The prokaryotes are organisms that are commonly called bacteria. They occupy
many environments such as soil, dust, water, air, and in or on animals and
plants (figure 1.3). Some are found in hot springs where temperatures may be
higher than 78 °C. Some can survive freezing in ice. Some have been found
in deep cracks in the ocean floor, at very high pressures and temperatures of
360 °C. They can be found in every part of the living world.
They are the most ancient group
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of organisms. They are also the
smallest organisms that have a cellular
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structure. Many exist as single cells,
others are found in groups (figure 1.4).
Their cells have a much simpler
JLSSTLTIYHUL
structure than those of the eukaryotes
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(figure 1.5).
Figure 1.5 Structure of a typical bacterium,
Prokaryotes are vital to all other
e.g. Escherichia coli. The chromosomes are
organisms since they cause decay
not enclosed in a nucleus and there is little
of dead plant and animal material
structure in the cytoplasm.
which releases nutrients back into the
1 • The Variety of Living Organisms
CHAPTER 16, CHAPTER 22
environment. They are essential to the nitrogen cycle. They are also important
to humans because they cause disease (e.g. cholera and TB – chapter 22) and
are used in biotechnology (e.g. in insulin production – chapter 16).
Protoctists
Most protoctists are unicellular, that is made of one cell. This cell shows all the
characteristics of life. Algae and protozoa are two kinds of protoctist.
• Protozoa are unicellular and feed on other organisms (heterotrophically).
They are found in all environments, especially in water, and examples
include Amoeba and Paramecium (figure 1.6 and figure 1.7). They are
important to humans because diseases such as malaria and sleeping sickness
are caused by protozoan parasites.
• Algae live in the sea and in fresh water, and some live on land where the
Figure 1.6 Amoeba proteus (×200).
surface is damp. They make their own food by photosynthesis (figure 1.8).
Some live as single cells, others are found in groups or colonies. A few, such
J`[VWSHZT
as the seaweeds, can grow extremely large. These have structures that look
JLSSTLTIYHUL
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like stems, roots and leaves, but they are much simpler than true plants.
Rapid growth (blooms) of algae can form scums on the surface of ponds,
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lakes and rivers, turning them green.
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Malaria infects millions of people each year and it is estimated that 2.7 million
people worldwide die from this disease each year.
Fungi
Figure 1.7 The structure of Amoeba.
ÅHNLSSH
Fungi range in size from unicellular yeasts to large toadstools. Some are
used by humans for medicinal and dietary purposes. They are heterotrophic
organisms and obtain their food from the environment. However, they do not
take in large particles of food that need to be broken down. They digest their
food outside the body using enzymes which make it soluble. Then they absorb
the food. So, they are usually found living in or on their food, which can be a
dead or living organism (figure 1.9).
cytoplasm
light-sensitive
spot
ZWVYLIVK`
nucleus
chloroplast
starch
storage
O`WOHVM
M\UN\Z
Figure 1.8 Chlorella, a photosynthetic
alga. Note the presence of the chloroplast,
where photosynthesis takes place.
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Figure 1.9 The hyphae of fungi extend into their food. Digestion occurs outside the body.
5
Living Organisms in the Environment
ITQ5
Using one named example of each,
describe one similarity and one
difference between algae and
protozoans.
Figure 1.10 Penicillin spores are made in
sexual reproduction (×600).
Fungi reproduce by producing spores asexually or sexually (figure 1.10).
These are dispersed by the wind and water and some rely on animals to take
them to new environments. Common fungi are:
• moulds (figure 1.10);
• yeasts (figure 1.11);
• mushrooms and toadstools (figure 1.12).
Figure 1.11 Yeast cells bud to make new cells in asexual reproduction.
Importance of fungi to humans
• Important in the making of the antibiotic penicillin.
• Essential to many fermentation processes, such as those used in making
bread, wine, beer and other alcoholic beverages.
• Used to make a range of chemical products, such as anaesthetics, birth
control pills and meat tenderiser.
• Moulds and rust are fungi that are important in damaging growing crops.
• Cause of spoilage of food.
• Source of food and used to make food, such as sufu in East Asia.
Plants (Plantae)
Figure 1.12 Mushrooms are the spore
bodies of some fungi.
ITQ6
Name three kinds of fungi and a
possible use of each.
6
The plant kingdom includes
mosses, liverworts, ferns,
conifers and flowering
plants. Almost all plants are
photosynthetic.
Many plants are a source
of food for humans and
other animals (figure 1.13).
Some provide a rich ad
diverse habitat (figure 1.14).
Some plants can be used as
medicines. Bidens is a weed
which has a small daisy-
Figure 1.13 Bananas,
a food source for many
animals.
Figure 1.14
rich habitat.
Mangroves, a
1 • The Variety of Living Organisms
like flower (figure 1.15).
The leaves and flowers are
steeped and used to ‘cool
the blood’ (prickly heat) and
to relieve a sick stomach.
Sometimes it is given to
children to cure worms.
Flowering plants
angiosperms ❯
Figure 1.15 Bidens – Shepherds needle, Spanish needle,
The flowering plants have
Beggar-ticks, sticktight.
true flowers and so make
seeds. They are also called
angiosperms and are divided into two groups:
• the monocotyledons;
• the dicotyledons.
Table 1.1 shows the distinguishing features of monocotyledons and dicotyledons.
ITQ7
(i) Plants range in size from
unicellular to giant. Put these
plants in order of size starting
from the smallest: fern, mango
tree, croton, moss and lettuce.
(ii) List five reasons why plants are
important.
Feature
Monocotyledons
Dicotyledons
seed
has one cotyledon or seed leaf
has two cotyledons or seed leaves
leaf
has parallel veins
has net-like or branching veins
example
corn (Zea mays)
Hibiscus
Table 1.1 Distinguishing features of monocotyledons and dicotyledons.
Angiosperms are the largest group of plants. They include most crop plants,
ornamental plants and plants used as herbs or medicinal plants. They vary
in size from the very small to gigantic (over 90 m tall) and are often very
beautiful (figure 1.16). They can live in a wide variety of habitats, from deserts
to rainforests.
Figure 1.16
Flame tree.
Phyla is the plural of phylum.
Animals (Animalia)
The animal kingdom contains multicellular, heterotrophic organisms. They are
grouped in phyla as shown in figure 1.17.
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Figure 1.17 Animals are placed in phyla. (Those shown in red are described in more detail
overleaf.)
7
Living Organisms in the Environment
phylum ❯
Table 1.2 shows examples of each animal phylum.
Phylum
Examples
Cnidaria
jellyfish, sea anemone, coral
Platyhelminthes
flatworms, e.g. tapeworm
Mollusca
slug, snail, mussel, octopus
Annelida
roundworm, earthworm, leech
Arthropoda
insect, spider, lobster, millipede, centipede
Nematoda
roundworms
Chordata
fish, amphibian, reptile, bird, mammal
Table 1.2
Examples of the animal phyla.
Arthropods (Arthropoda)
Figure 1.18 An invertebrate that lives on
land, a snail.
Arthropods dominate life on Earth. They include the crustaceans, millipedes,
centipedes, arachnids and insects. They all have an exoskeleton (outer skeleton
of chitin) and jointed limbs.
• The crustaceans are aquatic or live in damp places. They include woodlice,
crayfish, crabs, lobsters and barnacles.
• The arachnids include spiders, scorpions, mites and ticks. They have four
pairs of walking legs and are mainly terrestrial and carnivorous.
• The insects have a distinct head, thorax and abdomen, and three pairs of
walking legs. They include locusts, bees, ants, beetles, aphids and fleas.
Molluscs (Mollusca)
Figure 1.19 An invertebrate that lives in
water, a sea cucumber.
The molluscs have a soft body which is often covered by a shell. They include
conch, snails, slugs, cockles, mussels, octopus, squid, clams and oysters.
Figures 1.18 and 1.19 show examples of mollusccs.
Some molluscs like conch and oysters are important to Caribbean people as
a source of food and an exotic treat to locals and tourists. Farming of molluscs
is practised on some islands as demand exceeds supply from wild populations.
These animals are a renewable resource but populations can decline rapidly
because of over-harvesting from their natural habitat.
Chordates (Chordata)
Most chordates are also vertebrates because they have a vertebral column. The
vertebrates include the fishes (cartilaginous and bony), amphibians, reptiles,
birds and mammals (figure 1.20).
8
1 • The Variety of Living Organisms
frog (amphibian)
lizard (reptile)
scarlet ibis (bird)
monkey (mammal)
Figure 1.20 There are five groups of vertebrates: fish, amphibians, reptiles, birds and mammals.
Birds (Aves) have the following characteristic features:
• front pair of limbs modified to form wings;
• skin covered with feathers;
• produce hard-shelled eggs (reproduction);
• are warm-blooded.
fish
ITQ8
Name the five groups of vertebrates,
giving two examples of each.
Mammals (Mammalia) have the following characteristics:
• four limbs;
• skin covered with hair;
• most give birth to live young;
• feed their young with milk made by the mother (suckle);
• are warm-blooded.
Classification of organisms on the basis of
visible characteristics
Practical activity
SBA 1.1: To observe visible
characteristics of plants and animals,
page 333
artificial classification ❯
natural classification ❯
The simplest way to classify organisms is according to similarities in their visible
characteristics. For example, if we see a number of organisms, we could start to
group them by putting those with wings together. We can make another group
of those with eight legs. We could also put the hairy ones together. And so on.
However, where do we put those that are both hairy and winged?
There are two types of classification, artificial and natural. Artificial
classification is based on easily observed characteristics, like colour, shape or
number of legs. This is a convenient and easy method of grouping organisms
and is designed for a practical purpose. However, worms and snakes have the
same shape, but snakes have a backbone while worms do not.
Natural classification tries to use natural relationships between organisms
using both internal and external characteristics. For example, organisms with
backbones are grouped together because they all have backbones and many
other similarities. Similarities in anatomy, physiology and behaviour may all be
considered when grouping organisms in a natural classification.
Organisms are grouped by similarities that show descent from shared ancestors.
For example, a bird wing and a human arm show descent from a vertebrate
ancestor. A bird wing and an insect wing are derived from different structures.
9
Living Organisms in the Environment
Similarities in DNA (deoxyribonucleic acid) sequences are increasingly
being relied on to determine ancestry. The more alike the DNA sequences are
for two types of organisms, the recently they diverged from a shared ancestor.
Remember that each organism has its own DNA ‘fingerprint’. Biologists
can now construct new evolutionary tree diagrams that show how existing
organisms are related to one another using their DNA.
ITQ9
Classify these organisms according to similarities in their visible characteristics
into three groups.
Dichotomous keys
A dichotomous key is a tool that enables classification of organisms. It works
by asking a series of questions in a step-by-step fashion until you are led to
the name of the organism. Dichotomous means ‘divided into two parts’ and a
dichotomous key always offers two answers to each question.
Simple example of part of a dichotomous key
1
2
3
Does it have wings?
yes – go to question 2
no – go to question 5
Does it have feathers?
yes – it is a bird
no – go to question 3
Are the wings brightly coloured? yes – it is a moth or butterfly
no – go to question 4
And so on.
Dichotomous keys can be used to classify organisms according to both
artificial or natural criteria, including DNA information where it is available.
The binomial system
Carl Linnaeus was a scientist in the 18th century who first grouped organisms
together by a natural classification. Many people had tried grouping organisms
before, but they had all used artificial classification. Linnaeus’ classification
10
1 • The Variety of Living Organisms
binomial system ❯
Genera is the plural of genus.
made it easier to study organisms, since the enormous variety is organised into
closely related groups.
Carl Linnaeus also put forward a system for naming each species of
organism with a biological name, which is called the binomial system. He
did this because organisms may have many common names. For example the
plant called shadow benny, bandania and cilantro in Trinidad and Tobago, is
called sit weed or spirit weed in Jamaica, and in Martinique and Guadeloupe it
is known as bandanie. Each biological name has two parts which are the same
in all these countries and all over the world – the biological name for the plant
is Eryngium foetidum. The first word of this name is the genus name and always
starts with a capital letter. If you are writing it several times, the first word may
be shortened. For example Eryngium foetidum may be abbreviated to E. foetidum.
The second word is the species name.
Every known species has a place in this classification. It starts with
major groups of general features, which are broken down into smaller and
smaller groups that get more and more specific. Look at the example of the
classification of humans in figure 1.21.
Living organisms
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Kingdom
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Phylum
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Animalia
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Chordata
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Vertebrata
(possess a vertebral column)
Sub-phylum
Class
Order
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IPYKZ
Mammalia
(hairy, warm-blooded, suckle young)
*HYUP]VYH
Primates
(monkeys)
Hominidae
(human-like apes)
Family
Genus
Species
LYLJ[\Z
(well-developed brain)
Figure 1.21 The classification of humans.
11
Living Organisms in the Environment
Human beings belong in the kingdom Animalia because we are multicellular
and heterotrophic. We belong in the phylum Chordata and the sub-phylum
Vertebrata because we have a backbone. We are in the class Mammalia
because we have hair, are warm-blooded and suckle our young. We are in the
order Primates with all the other monkeys and apes. We belong to the family
Hominidae which are the human-like apes. In the past, this family has included
several genera including the genus Homo, grouped by the structure of the skull
and teeth. There have also been other species of Homo in the past, for example
Homo erectus. However, that species is separated from the modern Homo sapiens
because they had more body hair and a smaller brain. All people today belong
to the species Homo sapiens because they all have the same characteristics.
Table 1.3 shows how the ocelot starts in the same large groups as humans
but is placed in a different group from the level of Order down. It is grouped
with all the other kinds of cat.
Classification group
Humans
Ocelot
Kingdom
Animalia
Animalia
Phylum
Chordata
Chordata
Sub-phylum
Vertebrata
Vertebrata
Class
Mammalia
Mammalia
Order
Primates
Carnivora
Family
Hominidae
Felidae
Genus
Homo
Leopardus
Species
sapiens
pardalis
Table 1.3
Classification of humans and ocelot.
Chapter summary
• A huge variety of living forms exist on planet Earth.
• All living organisms show the seven characteristics of life: growth, respiration,
irritability, movement, nutrition, excretion and reproduction.
• Living organisms are grouped into five kingdoms: prokaryotes, protoctists, fungi,
plants and animals.
• The prokaryotes are bacteria.
• The protoctists include algae and protozoa.
• The fungi include yeasts and toadstools.
• The plants are mostly photosynthetic (make their own food).
• The animals need to get their food by eating plants or other animals.
• The phyla of animals are cnidarians, platyhelminths, molluscs, annelids, arthropods,
nematodes and chordates.
• The chordates include fish, amphibian, reptiles, birds and mammals.
• Each major group or phylum is broken down into smaller groups.
• Organisms can be classified according to similarities in their visible characteristics.
• A dichotomous key is a tool for classifying organisms by asking a series of yes/no
questions in a step-by-step fashion until you are led to the name of the organism.
• Each species has a common name and a scientific name.
• A species is a group of similar organisms that can interbreed.
12
1 • The Variety of Living Organisms
Answers to ITQs
ITQ1 The presence of water, suitable temperature range, the presence of
gases in the atmosphere, like oxygen and carbon dioxide.
ITQ2 (i) Most plants are able to make their own food in a process called
photosynthesis.
(ii) A plant moves by growing towards light from the environment.
ITQ3 Prokaryotes (bacteria), protoctists (algae and protozoans), fungi
(moulds, yeasts and mushrooms), plants (mosses, liverworts, ferns, conifers
and flowering plants), animals (invertebrates and vertebrates).
ITQ4 Microscopic means cannot be seen with the eye without the use of a
microscope because they are so small. Unicellular means made up of one cell.
A bacterium is a single cell which can carry out all the processes of life.
ITQ5 Algae: Chlorella; protozoan: Amoeba. Both organisms have ‘true’ nuclei;
the chromosomes are enclosed in a membrane which is called a nucleus (so they
belong to the eukaryotes). (Bacteria differ from this and are prokaryotes.)
A difference between Chlorella and Amoeba is that Chlorella has a chloroplast
and is able to photosynthesise or make its own food, while Amoeba cannot
photosynthesise and must feed on other organisms.
ITQ6 Yeast: to make bread. Mushrooms: for food. Moulds: to make the
antibiotic penicillin.
ITQ7 (i) Moss, lettuce, fern, croton and mango tree.
(ii) They produce oxygen which is need by animals for respiration.
They are a food source.
They can be used for medicinal purposes (herbs).
They hold topsoil in place.
They provide homes for animals.
ITQ8 Fish: shark, guppy. Amphibian: frog, toad. Reptile: snake, lizard. Bird:
parrot, duck. Mammal: lion, goat. (You may have thought of many other
examples.)
ITQ9 Two pairs of wings, three pairs of legs, body divided into three parts.
Examination-style questions
1
(i)
(a) List the characteristics of life.
(b) Describe the importance of two of these characteristics.
(ii) Explain the difference between:
(a) the growth of a crystal and the growth of a plant.
(b) the movement of a cloud and the movement of an animal.
(iii) Robots have been built that move, detect and respond to various stimuli.
(a) In what ways is a robot similar to a human?
(b) What are some differences between a robot and a human?
2
(i)
Living organisms can be classified into five kingdoms. List these five groups giving a
named example of each.
(ii) Describe two differences between vertebrates and invertebrates.
(iii) List the main characteristics of dicotyledons and monocotyledons in order to
distinguish between them.
(iv) Discuss the importance of microorganisms to humans.
13
Living Organisms in the Environment
3
(i)
Animals can be found almost anywhere on Earth. Describe how:
(a) a bird is adapted for flying.
(b) a fish is adapted for swimming.
(c) a bird is similar to a fish.
(d) a bird is different from a fish.
(ii) Humans are said to be closely related to chimpanzees.
(a) Explain why this is so by comparing visible differences and similarities between
humans and chimpanzees.
(b) Are there any similarities in their behaviour? Explain fully.
4
(i) List two features common to the organisms shown below.
(ii) Using each feature, classify the organisms. List the members of each group.
A
D
B
F
E
C
G
I
14
H
2
By the end of this
chapter, you should
be able to:
Ecology and the
Impact of Abiotic
Factors on Living
Organisms
understand the terms ‘ecology’, ‘ecosystem’ and ‘environment’
distinguish between abiotic and biotic factors
distinguish between habitat and niche
distinguish between community and population
distinguish between population and species
relate the distribution of species to abiotic factors
describe the components of soil
understand the advantages and disadvantages of the use of natural and
chemical fertilisers
ecology
biotic factors
ecological
study
environmental
abiotic factors
ecosystem
distribution of
plants and animals
community
population
habitat
species
niche
Practical activity
SBA 2.1: A simple ecological study,
page 334
ecology ❯
Ecology
Ecology is the study of the relationships of organisms with each other and
their environment. Together, all the external conditions in which an organism
lives constitute its environment.
Environmental factors
abiotic factors ❯
Environmental factors may be of two kinds:
• abiotic or physical factors (non-living);
• biotic factors (living).
15
Living Organisms in the Environment
Abiotic or physical factors
edaphic factors ❯
ITQ1
Examine figure 2.1 and its caption.
List: (i) two abiotic factors (ii) two
biotic factors you can deduce from
the images.
biotic factors ❯
(a)
Biotic factors
Biotic factors result from the activities of living organisms in the
environment. Factors like predation, symbiosis, competition and disease all
involve the living elements of the environment. All the relationships that exist
between the living organisms, including the feeding relationships (food chains
and food webs), camouflage, pollination and dispersal make up the biotic part
of the environment.
(b)
ITQ2
Why is a home aquarium not selfsustaining while a backyard pond
might be?
(c)
Figure 2.1 (a) The white-lip anole lizard lives in tropical rainforest and feeds on insects. (b) The
bottle-nosed dolphin is a fast-swimming marine mammal that feeds on big-eye scad. The scad
swim in big shoals and dart back and forth when attacked to try to confuse the dolphin. (c) The
Caribbean flamingo feeds on tiny algae and shrimp which it filters from soda lake water with its
specialised bill. Other birds cannot feed in these lakes because soda is caustic.
habitat ❯
Ecosystem, habitat, population, community
ecosystem ❯
An ecosystem is a self-sustaining system of organisms interacting with each
other and their environment. It is made up of all the plants and animals
sharing an environment. It is self-sustained when it can take care of itself – no
human intervention is needed to keep it going.
The area in which an organism lives is called its habitat, for example a small
pond, a swamp or a rocky shore. A very small habitat is called a microhabitat,
for example the soil at the bottom of a pond, the roots of a mangrove tree,
the crevice of a rock. A niche describes the role an organism plays within the
ecosystem. It how the organism lives in its habaitat.
A population is a group of organisms of the same species which live
in a particular habitat. For example, in a pond ecosystem, there may be a
population of beetles and a population of snails. A community consists of
all the populations which live in the same place and interact with each other.
The community in the pond ecosystem is made up of populations of different
species of organisms, feeding on each other, competing with each other,
hiding and protecting each other and also communicating with each other
(figure 2.2).
ITQ3
Distinguish between (i) habitat and
niche (ii) population and community.
niche ❯
population ❯
community ❯
ITQ4
Using figure 2.2, describe: (i) a
population (ii) a habitat (iii) a niche (iv)
a community.
16
• Climatic factors such as light, temperature, rainfall, wind and availability of
water.
• Edaphic factors (associated with the soil) such as pH, texture,
temperature, organic and mineral content.
• Aquatic factors such as salinity, wave action and dissolved oxygen.
• Topographic factors (associated with physical features of the Earth’s surface)
such as the angle of the slope.
2 • Ecology and the Impact of Abiotic Factors on Living Organisms
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Figure 2.2 The biotic and abiotic factors in a pond habitat, including the community and
populations of organisms living in the pond habitat.
Distribution of species
The distribution of species is related to the physical or abiotic factors of the
environment as well as the availability of food or prey. A species is adapted to
live in its environment. For example, camels are adapted to survive and live
in the desert, an extremely harsh environment. Other species simply cannot
live there. Only animals that can tolerate dehydration and survive extremes of
temperature can be found there.
Effects of water on distribution
ITQ5
What are some factors or qualities
of water that determine the types of
organism that live in water?
Water is an abiotic factor that affects the distribution of species. Organisms like
fish and jellyfish that live in water must be able to use oxygen dissolved in
water or take their oxygen from the air above the water, like whales. If they
do not attach themselves to rocks or bury themselves in the seabed, they must
also be adapted to move in water.
There are two main kinds of water found on Earth:
• fresh water found in lakes, rivers and ponds;
• salt water found in the oceans and seas.
17
Living Organisms in the Environment
Fresh water is low in salt and mineral content, but salt water can be very
concentrated. Where these two kinds of water meet, such as in estuaries, the
waters mix to give brackish water.
Most animal species are adapted to live in either fresh water or salt water
(figure 2.3). Only a very few that live in estuaries or regularly migrate from
sea to river or back again (such as salmon and eels), can cope with the
different conditions.
Fish in a marine environment
Fish in a freshwater environment
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Figure 2.3 Adaptations of bony fishes to live in marine or freshwater environments.
Freshwater animals, like Amoeba, have mechanisms to get rid of the excess
water that enters their body by osmosis (figure 2.4).
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Figure 2.4 Amoebae can live in fresh
water because they are adapted to get
rid of the excess water in their bodies.
(a)
Some species do not live in water, but it still determines their distribution.
Toads and frogs live and feed on land but must return to water to reproduce.
They are always found near rivers, ponds and lakes. Others return to water to
cool down and are found in or around areas with water.
The distribution of plants is also related to water. Plants need a constant
supply of water from the soil. Some actually live in water, like water lilies.
Plants that live in areas where water is in short supply are called xerophytes.
They have special features which help reduce transpiration and therefore water
loss (figure 2.5). Some of these features are:
• reduction of leaves to fine spikes (e.g. cacti);
• the stomata are sunken in grooves and reduced in number (e.g. oleander);
• the leaves roll into a cylindrical shape (e.g. marram grass).
(b)
Figure 2.5 (a) Cacti have leaves reduced to spines to reduce transpiration. (b) The leaves of
oleander have stomata sunken in grooves to reduce water loss.
18
2 • Ecology and the Impact of Abiotic Factors on Living Organisms
The change from water to land along the edge of water can create very clear
zones of vegetation. Plant species that are more tolerant of having their roots
submerged in water for long periods of time, such as the red mangrove, are
found at the edge of the water. These species are replaced further inland by
those which can tolerate some submersion, such as the black mangrove, and
even further inland by those which are adapted to cope with only a little
submersion, such as the white mangrove (figure 2.6).
Red mangrove
Black mangrove
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Button mangrove
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Mangrove zonation
Figure 2.6
Zonation of vegetation along the edge of a mangrove swamp.
19
Living Organisms in the Environment
Effect of light on distribution
ITQ6
(i) Name some herbivores that come
out at night to feed, hoping to
escape their predators.
(ii) Predators that hunt at night may
use mechanisms other than light
to detect their prey. Describe,
using examples, two other means
apart from light that can be used
to detect prey.
Light also affects the distribution of plants and animals throughout the Earth.
Animals use light mainly to see their prey (figure 2.7). Some use the absence of
light to escape predators.
Light is vital to plants because it is needed for photosynthesis. Without light
a plant will die. Plants are not found in those areas of the Earth without light,
like deep caves and deep ocean floors. Two aspects of light, its duration and
its intensity, are important for the distribution of species, particularly plants.
However, heat is usually associated with high light intensity or bright light, and
temperature is also an abiotic factor that affects species distribution.
Effect of temperature on distribution
Figure 2.7
Temperature also affects the distribution of species. Poikilothermic animals are
particularly affected because their body temperature reflects the temperature
of the surroundings. If it is too cold, they cannot generate
enough energy to move around to find food or escape
predators; if it is too hot, the proteins in their bodies start
to break down and they die.
Homeothermic animals, such as mammals and birds,
may be able to live in a greater range of temperatures
but they show adaptations to cope with extremes of
temperature. Camels are adapted for desert life. Desert
hares have long ears which give off heat to keep the
animal cool, but arctic hares have very short ears to
reduce heat loss. Other mammals that live in the polar
regions, like the polar bear, have thick layers of body fat
and fur to keep them warm. Mammals such as whales,
walruses and seals, are also able to live in cold polar
waters because they have a thick layer of fat, called
blubber, just beneath the skin. This insulates them from
Chameleon actively hunts its prey.
the cold: whale blubber can be up to 50 cm thick.
Effect of heavy metals on distribution
micronutrients ❯
20
Our environment, and in particular the sea, contains in large or smaller
amounts almost every metal known to humans. Life began in the sea and so
most living things, through the process of evolution, have acquired a tolerance
for small concentrations of these metals. Some of them, such as copper, are
essential in trace quantities and are called micronutrients.
Metals such as copper, mercury and lead (called the heavy metals) are not
tolerated in more than trace amounts. In larger concentrations they become
toxic to animal and plant life, and we think of them as pollutants. These large
concentrations often arise as a result of human activities. For example, some
slag heaps on the island of Anglesey in the United Kingdom are so rich in
copper that nothing, except a few clumps of horsetail grass, will grow on them.
Mercury and lead are particularly dangerous to humans. The poisonous
effects of mercury have been known since Roman times. By the 19th century,
mercury was widely used for ‘silvering’ mirrors, and for treating sexually
transmitted diseases. Makers of felt hats, who used mercury, suffered from
various nervous and mental disorders – hence the phrase ‘mad as a hatter’.
As the chemical industry developed, organic compounds of mercury were
discovered. These are even more toxic because they bind to proteins and fats
in body cells. The cells of the brain and the nervous system appear to be more
2 • Ecology and the Impact of Abiotic Factors on Living Organisms
affected by these compounds and nowadays many mercury compounds which
were once commonly used, for example as seed dressings, are prohibited.
Lead is hardly less dangerous. Lead compounds damage the brain,
particularly in young children, and lead poisoning can cause to serious
mental disorders. The three main ways in which lead was released into the
environment were from local water pipes, from lead compounds in paint and
from additives in petrol. In many countries all three are now prohibited.
Tolerance to heavy metals like lead, copper, zinc and mercury, is inherited
and passed on to offspring. Random mutations can result in some organisms
having greater tolerance to heavy metals than others. Plants may be able to:
• trap heavy metals in the cellulose cell walls;
• confine the metals to the vacuoles;
• excrete the metals back into the environment.
These heavy-metal tolerant plants are rarely found in unpolluted areas as they
are less competitive than other plants. They flourish in polluted areas as the
heavy metals kill the competing plants. Tolerant plants pass on their tolerance
to their offspring.
Effect of soil on distribution
Practical activity
SBA 2.2: Water-holding capacity of three
types of soil, page 338
Practical activity
SBA 2.3: Percentage of water in a soil
sample, page 339
Practical activity
SBA 2.4: Percentage of air in a soil
sample, page 340
Soil supports terrestrial life. For plants, it provides an anchor for roots and is a
medium for nutrients. It acts as a sponge for water, holding it for absorption by
the roots of plants. Plants are able to grow where the soil can provide all their
needs. This means that soil type is very important to the distribution of plants.
Animals depend on plants which depend on soil. Thus soil is also and so
important to the distribution of terrestrial animals. It provides shelter for
subterranean animals, but more importantly, thousands of microbes exist in
soil that replenish the microbes that live in the digestive tracts of herbivores.
Humans have adapted to life on land. We build homes on land and depend
on agriculture for our food. All crops require special types of soil. The soil
sustains all forms of life across the planet.
Chapter summary
• Ecology is the study of the relationships of organisms with each other and their
environment.
• There are two kinds of environmental factors: abiotic and biotic.
• Abiotic factors make up the non-living part of the environment.
• Biotic factors result from the activities of the living organisms in the environment.
• An ecological study involves looking at the biotic and abiotic factors of an area.
• Sampling methods include quadrats, line transects and sweep nets.
• A habitat is a place or area where an organism lives.
• A niche is the role an organism plays within the ecosystem.
• A species is a group of organisms that can interbreed and are adapted to live in their
environment.
• A population is a group of organisms of the same species living in an area.
• A community consists of all the populations living in the same area.
• The abiotic factors of an environment affect the distribution of the species found
there.
• Water and light are examples of abiotic factors that affect the distribution of species.
21
Living Organisms in the Environment
Answers to ITQs
ITQ1
Abiotic factors
Biotic factors
The temperature of water.
Feeding relationships, e.g. between the lizard
and insects that are its prey.
The amount of light available to the organisms.
Behaviour of scad when attached by dolphin.
You may have noted other examples from the pictures.
ITQ2 A home aquarium is a limited ecosystem; it doesn’t contain the
diversity of species that would be found in the naute. A backyard pond is more
likely to be a complete ecosystem with all the diversity necessary to sustain
itself.
ITQ3 (i) A habitat is the place where an organism lives. A niche is the role an
organism plays in an ecosystem. (ii) population is a group of organisms of one
species living together in one habitat. A community is all the populations of all
the organisms living together in an ecosystem.
ITQ4 (i) A population is a group of organisms, all of the same species living
together in one habitat. In this pond there are populations of many different
species of fish and plants. (ii) A habitat is the place where an organism lives.
The habitat is the pond. (iii) A niche is the role an organism plays in an
ecosystem. Each organism in the pond has its own niche. (iv) A community
is all the populations living together. This pond community includes the
populations of all the plants, fish and other animals found there.
ITQ5 Water may be salt water or fresh water. Salt water makes up the
oceans and seas. Fresh water includes the lakes, rivers and ponds. Water
can be stagnant or fast-flowing and all the stages in between. Rocky shores
have strong currents and wave action. Mangrove swamps have brackish
water, which is a mix of salt and fresh. Organisms are adapted to live in these
different habitats.
ITQ6 (i) Examples are fruit-eating bats, and agouti which feed on fruits
and seeds; there are many others that you might have thought of. (ii)
Snakes have heat sensors found in pits on their face which can determine
the presence of other living organisms. Snakes also use their forked tongue
to pick up tiny particles left by an organism in the air. The tongue is then
pushed into the pits of the mouth, and the snake ‘tastes’ the organism.
Many other organisms use scent to find food. Insect-eating bats use sonar, or
sound, to determine exactly what is around them and help them catch prey.
Examination-style questions
1
22
(i)
Explain, using examples, the meaning of the terms:
(a) abiotic factor;
(b) biotic factor.
(ii) Define:
(a) environment;
(b) habitat;
(c) population;
(d) community.
(iii) Describe, using examples, how abiotic factors of the environment affect the
distribution of species.
(iv) (a) Amoebae live in fresh and salt water habitats. Describe a major problem of
amoebae living in fresh water.
2 • Ecology and the Impact of Abiotic Factors on Living Organisms
(b) Explain how Amoeba is adapted to live in fresh water.
An ecological study was conducted in a cocoa estate and the data collected by a
student are seen below.
2
Animals caught in the
sweep net
Animals seen
Plants seen
spider
beetle
caterpillar
grasshopper
other (unidentified)
frog
kiskadee
lizard
worm
squirrel
dog
iguana
millipede
grass
mango tree
cocoa tree
unknown shrubs
coffee tree
pea plant
pomerac tree
Quadrat throw
Millipedes
Spider
1
40
4
2
30
0
3
10
0
4
5
0
5
23
1
6
28
2
7
51
3
8
19
4
9
37
0
10
40
1
(i) Construct a possible food web from the plants and animals recorded.
(ii) These organisms interact with each other in a number of ways. Suggest two possible
relationships that may exist between the organisms recorded. Using names examples,
describe fully each example.
(iii) Suggest some sources of error when using sweep nets.
(iv) Calculate the population density of the millipede and spider.
(v) The area studied was approximately 12 m wide and 20 m long. Calculate the
population size for the millipede and spider.
(vi) Describe fully how a quadrat can be used to estimate the number of organisms
present in an area.
(vii) Compare the use of the quadrat for these two organisms, millipede and spiders. Which
do you think are the more accurate results? Explain why.
23
3
By the end of this
chapter, you should
be able to:
Feeding Relationships
between Organisms
understand the meaning of the terms producers and consumers in a food chain
and relate the position in the food chain to the mode of feeding
understand the terms herbivore, carnivore and omnivore
identify a food chain
identify predator/prey relationships
construct a food web that includes different trophic levels
explain the role of decomposers
understand that special relationships exist and discuss the advantages and
disadvantages of such relationships
food chain
first
trophic level
second
trophic level
third
trophic level
fourth
trophic level
producer
primary
consumer
secondary
consumer
tertiary
consumer
plants
herbivore
carnivore
carnivore
symbiosis – relationships
between organisms of
different species
decomposers
food web – interlinking
of food chains
parasitism
commensalism
mutualism
predator/prey
CHAPTER 9
Phytoplankton are microscopic organisms,
like algae and blue–green bacteria that live in
the oceans. They are seen in rivers, lakes and
puddles of water. They are important since they
start food chains in the world’s oceans or seas.
Around deep-ocean hot water vents, there are bacteria which get their
nutrients and energy from the water. These bacteria are the food for animals,
and these food chains are the only ones we know on Earth which do not
depend on the Sun for their energy.
Life depends on photosynthesis which is carried out by plants (chapter 9).
Most animals get their nutrients (their source of energy) either directly or
indirectly from plants. Plants photosynthesise or make food from water and
carbon dioxide, using light energy from the Sun to carry out the process. So
the Sun is the ultimate source of energy for almost all life on Earth.
Producers and consumers
producers ❯
consumers ❯
24
Plants are called producers because they produce or make their own food.
They include mosses and green plants on land, and algae, aquatic plants and
phytoplankton in water. Organisms that consume the plants or producers,
mainly the animals, are called consumers (figure 3.1). Decomposers feed on
dead organic matter (figure 3.2).
3 • Feeding Relationships between Organisms
nutrients (humus) made
available by decomposers
producer
plant
consumer
caterpillar
consumer
small bird
they all die and their bodies are eaten
Sun
decomposers
return nutrients to the soil
in the form of humus
Figure 3.2
dead fruit.
Mould (a fungus) feeding on
producer
consumer
etc ......
Figure 3.1 The relationship between producers, consumers and decomposers.
Herbivores, carnivores and omnivores
herbivores ❯
carnivores ❯
omnivores ❯
Herbivores are organisms that feed only on plants. Examples are some insects
(like grasshoppers, locusts, butterflies, bees), some birds (such as seed-eating
and fruit-eating species) and some mammals (cows, horses, elephants, giraffes).
In water, herbivores may be very large like the manatee or very small like
a shrimp.
Carnivores are organisms that feed only on animals. They may hunt and
kill other animals for food. Examples include some insects (like the praying
mantis), some reptiles (such as snakes), some birds (eagles and hawks) and
some mammals (lions, dolphins and leopards).
Omnivores feed on both plants and animals. Examples are pigs and
humans.
Food chains
food chain ❯
A food chain is a simple diagram that shows how the food or nutrients (the
energy source) pass from one organism to another. For example:
leaf caterpillar small bird hawk
The arrows show the movement of energy along the food chain.
The leaf is a part of a green plant that is photosynthesising and is a producer.
The caterpillar eats the leaf to get food (energy) to live and is thus a consumer.
The small bird and the hawk are also consumers because they are getting their
food or energy from eating other organisms. Indirectly, their food comes from
the leaf, since the food made by the leaf is first taken into the caterpillar, then
into the small bird as it feeds on the caterpillar and finally to the hawk. So all
the consumers in the food chain ultimately get their food from the producer.
We can also describe the food chain in terms of herbivores and carnivores.
Herbivores feed on the plants or producers and then the carnivores feed on the
herbivores. An omnivore may feed on the producer or herbivore (and even
carnivore in some cases).
producer herbivore carnivore
(grass)
(chicken) (mongoose)
omnivore
(human)
25
Living Organisms in the Environment
Herbivores can only feed on the producers and are called the primary
consumers. Carnivores which feed on herbivores are secondary consumers.
Tertiary consumers feed on the secondary consumers and so on.
producer primary consumer secondary consumer tertiary consumer
Example: waterweed tadpoles small fish bigger fish
Producer
primary
secondary
tertiary
(1°ry)
(2°ry)
(3°ry)
consumer
consumer
consumer
trophic level ❯
A food chain is composed of trophic levels.
Each organism in the food chain represents a trophic level. The three food
chains below each consist of four trophic levels.
These are examples of terrestrial food chains.
Food chain I
leaf caterpillar toad snake
Food chain II
grass grasshopper insect-eating bird hawk
This is an example of an aquatic food chain.
Food chain III
algae snail leech fish
Table 3.1 Shows how the organisms of these three different food chains can be
classified.
Food chain I Food chain II Food chain
III
Type of
feeder
Consumer level Trophic level
leaf
grass
algae
producer
producer
first trophic level
caterpillar
grasshopper
snail
herbivore
primary
consumer
second trophic
level
toad
insect-eating leech
bird
carnivore
secondary
consumer
third trophic level
snake
hawk
carnivore
tertiary consumer fourth trophic
level
Table 3.1
CHAPTER 4
fish
Different ways to classify organisms in food chains
All food chains have certain characteristics in common, as seen in table 3.1.
The number of trophic levels in a food chain is normally limited to four or five,
since the amount of energy being passed on gets smaller and smaller at each
level (chapter 4).
Predators and prey
predators ❯
prey ❯
Animals also show predator/prey relationships. Predators are carnivores that
feed on other animals that are called their prey. Predators hunt, capture, kill
and eat other animals and those that are hunted and eaten are the prey. Food
chains therefore include predators. They are the higher order consumers.
rosebush aphid ladybird spider insectivorous bird
26
3 • Feeding Relationships between Organisms
Prey
Predator
aphid
ladybird
ladybird
spider
spider
bird
Table 3.2
Predator/prey relationships in the
rosebush food chain.
In this food chain, while the spider is a predator because it kills and eats the
ladybird, it is also prey to the insectivorous bird. The food chain shows three
predator/prey relationships(table 3.2)
Animals that are prey have evolved to hide and escape predators, using
characteristics such as camouflage, mimicry and speed. Predators, on the other
hand, have evolved characteristics to improve their chances of catching prey,
like speed, lures and traps.
When all these organisms die, decomposers return their nutrients to the
plants through the soil, and the nutrients return to other feeding animals in
the food chains.
Food webs
food web ❯
A food chain shows one organism feeding on one other organism only, but
feeding relationships are more complex than this. One organism may feed on
a number of organisms and in turn may be eaten by a number of organisms.
The interlinking of a number of food chains is called a food web (figures 3.3
and 3.4).
hawk
mongoose
snake
ITQ1
From the food web shown in figure 3.3:
(i) name (a) two herbivores, and (b)
two carnivores.
(ii) give the name of an organism
which is
(a) a primary consumer;
(b) a secondary consumer;
(c) a producer;
(d) a tertiary consumer;
(e) both a secondary and tertiary
consumer.
(iii) name (a) two predators, and (b)
two prey.
(iv) name an organism found in:
(a) the first trophic level;
(b) the third trophic level.
ITQ2
Construct a food web seen in a (i)
marine habitat (ii) a tree (such as a
mango tree).
frog
ladybird
aphid
tarantula
(spider)
kiskedee
(bird)
rat
hummingbird
butterfly
beetle
hibiscus plant
caterpillar
mango tree
grasshopper
snail
grass
Figure 3.3 A terrestrial food web.
warbine
coscorob
duck
water beetle
mayfly nymph
water boatman
water-flea
pond weed
algae
Figure 3.4 A freshwater (aquatic) food web.
27
Living Organisms in the Environment
Decomposers and detritivores
decomposer ❯
detritivore ❯
CHAPTER 5
ITQ3
Define the terms ‘producer’, ‘consumer’
and ‘decomposer’ and give two named
examples of each.
All living organisms eventually die. Their bodies are composed of complex
compounds like carbohydrates, lipids and proteins that they stored when
they were alive. Two groups of organisms called the decomposers and
detritivores obtain their food or energy from the remains of the dead
organisms. As they feed on the dead organisms they cause their decay or
decomposition (figure 3.5). They help in the recycling of nutrients (chapter 5)
since they return the nutrients trapped in the dead organisms back to the
environment. The nutrients become available again to living organisms.
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Figure 3.5 A dead organism decays or decomposes as fungi and bacteria feed on it.
humus ❯
saprophyte ❯
ITQ4
Draw a diagram to show the feeding
relationship between a producer, a
consumer and a decomposer using
examples from your answer to ITQ3.
symbiosis ❯
mutualism ❯
28
Decomposers include bacteria and fungi. They secrete enzymes which break
down dead plants and animal material into a substance called humus. Humus
enriches and improves the structure of soils in which plants grow and from
which they derive nutrients. Imagine the build-up of dead plants and animals
on the Earth’s surface if there were no decomposers. All the vital chemical
elements or nutrients trapped in these dead organisms would not be able to
return to living organisms or be recycled.
Detritivores also help in the removal and recycling of dead organisms by
feeding on small fragments of the dead material, which are called detritus.
Examples of detritivores include woodlice and earthworms.
Saprophyte is the name given to any organism that feeds on dead organic
material, so decomposers and detritivores are all saprophytes.
Special relationships
The environment supports a host of organisms all living together. But some
organisms live in very special relationships with each other. These relationships
may be advantageous to all the organisms involved but, sometimes, one
organism can cause harm to another.
Symbiosis describes any relationship that exists when different species of
organisms live together. There are three types of symbiosis:
• mutualism;
• commensalism;
• parasitism.
3 • Feeding Relationships between Organisms
Mutualism
CHAPTER 5
In this kind of association,
two organisms of different
species live closely together
and both benefit. Here are
some examples.
• Some sea anemones
and hermit crabs – The
anemone attaches itself
to the shell used by the
hermit crab and obtains
scraps of food as the crab
feeds. The crab gains
protection from predators
as it is camouflaged by the
Figure 3.6 A hermit crab and sea anemone.
anemone and protected
from predators by the
stinging tentacles (figure 3.6).
• Leguminous plants and the bacterium Rhizobium (chapter 5) – The
bacteria live inside swellings on the roots of the leguminous plants, like peas
and beans. These bacteria convert nitrogen gas into ammonia, which is then
converted into amino acids and used by the plants for growth. The plants
benefit because they can thrive in all types of soil, even soil where nitrate
is in short supply. The bacteria also benefit by having a place to live and an
energy supply which they get from the plant.
• Egret and cow – The egret perches on the cow’s back as it feeds on insects
and arachnids, especially ticks that can harm the cow. The egret is obtaining
food and the cow benefits by having blood-sucking insects removed from
its body.
Commensalism
commensalism ❯
ITQ5
Using named examples, distinguish
between mutualism and
commensalism.
Commensalism is a
relationship between two
species in which one clearly
benefits and the other is
not harmed. Here are some
examples.
• Some orchids or ferns
on trees – The orchids
or ferns are small plants
that grow high on the
tree to obtain sunlight
for photosynthesis
(figure 3.7). They use the Figure 3.7 An orchid growing on a tree.
tree for support but not as
a food source. The tree is not harmed, nor does it benefit.
• Egret and cow – When the egret walks behind the cow, it feeds on insects
that fly up as the cow shakes the grass while it walks. The egret benefits but
the cow does not.
• Shark and remora – The remora attaches itself to the shark and moves
around with it. As the shark feeds, the remora also feeds on scraps of food
that are floating around. The remora obtains food while the shark is not
harmed, but nor does it benefit.
29
Living Organisms in the Environment
Parasitism
parasite ❯
ectoparasite ❯
endoparasite ❯
A parasite is an organism which lives and feeds on or inside another
organism, which is called the host. The parasite gains while the host is harmed.
• Parasites which live on the outer surface of their hosts are called
ectoparasites. For example, ticks, lice, fleas and leeches feed on the blood
of their hosts such as dogs, humans, cattle and fish (figure 3.8).
• Parasites that live within a host are called endoparasites. An example in
humans is the organism which causes malaria. A protozoan of the genus
Plasmodium enters the human bloodstream through the bite of an infected
female Anopheles mosquito. Once in the body, the parasite multiplies,
causing bouts of fever, pain, shivering and sweating. Millions of people
die each year from malaria, although anti-malarial drugs like quinine and
choroquinine have been developed.
Chapter summary
Figure 3.8 A leech sucks blood from a
human.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
The Sun is the ultimate source of energy for most life on Earth.
Plants make food and are called producers.
Animals eat plants or other animals and are called consumers.
A diagram which shows the sequence in which organisms feed on each other is
called a food chain.
A food web shows the interlinking of a number of food chains.
Decomposers feed on dead plants and animals.
Herbivores feed on plants alone.
Carnivores feed on animals alone.
Omnivores feed on both plants and animals.
Symbiosis describes relationships between two different species.
Mutualism describes a relationship where both species benefit.
Commensalism is when one species benefits and the other is not harmed but nor
does it benefit.
In a parasitic relationship, one species benefits at the expense of the other.
Predators are carnivores that feed on other animals which are called their prey.
Answers to ITQs
ITQ1 (i) (a) Aphid, butterfly, hummingbird, beetle, caterpillar, grasshopper
or snail.
(b) Ladybird, frog, kiskedee, tarantula, rat, snake, mongoose or hawk.
(ii)
(a) Aphid, butterfly, beetle, hummingbird, caterpillar, grasshopper or
snail
(b) Ladybird, frog, kiskedee, tarantula, rat
(c) Hibiscus, mango tree, grass
(d) Frog, snake, mongoose, hawk
(e) Frog, hawk
(iii) (a) Ladybird, frog, kiskedee, tarantula, rat, snake, mongoose or hawk
(b) Snake, frog, ladybird, kiskedee, tarantula, rat, hummingbird, aphid,
beetle, caterpillar, grasshopper or snail
(iv) (a) Hibiscus, mango or grass
(b) Ladybird, hawk, kiskedee, frog, tarantula or rat
30
3 • Feeding Relationships between Organisms
ITQ2
killer whale
hawk
penguin
seal
insect-eating
bird
chicken
fish
krill
lizard
fruit-eating
bird
zooplankton
bachae
aphid
phytoplankton
ants
butterfly
mango tree
Foodweb for a marine habitat.
Foodweb for a mango tree.
ITQ3 A producer is an organism that produces or makes organic food. A
plant makes organic food during photosynthesis, so any plant is a producer.
Examples are mango tree and hibiscus plant, but you may have thought of
many others.
A consumer is an organism that eats or consumes organic food. Animals
cannot make their own food, so any animal is a consumer. Examples are
caterpillars and humans.
A decomposer is an organism that feeds on dead organic food (dead animals
and plants). The food is said to be decaying or rotting as the decomposer feeds
on it. Examples are bacteria, fungi.
ITQ4 hibiscus plant caterpillar
bacteria
ITQ5 Mutualism and commensalism are both relationships between two
species or partners that are beneficial or good. In mutualism, both partners
benefit. In commensalism, one partner benefits while the other, though not
benefitting from the relationship, is not harmed in any way.
An example of mutualism is between the pigeon pea plant (leguminous
plant) and Rhizobium bacteria that live in swellings of its roots. The pigeon pea
plant gets amino acids for growth, and the bacteria obtain shelter and energy.
An example of commensalism is seen with sharks and remora
fish. The remora fish obtain food and protection from the shark
which benefits nothing from the relationship and is also not harmed.
Examination-style questions
1
(i)
Construct a food web from the information given in the table.
Animal
What it was seen doing
small moth
feeding on nectar of a flower (morning glory)
lizard
feeding on insects
small bird
with a lizard in its beak
spider
feeding on insects trapped in its web
small butterfly
feeding on the nectar of a flower (Ixora)
31
Living Organisms in the Environment
(ii) Examine the food web constructed and describe three consequences of the removal
of the lizards.
(iii) Describe the relationship between:
(a) the moth and the morning glory;
(b) the spider and the moth.
(iv) Name one predator/prey relationship from the food web and describe:
(a) how the predator is adapted to catch its prey;
(b) any feature used by the prey to escape the predator.
2
(i)
Using named examples, describe a:
(a) parasite relationship;
(b) mutualistic relationship.
(ii) (a) Draw a food chain with four trophic levels. (Use named organisms.)
(b) Identify the producer.
(c) How does the organism in the fourth trophic level obtain energy from the Sun?
(d) Which organism is the primary consumer?
(iii) Which organism in the food chain is a:
(a) herbivore?
(b) carnivore?
(c) predator?
(d) prey?
(iv) Describe the role of the decomposers in the food chain.
(v) Copy the table below and use examples from these food chains to complete it.
root earthworm frog fox
pondweed mayfly nymph water beetle
Stages of food chain
producer
primary consumer
predator
prey
herbivore
second trophic level
third trophic level
first trophic level
32
Two examples of organisms, one from each food chain
4
By the end of this
chapter, you should
be able to:
Ecosystem,
Habitat, Population,
Community
understand that the Sun is the ultimate source of energy for life on Earth
explain why food is the source of energy needed by living organisms
understand that respiration is the process by which energy is released from
food
describe pyramids of energy
describe pyramids of numbers
describe pyramids of biomass
food chain
plant makes food using
light energy from Sun
some energy passed on
animal eats and obtains
food (chemical energy)
energy lost due to
respiration, in
urine and faeces
some energy passed on
animal
food – source of
energy for all organisms
feeding pyramids:
• energy
• numbers
• biomass
importance of photosynthesis
to food chains
All living organisms need energy to carry out life processes; for example your
body uses energy to grow, move, inhale and eat. The energy that your body
is using came from your food. If you made a food web for everything you eat,
you would find that all the energy you use was trapped by plants from the
Sun. Ultimately, all energy for life comes from the Sun.
Trapping the Sun’s energy
photosynthesis ❯
CHAPTER 9
Plants use the Sun’s energy to make food during photosynthesis (chapter 9).
During photosynthesis carbon dioxide and water are combined to make
glucose and oxygen.
energy from the Sun
carbon dioxide + water
CHAPTER 13
oxygen + glucose
The glucose is then used to make other carbohydrates, lipids and proteins
and everything else the plant needs. These become the components of food
(chapter 13) for consumers. The term ‘food’ can thus be used for the term
‘energy’, because energy is released from food.
33
Living Organisms in the Environment
respiration ❯
ITQ1
Why is the Sun considered to be the
ultimate source of energy for all life
on Earth?
So the energy in the light from the Sun is converted to chemical energy (as
glucose and other chemicals) in the plant. The chemical energy (as food) then
passes on to consumers as they feed on the plants (figure 4.1).
Respiration releases the energy trapped in the food so that it can be used
by the organism. Respiration also makes carbon dioxide and water.
Food (usually glucose) is ‘burnt’ during respiration by plants and animals
to release energy so that they can carry out all the processes necessary for life.
So, not all the energy gained by a plant is passed on to an animal that eats the
plant (figure 4.2). Likewise, not all the energy gained by an animal is passed on
to a predator (figure 4.3)
glucose + oxygen energy + carbon dioxide + water
Sun
energy from
Sun passed to
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During respiration this
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Some energy changed
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for life processes.
Heat lost to the environment
Figure 4.1 Energy from the Sun is
used by plants and by animals.
Figure 4.2 Only some of the energy taken up
by a plant can be passed on to a herbivore.
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predator.
34
Only some of the energy that an animal gains through eating can be passed on to a
4 • Ecosystem, Habitat, Population, Community
How a plant gains and loses energy
ITQ2
What happens to the energy that a plant
gains during photosynthesis?
• A plant gains energy when it converts light energy to chemical energy
during photosynthesis.
• It stores some of the energy by changing the glucose it made into other
chemicals.
• It uses up some of the food during respiration to release energy to grow and
carry out other life processes. Some of the energy that is released is lost as
heat energy from the plant.
How an animal gains and loses energy
For each animal at each trophic level:
• energy is gained as the organism feeds;
• some of this energy is stored as tissue as the animal grows;
• some energy is lost as faeces and urine straight out of the animal’s body;
• some of the stored energy is released during respiration for the organism to
stay alive and some of that energy is lost as heat to the environment.
ITQ3
What happens to the energy that an
animal obtains?
Movement of energy through a food chain
Energy flow through a food chain or web is related to the movement of food
through the chain. Figure 4.4 shows the movement of energy through a
food chain.
Sun
Energy lost as
heat due to
respiration
Energy lost as
heat due to
respiration
Energy lost as
heat due to
respiration
Energy lost as
heat due to
respiration
PLANT
HERBIVORE or
PRIMARY
CONSUMER
CARNIVORE or
SECONDARY
CONSUMER
CARNIVORE or
TERTIARY
CONSUMER
Energy stored
in tissue
Energy stored
in tissue
Energy stored
in tissue
Energy stored
in tissue
Energy lost
in urine
and faeces
Energy lost
in urine
and faeces
Energy lost
in urine
and faeces
Figure 4.4
ITQ4
How is energy transferred through a
food chain?
ITQ5
What is the importance of respiration in
a food chain?
Movement of energy through a food chain.
Figure 4.4 shows that energy is lost at every step in the food chain. This means
there is less energy at each level for the animals in that level than in the level
below. The length of a food chain is limited by the energy loss at each level.
There will come a point when there is not enough energy to support another
level. There are usually not more than five steps in any food chain.
When the plants and animals die, the energy stored in the dead bodies is
passed on to the detritivores and decomposers as they feed. They also feed on
the urine and faeces made by animals.
35
Living Organisms in the Environment
CHAPTER 5
biomass ❯
productivity ❯
Unlike energy, the elements of which organisms are made, such as carbon
and nitrogen, are recycled (chapter 5).
Energy is not recycled, it moves through and out of the food chains. Energy
enters a food chain as light energy from the Sun, and is lost from every trophic
level as heat energy to the environment. Its flow is non-cyclical, which means
that the energy cannot be returned to a living organism.
The length of a food chain depends on the energy in the biomass available
at each level. Ultimately this depends on how much energy is being trapped
by the producers (their productivity). If the whole ecosystem is highly
productive, then the food chains will be longer because there will be more
energy entering at the producer level of the chain. If there is only a small
amount of energy being trapped by the producers, then they can support only
a few trophic levels (figure 4.5). Ecosystems in equatorial regions are generally
more productive than those in higher latitudes because they get more light
(figure 4.6).
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Figure 4.5 The productivity of the producers in an ecosystem limits the length of food chains that
can be supported.
(a)
Figure 4.6
(b)
(a) Ecosystem of high productivity. (b) Ecosystem of low productivity
Crop plants are mass-harvested for human consumption. If these plants are
eaten directly by humans, a lot more energy can be obtained by the humans
than if the plants were fed to other animals and those animals then eaten by
humans (figures 4.7 and 4.8).
36
4 • Ecosystem, Habitat, Population, Community
energy loss
energy loss
energy loss
energy
energy
energy
Figure 4.7
humans.
energy
HUMANS
Efficient use of food chain for energy by
Figure 4.8
humans.
energy
OTHER
ANIMAL
HUMANS
Inefficient use of food chain; a lot of energy is lost that could be available to
Pyramids of energy
pyramid of energy ❯
A pyramid of energy is a good way of showing the energy relationships
between organisms in different trophic levels. Figure 4.9 shows the pyramid of
energy for a simple food chain. Each block in the pyramid shows the amount
of energy available to the next trophic level.
Using figure 4.9 as an example, 90 000 units of energy are available to the
grasshoppers. The grasshoppers consume that energy as food and lose some
of it to the environment as heat during respiration and activity, and some of
it as faeces. That leaves only 15 000 units for the insect-eating birds. The birds
consume that energy and lose some of it to the environment in faeces and as
heat. So only 2000 units are available to the next level, the cats. The cats lose
energy to the environment as faeces and as heat, leaving only 100 units of
energy in their bodies. This is not enough to support another trophic level, so
there are only four trophic levels in this chain.
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Figure 4.9 A pyramid of energy shows that less and less energy is available to higher trophic
levels in a food chain.
Pyramids of numbers
pyramid of numbers ❯
A pyramid of numbers is like a pyramid of energy but shows the numbers
of all the organisms at each trophic level of a food chain within a given area.
Look at the pyramid in figure 4.10 (overleaf). The pyramid shows that, within
the area being studied there were 80 leaves. On these leaves, 8 caterpillars
were feeding. Two birds were seen feeding on the caterpillars and one cat ate
both birds.
Ecosystems usually contain a large number of small organisms and a smaller
number of large animals. Predators are usually larger than their prey and must
eat a number of them to stay alive.
37
Living Organisms in the Environment
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
10 leaves
grasshopper
Each grasshopper eats
10 leaves each day
bird
cat
bird
Each cat eats 2 birds
a day
Each bird eats
4 grasshoppers a day
cat
bird
grasshopper
leaves
Figure 4.10 A pyramid of numbers is obtained by counting all the individuals at each trophic level.
With this type of ecological pyramid, no allowance is made for the size
of the organism. Each cat and each caterpillar is each counted as one. So
sometimes we can see different shapes in pyramids of numbers (figure 4.11).
One tree may be eaten by many caterpillars, though we could have counted
each leaf separately to get a ‘normal’ pyramid shape. One dog is host to many
ticks, and each tick may have several parasites, but in this case each ‘predator’
is actually smaller than its ‘prey’.
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Figure 4.11
pyramid of biomass ❯
Some pyramids of numbers are of different shapes.
Pyramids of biomass
Instead of estimating the numbers of organisms at each trophic level we can
estimate their biomass or dry weight. From this we can construct a pyramid
showing the biomass of organisms at a given time in each trophic level. The
width of the boxes indicates the relative amounts of biomass present at each
trophic level.
At the start of the food chain in figure 4.12 is a large biomass of green
leaves. The pyramid shows that a large amount of plant material supports a
smaller mass of herbivores and an even smaller mass of carnivores.
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Figure 4.12 A pyramid of biomass.
38
4 • Ecosystem, Habitat, Population, Community
Bioaccumulation
Pesticides can spread through the environment in a food chain. Pesticides (such
as fungicides, herbicides and insecticides) are chemicals that are toxic to some
organisms. They work in one of two ways, on contact or once the chemical has
entered the organism. A grasshopper feeding on plants sprayed with insecticide
will only need to take in a small amount to kill it.
But this can harm other animals in the food chain. For example, a bird
feeding on the grasshoppers will accumulate in its body all the insecticide
that the grasshoppers have ingested. Remember that the bird will eat a large
number of grasshoppers every day. So the bird may end up with levels of
insecticide high enough to poison it or harm it in some way. A hawk or other
predator feeding on the small birds could end up with even higher levels
of pesticide in its body, again enough to poison or harm it. This is called
bioaccumulation or biological magnification.
DDT (dichlorodiphenyltrichloroethane)
DDT provides a well-known example of bioaccumulation (figure 4.13). It is
a very effective insecticide that was used in many countries in the 1950s and
1960s to control mosquitoes, which carry malaria, and to control other insect
pests. However, DDT is stored in fatty tissue so predators absorb the chemical
when they eat prey that contains it. Levels of DDT that accumulate in the
bodies of top predators may be enough to kill them or to harm them in other
ways. In a study of ospreys (North American birds) adult birds were found
to contain 8 million times more DDT than organisms at the bottom of the
food chain. These high concentrations did not kill the birds, but caused the
females to lay eggs with very thin shells. Many eggs broke and so numbers of
these birds dropped rapidly. Since 1972 the use of DDT has been banned in
many countries.
++;HJJ\T\SH[LZ
PU[OL[VWJVUZ\TLYZ
MPZOLH[ZOLYIP]VYL
HUKHJJ\T\SH[LZ++;
OLYIP]VYLLH[Z
WO`[VWSHUR[VUHUK
HJJ\T\SH[LZ++;
;VWWYLKH[VY
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/LYIP]VYL
WWT++;
7YVK\JLYWWT++;
++;LU[LYZ
WO`[VWSHUR[VU
Y\UVMMMYVTHNYPJ\S[\YHSSHUK
JHYYPLZHKPS\[LZVS\[PVUVM
WLZ[PJPKLZLN++;
Figure 4.13
Pesticides like DDT accumulate in the tissues of each trophic level of a food chain.
39
Living Organisms in the Environment
Chapter summary
• Energy from the Sun is used by plants to make food during photosynthesis.
• The equation for photosynthesis is:
carbon dioxide + water + light energy glucose (food) + oxygen
• The energy that is stored in a plant is passed on to other organisms when they feed
on the plant.
• Respiration releases energy in plants and animals for life and growth.
• The equation for respiration is:
food (glucose) + oxygen energy + carbon dioxide + water
• Most of the energy released in respiration is lost as heat to the environment and
cannot be passed on to the next trophic level.
• A pyramid of energy shows that less and less energy is passed on to the higher
trophic levels of a food chain.
• A pyramid of numbers shows the number of organisms found in each trophic level of
a food chain.
• If the dry mass of the organisms at each tropic level of a food chain is measured, a
pyramid of biomass can be produced.
Answers to ITQs
ITQ1 The energy from the Sun is used by plants or producers to make
organic food that is used directly and indirectly by all animals, including
humans. Without the Sun, plants would die so there would be no food for the
animals. They would also die and life, as we know it, would cease to exist.
ITQ2 A plant stores some of the energy in its tissues as it grows and uses
some energy to stay alive. Some energy is lost as heat energy. Some energy is
thus lost to the environment and some is kept in the plant’s body.
ITQ3 An animal uses some of the energy from respiration to stay alive.
Much of the energy is lost to the environment as heat. The animal may use up
more energy than a plant since it is more active. It also stores some energy as
chemical energy in its tissues as it grows.
ITQ4 Energy is transferred from one trophic or feeding level to another
when an organism feeds. Energy is transferred in the form of food. The food
is needed for respiration which makes energy available to the organism. So
energy moves through a food chain when the organisms eat.
ITQ5 Some of the energy that is released during respiration is lost to
the environment in the form of heat from the organism. Respiration is
important in a food chain because at each level in the food chain energy
is lost. Only a proportion of the energy entering one trophic level is stored
in the organism’s body and is thus available to the next trophic level.
Examination-style questions
1
grass
(i)
grasshopper
bird
Copy the diagram above and, using arrows, annotate it to show the movement of
energy into and out of each organism.
(ii) What is the importance of the following in a food chain:
(a) respiration?
(b) photosynthesis?
40
4 • Ecosystem, Habitat, Population, Community
(c) digestion?
(iii) How is light energy converted to chemical energy?
(iv) Most animals spend a great percentage of their day looking for food. Why must
animals eat food?
(v) On the TV programme Sesame Street, there is a story about a boy who ate the Sun.
What do you think of this story? Give details.
2
Food chain A
grass cow tick egret
Food chain B
grass grasshopper izard
(i)
Construct possible pyramids of numbers for food chains A and B. In each case,
discuss the shape of the pyramid.
(ii) Construct possible pyramids of energy for the same two food chains, A and B. Discuss
the shapes of the pyramids.
JH[
IPYK
NYHZZOVWWLY
SLH]LZ
(iii) Look at the pyramid of energy above. Why do leaves contain the greatest amount of
energy?
(iv) What happens to the energy that is not passed on to the grasshoppers?
(v) What will happen to the cats if all the grasshoppers were killed by the use of
insecticide?
41
5
By the end of this
chapter, you should
be able to:
The Cycling of
Nutrients
explain the carbon cycle
understand what is meant by the greenhouse effect and global warming
explain the importance of nitrogen to plants and animals
explain the nitrogen cycle
describe the causes and effects of acid rain
atoms in
plants
atoms in
animals
carbon
hydrogen
oxygen
nitrogen
others
carbon
hydrogen
oxygen
nitrogen
others
biogeochemical cycles
atoms in the
environment
carbon
hydrogen
oxygen
nitrogen
others
greenhouse effect
acid rain
leaching
global warming
Biogeochemical cycles
Living organisms are made up of different kinds of atoms. The most common
atoms are carbon, hydrogen and oxygen, with nitrogen following closely
behind. Smaller amounts of other atoms, such as iron, calcium and sodium, are
also found in living organisms.
These atoms bond together to form larger structures such as protein,
carbohydrates and lipids. These larger structures are then arranged in particular
ways to make up all the tissues needed to build a living organism. All living
organisms are, in essence, complex structures of organic molecules. If we look
at a person, we see skin, hair and nails – it is difficult to imagine that basically
we are just atoms of carbon, hydrogen, oxygen and nitrogen.
42
5 • The Cycling of Nutrients
A carbon atom that was present in Einstein’s
body could be present in your body right now.
biogeochemical cycles ❯
Remember that carbon is found in carbon dioxide
(CO2), carbohydrates, lipids and proteins, since
carbon is an integral part of those compounds.
As an animal grows from birth to adulthood, the growing tissues come from
the food it eats. The animal increases its store of these atoms as it eats and
muscle, bone and all the tissues that make up the organism increase in mass.
Then, when the organism dies, the body is broken down or decomposed, and
the atoms are released back into the environment.
The atoms become part of the soil as the organism’s decomposed body
becomes mixed into the soil. They may then be taken up by plants and built
into the plant’s tissues as the plant absorbs them from the soil with water.
These plants are then eaten by animals and the atoms thus become part of an
animal once again.
The cycling processed by which these essential atoms are released and
reused in nature are called biogeochemical cycles. The carbon and nitrogen
cycles are examples of such cycles.
The carbon cycle
The carbon cycle shows how carbon atoms are passed from one organism to
another and to their environment as they live, breathe, eat, die and decay. The
numbers of the following paragraphs refer to numbers in figures 5.1 and 5.2.
carbon dioxide (CO2)
in the air (0.04%)
3
1 photosynthesis
organic
compounds
in green
plants
respiration
3
7 combustion
5 respiration
2 eaten by animals
organic
compounds
in fossil
fuels
organic
compounds
in animals
4 death and decay
4 death and decay
organic
compounds
in bacteria
and fungi
6 fossilisation
6 fossilisation
Figure 5.1 The carbon cycle shown in diagrammatic form.
Equation for respiration:
food (glucose) + oxygen energy +
carbon dioxide + water
1
2
ITQ1
(i) What atoms are living organisms
made up of?
(ii) How do they obtain these
components?
(iii) What happens to these
components after the organism
dies?
(iv) What is a biogeochemical cycle?
3
4
5
6
The atmosphere contains about 0.04% carbon dioxide. During
photosynthesis, plants use carbon dioxide from the atmosphere to make
carbohydrates, proteins and lipids. This is the first source of carbon in living
organisms – as a part of the plant’s body.
Animals then obtain their supply of carbon by eating plants or other
animals that have eaten plants.
As plants and animals respire, molecules of carbon dioxide are released
back into the atmosphere.
Waste materials from living organisms (like urine and faeces) and their
dead bodies (all organisms die), are used as food sources by decomposers.
Decomposers, like bacteria and fungi, feed on dead organic matter. Carbon
atoms then become incorporated into the bodies of the decomposers.
Respiration of the decomposers releases carbon dioxide into the
atmosphere.
In waterlogged soils where oxygen is in short supply, decomposers are
not able to break down tissues completely in dead bodies and the remains
43
Living Organisms in the Environment
fossil fuels ❯
7
accumulate. For example, in the Carboniferous period (about 290 million
years ago) huge areas of waterlogged swamps covered many parts of the
world. When the swamp plants died, partially decomposed plant material
accumulated and eventually turned to coal, a solid fossil fuel. Oil and
natural gas are liquid fossil fuels that formed in a similar way from the
remains of plants and animals that died in oceans. Fossil fuels contain a
large proportion of carbon.
The burning of fossil fuels (combustion) releases carbon dioxide into the
atmosphere.
carbon dioxide
in the air
3 respiration
1 photosynthesis
7 combustion
2 eaten
4 death and decay
gas
.(:
4 urine and
faeces
6 fossilisation
coal
6 fossilisation
decomposers
in the soil
5 respiration
of decomposers
Figure 5.2 The carbon cycle in more detail.
ITQ2
(i) What is the importance of
photosynthesis in the carbon
cycle?
(ii) What is the importance of
respiration in the carbon cycle?
(iii) What is combustion?
(iv) What role do decomposers play in
the carbon cycle?
44
And so the cycle continues, carbon dioxide in the atmosphere is taken up by
plants, which are eaten by animals, and returned to the atmosphere through
respiration, decomposition or combustion of fossil fuels.
Note the importance of plants in this cycle. Without plants, the carbon stays
in the atmosphere and cannot be reused and incorporated into the bodies of
animals. If there were no plants, there would be no animals.
The human effect on the carbon cycle
Figure 5.3 shows how the level of carbon dioxide in the air has been rising.
The rise in human population has been supported by an increase in
manufacturing and other types of industries. Since the Industrial Revolution,
humans have been burning fossil fuels to release energy for machines. This has
added carbon dioxide to the air at an alarmingly fast rate. The carbon was locked
away in the solid or liquid forms of fossil fuel for millions of years. Increased
combustion of these fossil fuels increases the carbon dioxide concentration
in the air. Increased concentration of carbon dioxide in the atmosphere is
associated with the environmental problem known as global warming.
5 • The Cycling of Nutrients
The Industrial Revolution is a term used to describe the time when people
started to make and use machines to do a lot of their work. It began about 200
years ago. Machines need energy to make them work, and most of this energy
comes from burning fossil fuels.
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IPSSPVU[VUULZ
([TVZWOLYPJJHYIVUKPV_PKL
WHY[ZWLYTPSSPVU
@LHY
@LHY
Figure 5.3 The levels of carbon dioxide in the atmosphere over the last 160 years.
The greenhouse effect and global warming
greenhouse gases ❯
greenhouse effect ❯
global warming ❯
When heat from the Sun reaches the Earth’s surface much of it bounces
straight back into the atmosphere (figure 5.4). Within the Earth’s atmosphere
there are gases like carbon dioxide and methane that absorb some of the
escaping heat and send it back to the Earth’s surface, keeping it trapped around
the Earth. They act like a greenhouse around the Earth and thus are called
greenhouse gases.
This is a natural process which helps keep the surface of the Earth warm.
Without this natural greenhouse effect, the Earth would be too cold for most
of the organisms living on it.
A problem arises when the proportions of these gases in the atmosphere
increase. They bounce more of the heat back to the Earth’s surface. This is
called the ‘enhanced’ greenhouse effect. As a result the temperature of the
Earth increases, which is known as global warming.
reflected back
to space
infrared radiation
(heat) radiated
back towards space
absorbed by
'greenhouse gases'
Sun
incoming solar
radiation (ultraviolet,
visible and infrared)
reradiated
into space
atmosphere
atmosphere heated
– raising Earth's
temperature
reflection
from clouds
Earth
Figure 5.4
Some solar radiation that reaches the Earth is absorbed by the atmosphere rather than going back out to space.
45
Living Organisms in the Environment
Carbon dioxide concentration in the Earth’s atmosphere has increased
by about 20% over the last 100 years. This effect has also been worsened by
deforestation. Trees (forests) remove carbon dioxide from the atmosphere
during photosynthesis, but large areas of forests are being cut down.
It is not proven that higher carbon dioxide levels cause temperature
increase, but scientific research suggests that the two may be associated.
Some people think that global warming might cause the Earth’s temperature
to rise between 1.5 °C and 4.5 °C by the end of the 21st century.
Possible effects of global warming
• The polar ice caps may melt which could cause sea levels all over the world
to rise significantly. Many millions of people now live in lowland areas and
these may be flooded, driving people from their homes.
• Fertile, crop-producing land would be lost by flooding.
• The distribution of organisms over the face of the Earth may change as land
floods and temperature and rainfall patterns change.
• Changes in the amount of land and sea could change weather patterns. This
could increase rainfall in some places and increase periods of drought in
others. Natural storms like hurricanes and typhoons may be more severe.
• Cold countries may become more temperate and fertile.
ITQ3
(i) Why are the carbon dioxide levels
in the atmosphere rising?
(ii) What might be some consequences
of this rise?
We must be very careful not to say that every example of extreme weather
is due to global warming. There have always been variations in climate over
the years and over centuries. Also, we must be careful not to make unjustified
assumptions about future changes. For example, on the island of Svalbard in
the Arctic Ocean, one of the glaciers is retreating, but a neighbouring glacier
has advanced by more than a mile in seven years. Some sea levels are said to
be lower now than in the 18th century – for example mean sea level in the
Cook Islands has apparently dropped by about 20 cm in 200 years. Globally,
mean sea level is rising at about 3 mm per year. So although global warming
is a reality, and many experts attribute this to the enhanced greenhouse effect,
we should not be too quick to predict catastrophe.
The nitrogen cycle
About 79% of the air around us is nitrogen gas. This gas is very unreactive – it
passes in and out of animal’s bodies unchanged when they breathe. However,
nitrogen is an essential component of biological molecules such as proteins and
DNA. Muscle is composed of long strands of protein and DNA is the molecule
in each nucleus of a cell which contains the information about how to build
that cell and make it work.
Plants manufacture protein by absorbing nitrogen from the soil mostly as
nitrate ions. These are combined with carbon, hydrogen and oxygen taken
from glucose that was made during photosynthesis. The elements are then
arranged in another way as they combine with the nitrogen, to make the
building blocks for proteins and DNA.
Remember that glucose is made during photosynthesis and is composed of
carbon, hydrogen and oxygen.
Animals obtain their nitrogen from the protein in their diet, through
eating plants or other animals. The protein they eat is digested, absorbed
and reused as needed in the feeding animal. That is, the nitrogen obtained
from the protein of a piece of plant material or meat can be used to build
growing muscles, make DNA, enzymes and other proteins, and everything else
requiring nitrogen.
The numbers of the following paragraphs refer to numbers in figure 5.5.
46
5 • The Cycling of Nutrients
nitrogen in the air
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IHJ[LYPH
SPNO[UPUN
4KLUP[YPM`PUN
IHJ[LYPH
animal protein
1
5
nitrogen
oxide
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9OPaVIP\T
WSHU[Z
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ammonium
compounds
plant protein
3
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UP[YPM`PUNIHJ[LYPH
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nitrates in the soil
nitrites
in soil
7
6
SLHJOPUN
VM[OLZVPS
MLY[PSPZLYZ
Figure 5.5 The nitrogen cycle shown in diagrammatic form.
nitrogen fixation ❯
1
2
nitrification ❯
3
denitrification ❯
4
5
Nitrogen fixation – This occurs in nitrogen-fixing bacteria that convert
nitrogen gas in the air to nitrate. Some of these bacteria, like Azotobacter
and Clostridium, live in in the soil and convert the nitrogen gas found in the
air in the soil to nitrate. Plants cannot absorb nitrogen gas, only substances
that contain it, like nitrates. So nitrogen-fixing bacteria thus make nitrogen
available to plants in a form they can absorb. Plants use the nitrogen from
nitrates in the soil to make proteins and DNA.
Other kinds of nitrogen-fixing bacteria, called Rhizobium, live in the roots
of legumes (plants of the pea family). There, nitrogen gas is converted to
nitrates and used directly inside the plant to make protein.
Decay – When plants and animals die, their bodies are decomposed by
decomposers to make ammonium compounds in the soil. Animal wastes,
like faeces and urine, are also decomposed by bacteria living freely in
the soil.
Nitrification – The ammonium compounds formed during decay are
converted to nitrites and then nitrates. The processes that lead to the
formation of nitrates in the soil are called nitrification and are carried
out by nitrifying bacteria like Nitrosomonas and Nitrobacter. Plants take up
nitrate ions from the soil and make proteins.
Denitrification – The nitrogen cycle is completed by denitrifying bacteria.
They convert nitrates in the soil back to nitrogen gas. The activities of these
bacteria reduce soil fertility, since they take nitrates out of the soil which
the plants need to grow well.
Lightning – This provides energy to convert a little nitrogen to nitrogen
oxides which dissolve in rain to form nitrates.
47
Living Organisms in the Environment
6
leaching ❯
7
Fertilisers – To make crops grow better, we add artificial and natural
fertilisers to the soil to increase the levels of nitrates.
Leaching – As rain water passes through the soil on its way to the rivers,
lakes or seas, it carries with it dissolved nitrates and other soil nutrients. So
the nitrates can be washed out of the soil. This is called leaching (figure 5.6).
YHPU
ITQ4
Copy and complete this table.
Process in
Importance Examples
nitrogen cycle
of bacteria
involved
YP]LY
nitrogen
fixation
ZVPS^H[LY[VYP]LY
[HRLZU\[YPLU[Z^P[OP[
decay
Figure 5.6
Diagram showing how nitrates can be leached from soil.
nitrification
denitrification
CHAPTER 3
The nitrogen cycle is thus essential to life as nitrogen is a vital component of
every living organism (figure 5.7).
This biogeochemical cycle allows nitrogen to be used over and over by
living organisms. Nitrogen atoms cannot be created and there is only a certain
amount on Earth. The importance of bacteria should be noted because they
are an integral part of this cycle. Nitrifying bacteria can be considered ‘good’
bacteria, without which living organisms would slowly become extinct. The
relationship between the plants and the nitrogen-fixing bacteria is an example
of mutualism (chapter 3).
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MHLJLZ
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UP[YP[LZPUZVPS
KLJH`
IHJ[LYPH
HTTVUP\T
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PUZVPS
Figure 5.7 The nitrogen cycle in more detail.
48
5 • The Cycling of Nutrients
Acid rain
acid rain ❯
Combustion of fossil fuels in industry and from motor vehicles releases acidic
gases such as sulfur dioxide and nitrogen dioxide. These gases dissolve in
atmospheric water vapour in clouds and later fall as acid rain (figure 5.8).
Sulfur dioxide dissolves in atmospheric water to give, eventually, dilute sulfuric
acid. Oxides of nitrogen dissolve to form dilute nitric acid.
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MYVTWVSS\[PVUKPZZVS]LPU^H[LY
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MYVT]LOPJSLZWV^LY
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¶
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Figure 5.8 The formation of acid rain.
pH is a measure of how acidic or how alkaline a
solution is. A pH of 7 is neutral. A solution with a
pH less than this is acidic. If it has a pH above 7,
it is alkaline.
The acid clouds may be carried hundreds of miles away from the source of the
pollution by air currents. It has been recorded that rain with a pH as low as 4
has fallen over Scandinavia, Germany and Canada.
• Acid rain may kill plants and
trees. Some forests, like the Black
Forest in Germany, have been
severely damaged (figure 5.9).
But it has been found that acid
rain enhances the growth of pine
forests in Scandinavia.
• Acid rain also dissolves some
compounds of poisonous metals
thus introducing them into
lakes and rivers. This poisons
organisms living in the water.
Figure 5.9 These trees have been killed by acid
About 400 lakes in Norway are
rain.
now rendered fishless because of
acid rain.
• In cities, stone (statues and carvings) and metal structures have been
damaged because of erosion due to acid rain.
Governments are trying to reduce acid rain by introducing regulations that
demand that industries do not release atmospheric pollutants. The design of
engines for motor vehicles is also important to reduce the amount of pollutant
gases that they make.
49
Living Organisms in the Environment
Chapter summary
• Living organisms are built up from single atoms, mostly carbon, hydrogen, oxygen,
with some nitrogen, iron, calcium, sodium, sulfur and other elements.
• Biogeochemical cycles show how materials are reused in nature.
• The carbon cycle shows how carbon passes between the air, soil, plants and animals
and back again.
• The greenhouse effect is an important natural process, caused by greenhouse gases
in the atmosphere that absorb heat energy from the Sun and keep the surface of the
Earth warm enough for life as we know it.
• Increasing levels of carbon dioxide in the air could lead to global warming which
could affect sea levels and weather, with devastating consequences.
• The nitrogen cycle shows how nitrogen passes between air, soil, plants and animals
and back again. Bacteria are very important in this cycle.
• Acid rain forms when acidic gases such as sulfur dioxide and nitrogen dioxide
dissolve in atmospheric water vapour. It can be very damaging to life.
Answers to ITQs
ITQ1 (i) A living organism is composed of different forms of proteins,
carbohydrates and lipids. These are made up of atoms of carbon, hydrogen,
oxygen, nitrogen and other atoms such as sodium, calcium and iron. (ii)
An animal obtains these components when it feeds. Food is organic and
contains carbon, hydrogen, oxygen, nitrogen, sodium, calcium and iron, etc.
Food is ingested, digested, absorbed into blood and transported to all parts of
the body to build tissues. Plants take in simple inorganic molecules, carbon
dioxide and water from the atmosphere, and nitrates form the soil to build
their tissues. (iii) The large organic molecules in dead bodies are broken
down by decomposers and detritivores into their smaller components. Then
the components can return to the environment and be used again by other
organisms. They are recycled through living tissue in different organisms in
food chains. (iv) A biogeochemical cycle is a cycling process by which an atom
is released and reused in nature.
ITQ2 (i) Photosynthesis is an important part of the carbon cycle because
it is the only means by which carbon from the air is taken into an organism.
Plants take in carbon dioxide and turn the carbon into glucose and other
chemicals in the plant’s tissues. When animals eat the plant, the carbon atoms
can then become part of the animal’s tissues. (ii) Respiration is the means
by which carbon atoms get back into the air (as carbon dioxide) from living
organisms. (iii) Combustion is the burning of fuels, a process which uses
oxygen. When fuels, such as wood, gas and coal, are burnt, carbon dioxide is
produced, thus returning carbon atoms to the air. (iv) Dead plants and animals
have carbon molecules, in carbohydrates, proteins and fats, trapped in their
bodies. Decomposers feed on the dead bodies and release the carbon to the
environment as carbon dioxide when they respire.
ITQ3 (i) Carbon dioxide levels in the atmosphere are rising because of
the vast amount of combustion of fossil fuels to release energy, especially in
industry. An increase in human population leads to a greater demand for
energy. Widespread deforestation adds to the problem. (ii) Global warming
(or the enhanced greenhouse effect) which could lead to higher temperatures,
melting of polar ice caps, flooding and changes in weather patterns.
50
5 • The Cycling of Nutrients
ITQ4
Process in nitrogen
cycle
Importance
Examples of
bacteria involved
Nitrogen fixation
Nitrogen gas is converted to nitrates in the soil and Azotobacter
absorbed by plants; or to amino acids in the root
nodules and used by the plant to make protein.
Rhizobium
Decay
Tissues of plants and animals are broken down and Decay bacteria
their components can be reused. They are broken
down to ammonium compounds.
Nitrification
Ammonium compounds are converted to a more
usable form, nitrates. Nitrates are absorbed by
plants and used to make proteins.
Nitrosomonas
Nitrobacter
Denitrification
Nitrates are converted back to nitrogen gas in
the air.
Denitrifying bacteria
Examination-style questions
1
(i) Using only an annotated diagram, describe the carbon cycle.
(ii) In the carbon cycle, carbon ‘moves’ as it becomes incorporated into the bodies of
organisms or is released into the environment during various processes. Copy and
complete the table below to show the movement of carbon in the processes listed.
Process
Movement of carbon
From
To
respiration in an animal
combustion of coal
photosynthesis
decomposition
(iii) State three ways human activities add carbon to the atmosphere.
(iv) State four possible effects of global warming.
2
(i)
Copy and complete the diagram of the nitrogen cycle shown below.
UP[YVNLUPU[OLHPY
KLJH`
*
)
HTTVUP\TJVTWV\UKZ
KLJH`
(
UP[YH[LPUZVPS
(ii) Describe what happens at A, B and C.
(iii) How are some plants like the garden pea able to survive in soil deficient in nitrates?
(iv) Describe how nitrates are leached from the soil.
(v) Describe an example of symbiosis as seen in the nitrogen cycle.
(vi) Nitrogen is a vital component of every living organism. Describe its importance.
51
6
By the end of this
chapter, you should
be able to:
Population Growth,
Natural Resources
and their Limits
understand that factors affect the growth of natural populations
understand why humans are not subject to the same constraints as other
organisms
describe various resources and their limits
understand the advantage and difficulties of recycling manufactured materials
consider biodegradable and non-biodegradable materials
population growth
population growth of humans
natural population growth
renewable
depletion of natural resources
non-renewable
manufactured
materials
biodegradable
non-biodegradable
recycle
reuse
conservation
reduce
Growth of natural populations
exponential growth phase ❯
52
A population is composed of all the members of the same species living
together in the same place. Many populations live together as a community
occupying the same habitat. A population size may grow or decline depending
on conditions at the time. If food is readily available, or there is adequate
space, then the population may grow (the number of individuals or members
of the species may increase).
Consider a population colonising a new habitat in which conditions are
initially ideal.
1 At first, there are few reproducing individuals and population growth rate
is slow.
2 Then, since there is an abundant food supply, no competitors, no predators
or disease, population growth rapidly reaches its maximum rate. Birth rate
exceeds death rate and the population size doubles at regular intervals. This
phase is called the exponential growth phase or log phase.
3 Exponential growth cannot, and does not, go on forever. Eventually
the population growth slows down. This is because of various factors in
the environment such as lack of food or space, increase in numbers of
predators, increased competition or an increase in the incidence of disease.
6 • Population Growth, Natural Resources and their Limits
4
sigmoid growth curve ❯
The population growth rate slows down and stops and the population size
remains fairly constant.
Figure 6.1 shows the typical growth curve resulting from steps 1–4 above. It is
called a sigmoid growth curve (or S curve).
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Figure 6.1 A typical growth curve.
Factors which reduce population size
carrying capacity ❯
ITQ1
What is meant by ‘an environment can
carry a certain number of organisms of
a population?’
The maximum population size that can be sustained over a period of time by
the environment is called the carrying capacity of the environment. The
environment has enough space, food and whatever is needed to sustain or
‘carry’ a certain number of individuals. Some individuals die from disease or
predation (eaten by predators). However, the death rate is more or less equal
to the birth rate as the population stabilises. Disease and predators help to keep
the population size ‘in check’ or constant or stable.
This will continue until there is a major change in the environment. For
example, a natural disease may develop in a population that could wipe out or
kill most of the individuals. The population size would then decrease drastically
(figure 6.2).
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Some of the factors affecting population size.
Alternatively, another organism may arrive in a habitat. It may be ‘fitter’ (more
able to adapt to small environmental changes), or a better competitor for space
and food, or it may reproduce at a faster rate than already existing populations.
53
Living Organisms in the Environment
ITQ2
How is it possible for the planet Earth
to carry all of the different kinds of
animals and plants known to exist
on it?
This organism could ‘take over’ the habitat as its population size increases
causing others to decrease. Such an organism is called an invasive species.
Or, a natural disaster could drastically reduce population size as many
individuals are killed, damaged or left homeless. For example, a fire blazing
through a forest could kill many of the organisms there.
Growth of the human population
Humans are subject to the same
constraints as other organisms. They
need space for homes and adequate
food for their families. They are also
susceptible to many types of disease:
hereditary, deficiency, physiological and
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At present, it is estimated that there
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are about 7.2 billion people on Earth.
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The human population growth curve in
figure 6.3 shows that the population is
now doubling about every 44 years.
Human population growth depends
Figure 6.3 The human population growth curve 1950–2050.
on the carrying capacity of the Earth, or
the maximum number of people that the Earth could sustain over a long time.
United Nation analysts predict that the world population may stabilise at about
12 billion in about 120 years’ time. Although humans are subject to the same
constraints as other populations, they have actively worked on overcoming them.
• Space – Humans have developed the equipment to move into and inhabit
most places in the world. Forests are cleared, coastal waters are filled and
developed for houses, and deserts are made inhabitable. Some apartment
blocks are over 100 storeys high, to increase the possible living space.
Some people live permanently in boats on rivers and coasts. Humans need
space for homes and also for factories and industries to support their needs
(figure 6.4).
• Food – Humans practise agriculture, which is the mass production of
food. Farming techniques and, more recently, developments in genetic
engineering, have increased agricultural and livestock outputs.
• Disease – Humans are constantly studying diseases, their causes,
symptoms, prevention and cures. Prenatal and postnatal care, and welldeveloped immunisation programmes, prevent the death of millions of
children. Education about disease, technology to prolong life, development
Figure 6.4 Humans can adapt how they
of vaccinations and genetic engineering help prevent death from disease.
live in order to create more space.
• Predators – Humans invented gunpowder, which gave them an advantage
over all other animals, even those much larger and fiercer than themselves.
Humans no longer have any effective predator.
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ITQ3
The activities of humans can cause the
sizes of populations of other organisms
to change. Using examples of animals
or plants, discuss how humans can
cause these population increases and
population decreases.
54
Some environmentalists believe that we need to do something now to curb
our population growth because of the way we are exploiting the environment.
Imagine having to feed 7.2 billion people every day. They say that food
resources may be used up, but there is still an abundance of food in the world.
However, agricultural practices encourage pathogens, pests and parasites
to flourish. Others point out that as a population becomes more advanced,
population growth slows naturally.
For the moment, disease is still the greatest controlling factor on human
mortality (death rate). Even with all our research and technology, there is
still a prevalence of disease. In developing countries, such as Africa, Central
6 • Population Growth, Natural Resources and their Limits
ITQ4
Discuss four ways humans have
‘conquered disease’.
America and India, overcrowding and poor living conditions and medical care
have led to prevalence of infectious diseases. In more developed countries, like
USA, Canada, Japan and the UK, a far smaller percentage of people die from
infectious disease. Most deaths in these areas are due to degenerative diseases
(those that get worse with time), some of which are social and self-inflicted
in nature. Smoking can cause harm in a number of ways (cancer, bronchitis,
asthma). Misusing drugs like alcohol and heroin may lead to the development
of physical and mental disease. Eating large quantities of salty and fatty foods
puts people at risk of becoming obese and then at risk of obesity-related
diseases (diabetes, hypertension, heart disease). People in developed countries
generally live longer than those in less-developed countries, and so there is a
greater prevalence of diseases related to old age.
Population growth is a function of how many individuals that are born
survive to adulthood and reproduction. Birth rate and death rate are therefore
important controlling factors. In natural populations, these are not usually
under the control of the individual, but in humans we have a greatly reduced
death rate. We can also do something that very few other species can do:
control birth rate. In many developed countries, birth rate has fallen to
around the same level as the death rate because of the use of contraception,
so population numbers in those countries are stabilising. In a few countries,
population size is actually falling. But there are still large areas of the world
where population size in increasing rapidly.
Resources and their limits
ITQ5
Name two renewable resources and
two non-renewable resources.
renewable resource
non-renewable resource ❯
Figure 6.5 The solar energy panel provides
a renewable electricity supply to the house.
Resources are features of the environment that can be used by human society.
There are many different types of resource which include:
• mineral resources like bauxite and other metal ores;
• soil resources for agriculture;
• biotic resources, like fish and plants for food and other purposes;
• water;
• fuel and other energy resources, like petroleum and natural gas.
Resources can also be classified as renewable or non-renewable. A
renewable resource is one which can be reused or quickly replaced. Nonrenewable resources are in limited supply and once they are used up, they are
gone forever.
• Mineral resources are non-renewable: once they have been removed from
the ground they cannot be replaced.
• Soil resources remain renewable so long as the soil is cared for properly, but
if damaged by pollution or washed away by rainfall, soil is non-renewable.
• Biotic resources (including for food and timber for paper) are renewable so
long as they are cared for and managed properly.
• Water resources are renewable so long as we prevent the water from
becoming contaminated with pollution.
• Fuel resources based on fossil fuels (e.g. oil, coal and natural gas) are
non-renewable; others (e.g. wind, sunlight and water) are renewable
(figure 6.5).
Energy resources
Some form of energy is used for every form of human activity. In the past,
renewable energy sources like wind, water and firewood were used. Today,
most energy is derived from fossil fuels, like coal, oil and natural gas.
55
Living Organisms in the Environment
At present, the most important commercial energy resources in the
Caribbean are oil and natural gas which are found mainly in and around
Trinidad and Tobago (figure 6.6).
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(a)
(b)
Figure 6.6 (a) On-shore and in-shore gas and oil fields of Trinidad; Large oil and gas fields are also found to the north-west and east
of Trinidad. (b) Percentage share of Trinidad and Tobago energy source.
Local mineral resources
Bauxite from Kwakwani is transported
to Everton by barge for processing.
Bauxite from Ituni is transported to
Linden by rail. Bauxite and alumina
from Linden and Everton are exported
by ship.
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Georgetown
New Amsterdam
Everton
Linden
Ituni
Kwakwani
Berbice
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Ess
processing plant
bauxite bearing area
Berbice Deepwater Facility
mining area
alumina plant (closed)
rail transport of Bauxite
water transport of Bauxite
Figure 6.7 The bauxite industry of Guyana.
56
Bauxite is a red clay, an ore from which aluminium is
obtained. Bauxite is important in the economies of Guyana
and Jamaica (figure 6.7). Aluminium and its alloys are used
in the manufacture of aircraft, trains, buses, cooking foil
and many other items. Bauxite mining uses a large area of
land. In Jamaica, there are regulations which ensure that:
• land remains in agricultural use until mining begins;
• after mining, land is restored for other uses.
When land is cleared for mining, the topsoil is removed
and preserved for later use since it contains most of the
nutrients and organic matter. After mining, the land is
smoothed, reshaped and the topsoil replaced. Fertilisers
can be added. The reclaimed land can be used for pasture,
housing or small-scale farming.
Reducing resource consumption
By definition, non-renewable resources will eventually
run out if we continue to use them. In order to preserve as
much of these resources as possible for future generations,
people are being taught to:
• reuse;
• reduce;
• recycle.
This is particularly important in relation to discarded
manufactured materials such as paper, glass, metals,
plastics and textiles. Humans are the only species to use
these materials.
6 • Population Growth, Natural Resources and their Limits
biodegradable ❯
These manufactured materials are used at home, school offices and factories
and after a while they are discarded because they are worn out, used up or
no longer needed. The average composition of domestic waste is shown in
figure 6.8.
When discarded into the environment, some of these materials break
down naturally into simpler, usually harmless forms, by the action of
microorganisms. These are called biodegradable materials, and include
organic material, paper, some textiles and plastics.
Non-biodegradable materials, such as metals, glass and other kinds of
plastic, cannot be broken down or take a very long time to do so. As a result,
these wastes accumulate in the environment, leading to all kinds of pollution
of soil and water. The useful life of a fast-food package is often minutes, but it
may persist in the environment for years or centuries.
Reuse
compost ❯
Some types of waste can be reused in the same or other ways. For example,
tins and jars can be reused as containers, soda bottles may be returned to the
company for refilling and organic waste can be made into compost. Compost
is used as a natural fertiliser and soil conditioner (figure 6.9, overleaf).
Reduce
Table 6.1 list some manufactured materials together with their sources and
uses. By simply reducing the accumulation of these manufactured materials,
the amount of discarded waste will be reduced, and less pollution will take
place. We can do this by buying only what is needed and choosing products
that are not over-packaged.
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Manufactured
material
Source
Uses
paper
pulp from wood
writing, printing, wrapping
glass
molten mixture of soda ash,
silica,sand and lime
bottles, windows, spectacles, drinking
utensils, containers, ornaments
metals
iron, gold, tin, aluminium
cars, ships, buildings, containers,
electrical appliances, cables (and
many others)
plastics
petroleum
bottles, bags, containers, kitchen
utensils, cases for appliances, fibres
Table 6.1
Some manufactured materials, their sources and uses.
Figure 6.8 Pie chart showing the average
composition of domestic waste.
57
Living Organisms in the Environment
(a)
Figure 6.9
(b)
Recycling in the Caribbean. (a) Collecting glass for recycling. (b) Collecting plastic for recycling.
Recycle
This is the process of collecting materials from the waste stream, separating
them by type, remaking them into new products, and marketing and reusing
the materials as new products.
Advantages of recycling
• Resources will not be used up as quickly, so there will be more for later
generations.
• Less land is needed for disposal of waste.
• Less pollution of soil and water occurs as waste decomposes.
• Less toxic waste is generated.
• Harm to animals is prevented (for example, plastic bags and glass are a
danger to animals).
• In many cases, less energy is used to recycle than to make a new product.
Difficulties of recycling
Collection of recyclables is time-consuming. Each household must sort and have
suitable containers for rubbish and recyclables. The recyclables must be further
separated into glass, metal, plastic and so on, and placed in separate containers.
A problem is that recycle bins or containers must be provided and placed at
strategic places that are accessible to all. People must be educated about the
importance of separating their garbage and how to separate their garbage.
The recyclable materials must then be transported to recycling factories by
trucks or vans. Money must be spent on vehicles, maintenance of the vehicles
and gas. This uses fuel resources which must be taken into account when
trying to judge whether it is worthwhile recycling a material.
Tonnes of recyclables are collected weekly and there must be vast amounts
of storage space. There must also be space for the recycling factories and for
storage of the new product (figure 6.10).
58
6 • Population Growth, Natural Resources and their Limits
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Recycling of household waste.
Ideas for making less waste
Shopping and the environment
ITQ6
Define these terms associated with
preserving and conserving the
environment: (i) reuse (ii) reduce
(iii) recycle.
ITQ7
What are some advantages of reusing?
ITQ8
What are the difficulties of recycling?
• Reduce – Buy only what you need. Can you make do with what you have
already? Buy long-lasting goods whenever you can afford them – you may
save in the long run. Consumer magazines can help you make informed
choices.
• Recycle – Choose containers that can be recycled (such as cans, glass and
recyclable plastic) and recycle them. If you can, buy drinks in returnable
bottles.
• Avoid disposables – The ‘throw-away convenience’ of some products may
not be worth the environmental cost. Paper plates and cups, throwaway
lighters and razors, and disposable diapers are handy but why not buy the
sorts you can use and use again?
• Avoid products that are hazardous to the environment – Baking soda
can be used to scrub out tubs and sink; warm water and vinegar can be used
to clean windows; twice-weekly boiling water rinses keep drains open – and
if they do clog, use a metal ‘snake’ or a plunger to unblock them.
Reducing waste
• Save and reuse things like string, gift-wrap, shopping bags.
• Give magazines and books to friends, hospitals, doctors’ offices.
• Help nursery schools; they love to have egg cartons, yogurt containers, toilet
paper rolls, apple baskets.
• Give old clothes or furniture to charitable organisations such as Goodwill,
The Salvation Army, Saint Vincent de Paul.
59
Living Organisms in the Environment
• Cut down on food waste; 20% of food we buy ends up as garbage. Keep
track of what you have and use groceries while they are still fresh.
• Repair broken toys, appliances, furniture instead of buying new ones.
• Start a compost heap with kitchen and yard waste. You can use things like
banana peel, shells, coffee grounds, leaves, grass clippings. You will reduce
your garbage by a third and make a good soil conditioner for your garden.
Chapter summary
• A population is composed of all the members of the same species living together in
the same place.
• The population grows if food is readily available and there is adequate space.
• Many factors like disease, predation, competition, availability of food and space keep
a population size constant.
• A typical growth curve is sigmoid in shape (S-shaped).
• The maximum population size that can be sustained in an area is called the carrying
capacity of that area.
• Humans, though subject to the same constraints as other organisms, have devised
many ways to overcome such constraints and, as a result, human population growth
is still increasing.
• Humans can make use of many natural resources such as minerals, biotic resources,
water, soil and fuel.
• Resources can be defined as being renewable and non-renewable.
• A renewable resource can be reused.
• A non-renewable resource is limited in supply and once used up is no longer available
for use.
• Oil and natural gas are important resources in Trinidad and Tobago.
• In Guyana and Jamaica, bauxite mining is important.
• Manufactured materials like paper, plastic, textures and metals are used by humans.
These materials can be biodegradable or non-biodegradable.
• Biodegradable materials can be broken down and recycled back into the environment.
• Non-biodegradable materials accumulate in the environment.
• People are being taught to reduce, reuse and recycle these materials.
• There are many advantages to recycling.
• There are also many difficulties involving in recycling.
Answers to ITQs
ITQ1 An environment has a certain amount of space; this will support
only a specific amount of organisms, depending on how they use it. It can
also provide food for a specific number of organisms. Many offspring may be
produced, but some will be eaten by predators and some may die of disease.
At any time, the environment will make it possible for some to live and so the
size of the population remains more or less constant unless there is a change
to the environment. This constant point is called the carrying capacity for the
population in that environment.
ITQ2 Earth has many different environments in which a diversity of
organisms exists. Animals and plants show adaptations for living in the
different habitats and changing environments. Natural disasters, diseases and
predators help to keep population sizes under control. As long as there is
physical space and food for all these organisms, they will survive on Earth.
60
6 • Population Growth, Natural Resources and their Limits
ITQ3 Animals and plants that humans use for food are encouraged to grow,
by creating ideal conditions in which they can live, and removing their pests
and predators.
Humans will cause population decreases by taking too much of a population
of plant or animal for food or other purposes, such as over-fishing of whales
for oil; clearance of an area of natural animal and plant populations so that the
area can be used for human purposes, such as housing, industry or growing
food crops; or accidentally by introducing a species from one environment to
another where it outcompetes the natural populations.
ITQ4 The invention of vaccines and immunisation against deadly diseases
prevents thousands from dying each year.
Diseases that prevent couples from having children have been researched
and studied and now many couples who would have been childless can have
children of their own.
Many more babies are born daily because of technology, drugs and
healthcare services. The baby and mother are treated and cared for during the
pregnancy, at birth and after birth.
Many people live longer because of the research and technology applied to
the treatment and cure of diseases like pneumonia and cancer.
ITQ5 (i) Renewable resources: forest trees and fish. (You may have
mentioned others.)
(ii) Non-renewable resources: fossil fuels and precious metals (e.g. gold and
silver). (You may have mentioned others.)
ITQ6 (i) Reuse – find another use for a product which has already beed
used, so that there is no waste to pollute the environment.
(ii) Reduce – decrease the amount of products being used, so that there is less
to pollute the environment.
(iii) Recycle – utilise an already-used product in the manufacture of another
product that will be used again. This reduces the amount of pollution in the
environment.
ITQ7 Advantages of reusing include:
• less waste is generated;
• there is a reduction in the rate at which the resource which makes the
product is used, so the resource is not depleted as quickly;
• it allows for less pollution of the environment;
• fewer organisms will be affected by pollution.
ITQ8 The difficulties of recycling include:
• it is expensive;
• the public may not cooperate, for example in separating their garbage;
• waste may not be generated in large enough amounts and rates to keep the
recycling plant functioning;
• recycling plants cover a large area, so there is often not enough land
available for the plant.
61
Living Organisms in the Environment
Examination-style questions
1
(i) List four factors that could lead to an increase in population size.
(ii) The graph below shows a typical sigmoid growth curve.
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(a) Describe each phase of growth.
(b) What factors influence the carrying capacity of the environment?
(c) A disease developed that wiped out all the individuals of the population. Copy the
graph above and continue the line to show how the population size is affected.
7VW\SH[PVUZPaL
;PTL
(iii) The growth curve above shows the growth of the population of humans over the last
two centuries.
(a) How is this curve different from the typical sigmoid growth curve of a population?
(b) humans are not subject to the same constraints as other populations. Describe
how humans has overcome two factors that keep other populations ‘in check’.
(c) What are some social and economic consequences of over-population by
humans?
(d) What are some implications to the environment of human over-population?
62
2
(i)
Explain the following terms giving examples of each:
(a) renewable resources;
(b) non-renewable resources;
(c) biodegradable waste.
(ii) In an effort to reduce the production of waste, people are being taught to (a) reduce,
(b) reuse, and (c) recycle. Explain each of these terms using examples.
3
(i) Describe the impact of agriculture on the environment.
(ii) (a) What is compost?
(b) Every home should practice backyard composting. What do you think of this
statement? What are some advantages to the environment of composting?
(iii) Discuss some difficulties of recycling.
7
By the end of this
chapter, you should
be able to:
The Effects of
Human Activity on
the Environment
understand that human activities have great effects on the environment
describe the origin and effects of air, soil and water pollution
discuss deforestation and its consequences on the environment
understand the impact of industrialisation
understand the impact of human activities on marine and wetland environments
discuss current and future trends regarding climate change
understand how the environment can be conserved and restored
human activities
destruction of
the environment
industrialisation
deforestation
pollution
climate change
soil
marine
water
wetland
air
Humans and the environment
Figure 7.1 The planet Earth.
The planet Earth is a beautiful and green place brimming with life-sustaining
water and ideal for life as we know it (figure 7.1).
Millions of different species of plants and animals inhabit the Earth, living in
balance with each other and the various resources that they use.
One species, Homo sapiens (humans) is able to dominate life on Earth.
Humans have been able to use their intellectual abilities to ‘conquer’ the
Earth, to exploit all kinds of environments and harness its natural resources
for their own use. Humans are very
successful, as seen by their great
population size; some parts of the
world are very densely populated by
humans (figure 7.2).
The human environment is
where humans live. The whole Earth
can be considered as the human
environment since humans can
be found almost everywhere. The
Earth itself and other living species,
plants and animals, make up the
Figure 7.2 The human population is increasing.
environment for humans.
63
Living Organisms in the Environment
CHAPTER 6
ITQ1
List three reasons why it is believed
that humans are ‘successful’.
ITQ2
Food, disease, predators and space are
all factors that control population size
and keep populations in check or at a
constant number. List three practices
of food production that have helped
humans to feed billions of people
worldwide.
Humans have been able to overcome those factors which naturally keep
populations in check, as we saw in the chapter 6. Each year, nearly 100 million
people are added to the Earth’s population. This increase in population has led
to many effects on the environment.
• Humans are constantly looking for new space, and have taken over areas
already inhabited by other species. They need space for homes, industries,
agriculture and other activities. As a result, many other species are losing
their living space and are becoming extinct. Changing the use of the land for
human purposes changes its suitability for other species and they die out.
• Humans are constantly producing waste from their various activities and
this has led to all kinds of pollution of land, sea and air.
• The great increase in human population size, together with the
development of global travel and communications, leads to a greater need
for factories and industries. The impact of industrialisation includes the
generation of more waste and increasing use of natural resources on an ever
larger and more dangerous scale.
• Although humans do not live in water, their activities have polluted the
seas, over-fished marine life, caused oil spills and other disasters, and led to
severe stress on marine environments all over the world.
• More humans means more carbon dioxide in the atmosphere; many people
consider this is leading to global warming. The release of other chemicals is
also thought to cause depletion of the ozone layer which protects us from
harmful ultraviolet radiation.
• Human use and management of crops and food species changes the balance
of nature. In trying to control pests, many natural populations may be
destroyed. The development of genetic engineering makes these possibilities
and the risks of affecting the environment even greater.
Endangered and vulnerable organisms
biodiversity ❯
endemic species ❯
There is an extraordinary variety of life on Earth. Biodiversity refers to this
biological diversity of the millions of microorganisms, plants and animals that
inhabit the Earth. The Caribbean islands, like most places, have a rich heritage
of biodiversity. In particular, there are a large number of endemic species
which are only found in a single geographic area, maybe just on one island.
People are concerned over the rapid decline of the Earth’s biodiversity.
Extinction can be seen as a natural process – as environments change naturally
over time, some organisms will not be as well suited to the environment and
will die out. However, human activities have increased the rate of extinction
to several hundred times greater than the natural rate. For example, there are
believed to be less than 1000 giant pandas in the wild in China. Since 1974, their
bamboo forests have died out at an alarming rate. This is part of the natural life
cycle of the bamboo, but increased population of the area by humans has made
the possibility of the panda becoming extinct much more likely.
We now classify organisms that are at risk for extinction as follows:
• endangered species are species whose numbers have been reduced to the
point that the survival of the species is unlikely;
• vulnerable species are those that may become endangered in the near
future because their populations are decreasing at alarming rates;
• extinct species are no longer known to exist.
Much research is being done on endangered species, to find ways to save
them from extinction. The International Union for the Conservation of Nature
(IUCN) regularly publishes lists of endangered organisms.
64
7 • The Effects of Human Activity on the Environment
Reasons for increasing extinction rates
The increased rate of extinction is the result of many kinds of human activity:
• habitat destruction, including deforestation;
• pollution;
• introduction of more competitive species;
• hunting or cropping more than the population can sustain.
Habitat destruction
The manatee is also called sea cow because it
grazes on plants that grow on the sea bed.
Habitat destruction is the leading cause of species extinction around the world.
For example, all species of gorilla and many of the other apes are endangered
because they live in areas where human population is increasing. Many
large predators, such as tigers, and large herbivores, such as the West Indian
manatee, are also endangered because they need space that competes with
humans’s need for space.
Deforestation
(a)
Deforestation is of particular concern at present. Large areas of rainforest are
being destroyed, to provide hardwood timbers and to create space for roads
and mining. These forests are not only important for their contribution to
carbon dioxide removal in the carbon cycle, but contain thousands of species
that could easily become extinct.
Introducing competitive species
(b)
The introduction of more competitive species has a major effect on endemic
island species. For example rats, cats, dogs and mongoose have caused the
decline of many Caribbean species, especially reptiles and birds that are killed
or lose their eggs through predation by the new species.
Hunting
(c)
Hunting may directly cause the extinction of a species, for example, the
Tasmanian Wolf may have been hunted to extinction in Australia by the
middle of the last century, and the Dodo is a famous example of extinction
through hunting which happened around 1680. The polar bear and black
bear are killed for their fur and are under threat of extinction from hunters.
In addition, food fish, like populations of tuna, shark and card, are over-fished
and are at risk.
Hunting may also indirectly affect other species. For example, drift nets
are enormous nets used for commercial fishing. The nets trap and kill many
creatures apart from fish, especially dolphins, porpoises, sharks and turtles.
Today, many animals are at risk in the Caribbean (figure 7.3). These include
many birds, like the West Indian Whistling-Duck, the West Indian Flamingo,
the Barbuda Warbler and the Cayman Parrot. Of the reptiles, the Hawksbill
Turtle, the Leatherback Turtle and the Loggerhead Turtle are all at risk. The
American Crocodile, the Cuban Tree Boa and many iguanas are also at risk.
Reducing the risk of extinction
Figure 7.3 Endangered species
in the Caribbean. (a) Whistling duck
(b) Leatherback turtle (c) Manatee.
There are ways to reduce the risk of extinction of other species. Wildlife
parks and reserves set aside land where the activities of humans are strictly
controlled. This can help to increase the numbers of endangered species and
prevent extinction. Zoos often specialise in looking after particular animals.
However, there are many problems in the breeding of animals in captivity.
65
Living Organisms in the Environment
Although much research is being done on maintaining the Earth’s
biodiversity, we need to be sensitive to the needs of all the species living
around us and the effect that we are having on them.
Other effects of human activity
Shortage of water
Most living organisms must have a daily supply of water to live. Humans often
use more water than is strictly necessary and the shortage of fresh water is now
a worldwide problem. A number of factors working together have resulted in
this water shortage problem.
• Deforestation – This can remove large areas of densely growing trees,
like rainforests. Water is transferred to the atmosphere by evaporation
and transpiration by plants. This is an important part of the water cycle as
it leads to the formation of rain. Rain falling into rivers, lakes and dams
provide our supply of water. Removing large areas of trees can change the
pattern of rainfall all around the area and possibly beyond.
• Industrialisation – Industry uses a lot of water most often simply to cool
equipment. For example, water ensures the constant functioning of the
iron, steel and oil industries.
• Pollution – Water pollution results in less water being suitable for drinking.
Each individual needs water for hygiene and food production. Billions of
individuals use and waste water every day, though much of this can be
recycled.
• Depletion of ground water – Much water is trapped in lakes
underground after it has seeped through the soil and rocks above. These
lakes are known as aquifers and we get a lot of our water from these. It
can take many years, even hundreds or thousands of years, for the water
to seep into them, so they can be used up faster than they are replenished
by rainfall.
Over 90% of the Earth’s water is salt water, which we cannot drink but which
could be desalted and made available for use by humans. Desalination plants
convert salt water of the oceans and seas into drinkable water. However,
desalination plants are costly to install.
At national level, efforts to reduce pollution (especially of the rivers,
lakes and seas) may result in increased water supplies, but also many simple,
everyday practices could add up to conserve water and reduce shortages.
These include:
• turning off taps while brushing teeth;
• fixing leaks in water pipes immediately;
• recycling-reusing water when possible;
• minimising use of water in the garden;
• not having over-long showers;
• using a bucked instead of a hose to wash cars;
• being mindful of how valuable a commodity water is.
Pollution
Figure 7.4 Chemical pollution from an
oil refinery.
66
Pollution is the contamination of land, water or air by the discharge of harmful
substances (figure 7.4). Humans are constantly polluting the environment.
Tables 7.1, 7.2 and 7.3 describe the origin of these pollutants and the effects on
the environment and on humans.
7 • The Effects of Human Activity on the Environment
Pollutants
Origin
non-biodegradable
waste
Careful removal of
household and industry Harmful substances
can accumulate on land toxic materials from
( e.g. mercury, lead,
and enter air and water, waste to be disposed.
plastics, cyanide)
where they can poison Replacement with nontoxic alternatives where
living organisms.
possible.
Insecticides and
herbicides
agriculture (e.g. DDT)
Table 7.1
biological control ❯
Effects
Control of pollutant
Accumulate in
organisms through food
chains, even killing top
consumers. May cause
mutation. May cause
eutrophication in rivers
and lakes. May upset
the balance of food
chains.
Replacement with
less toxic alternatives.
Replacement with
biological control.
Land pollution.
Biological control is the use of living organisms to control pests, often in
horticulture and agriculture. Introducing a predator or parasite of a pest can
greatly reduce the population size of the pest to the point where the level
of damage is economically acceptable. However, great care must be taken to
make sure the introduced predator or parasite does not become a pest itself by
damaging populations of other organisms in that environment as happened
when the cane toad was introduced to Australia.
Pollutants
Origin
Effects
oil
spilled from
tankers or offshore rigs
Forms ‘slicks’ on the sea which Legislation to prevent cleaning
blocks oxygen and light, killing of tankers near land, and
marine life. Stops birds flying reduce the risk of loss from
and feeding. Ruins beaches. Is rigs. Spills can be localised
toxic to some marine life. Clogs with floating booms and
dispersed with detergent. The
respiratory systems of fish.
dispersed material may still be
harmful to marine life.
hot water
power stations
Changes the temperature of
the habitat which results in
death or migration of marine
life, especially death of coral.
Some organisms may flourish.
Cool the water before it is
released into the environment.
(Some of this heat can be used
to heat or power local homes.)
Bacteria multiply and use
up oxygen. Can lead to
eutrophication of water. Can
cause disease.
Treat all sewage to remove the
biological risk before release
into the environment.
organic waste untreated
sewage from
housing, ships,
farms
mineral
salts (e.g.
phosphate,
sulfate and
nitrate ions)
detergents,
fertilisers in
sewage and
water from
housing and
farms.
Control of pollutant
Use detergents which contain
Eutrophication as increased
minimal amounts of these
nutrients in water encourage
algal growth. Light is blocked substances. Use fertilisers in
because the growth is so thick. controlled amounts, only as
needed, to reduce leaching into
Algae die resulting in rapid
growth of anaerobic bacteria. water systems.
Oxygen used up so other
organisms die.
(continued)
67
Living Organisms in the Environment
Pollutants
Origin
Effects
toxic
chemicals
(e.g. organic
mercury
acid wastes,
heavy-metal
products)
industrial plants May be toxic to aquatic
organisms. Concentrated in
organisms as they move up
food chains. May change
the behaviour of aquatic
organisms.
Control of pollutant
Screen all wastes from
industrial processes to remove
toxic materials before release
to environment.
Table 7.2 Water pollution.
Pollutants
Origin
Effects
carbon dioxide burning fossil Builds up in the atmosphere
fuels, increased trapping heat which could lead
to global warming.
human
population
(deforestation
increases
problem)
car exhaust
Control CO by use of more
Carbon monoxide combines
more readily with haemoglobin efficient engines and catalytic
converters. Use lead-free fuel.
than oxygen does. Causes
headaches, unconsciousness,
death. Lead may cause mental
retardation.
smoke
burning fossil
fuels
Combines with fog to form
smog. Particles cause
respiratory disorders, increase
the risk of lung cancer.
Figure 7.5 This vehicle is run on biogas.
sewage ❯
biochemical oxygen demand ❯
68
For energy, use renewable
energy resources. For
vehicles, use mass transport
where possible, use biogas
replacements for fossil fuel.
carbon
monoxide
(CO), lead
sulfur dioxide burning fossil
fuels
Biogas is a fuel made from the fermentation of
waste plant products, such as sugar cane after
sugar extraction. In Brazil and parts of the US,
this is now being used to fuel vehicles instead
of gasoline (figure 7.5).
Control of pollutant
Cleaning of waste gases
from industrial processes can
remove these and prevent their
release into the environment.
Dissolves in rain water forming Cleaning (‘scrubbing’) of waste
acid rain. Affects plant growth, gases from industrial processes
can remove sulfur dioxide and
damages leaves, corrodes
surfaces of rocks and buildings, prevent its release into the
environment.
causes the death of fish and
plant life. Crop yields may be
reduced. Aggravates asthma.
Table 7.3 Air pollution.
Sewage treatment
Sewage consists of liquid and solid waste from urine and faeces, water from
domestic use and industrial waste.
When raw sewage is discharged into rivers and sea, bacteria in the water
decompose the organic matter in the sewage. These aerobic bacteria multiply
rapidly and need large amounts of oxygen dissolved in the water. This is called
biochemical oxygen demand (BOD). The release of nutrients by the bacteria
into the water encourages the rapid growth of algae. The algae add oxygen
to the environment as they photosynthesise. But they are a problem because
they blanket the surface of the water and restrict the entry of oxygen from the
atmosphere. They also restrict the entry of light and this causes submerged
plants to die thus adding to the organic matter available to the bacteria. When
all this takes too much oxygen from the water, other aerobic organisms like
7 • The Effects of Human Activity on the Environment
eutrophication ❯
domestic sewage
plants and fish cannot respire and so die. The decomposition of the dead
material and sewage by anaerobic bacteria continues.
This process, whereby large amounts of nutrients are added to a water
system and lead to the death of many of the organisms living in it, is known
as eutrophication.
To prevent pollution of water, raw sewage must be treated to remove solid
waste and other harmful substances, and decomposed aerobically before it is
disposed of into the rivers and the sea. Most sewage works use a biological
method of sewage treatment (figure 7.6).
water drainage from streets
Grit tank
Large tanks where grit, stones
and other heavy objects
sink to the bottom.
Coarse screen
Has wire nets to strain
out solid litter.
Aeration tank
Air is bubbled through the liquor
to encourage growth of aerobic
bacteria. These bacteria
decompose organic waste into
harmless substances and
carbon dioxide.
industrial sewage
liquid sewage
Primary sedimentation tank
Lighter matter such as faeces
settles to form sludge.
Sludge digester
Sludge undergoes anaerobic
treatment to form methane gas
(biogas) which is a good fuel.
Residual matter is dried
and used as a fertiliser.
Biological filter
The liquor is sprinkled onto large
tanks from a rotating arm. The
liquid sewage percolates down
through a bed of stones covered
with a film of aerobic microbes
(bacteria and protozoa) which
digest the organic matter
in the sewage.
Effluent
• Discharged into the river
and sea.
• Used to water plants.
• Used for flushing toilets.
Figure 7.6 The sewage treatment process.
Deforestation
Figure 7.7
Forest clearance.
Every week, at least 1 million acres of forest are cleared or degraded
worldwide. Although a forest is a renewable resource, removal at that rate is
much greater than the rate at which the trees can be replaced. Forests are not
being managed and maintained, but are being exploited in a destructive way
(figure 7.7). Rainforests are being cleared for farming, timber, mining, largescale cattle ranching, housing and industry.
When a small area is cleared, the forest recovers quite quickly, but when
large areas are cleared, there are many consequences on the environment.
• Soil erosion – Deforested slopes encourage surface run-off during
rainfall. When the forest is in place, it intercepts the rain water and lets
it trickle slowly to the soil. When the canopy of trees and smaller plants
is removed, rain falls directly on the soil causing erosion of the topsoil as
it runs off the surface of the land. The soil below is not as fertile, so will
69
Living Organisms in the Environment
•
•
•
not be able to sustain growth as well as the lost topsoil. The land can be
permanently damaged.
Soil degradation – If the land is cleared for agriculture, the soil becomes
infertile due to the removal of minerals by the crops. Leaching also occurs
which removes minerals that would otherwise have been taken in by
the trees.
Increased flooding – The increased run-off takes silt with it which blocks
rivers and causes flooding in low-lying areas. Flash floods also occur because
of the increased run-off.
Species destruction – The plants removed may become extinct. The
habitats of many organisms are destroyed, and the food chains that
dependent on those plants will break down Many organisms, plants and
animals, may become extinct.
Destruction of natural resources – Many crop plants originated as
rainforest species (cocoa, banana, rubber). Many forest plants also produce
medicinal drugs. Disease-resistant varieties of plants are usually found in
the wild. If we destroy these forests we may lose these resources before we
have even learned about them.
ITQ3
(i) What is deforestation?
(ii) Why do people practise
deforestation?
(iii) What are two consequences of
deforestation?
•
ITQ4
Look back through this chapter
and make a list of the results of
industrialisation.
Industrialisation
ITQ5
For each item on your answer to ITQ4,
give an example of how humans are
attempting to reduce the impact of
industrialisation on the environment.
ITQ6
(i) What is industrialisation?
(ii) Discuss one advantage and one
disadvantage of industrialisation.
Industrialisation is a sign of human success. The word industry is used to cover
all forms of economic activity: primary (farming, fishing, mining and forestry);
secondary (manufacturing and construction); tertiary (back-up services such as
administration, retailing and transport); and quaternary (high technology and
information services).
Industry requires a workforce and thus provides jobs for people who can
enjoy a ‘good’ standard of living. It generates income for the country which
can be spent on improving education, healthcare and public utilities for
its people.
There is, however, a price to be paid for industrialisation. If not managed
properly, it can lead to serious environmental consequences (discussed above)
including pollution of land, air and water, water shortages and deforestation.
Industrialisation is two-sided. On one side, we see successful humans
harnessing resources and living a comfortable life. On the other side, we see
widespread destruction of the environment. The Earth is our only home and
we should take care of it.
Impact of human activities on marine and
wetland environments
Water is important to us. We love to go to the beach, snorkel, lie by the river,
fish and do many other activities which involve water. On the physiological
side, over 75% of the human body is water. We cannot exist without water
and die faster from dehydration than from starvation. Other living organisms
depend on water as much as, or even more than, we do. Our planet is able to
sustain life because of the presence of water.
Yet many human activities lead to a hotter and drier Earth, and to pollution
and the destruction of aquatic environments. Belwo are examples of the
specific effects of human activity in marine environments in the Caribbean.
70
7 • The Effects of Human Activity on the Environment
ITQ7
Humans do not live in water, yet their
activities and practices have affected
the marine environment. Describe two
effects on the marine environment that
humans have had, and how they were
brought about.
ITQ8
Why are coral reefs important to
Caribbean people?
ITQ9
Discuss two reasons why mangrove
swamps should be conserved and
protected.
Some effects of human activities in the
Caribbean
• Destruction of mangrove swamps – Using the swamps as dumping
grounds or for building rice farms or other development, leads to many
problems, such as:
– nursery grounds for all kinds of fish are affected;
– pollution of water with toxic materials, or those which cause
eutrophication;
– pollution of water with human and animal waste can spread diseases like
cholera and diarrhoea;
– breeding grounds and nesting and roosting grounds for birds are reduced;
– the natural habitat of many invertebrates, like oysters and crabs, is
destroyed;
– money generated from eco-tourism (tourists visiting the area to see the
wildlife) is reduced;
– money generated from fishing crabs, oysters and other local fish is
reduced.
• Over-fishing – In the seas of the Carinbbean this is a serious problem.
Much money is spent on catching fish and too little on managing and
producing them.
• Destruction of coral reefs – This occurs by:
– collecting coral for sale;
– dynamiting for fish;
– pollution with raw sewage, garbage, industrial and kitchen waste;
– pollution with hot water which kills the coral organisms;
– pollution with insecticides and fertilisers;
– smothering with silt from soil erosion;
– smothering with red mud bauxite waste, dust and cement from
construction;
– removal to allow the building of harbours, channels, etc.;
– damage from anchors;
– damage by visitors to the reef.
• Damage to wetland environments – this occurs by:
– allocation for rice and other farming;
– fertilisers and other dangerous chemicals in the water;
– noise pollution from use of heavy rice equipment;
– damage by visitors to the wetlands;
– pollution from visitors;
– hunting of animals;
– noise pollution from hunting;
– collection of animals for sale;
– squatters or development of housing settlements;
– over-fishing.
71
Living Organisms in the Environment
The coral reef is a natural resource important to the Caribbean islands and, as
such, needs to be conserved. Damage such as that shown in figure 7.8 is to be
avoided if at all possible.
Figure 7.8
Coral reef smothered by land run-off.
Impact of increase in greenhouse gases
Global climate change has already had observable effects on the environment.
Effects that scientists have predicted are now occurring. These changes are
largely due to an increase in the levels of greenhouse gases; they include:
• increase in Earth’s average temperature;
• changes in the pattern and amount of rainfall;
• reduction in ice and snow cover;
• rise in sea level;
• increase in the acidity of the oceans.
Conservation and restoration of the
environment
conservation ❯
sustainable ❯
Since humans are responsible for widespread destruction of the environment
on the Earth, they should take responsibility for the widespread restoration
and conservation needed to ‘heal’ this damage to our planet. We need to
manage the environments that we live in and the resources that we use in a
sustainable way. That means using them in a way that they are not damaged
and depleted, so they are available for future generations. There are many
ways that this can be done.
Reduce pollution
• Replace fossil fuels with alternative, non-polluting energy sources like solar
energy, wind energy, wave energy, biogas and hydroelectricity.
• Treat all sewage.
• Use unleaded petrol and catalytic converters on vehicles to reduce all
emissions of pollutants in exhaust gases.
• Replace harmful insecticides and herbicides with biological control or
biodegradable insecticides.
72
7 • The Effects of Human Activity on the Environment
• Use cleaners of all exhaust gases from industry.
• Improve effluent release standards of industries to purify, treat and reduce
effluent release.
• Use recyclable materials at home and in industry; for example, paper and
biodegradable plastics rather than plastics, which are not biodegradable.
• Ban the disposal of garbage in rivers, swamps and seas.
Conserve natural resources
•
•
•
•
Recycle resources such as glass, metal and paper.
Use alternative energy sources.
Replace renewable resources, such as forests.
Manage food species, plants and animals, in a sustainable way by imposing
closed seasons and strict restrictions on how heavily populations are cropped.
• Limit deforestation to a rate at which forest recovery can be maintained.
Protect endangered species
• Ban the killing of species in danger of extinction, such as turtles and the
West Indian manatee.
• Set up breeding programmes for species in danger of extinction.
• Set up National Parks to provide areas for species to live and breed
undisturbed by human activity.
Conserve soil
• Replant trees immediately after harvesting to prevent run-off when it rains.
• Use crop rotation to maintain a balance of soil nutrients; different crops
remove different minerals from the soil.
• Use natural rather than artificial fertilisers to preserve soil structure, but
only as needed so as to prevent eutrophication of nearby water sources.
• Use terracing and contour ploughing around hillsides to prevent erosion by
soil run-off when it rains.
• Prevent over-grazing by animals like cows and sheep that remove plants
and their roots which hold the soil when it rains and prevent erosion.
Preserve clean water
• Prevent eutrophication by treating sewage properly and using only as much
fertiliser as is needed on farm land.
Improve land used in mining
• Replace vegetation as soon as possible after mining has finished.
• Fill the mined areas with soil and use for farming, housing or industry, so
that new areas do not need to be used.
Earth is home not only for humans, but for millions of species of plants and
animals. The Earth can successfully ‘carry’ all these organisms when there is
balance in population size and the natural cycles can take place efficiently. It is
our responsibility to maintain that balance.
73
Living Organisms in the Environment
Chapter summary
• Over-population of the planet Earth by humans has led to many negative effects on
the environment.
• Each year over 100 million people are added to the Earth’s population.
• Humans are constantly polluting the land, water and air of the environment.
• Deforestation is the removal of trees. This is done for farming, timber, large-scale
cattle ranching, mining and industries.
• Deforestation has many negative effects on the environment.
• Industrialisation, though a sign of human success, has many serious consequences
on the environment.
• The marine and wetland environments have also suffered from human activity.
• Humans have devised many ways to conserve and restore the environment.
Answers to ITQs
ITQ1 (i) They have inhabited most places on Earth and continue to colonise
new areas.
(ii) They have been able to overcome many natural diseases.
(iii) They mass-produce food, and so do not have to search for food each day.
ITQ2 (i) Agriculture – mass cultivation of plants that can be used as food.
(ii) Livestock production – mass production of animals (cows, chickens) and
their products, (eggs, milk) that can be used as food.
(iii) Use of technology to intensify production to increase the yield (e.g. the
use of artificial selection) and, in the future, possibly increasing use of genetic
engineering to speed this up.
ITQ3 (i) Deforestation is the removal of a large number of trees from an
area.
(ii)
• For quarrying – to obtain gravel, soil, sand, etc. for building.
• To clear land to build homes.
• To provide lumber for housing, furniture, etc.
• To plant crops or for ranching.
(iii) Any of the consequences mentioned on pages 69–70 would be
approrpiate.
ITQ4 Your list should include examples for each of the following: pollution
of land, air and water, water shortages and deforestation (see tables 7.1–7.3).
ITQ5 Your answer should include examples for each of the following:
control of pollution of land, air and water, water shortages and deforestation
(see tables 7.1–7.3).
ITQ6 (i) Industrialisation is the spread of industries. As a country develops,
it becomes more industrialised.
(ii) As more industries develop, fewer foreign products are imported, so more
money is generated by the country and more jobs are available. This is the
major advantage of industrialisation.
A major disadvantage is that as industrialisation increases, damage to the
environment is more likely.
ITQ7 (i) Hot water from power stations is poured into the sea, changing the
temperature and thus the environment of marine life. Plants and animals may
not be able to adapt fast enough and are killed.
(ii) Oil spills from tankers form a layer of oil on the surface covering many
miles of ocean. This can have disastrous effects on the marine environment as
shown in table 7.2.
ITQ8 Coral reefs have many important functions.
74
7 • The Effects of Human Activity on the Environment
• As a tourist attraction – A lot of money is generated from tourists who wish
to visit the area to see the reefs.
• As a source of job opportunities in tourism and conservation.
• In preserving biodiversity – Many different species live in the reefs.
• As a spawning ground – Many species of fish breed in coral reef areas,
including fish that we use for food.
• In protecting the coastline from erosion – The reefs absorb bursts of wave
energy, especially during a storm.
• As a sensitive indicator of global environmental changes – Corals are very
sensitive to environmental conditions and any change results in a change in
the population of corals.
ITQ9 Mangrove swamps are spawning grounds for large fish and ensure the
continuation of many fish species.
Mangrove swamps support biodiversity: there are many species that live in
that one ecosystem.
Examination-style questions
1
(i)
Humans are constantly looking for and occupying new space.
(a) List some reasons why humans needs new space constantly.
(b) Suggest three consequences of occupying new space constantly.
(c) Discuss the greenhouse effect with particular reference to the over-population of
humans.
(ii) (a) What is a pollutant? Give some examples.
(b) Account for the presence of lead as a pollutant in the atmosphere.
(c) How can lead pollution be controlled?
(d) Describe the causes and effects of eutrophication.
(iii) (a) List four effects of sulfur dioxide pollution.
(b) Account for the presence of chlorofluorocarbons in the atmosphere.
(c) What are some of the effects of the build-up of these pollutants?
2
(i) What is deforestation and why is it practised?
(ii) State some consequences of deforestation.
(iii) State some consequences of industrialisation.
(iv) Why are conservation efforts so important?
75
Living Organisms in the Environment
76
Section B:
Life Processes and
Disease
8
By the end of this
chapter you should
be able to:
Cells
draw diagrams to show the structure of typical plant and animal cells
understand the functions of the cell wall, cell membrane, cytoplasm,
mitochondrion, chloroplast, nucleus and vacuole
compare plant and animal cells
understand that most microbes are unicellular
understand why specialisation is important in multicellular organisms
understand how some substances move into and out of cells
plant
animal
microbes – bacteria,
Amoeba
cell – basic
unit of life
microscope
tissue
calculating size
of cells
organ
6PTHNLZLLUPZ[PTLZIPNNLY
[OHU[OLHJ[\HSZWLJPTLU
system
movement into
and out of cell
diffusion
osmosis
5SPNO[WHZZLZ[OYV\NOL`LWPLJLSLUZ
THNUPMPJH[PVU_
organism
Why we need microscopes
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THNUPMPJH[PVU_
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Figure 8.1
78
Diagram of a light microscope showing how it works.
The cell is the basic unit of life. A cell cannot be
viewed by the naked eye since it is too small. It
can only be seen with a microscope. Cells are thus
described as being microscopic.
A microscope is used to produce a magnified
image of an object. There are different kinds of
microscopes, for example light and electron. When
looking through the microscope at a piece of tissue,
separate cells can be distinguished which would
not have been seen with the naked eye. How much
you can see with a microscope depends on how
powerful its magnification is. A light microscope
typically magnifies between 10 and 400 times real
size (figure 8.1). An electron microscope is more
powerful and can magnify tens of thousands of times
actual size.
8 • Cells
Calculating the size of cells
The actual size of an object in a photograph can easily be calculated from the
image and the magnification given. If the length of the object in the photo is
measured as Z, and the magnification is given as =100, that means the object is
100 times larger than in real life. So, the actual size of the object is Z ÷ 100.
ITQ1
What is the purpose of a microscope?
Plant and animal cells
organelles ❯
Syllabus reference C1.1
Plant and animal cells have the same basic structure but each has its own
characteristics that make it typically plant or typically animal.
The structures found within a cell are called the cell organelles. They
have different functions and, as they work together, they keep the cell (and
therefore the organism) alive. Figure 8.2 shows diagrams of typical plant and
animal cells; figure 8.3 shows plant cell. Table 8.1 describes the functions of
some cell organelles.
cell wall
cell membrane
cytoplasm
nucleus contains
chromosomes which
carry genetic information
vacuole
chloroplast
mitochondrion
starch grain
glycogen
granule
Plant
cell
Animal
cell
Figure 8.2 Typical plant and animal cells.
ITQ 2
Measure the width of the largest
chloroplast in the cell in figure 8.3,
and calculate its actual size using the
magnification given.
ITQ 3
Make a list of (i) all the organelles
which are found in both plant and
animal cells and (ii) organelles which
are only found in plant cells.
Figure 8.3 Photomicrograph of a plant cell
(magnification ×5000). Compare this with
the plant cell drawn in figure 8.2.
Organelle
Function
cell wall
prevents bursting of a plant cell and gives it a fixed shape
cell membrane
a selectively permeable barrier which controls exchange between the cell
and its environment
cytoplasm
site of many of the chemical reactions of life
nucleus
controls the activities of the cell, contains chromosomes
chromosome
carries genetic information in the form of DNA
mitochondrion
site of energy production
permanent vacuole important during exchange of water and minerals, and stores various
substances including waste products
chloroplast
where photosynthesis takes place
Table 8.1 The functions of some cell organelles. (Organelles shown in green are only found in
plant cells.)
79
Life Processes and Disease
Table 8.2 describes differences between plant and animal cells in more detail.
ITQ 4
Name two organelles found in a plant
cell but not in an animal cell. What is
the importance of these two organelles
to a plant cell?
ITQ 5
(i) Distinguish between cell wall and
cell membrane.
(ii) Distinguish between
mitochondrion and chloroplast.
Plant cell
Animal cell
Cytoplasm is surrounded by a cell membrane as Cytoplasm is surrounded by a cell membrane
well as a cell wall.
only.
Chloroplasts are present.
Chloroplasts are absent.
Carbohydrates are stored as starch.
Carbohydrates are stored as glycogen.
A large, permanent vacuole is present in most
plant cells. It has a definite, fixed shape.
Many small, temporary vacuoles are present at
a time. These have no fixed shape.
Cytoplasm is pushed to the edges of the cell by Cytoplasm is present throughout the cell.
the vacuole, so it is normally confined to a thin
layer.
Table 8.2 The main differences between plant and animal cells.
Unicellular microbes
Microbes are microscopic organisms (microorganisms) that cannot be seen
by the naked eye, only by using a microscope. Most, but not all, are singlecelled organisms, and are so tiny that millions could fit in the eye of a needle.
Microbes are everywhere, in the air we breathe, the ground we walk on and in
the food we eat. They are even inside us. They include:
• viruses
• bacteria
• protozoa.
Viruses
These are very small and can only be seen with an electron microscope. They
are not made of cells and are sometimes referred to as virus particles or virions.
They cannot be killed by antibiotics such as penicillin. Examples of diseases
they cause include influenza, common cold, measles, mumps, german measles
(Rubella), smallpox, chickenpox, HIV (can lead to AIDS) and rabies.
Bacteria
glycogen granules,
lipid droplets
mesosome*
cell surface membrane
small ribosomes
cell wall
flagelium*
plasmids*
capsule or
slime layer*
* = not pesent in all bacteria
Figure 8.4
80
Diagram of a bacterium.
photosynthetic
membranes*
circular DNA
Bacteria are single-celled
organisms (Figure 8.4). Many
of us refer to them as ‘germs’,
but some are very useful. For
example, they decompose dead
organisms and digest cellulose.
Examples of diseases they cause
include cholera, tuberculosis,
septicaemia (blood poisoning),
pneumonia and gastroenteritis.
8 • Cells
Protozoa
These are generally single-celled organisms (figure 8.5). Amoeba is very
common and can be found in back-yard ponds and drains. Examples of
diseases they cause include malaria, sleeping sickness and dysentery
amoeba
food particle
pseudopodium
peeudopodium (false foot) is sent out
in the direction of the food particle
the food particle is engulfed
food vacuole
food vacuole is formed inside the amoeba
enzymes are secreted into the food
vacuole and the food is digested
nutrients are absorbed into the
cytoplasm of the amoeba
unwanted (undigested) substances are
released from the amoeba into the
enviroment
Figure 8.5
Movement and feeding in Amoeba.
Cell specialisation in multicellular
organisms
Organisms can be described as unicellular or multicellular. Unicellular
organisms like Amoeba (animal) and Chlorella (plant) are just one cell in size.
Multicellular organisms, like all the larger animals and plants are made up of
many (sometimes millions) of cells.
The cells of unicellular organisms (e.g. Amoeba and bacteria) are
independent but are still able to carry out all characteristics of life. Multicellular
organisms, however, are made up of millions of cells. These cells work together
and are often dependent on each other to carry out all the characteristics
of life.
81
Life Processes and Disease
Figure 8.6
In multicellular organisms, each
cell has the same basic structure,
but there are variations in the
U\JSL\Z
design. Within a single organism,
such as a human, there are great
differences between the cells.
[YHUZTP[Z
Each type of cell is specialised to
ULY]LW\SZLZ
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carry out a particular function
well. For example, a muscle cell
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is concerned with contraction of
the muscle, while a nerve cell
is specialised to transmit nerve
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impulses (figure 8.6).
ULY]LJLSS
In a multicellular organism,
cells are arranged in groups to
tissue ❯ form tissues. A tissue is a structure
made up of many similar or
identical cells which are adapted
to perform one specific function.
Skeletal muscle cells
Muscle cells make up muscle tissue
Nerve cell
make up a muscle fibre
and all these cells are concerned
with the muscle function
of contraction.
Several different kinds of
Specialised cells that are found in nerves and muscle.
tissue may be grouped to form an
organ ❯
organ. For example, intestines
contain epithelial tissue and muscle tissue and a blood supply (figure 8.7). In
system ❯ animals, organs form parts of even larger functional units called systems. The
digestive system is made up of several organs, including the stomach, intestines
and liver.
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Figure 8.8
*,33:
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Figure 8.7 Tissues in the intestine.
82
ZWVUN`
TLZVWO`SSJLSSZ
Grouping of cells to form tissues in the organ of a leaf.
Cells in plants are also grouped into tissues, and tissues grouped into organs
(figure 8.8). Table 8.3 shows examples of tissues, organs and systems that are
found in plants and animals.
8 • Cells
Structure
Examples in plants
Examples in animals
tissue
palisade mesophyll (chapter 9)
nerve tissue (chapter 18)
phloem tissue (chapter 14)
muscle tissue (chapter 17)
xylem tissue (chapter 14)
CHAPTERS 9, 10, 12, 14, 17, 18
organ
leaf, root
stomach, lung, brain, eye
system
(not organised into systems)
digestive system (chapter 10)
respiratory system (chapter 12)
nervous system (chapter 18)
Table 8.3
ITQ6
Give an example of each of the
following: cell, tissue, organ, system.
ITQ7
Distinguish between unicellular and
multicellular organisms, giving two
examples of each.
Examples of tissues, organs and systems in plants and animals.
A healthy organism is made up of all these parts working efficiently together,
enabling it to do many things at the same time, such as use its energy source
and make the energy available for movement, reproduction, growth, response
and excretion. A total breakdown in the normal functioning of any one of
these systems can lead to the death of the organism, such as a heart attack
when the circulatory system breaks down.
Most animals are either predator or prey in food chains. A healthy organism
has all its systems functioning efficiently and so is able to survive in the
environment or wild. Unhealthy organisms may be unable to capture food or
fall prey to predators more easily. Survival is for the fittest, meaning that an
organism with all its systems functioning efficiently and continuously has an
advantage for survival – an advantage for life.
Movement of substances into and out of
cells
secretion ❯
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SPRLOVYTVULZHUKLUa`TLZ
All kinds of reactions take place within a cell. The organelles within a cell
require many different substances to carry out these reactions. Waste products
are formed during these reactions and must be removed. The substances,
needed and produced, must pass into and out of the cell.
There is thus a constant movement of substances into and out of cells.
• Substances needed by the cell, like glucose and oxygen, must pass into the
cell.
• Substances produced by the cell must pass out of the cell. These may be
waste products, like carbon dioxide and urea, or substances needed by
another cell, like enzymes. This is called secretion.
Z\IZ[HUJLZTV]PUN
PU[VJLSS
^HZ[LWYVK\J[ZTV]PUN
V\[VMJLSS
Figure 8. 9
Substances can be taken in within small
vesicles made from the cell membrane.
Amoeba takes its food in this way.
Substances can also be released from cells
when vesicles containing the substance
join with the cell membrane (figure 8.9).
Hormones are released from cells like this.
Substances may also enter and leave
cells as individual molecules. They do this
by various mechanisms including diffusion.
Water enters and leaves cells by osmosis.
Diagram showing substances moving into and out of a cell in small vesicles.
83
Life Processes and Disease
Movement by diffusion
diffusion ❯
concentration gradient ❯
Practical activity
SBA 8.1: Diffusion in a solution,
page 341
permeable ❯
Diffusion is the movement of molecules from a region of high concentration
of those molecules to a region of lower concentration of those molecules.
Diffusion can happen in gases and in liquids.
A diffusion gradient or concentration gradient occurs when there is
a difference in the number of molecules, or the concentration of molecules
between the two regions. For example, when a drop of dye is added to
water, the dye molecules move around and between the water molecules
and eventually are spread evenly, even when not stirred. In other words, the
dye molecules move from where they are plentiful to where they are not so
plentiful. We say these diffuse (figure 8.10).
Substances can also diffuse across membranes if the concentrations are
different on both sides and the membrane is permeable to those molecules
(figure 8.11).
Figure 8.10
solution.
Over time, the dye molecules diffuse so they are evenly spread throughout the
molecules at
a higher
concentration
more molecules move from
left to right than from right to left
Figure 8.11
molecules at
the same
concentration
on both sides
molecules at
a lower
concentration
no net movement of molecules
Diffusion can occur across permeable cell membranes.
Some examples of diffusion in the human body
• After a meal, the end-products of digestion are at a high concentration
in the gut. They diffuse down their concentration gradient into the blood
where they are at a lower concentration (figure 8.12).
84
8 • Cells
blood rich
in the endproducts of
digestion
blood capillary
ileum of the gut
end-products of digestion
at a high concentration
Figure 8.12
blood rich
in oxygen (O2)
oxygen at a
higher concentration
in the alveolus
O2
O2
O2
O2
O2
O2
O2
CO2
O2
CO2
Diffusion of small food molecules from gut to blood.
• Diffusion occurs in the lungs (figure 8.13). Carbon dioxide diffuses from
the blood where it is at high concentration into the lungs where its
concentration is lower. Oxygen diffuses in the other direction because it has
a higher concentration in the lungs and a lower concentration in the blood.
• When the blood gets near the cells, the oxygen concentration in the blood is
higher than in the cells. The blood came from the lungs where it picked up
oxygen. The oxygen concentration in the cell is low, since the oxygen that
was in the cell was used for respiration. The oxygen in the blood diffuses
into the cell, where it can be used for energy production during respiration
(figure 8.14).
JVUJLU[YH[PVUNYHKPLU[
carbon dioxide
at a higher
concentration
in the blood
OPNOLYJVUJLU[YH[PVU
VMNS\JVZLPU[OLN\[
blood capillary
SV^LYJVUJLU[YH[PVU
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VULJLSS[OPJR
Figure 8.13 Diffusion of gases between
the lungs and the blood.
ISVVKJHWPSSHY`
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ISVVKJHWPSSHY`^P[OISVVK
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6
6
6
6
6
6
6
6
6
6
6
Figure 8.14
6
6
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ILPUJYLHZLKPU
THU`^H`Z¶TVYL
Z\YMHJLHYLHTVYLKPMM\ZPVU
IVK`JLSSZ¶
V_`NLU\ZLK\W
K\YPUNYLZWPYH[PVU
HUKP[ZJVUJLU[YH[PVU
PZSV^
Diffusion of oxygen from blood into cells.
Figure 8.15 Adaptations that help to speed up the rate of diffusion.
• In the cells, carbon dioxide builds up as a waste product of respiration. It is
at a higher concentration than in the blood. Thus it diffuses out of the cell
and into the blood.
• Other wastes made by cells, such as ammonia, are at a higher concentration
in the cell than in the blood. They also diffuse out of the cell to the blood
and are taken away and expelled from the body.
Diffusion is a very slow process unless there is a large concentration gradient
over a short distance. Tissues like the lungs and small intestine are especially
adapted to maximise the rate (figure 8.15). Adaptations include:
• keeping the difference between the concentration on each side as high as
possible (maintaining a steep concentration gradient);
85
Life Processes and Disease
• having a large surface area to volume ratio so that molecules have as large a
surface area of cells as possible to diffuse through;
• being very thin and thus minimising the distance over which diffusion must
take place.
Movement by osmosis
osmosis ❯
Practical activity
SBA 8.2: Some effects of osmosis,
page 342
Osmosis is a special kind of diffusion. It is the diffusion of water molecules
across a selectively permeable membrane. Cell membranes are all selectively
permeable membranes. ‘Selectively permeable’ means that water and some
substances can pass through the membrane but other substances do not.
Osmosis in plant cells
isotonic ❯
net flow ❯
When a plant cell is put into a solution which has the same concentration as
the cell contents (isotonic), some water molecules will move into the cell
through the cell membrane and some will move out. There is no concentration
gradient so the movements each way are the same and balance each other out.
We say there is no net movement, or net flow, or water (figure 8.16).
‹JVUJLU[YH[PVUVMZVS\[PVU
V\[ZPKLPZV[VUPJ^P[O
ZHTLHZPUZPKLJLSS
‹UVUL[MSV^VM^H[LY
turgid cell
‹V\[ZPKLO`WV[VUPJ[V
SLZZJVUJLU[YH[LK
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‹UL[MSV^VM^H[LY
intoJLSS
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flaccid cell
‹V\[ZPKLO`WLY[VUPJ[V
TVYLJVUJLU[YH[LK
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of[OLJLSS
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H^H`MYVTJLSS^HSS
‹JLSSPZflaccid
Figure 8.16 The effect of different concentrations of solution on a plant cell.
hypotonic ❯
turgid ❯
hypertonic ❯
86
When a plant cell is put into a solution that is less concentrated (hypotonic)
than the cell contents, there is a greater concentration of water molecules
outside than inside. Some water molecules move out of the cell but more move
into the cell, so there is a net flow of water into the cell. The cell becomes full
of water and is described as being turgid.
When a plant cell is put into a solution that is more concentrated
(hypertonic) than the cell contents, there are fewer water molecules outside
than inside. A few water molecules move into the cell but many more move
8 • Cells
flaccid ❯
out of it, so there is a net flow of water out of the cell. The cell loses water and
is described as being flaccid. Flaccid cells are easy to distinguish under the
microscope because the cell membrane and contents pull away from the cell
wall.
Osmosis in animal cells
ITQ8
An animal cell placed in water will
burst. Explain fully why a plant cell will
not burst when placed in water.
An animal cell has no cell wall like a plant cell, so hypotonic and hypertonic
solutions have different effects. In a hypotonic (dilute) solution there is a net
flow of water into the cell. With no strong cell wall to prevent the membrane
from stretching too far, it eventually bursts. In a hypertonic (concentrated)
solution there is a net flow of water out of the cell and the whole cell shrinks
(figure 8.17).
‹JLSSPUPZV[VUPJ
ZVS\[PVU
‹UVUL[TV]LTLU[
VM^H[LY
‹JLSSPUO`WV[VUPJ
ZVS\[PVU
‹UL[MSV^VM^H[LY
PU[VJLSS
‹UVZ[YVUNJLSS^HSS
ZVJLSSI\YZ[Z
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ZVS\[PVU
‹UL[MSV^VM^H[LY
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HUKZOYPURZ
Figure 8.17 The effect of different concentrations of solution on an animal cell.
CHAPTER 16
It is important for cells to be protected from large changes in concentration of
the solutions around them. Animal bodies have complex mechanisms to do
this called osmoregulation and homeostasis (chapter 16).
Chapter summary
• The cell is the basic unit of life.
• A cell contains smaller parts called organelles.
• The nucleus, cell membrane, cytoplasm and mitochondrion are some organelles
found in typical plant and animal cells.
• Plant cells also contain cell walls, chloroplasts and large central vacuoles.
• Most microbes are unicellular.
87
Life Processes and Disease
•
•
•
•
•
•
•
•
•
•
Cells in multicellular organisms are often specialised for a particular function.
A group of specialised cells that have the same function is called a tissue.
An organ is a group of different tissues that work together.
Organs working together make up a system.
Systems coordinate with each other and work together in a living organism.
Many substances can move into and out of a cell through the cell membrane which is
selectively permeable.
Diffusion is the movement of a substance from a high concentration to a low
concentration.
Osmosis is the movement of water across a selectively permeable membrane from a
solution where there is a high concentration of water molecules to a solution where
the concentration of water molecules is lower.
Diffusion and osmosis occur at many places in a living organism.
Different concentrations of solution have different effects on plant and animal cells.
Answers to ITQs
ITQ1 A microscope is an instrument used to produce a magnified image of
an object. Organisms and objects that cannot be seen by the naked eye may be
visible under a microscope.
ITQ2 The measured width of the chloroplast in the photograph is
14 mm (or 14 × 10–3 m). The magnification is ×5000. This means that
the measured size is 5000 times larger than in reality. So the actual size is
(14 ÷ 5000) × 10–3 m = 0.0028 × 10–3 m (or 2.8 × 10–6 m or 2.8 μm).
ITQ3 (i) Plant and animal cells have: cell membrane, nucleus, cytoplasm,
mitochondria, small vacuoles.
(ii) Plant cells have a cell wall*, chloroplasts, large central vacuole. (*Fungal
cells and some bacteria also have cell walls,but these have a completely
different structure from those in plants.)
ITQ4 The plant cell wall has protective and structural functions. It protects
the plant by protecting each plant cell from bursting when the plant takes up
water. It also helps to support stems and leaves of the plant when the cells are
full of water, because plants have no skeleton like many animals.
The chloroplast contains the pigment chlorophyll which collects the light
energy of the Sun. Chloroplasts are the sites of photosynthesis, so animals do
not need them.
The large plant vacuole is important during exchange of water and minerals,
and stores various substances including waste products.
ITQ5
(i) The cell membrane is a partially permeable barrier that controls
the passage of substances into and out of the cell whereas the cell wall provides
support and protection and allows the free passage of water.
(ii) The mitochondrion is the site of respiration during which energy is
released from sugar. All cells have mitocchondria. The chloroplast is the site of
photosynthesis where sugar is made. Chloroplasts are found only in plant cells.
ITQ6 Cell: e.g. muscle cell.
Tissue: any group of one kind of cell working together e.g. muscle cells in
muscle tissue.
Organ: any group of tissues working together e.g. stomach, made up of
secretory tissue, muscle tissue and other tissues; leaf, made of palisade tissue,
xylem tissue.
System: any group of one kind of organs working together e.g. digestive
system, made up of stomach, liver, intestines and other organs.
88
8 • Cells
ITQ7 A unicellular organism is an organism that has only one cell. This
small organism shows all the characteristics of life and lives an independent
life. For example, Amoeba and Chlorella. A multicellular organism is made up
of many cells. These cells work together, and the organism is able to show all
the characteristics of life. For example, a human and a worm (there are many
other examples you could have chosen).
ITQ8 A plant cell has a cellulose cell wall around the cell membrane.
The wall is strong and cannot stretch. When placed in water, the cell will
take up water, but the cell membrane will not burst because the cellulose
cell wall stops it stretching to bursting point. An animal cell does not
have a cellulose cell wall and so can stretch to the point where it bursts.
Examination-style questions
1
(i)
The drawing below was constructed by a biology student after viewing a slide under
the microscope. The drawing made was magnified 2500 times. What is the actual size
of the cell labelled A?
(
(ii) The figure below shows how a section of a root or stem is mounted for microscopic
investigation.
]LY`[OPUZLJ[PVU
KYVWVM^H[LY
Explain why it is necessary to cut a very thin section of the material which is to be
observed under the microscope.
(iii) (a) Name two types of microscope.
(b) Why are cells described as being microscopic?
2
(i) Make labelled drawings of typical plant and animal cells.
(ii) Use a table to compare typical plant and animal cells.
(iii) Give one advantage of being multicellular.
(iv) Name one difference between a tissue and an organ.
(v) Give one named example of:
(a) a tissue;
(b) an organ to be found in:
• an animal;
• a plant.
89
Life Processes and Disease
3
The figure below shows onion rings A, B, C and D before and after immersion in water and
a salt solution.
VUPVUYPUN(
VUPVUYPUN*
VUPVUYPUNILMVYL
PTTLYZPVUPU^H[LY
VUPVUYPUNILMVYL
PTTLYZPVUPUZHS[ZVS\[PVU
VUPVUYPUN)
VUPVUYPUN+
VUPVUYPUNHM[LYPTTLYZPVU
PU^H[LY
(i)
VUPVUYPUNHM[LYPTTLYZPVU
PUZHS[ZVS\[PVU
Copy and complete the table below to show the measurements of the rings.
Onion ring
Outer diameter
Inner diameter
Mean diameter
A
B
C
D
(ii) (a) Using measurements from the table, describe what happened to the onion ring
placed in:
• water;
• salt solution.
(b) Explain fully the results seen in:
• water;
• salt solution.
(iii) (a) What process is taking place?
(b) Give an example of the occurrence of this process in living organisms.
(iv) Describe two examples of diffusion as it occurs in living organisms.
90
9
By the end of this
chapter, you should
be able to:
Photosynthesis
understand the difference between heterotrophic, autotrophic and saprophytic
nutrition
describe photosynthesis in green plants
relate the structure of the leaf of a flowering plant to its function in
photosynthesis
explain how environmental factors affect the rate of photosynthesis
photosynthesis
autotrophic nutrition
inorganic substances
converted to
organic substances
heterotrophic
nutrition – animals
saprophytic
nutrition
leaf structures
limiting factors – light,
temperature, carbon dioxide,
water
conditions
adaptations for
photosynthesis
Practical activity
SBA 9.1: Starch in a green leaf,
page 343
autotroph ❯
Plants are the food supply for animals
The relationship between autotrophs, heterotrophs and sprophytes is shown
in figure 9.1.
AUTOTROPH
‘self-feeders’
e.g. plants that make
their own food
during photosynthesis
HETEROTROPH
feed on other organisms
e.g. consumers that
feed on plants and
other animals
SAPROPHYTE
feed on dead organic
material
e.g. decomposers that
feed on the dead
autotrophs and
heterotrophs
Figure 9.1
Relationships of autotrophs, heterotrophs and saprophytes.
91
Life Processes and Disease
heterotroph ❯
ITQ1
Distinguish between an autotroph and a
heterotroph.
ITQ2
Why must autotrophic nutrition occur
before heterotrophic nutrition?
ITQ3
Why is saprophytic nutrition important?
In the study of food chains we saw that plants are producers and are at the
start of almost all food chains. Animals are consumers and feed on the plants or
on other animals. Plants do not eat, yet they are full of food. They are rich in
carbohydrates, fats and proteins. This is because they are able to manufacture
their own food. We call them autotrophs (self-feeders) because they are able to
make organic substances (glucose) from simple inorganic substances (carbon
dioxide and water). This process is called photosynthesis and requires light
from the Sun to provide the energy needed to carry it out. From glucose, the
plant makes all the other carbohydrates, fats and proteins it needs.
Consumers feed on the organic substances made by the plants. Consumers
are heterotrophs (other or different feeders). Heterotrophic nutrition is
the intake of complex organic substances when animals feed. Autotrophic
nutrition is the intake of simple inorganic substances by plants during
photosynthesis and must occur before heterotrophic nutrition (figure 9.2).
*6
/6
PUVYNHUPJ
Z\IZ[HUJLZ
Autotrophic nutrition
Heterotrophic nutrition
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Z\IZ[HUJLZHUKTHRL
VYNHUPJZ\IZ[HUJLZ
HUPTHSZ[HRLPUVYNHUPJ
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[OL`MLLK
Figure 9.2 Autotrophic nutrition must occur before heterotrophic nutrition can occur. Food chains
start with plants, then animals feed on the plants.
When plants and animals die, saprophytes feed on the dead bodies which
are full of organic substances such as carbohydrates, fats and proteins.
Saprophytes are also called decomposers and they are very important to the
cycling of these materials back to the earth, from where they are then available
to plants again.
Photosynthesis
Practical activity
SBA 9.2: Is light needed for
photosynthesis? page 344
Photosynthesis can be summarised in words or by the simple equation:
light
carbon dioxide + water
glucose + oxygen
chlorophyll
photosynthesis equation ❯
light
6CO2 + 6H2O
C6H12O6 + 6O2
chlorophyll
light-dependent stage ❯
light-independent stage ❯
Chlorophyll is a complex green pigment. At the centre of a chlorophyll
molecule is a single atom of magnesium chemicaly bonded to four atoms of
nitrogen. Without supplies of nitrogen, a plant cannot make chlorophyll and so
cannot photosynthesise successfully.
Experiments show that there are two main stages in photosynthesis
(figure 9.3), namely:
• the light-dependent stage;
• the light-independent stage.
Light-dependent stage
Chloroplasts are organelles seen in green plants cells. They contain the green
pigment chlorophyll which ‘traps’ the light energy from the Sun. The energy
is used to ‘split’ water (H2O) into hydrogen and oxygen. The oxygen is a waste
product and diffuses out of the leaf.
92
9 • Photosynthesis
Light-independent stage
The hydrogen then combines with carbon dioxide (CO2) to make glucose
(C6H12O6). This stage of photosynthesis does not need light and can happen
when it is dark.
KPMM\ZLZV\[VM[OLSLHM
Sun
oxygen
chlorophyll
water
carbon dioxide
KPMM\ZLZPU[V[OLSLHM
hydrogen
SPNO[KLWLUKLU[Z[HNL
glucose
SPNO[PUKLWLUKLU[Z[HNL
Figure 9.3
Light-dependent and light-independent stages of photosynthesis.
The organ specialised for photosynthesis is the leaf. The transverse section
of a leaf reveals many cells, arranged in a manner that is ideally suited for
photosynthesis.
Adaptations of the leaf for photosynthesis
Practical activity
SBA 9.3: Is chlorophyll needed for
photosynthesis? page 345
stomata ❯
palisade cell ❯
Leaves are adapted to carry out
SPNO[
photosynthesis in a number of ways
SPNO[
(figures 9.4 and 9.5).
• They are generally broad and flat with
SPNO[
a large surface area to absorb a lot of
light and carbon dioxide.
• They lie at 90° to the sunlight and are
spaced around the stem to catch as
much light as possible.
Figure 9.4 How leaves catch as much
• The leaves are thin to allow light
sunlight as possible.
and carbon dioxide to reach all cells
rapidly.
• Stomata (small holes) are present in the lower epidermis to allow gases to
get in and out easily. (One hole is a stoma. Stomata is the plural.)
• Air spaces around the cells in the lower half of the leaf allow carbon dioxide
to get to the chloroplasts as quickly as possible.
• Chloroplasts are most numerous in cells in the palisade layer, which is in
the top part of the leaf, closest to the sunlight.
• Xylem vessels transport water to the leaf cells.
• Phloem sieve tubes carry away the food made in the leaf cells to the rest of
the plant.
• A waxy cuticle prevents water loss form both surfaces of the leaf; it is
transparent to let light through.
93
Life Processes and Disease
HWL_
(
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Figure 9.5 A section of a leaf.
Guard cells
CHAPTER 14
94
epidermal
cell
guard cell
(turgid)
A stoma is surrounded by a pair of specialised
epidermal cells called guard cells. The guard
cells vary the size of the opening of the stoma
stoma open
by changing their shape, thus the size of
the stomatal pore is regulated by the guard
cell. The stoma is the route by which water
is lost from the plant during transpiration
(chapter 14), and also by which the gaseous
guard cell
exchange necessary for photosynthesis
(flaccid)
occurs. By controlling stomatal opening and
closing, a plant controls the balance between
stoma
the need to conserve water and the need to
closed
exchange gases.
Stomatal opening varies as a result of
changes in the turgidity of the guard cells
Figure 9.6 The guard cells control the
(figure 9.6)
opening and closing of the stomatal pore.
9 • Photosynthesis
ITQ4
Why do you think that the stomata of
some desert plants close during the
day?
• when they are turgid, the stoma opens;
• when they are flaccid, the stoma closes.
The following observations have been made:
• most stomata open during the day and close at night;
• stomata generally close when a plant suffers water stress, or when
transpiration rate exceeds the rate of water absorption by the roots;
• the stomata of some desert plants close during the day and open at night.
How everything gets to the chloroplast
Practical activity
SBA 9.4: Is carbon dioxide needed for
photosynthesis? page 346
Photosynthesis takes place in the chloroplasts of specialised leaf cells. The
following numbered paragraphs refer to Figure 9.7.
3SPNO[MYVT[OL:\U
ITQ5
Describe how carbon dioxide gas in the
atmosphere gets to a photosynthesising
cell inside a leaf.
4[`WPJHS
WOV[VZ`U[OLZPZPUNJLSS
JOSVYVWO`SSPZWYLZLU[
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^H[LYTV]LZI`VZTVZPZ
MYVTJLSS[VJLSS
2^H[LY[YH]LSZ[V[OL
SLHM]PH[OL_`SLTMYVT
[OLZVPSZ\YYV\UKPUN
[OLYVV[Z
‹*6\ZLK\WK\YPUN
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‹0[ZJVUJLU[YH[PVUPZ
[O\ZSV^PU[OLJLSS
JHYIVUKPV_PKLPU[OL
1 HPYZ\YYV\UKPUN[OLSLHM
‹*6PU[OLHPYZWHJL
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OPNOLY[OHUPU[OLJLSS
‹*6KPMM\ZLZPU[V[OL
JLSSMYVT[OLHPYZWHJL
*6
Figure 9.8 Carbon dioxide diffuses down
its concentration gradient into the leaf.
Figure 9.7 All the requirements for photosynthesis must get to all the photosynthesising cells.
1
[OLSLH]LZHYL[OPU
HUKMSH[HUKSPL
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ITQ6
Look at the tomato plant and describe
four ways in which the plant is adapted
for photosynthesis.
SLH]LZHYL
ZWYLHKHYV\UK
[OLZ[LT
2
Carbon dioxide diffuses from
the surrounding air into
the stomata or pores on the
underside of the leaf. It moves
into the air space surrounding
the mesophyll cells, and then
into the cells themselves. As
the carbon dioxide is used
up during photosynthesis, its
concentration drops. There is
thus a greater concentration of
carbon dioxide outside the cells
than inside and carbon dioxide
diffuses into them (figure 9.8).
Water moves by osmosis from
the soil into the roots of the
plant. It then travels up the
xylem vessel in the stem and
into the leaves. From the
95
Life Processes and Disease
3
4
xylem in the leaf, water moves by osmosis to the palisade cells where it is
used during photosynthesis.
Light rays pass into the leaf from all around, especially from above.
Chloroplasts are found mainly in the palisade cells where the chlorophyll
can easily intercept and trap the light energy.
Within the chloroplasts the light energy splits the water which then reacts
with the carbon dioxide.
Products of photosynthesis
Practical activity
SBA 9.5: Is oxygen produced during
photosynthesis? page 347
‹6_`NLUPZWYVK\JLK
K\YPUNWOV[VZ`U[OLZPZ
‹0[ZJVUJLU[YH[PVUPZ
[OLYLMVYLOPNOPU[OL
JLSS
‹6_`NLUKPMM\ZLZPU[V
[OLHPYZWHJL^OLYLP[Z
JVUJLU[YH[PVUPZSV^LY
‹6_`NLUKPMM\ZLZV\[VM
[OLJLSS[OYV\NOZ[VTH[H
6
Figure 9.9
Oxygen moves out of a leaf.
limiting factor ❯
The glucose produced during photosynthesis is used in several ways.
• It is broken down during respiration to release energy so the plant can carry
out all the processes of life.
• It is converted to starch and stored in the leaf to be used in the night when
the plant is not photosynthesising.
• It is converted to sucrose and transported to other parts of the plant. It can
then be converted to other carbohydrates, lipids and proteins and used for
growth, or it can be converted to starch and stored, as in potatoes.
Oxygen is a waste product of photosynthesis. The cells in the leaf will use some
for respiration, but the rest of the oxygen is not needed by the plant. Inside the
leaf, photosynthesis is taking place and oxygen is being produced. It is thus at
higher concentration inside the leaf than outside. So oxygen diffuses out of the
leaf through the stomata (figure 9.9).
Limiting factors in photosynthesis
Photosynthesis is a chemical reaction, and the rate at which a reaction can
happen depends on how fast the chemicals that are reacting can get together.
In photosynthesis, a plant requires water, carbon dioxide and light. If
any one of these is in short supply, the rate of the reaction will slow down.
For example, a plant may have sufficient carbon dioxide and water, but not
enough light for photosynthesis to take place at its maximum rate. Light is
then said to be the limiting factor, since the rate of photosynthesis is limited
by the amount of light. The reaction will take place at a rate that is limited by
the factor which is at its least favourable value (light, in this example). Water,
light and carbon dioxide may all be limiting factors for photosynthesis at
different times.
The limiting factors which affect photosynthesis are:
• temperature;
• light intensity;
• carbon dioxide concentration;
• availability of water.
Temperature
CHAPTER 10
96
The rate of a reaction increases as temperature increases. With heat, the
molecules move about and come together faster.
Photosynthesis also involves a series of enzyme-catalysed reactions.
Enzymes have an optimum temperature or temperature at which they work
best (chapter 10), so this will also affect the rate of the reaction.
Temperature is often the limiting factor on the rate of photosynthesis in cool
seasons in temperate regions.
9 • Photosynthesis
Carbon dioxide concentration
The concentration of carbon dioxide is relatively low in the atmosphere. So
carbon dioxide is usually the limiting factor when temperature and light levels
are high. Commercial growers who grow their crops in large greenhouses often
pump in extra carbon dioxide to increase the rate of photosynthesis in the
crops (figure 9.10).
Light intensity
The amount of light in the environment varies greatly between night and day.
Light is usually the limiting factor from dusk until dawn (figure 9.10).
9H[LVMWOV[VZ`U[OLZPZ
V
YH[LZSV^ZKV^UZVTLMHJ[VYPZSPTP[PUN[OLYH[L
*6H[ *
NYLH[LY*6JVUJLU[YH[PVUYH[LPUJYLHZLZ
*6H[ V *
ITQ7
Which factor will most likely be limiting
photosynthesis in each of these cases?
(i) Middle of the day after plenty of
rain in Jamaica.
(ii) Cool autumn day in Britain.
(iii) Dry season in Australia.
YH[LVMWOV[VZ`U[OLZPZZSV^ZKV^UILJH\ZLVM*6
JVUJLU[YH[PVU¶*6PZ[OLSPTP[PUNMHJ[VYUV[SPNO[
YH[LVMWOV[VZ`U[OLZPZPUJYLHZLZHZSPNO[PU[LUZP[`
PUJYLHZLZ¶SPNO[PZ[OLSPTP[PUNMHJ[VY
3PNO[PU[LUZP[`
Figure 9.10
How light and carbon dioxide may limit the rate of photosynthesis.
Availability of water
The availability of water varies in the environment. If the soil is dry, water may
be the limiting factor on photosynthesis.
Etiolation
etiolation ❯
If the plant cannot get sunlight, for
example it is shaded by a rock or another
plant, it cannot photosynthesis. Without
photosynthesis it cannot make food.
But this does not mean that it cannot
continue to grow. For a short while, it can
use some of the food stored within the
plant to grow and lengthen. This gives it
a chance to get some leaves into the light
and so start to photosynthesise again.
The form of growth a plant shows
when it is out of light is different from
normal. All the energy is used to make
long thin cells, so the stem becomes
elongated and thin, and leaves are kept
very small. The stems and leaves are also
pale yellow as no chlorophyll is made.
This form of growth is called etiolation
(figure 9.11). If it does not reach light
Figure 9.11 The etiolated plants on the
quickly the plant will run out of food
right have long thin, white stems and small
reserves and die.
yellow leaves.
97
Life Processes and Disease
Chapter summary
• Plants make food in a process called photosynthesis.
• Photosynthesis is the process whereby food is made from simple inorganic
substances.
• Photosynthesis is an example of autotrophic nutrition.
• Heterotrophic nutrition is the intake of organic food. Animals feed heterotrophically.
• Photosynthesis is made up of two stages. In the light-dependent stage, light ‘splits’
water into hydrogen and oxygen. In the light-independent stage, the hydrogen
combines with carbon dioxide to make glucose.
• Photosynthesis occurs in the leaves of plants.
• Plants show many adaptations for photosynthesis.
• The food made in a plant is used in many ways.
Answers to ITQs
ITQ1 An autotroph is an organism that is able to make its own food (organic
substances) from simple substances (inorganic substances). A plant is an
autotroph – when it photosynthesises it makes glucose from carbon dioxide
and water.
A heterotroph is an organism that takes in organic food when it feeds. It
must have a supply of organic food since it cannot manufacture it for itself.
ITQ2 Autotrophs make organic food which is eaten by heterotrophs.
Autotrophic nutrition must therefore take place first so that heterotrophs can
have something to eat.
ITQ3 Saprophytic nutrition is important for the recycling of nutrients in the
environment. Nutrients trapped in an organism are made available when that
organism dies. Saprophytes can digest cellulose and lignin and can decompose
all plant remains.
ITQ4 Some desert plants close their stomata during the day to prevent loss
of too much water from the leaf when it is hot. They open their stomata at
night to exchange gases for photosynthesis. (They have a special mechanism
which allows them to trap the energy from sunlight during the day and
store it, until the stomata open at night and the energy can be used to make
glucose.)
ITQ5 Carbon dioxide is in the atmosphere around the leaf and gets to the
photosynthesising cell by diffusion. A photosynthesising cell uses carbon
dioxide, and so the carbon dioxide concentration decreases within the cell.
Carbon dioxide diffuses into the cell from the surrounding air space where its
concentration is greater. The carbon dioxide concentration is thus lowered in
the air space. Carbon dioxide from the atmosphere can now diffuse into the air
space through the stomata.
ITQ6 •
The leaves are spread around the stem and lie at right angles to
the Sun’s rays so that they can intercept as much light as possible.
• Leaves are green because the cells contain chlorophyll. This captures light
energy which is needed in photosynthesis.
• Xylem vessels in the stem transport water to the leaf.
• The leaves are thin and flat so gases can diffuse in and out as quickly as
possible.
ITQ7 (i)
Carbon dioxide
(ii) Temperature
(iii) Water
98
9 • Photosynthesis
Examination-style questions
1
The diagram below shows a transverse section of a leaf as seen under a microscope.
(
)
*
+
,
-
(i) Label the parts A to F.
(ii) Which cell is most actively photosynthesising?
(iii) (a) Write the equation that summarises the process of photosynthesis.
(b) From the equation, identify three factors/conditions necessary for photosynthesis
to take place.
(c) Describe how two of these factors reach a typical photosynthesising cell.
(d) Describe the role of the cell labelled E.
2
(i)
Define:
(a) autotrophic nutrition;
(b) heterotrophic nutrition.
(c) saprophytic nutrition
(ii) Photosynthesis is summarised in one equation, but described as two stages (a) lightdependent, and (b) light-independent. Describe the two stages of photosynthesis.
(iii) List five ways a plant is adapted for photosynthesis.
3
The diagram below shows a leaf in its actual size.
(i) Making a drawing of the leaf.
(ii) Write a heading for the drawing.
(iii) Calculate the magnification of your drawing.
(iv) Label the parts of the leaf.
99
10
By the end of this
chapter, you should
be able to:
Feeding and
Digestion
understand the importance of minerals in plant nutrition
understand the importance of a balanced diet to humans
describe food tests for carbohydrates, proteins and fats
relate a balanced diet to age, sex and activity of an individual
explain the meaning of the term ‘malnutrition’
describe hypertension and diabetes
describe health problems associated with food additives
describe the role and structure of teeth
understand the action of enzymes
understand how the alimentary canal of humans works
describe what happens to the products of digestion
age
minerals in plants
pregnancy
food additives
diet
balanced diet
sex
malnutrition
alimentary system
activity
ingestion
physical digestion
– teeth
digestion
chemical digestion
– enzymes
absorption
villus
assimilation
liver
egestion
constipation
All organisms, plants and animals, must be supplied with a source of energy
for metabolism. This energy is used for maintenance, growth, and repair of
their bodies to sustain their lives. Plants (autotrophs) are able to make their
own food using energy from the Sun. They take in only very simple inorganic
substances like water, carbon dioxide and also magnesium and nitrate ions.
Nitrogen and magnesium are basic components of chlorophyll. Choprophyll
allows a plant to grow more rapidly and produce large amounts of succulent
green leaves. These minerals also strengthen and support the roots thus
enabling plants to take in more water and nutrients from the soil. Nitrogen is
100
10 • Feeding and Digestion
also important in the formation of proteins. Animals (heterotrophs) can only
obtain energy when they feed on other living organisms made up of complex
materials such as carbohydrates, proteins and lipids.
Diet
diet ❯
balanced diet ❯
fibre ❯
ITQ1
Define the terms ‘diet’ and
‘balanced diet’.
To maintain their bodies in good health, animals need various materials. These
include carbohydrates, proteins, lipids, vitamins and minerals. Animals eat food
that contain these materials or nutrients. The term ‘diet’ is used to describe the
quantity and quality of food eaten (i.e. which nutrients and how much of each
is present in the food being eaten every day).
A balanced diet is a diet which has the quality and proportions of
nutrients needed to maintain good health. This includes water and fibre.
Water is essential because around 70% of our body mass is water. If we do not
get enough water, systems in the body soon stop functioning properly. Fibre, or
roughage, is the tough fibres that come from plant material. We cannot digest
and absorb them, but they are essential to the healthy working of the gut.
Without enough fibre, we soon suffer from constipation. Eventually this can
lead to bowel disease.
Some nutrients that are needed are organic and some are inorganic
(table 10.1).
Organic nutrients
Inorganic nutrients
Carbohydrates
Minerals
contain carbon (C), hydrogen (H) and oxygen (O) calcium, iron, potassium, sodium, iodine,
phosphorus
Proteins
contain C, H, O and also nitrogen (N) and small
amounts of sulfur (S)
Lipids
contain C, H and O
Vitamins
contain C, H and O and other essential elements
Table 10.1 The organic and inorganic nutrients needed by living organisms.
Organic nutrients
Figure 10.1 Three-dimensional ball-andstick model of a glucose molecule.
monosaccharide ❯
disaccharide ❯
These are required in the diet in relatively large amounts (tables 10.2 and 10.3,
overleaf).
Carbohydrates are compounds of carbon, hydrogen and oxygen in the
radio 1 C : 2 H : 1 O. An example is glucose. Figure 10.1 shows a ball-and-stick
model of a molecule of glucose. It can also exist as a ring formed from five
carbon atoms and one oxygen atom. The sixth carbon atom in a –CH2OH group
is attached to a ring carbon.
Compounds with one such ring structure are called monosaccharides.
6
The formula can be shortened to a symbol which can be either
or, for
diagrams, just
.
Glucose and fructose are examples of monosaccharides. Monosaccharides
are often called simple sugars.
Two monosaccharides can combine to form a disaccharide (figure 10.2,
overleaf). This happens in a condensation reaction as a water molecule is
removed. Disaccharides can be broken back down to monosaccharides by
hydrolysis which is a chemical reaction involving recombination with water.
101
Life Processes and Disease
Monosaccharides and most disaccharides reduce Benedict’s solution to an
orange/red compound. Sucrose is the only common disaccharide which
does not react in this way. This provides a distinguishing test for sucrose.
Disaccharides are called complex sugars.
/6
/
/
/
/
/6
6/
/6
6/
JVUKLUZH[PVU^H[LYYLTV]LK
O`KYVS`ZPZ^H[LYHKKLK
/
/6
JVUKLUZH[PVU
TVUVZHJJOHYPKLZ
/
6
KPZHJJOHYPKL
6/
O`KYVS`ZPZ
/
/6
/
6
6
6
6
WVS`ZHJJOHYPKL
6/
Figure 10.2 Disaccharide molecules are made when two monosaccharide molecules join
together. Polysaccharide molecules are made of many monosaccharide molecules.
Organic nutrient Major groups
Structure
Characteristics
Importance
Carbohydrate
monosaccharide (e.g.
glucose, fructose)
five carbon atoms and an
oxygen atom form a ring
called simple sugars
small molecules, soluble,
sweet taste
major energy source
disaccharide (e.g.
maltose, sucrose)
two rings join together
called complex sugars
soluble, sweet taste
major energy source
polysaccharide (e.g.
starch, cellulose,
glycogen)
many rings join together
long chains of simple sugar
(glucose) joined together
insoluble and do not have a
sweet taste
starch is used as the energy store
in plant cells and as a food source
for animals
cellulose is found in plant cell
walls
glycogen is used as the energy
store in animals cells
Protein
made up of long chains of
amino acids
there are about 20 different
amino acids
they can be arranged in the
protein chain in any order
a difference in the order of
amino acids in the chain
results in different protein
there are millions of
proteins, some soluble e.g.
haemoglobin, red pigment in
blood), and some insoluble
(e.g. keratin, from which hair
and nails are made).
used for making new cells, growth
and damaged parts of the body
antibodies, hormones and enzymes
are also proteins
Lipids
(fats and oils)
four moelcules (three fatty
acids and one glycerol)
joined together
insoluble in water
secondary energy supply after
carbohydrates have been used up
important for storage (oils in seeds)
also function as insulation (fat
under skin) especially for animals
living in cold regions
foods like butter, oils and nuts are
rich in lipids
NS`JLYVS
MH[[`HJPKZ
(continued)
102
10 • Feeding and Digestion
Organic nutrient Major groups
Vitamins
Structure
A, B, C, D, E and K
each vitmain has many
funcitons
Characteristics
Importance
small amounts needed for
good health
A – aids vision in dim light
B – asissts in respiraiton
C – keeps tiossues helathy
D – aids absopriton of calcuim
K – aids in blood clotting
Table 10.2 The major organic nutrient groups.
Vitamin
Sources
Functions
Symptoms of deficiency
A
carrots, spinach, egg yolk, cod liver oil,
butter
keeps skin and mucous membranes
healthy, aids vision in dim light
dry skin, mucous membranes
degenerate, poor night vision
B1
liver, rice, cereals, whole wheat flour,
yeast
helps in respiration
beriberi – muscles become weak and
painful, nervous system affected
B6
leafy vegetables, eggs, liver, fish, kidney
helps in metabolism
depression and irritability
C
citrus fruits, green vegetables
keeps tissues healthy
scurvy – guns bleed, wounds take
longer to heal, heart failure
D
egg yolk, dairy products, cod liver oil,
also made by the action of sunlight on
the skin
controls calcium and phosphorus
absorption, important in bone and tooth
formation
rickets – growing bones do not calcify,
results in ‘bow’ legs in young children,
and ‘knock-knee’ in older children
Table 10.3
polysaccharide ❯
NB Both Benedict’s and Fehling’s solutions
contain copper sulfate. Reducing sugars reduce
the copper(II) ions (CU2+) present in the copper
sulfate to insoluble red-brown copper(I) oxide
which contains CU+ ions and is a precipitate.
Practical activity
SBA 10.1: Which food groups are
present in a food sample? page 348
Some vitamins needed by humans for healthy growth.
Many monosaccharides can be joined to form or synthesise a very large
molecule called a polysaccharide. Since condensation (dehydration) reactions
are involved in the synthesis of these polymers, these reactions can be
called dehydration synthesis. Starch, cellulose and glycogen are examples of
polysaccharides. They can form very large molecules.
Food tests
Table 10.4 shows the standard tests which can be made on a sample of food to
indicate each of the main food groups.
Substance to be tested
Test
Observations
Reducing sugars – all
monosaccharides (e.g.
glucose, fructose) and some
disaccharides (e.g. maltose)
(i) Benedict’s test: 2 cm3 of the solution to be tested
The initial blue colour of the mixture turns green and
then yellow and may form a brick-red precipitate.
is put into a test-tube. 2 cm3 of Benedict’s solution is
then added. The mixture is shaken and brought gently to
the boil.
(ii) Fehling’s test: 2 cm3 of the solution to be tested is Same as Benedict’s test.
put into a test-tube. 1 cm3 of Fehling’s A is added. 1 cm3
of Fehling’s B is then added. The mixture is shaken and
brought gently to the boil.
Non-reducing sugars
(e.g. sucrose)
1 cm3 of the solution is put into a test-tube and 1 cm3 of
dilute hydrochloric acid (HCl) is also added. The mixture
is boiled for 1 minute. 1 cm3 of aqueous NaOH (NaOH
solution) is added, followed by 2 cm3 of Benedict’s
solution. The mixture is then shaken and boiled gently.
A red-brown precipitate results as the sucrose is
hydrolysed to fructose and glucose by the acid. Fructose
and glucose are reducing sugars, so Benedict’s test then
can be carried out.
Starch
2 cm3 of 1% starch solution is added to a test-tube. A
few drops of iodine in potassium iodide (I2KI) solution is
added.
A blue–black precipitate results.
(continued)
103
Life Processes and Disease
Substance to be tested
Test
Observations
Protein
Biuret test: 2 cm3 of protein solution is put into a testtube, 2 cm3 of 5% potassium hydroxide (KOH) is then
added. The mixture is stirred and 2 drops of 1% copper
sulfate (CuSO4) is added.
A mauve or purple colour slowly develops.
Fats
A cloudy white suspension can be seen when the water
Ethanol test: 2 cm3 of fat solution or oil is put into a
is added.
test-tube. 2 cm3 of absolute ethanol is then added.
The mixture is shaken vigorously and 3 cm3 of water
is added.
Grease spot test: a drop of the sample is dropped onto A permanent translucent spot is seen on the paper.
a piece of paper.
Table 10.4 Tests for the main food groups.
Inorganic nutrients
trace element ❯
ITQ2
Give three named examples of foods
which can be eaten to obtain (i) organic
nutrients (ii) inorganic nutrients.
Minerals are inorganic nutrients that are required in small amounts for good
health and development. Some are required in only trace (very small) amounts
for good health and thus are called trace elements. Table 10.5 shows some
mineral elements required by plants and table 10.6 shows some minerals
required by humans.
Element
One function
Deficiency effects
nitrogen (N) (absorbed as necessary for proteins
nitrates)
small yellow leaves and poor
growth
magnesium (Mg)
necessary for chlorophyll
leaves yellow between the veins
iron (Fe)
necessary for chlorophyll
new leaves yellow between veins
calcium (Ca)
necessary for cell walls
poor stunted growth, leaves
yellow, terminal buds die
potassium (K)
maintains the salt balance in cells yellow/brown edges on leaves,
edges wither, plant dies early
sulfur (S)
makes proteins
young leaves small, thin, yellow
between green veins
phosphorus (P)
makes some proteins
poor growth, small reddish-brown
leaves
Table 10.5
Some elements needed by plants for healthy growth.
Mineral
Sources
Functions
Symptoms of deficiency
calcium
milk, cheese
formation of bones and brittle bones and teeth
teeth
iron
red meat, green leafy
vegetables
formation of
haemoglobin
anaemia – tiredness, lack of
energy because of a reduction
in the number of red blood cells
(continued)
104
10 • Feeding and Digestion
Ingredients: sugar, enriched bleach flour
(wheat flour, niacin, reduced iron, thiamine
mononitrate, riboflavin, folate) food
starch-modified, partially hydrogenated
soybean and cottonseed oils, leavening
(sodium bicarbonate, sodium aluminium
phosphate), emulsifier (propylene glycol
monoester, monoglyceride, sodium
stearoyl lactylate), salt, natural and artificial
flavours, citric acid, guar gum, xanthan
gum, isolated soy protein, whey.
Blueberries: blueberries, water.
Figure 10.3 There are many additives in
the ingredients of manufactured food as
seen in this list of ingredients for a blueberry
muffin mix.
Mineral
Sources
iodine
sea foods, iodised table formation of the
salt
hormone thyroxin
goitre (adults) – reduced
metabolic rate, swelling of the
thyroid gland
cretinism (children) – physical
and mental retardation
phosphorus
meat, fish
brittle bones and teeth
Table 10.6
Functions
combine with calcium
in the formation of
bones and teeth
Symptoms of deficiency
Some elements needed by humans for healthy growth.
Food additives
Many additives are used in preparing food, for many different reasons
(figure 10.3). Food additives may be natural or artificial. Common natural
additives include sugar, corn syrup and pepper. Common artificial additives
are some flavours and sweeteners. The major groups of additives include the
following.
Dyes and colourings
These are purely cosmetic and rarely add nutritional value. Tartazine is used to
give a yellow colour to foods and drinks, for example, orange juice, fish fingers.
It does, however, have some adverse effects as it is associated with:
• hyperactivity in children;
• allergic reactions;
• adverse effects on asthmatics.
Preservatives
These make food less susceptible to bacterial infection, so food can be kept for
longer periods of time in tins, packets, spreads and bottles without spoiling. When
food is produced and packaged it may travel thousands of miles, over several
months, before it is used. The health of the general population has improved
because preservatives reduce the risk of bacterial poisoning. They are perhaps
the most easily justified additive, but only make up 1% of all additives used.
Synthetic flavourings
During preparation, food can lose some of its flavour, so these are added to
improve or even change the flavour.
Flavour enhancers and sweeteners
Saccharin is often used to sweeten prepared foods. Monosodium glutamate
(MSG, Ali-jo-moto, Vet-sin) is a commonly used flavour enhancer found in
processed foods including soups, fast foods and Chinese foods. Young children
and pregnant and lactating women are advised not to eat foods containing
MSG as it may be related to asthma, attention deficit disorder, acute headaches,
extreme mood swings, depression and paranoia.
Propellants
Carbon dioxide and nitrous oxide may each be used to form an aerosol, forcing
food out of containers.
105
Life Processes and Disease
Acids
These are added to give a sour taste to prepared food.
Firming agents
Aluminium salts are used to retain crispness; gums increase the thickness of
sauces and soups.
A balanced diet
HU
KZ MH[
\I Z
Z[P[
\[L
Z
Z[HWSLZ
HJLYLHSNYHPUZ
IZ[HYJO`MY\P[Z
YVV[ZHUK[\ILYZ
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SLN\TLZ
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MVVKMYVT
HUPTHSZ
Figure 10.4 Pie chart showing the relative
proportions of foods in a balanced diet.
ITQ3
Describe a meal which includes all the
nutrients necessary for good health.
We can group all the foods available to humans into six food groups.
• Staple foods – These include cereal grain (e.g. rice), cornmeal, wheat flour,
oats, starchy fruits, roots, tubers.
• Peas and beans (legumes) – These include red beans, pigeon peas, black
eyed peas, broad beans.
• Dark green, leafy vegeables and yellow vegetables – Cabbage, pak
choi, lettuce, spinach are leafy examples; pumpkin and carrot are examples
of yellow vegetables.
• Foods from animals – Fish, poultry, meat, milk, eggs, cheese are all foods
from animals.
• Fruits – Citrus fruits, bananas, apples are all examples.
• Fats – These include oils, butter, margarine and food with a high proportion
of fat such as cakes, biscuits.
Figure 10.4 shows the components of a balanced diet. Each block represents a
food group and the size of the block indicates the proportion of the diet which
that food group should constitute.
Balanced diet related to age, sex and activity of
an individual
Nutritional requirements vary with age, sex and activity.
Energy requirement
Energy requirements are generally greater for men (figure 10.5). They
usually have more muscle, relatively less fat and weigh more than women.
In women, the energy requirements are
higher during the last three months of
,ULYN`RJHSWLYKH`
THSLMLTHSL
pregnancy. Extra energy is needed for
growth of the fetus and deposition of fat
in preparation for breast feeding. This
requires extra energy because the energy
needed by the baby for rapid growth
in early postnatal life comes from its
mother’s milk.
Energy requirements for a growing
individual increase up to about the age of
18 years, when the energy requirements
are the greatest. The requirement for
energy then decreases as the person
¶
¶
¶
¶
¶
¶
¶
¶
gets older.
¶
¶
¶
¶
¶
¶
¶
¶
Physical activity of an individual
(NL`LHYZ
varies with both occupation and leisure.
Figure 10.5 Energy requirements for men and women vary as they get older.
Some people are mostly sedentary
106
10 • Feeding and Digestion
(sitting for much of the time, such as in an office) and others are very active.
Energy requirements for different levels of activity can vary greatly.
Protein requirement
Men require more protein than women from around the age of 11 years
onwards. This is when the muscle-to-fat ratio starts to differ because of the
development of secondary sexual characteristics. Women start to store fat
in their hips and breasts and men develop more muscle, especially on their
shoulders and legs. Extra protein is required by women during pregnancy and
breastfeeding.
Requirements of minerals and vitamins
Mineral intake is especially important during pregnancy and lactation. The
mother’s diet must contain sufficient iron, calcium, vitamin C, folic acid, and
everything needed to make the baby’s tissues inccluding blood, bone and
muscle. Extra folic acid may be given to the mother to reduce the risk of spina
bifida in the baby.
Malnutrition
malnutrition ❯
Malnutrition means bad nutrition, and can be applied to under-eating, overeating and bad eating habits. Malnutrition is the cause of many diseases like
deficiency diseases, obesity, heart diseases and anorexia. Education on balanced
diet and good health is very important in preventing the occurrence of many
diseases.
Under-eating
Figure 10.6 A child suffering from
kwashiorkor and marasmus.
anorexia ❯
Starvation is one kind of under-eating and is most often associated with
developing countries. It means not eating enough food to supply the energy
requirements for daily activities. Also, not enough protein and vitamins are
eaten which are necessary for growth, development, resistance to infection and
a healthy life. Marasmus and kwashiorkor are common conditions caused by
under-eating (figure 10.6).
Some signs and symptoms of marasmus and kwashiorkor:
• very underweight (less than 60% for age);
• thin muscles, thin arms and legs;
• reduced growth may lead to reduced mental development;
• reduced resistance to infection;
• sometimes swelling of the body tissues with fluid (oedema);
• shrunken features giving the face the appearance of an old person;
• hair becomes thin, sparse and easily removed;
• rough skin;
• little interest in surroundings.
Anorexia is another kind of under-eating but is associated with developed
countries. It is the voluntary refusal to eat and is most commonly found in
teenage girls, though teenage boys can also get this illness. It is as much a
psychological illness as a physical one, because the refusal to eat is based on
a poor self-image. The patient continues to think that they are fat even when
they are underweight. Recovery requires treatment for the psychological
condition as well as an improved diet.
107
Life Processes and Disease
Obesity
obesity ❯
diabetes
hypertension ❯
CHAPTER 13
Obesity results from over-eating, especially of fatty foods, and a lack of
exercise. Excess fat accumulates in the body and body mass increases to well
above normal (figure 10.7). Obese people are predisposed to many diseases
like diabetes (see below), hypertension (high blood pressure, chapter 13),
coronary heart disease, arthritis, cancer and stroke. Over-eating can be
prevented by eating sensibly, and engaging in regular aerobic exercise.
Heart diseases and cardiovascular disease
coronary heart disease ❯
Some diseases of the heart and cardiovascular system develop slowly after
years of living on a diet of fatty foods and not much exercise. Atherosclerosis
is a disease of blood vessels. It is a thickening of the inner layers of artery
walls, eventually the artery may become blocked. If the affected artery is the
coronary artery, the heart muscle is not supplied with food and oxygen and
that part of the heart dies. This could result in a heart attack (coronary heart
disease). A similar blockage in a blood vessel in the brain results in a stroke.
The rough surface of the thickened wall could also encourage formation of a
blood clot which may block blood vessels.
Diabetes
Figure 10.7 A person is described as
‘obese’ if they weigh at least 20% more than
the average for someone their height.
holozoic nutrition ❯
Diabetes is a group of metabolic diseases in which a person has high blood
sugar, either because the pancreas does not produce enough insulin, or the
body cells do not respond to the insulin that is produced. Management of
diabetes concentrates on keeping blood sugar levels as close to normal as
possible, which can usually be accomplished with diet, exercise and appropriate
medication. Obesity, high blood pressure and lack of regular exercise accelerate
the harmful effects of diabetes.
Holozoic nutrition
Mammals, including humans, feed by taking in or ingesting organic food. This
particular type of heterotrophic nutrition is termed holozoic nutrition, and
includes ingestion, digestion, absorption, assimilation and egestion.
• Ingestion – The act of taking in food (into the mouth in humans).
• Digestion – The process of breaking down large, complex, insoluble
material into small, simple, soluble molecules. The teeth physically break
the food into pieces, and enzymes then chemically break down the large
molecules into smaller ones.
• Absorption – The diffusion of soluble food molecules (glucose, amino
acids, fatty acids, glycerol, vitamins, minerals and water) into the
bloodstream.
• Assimilation – When these food molecules are taken from the blood and
used by the body cells for respiration, growth and development.
• Egestion – The process by which the undigested part of the good is
removed from the body. It is also known as defecation.
Digestion
The physical action of teeth
physical digestion ❯
108
Teeth help with the physical breakdown or mechanical breakdown of food.
This is called physical digestion. The structure of a typical tooth is shown in
figure 10.8.
10 • Feeding and Digestion
LUHTLSOHYKTH[LYPHS
JYV^U
KLU[PUL
N\T
W\SWJH]P[`JVU[HPUZ
ISVVK]LZZLSZHUK
ULY]LLUKPUNZ
YVV[
JLTLU[OVSKZ[OL
[VV[OPU[OLIVUL
MPIYLZ
QH^IVUL
ULY]L
Figure 10.8 A section through a tooth showing the general structure.
Mammals differ from other animals in that they have more than one type
of tooth. In humans, there are four kinds and Table 10.7 summarises their
structures and functions. Figure 10.9 shows the position of the different teeth
in the mouth.
Type
incisor
canine
premolar
molar
Shape
Function
chisel-shaped for cutting
1 root
cutting food
pointed or dagger-shaped
1 root
grasping and tearing food
flat with cusps or bumps on
the fairly broad surface
2 pointed cusps
2 roots
flat, with cusps on the
broad surface
4 or 5 cusps
2 or 3 roots
biting off bits of food
(well developed in carnivores for tearing
flesh)
crush and grind food
large back teeth to crush and grind food
Table 10.7 The shape and function of human teeth.
milk teeth ❯
Milk teeth are the first set of teeth in humans. They appear singly or in pairs
from the time a child is approximately 3 months old. By age 3 years, most
children have about 20 teeth. These begin to fall out when a child is about 7
years old.
109
Life Processes and Disease
permanent teeth ❯
wisdom teeth ❯
Permanent teeth are
the teeth which replace
the ones that have fallen
out. An additional 12 new
teeth also erupt, which
make up the complete
set of permanent teeth by
about age 17. Most adults
have 8 incisors, 4 canines,
8 premolars and 12 molars.
The 4 molars at the end of
the jaw are the last set to
grow through the gum and
are called wisdom teeth.
PUJPZVYZ
JHUPUL
WYLTVSHYZ
[VUN\L
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TVSHYZ
K\J[MYVT
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The use of fluorides
Figure 10.9 There are four types of teeth in humans.
The use of fluorides in
toothpaste or in water
supplies helps to prevent tooth decay in humans. Fluorides are compounds of
the element fluorine which improve resistance to tooth decay by hardening the
enamel. When permanent teeth are developing in children, the use of fluorides
is effective in helping these ‘new’ teeth resist decay.
Compounds of fluorine can act as serious pollutants in the environment. In
the production and extraction of aluminium from bauxite, sodium aluminium
fluoride (Na3AIF3) is used to lower the melting temperature of alumina from
2050 °C to 950 °C, so that less energy is used. The exhaust gases from the
manufacture of aluminium then contains fluorides. Fluorides seem to affect
trees, and on grass they can enter food chains. The teeth and bones of grazing
animals are affected badly.
Although fluoride provides resistance to tooth decay in humans, an excess
in the environment can be harmful. Also fluorides can be dangerous to young
children and they should never swallow fluoridated toothpaste.
The chemical action of enzymes in digestion
chemical digestion
enzymes ❯
catalyst ❯
Practical activity
SBA 10.2: The action of an enzyme,
page 349
Despite the action of teeth in breaking down food physically, food must
also be chemically broken down. Chemical digestion involves enzymes.
Enzymes are organic catalysts, which means they speed up chemical reactions
occurring in living cells. During digestion, enzymes speed up the rate at which
the large, insoluble food molecules are broken down into small, soluble
food molecules.
enzymes (amylase)
disaccharides + monosaccharides
polysaccharides
enzymes (protease)
amino acids
proteins
enzymes (lipase)
lipids
ITQ4
Define the terms ‘physical digestion’
and ‘chemical digestion’.
110
fatty acids + glycerol
There are thousands of enzymes but all have similar properties.
• They are all proteins.
• Each enzyme is specific for the type of chemical reaction it speeds up.
• They are required in small amounts.
• They are inhibited or prevented from working by poisons like cyanide and
arsenic.
10 • Feeding and Digestion
• They work best at a particular
temperature called the optimum
temperature (Figure 10.10).
• They are denatured or destroyed by high
temperatures.
• They work best at a particular pH, called
the optimum pH (Figure 10.11).
substrate ❯
products ❯
W/H[^OPJO
LUa`TL^VYRZILZ[
VW[PT\T[LTWLYH[\YL
¶LUa`TL^VYRZILZ[
H[[OPZ[LTWLYH[\YL
YH[LPUJYLHZLZHZ[OL
[LTWLYH[\YLPUJYLHZLZ
The substance that the enzyme breaks
down is called the substrate and the
substances that are made are known as the
products.
Digestion and
absorption along the
alimentary canal
9H[LVMYLHJ[PVU
9H[LVMYLHJ[PVU
LUa`TLIYLHRZKV^U
H[OPNOLY[LTWLYH[\YL
;LTWLYH[\YLV *
Figure 10.10 The effects of
temperature on an enzyme-catalysed
reaction. The activity of the enzyme is
small below 20 °C, rises steadily to a
maximum near 50 °C, then falls sharply.
The alimentary canal (gut) is a long
muscular tube, which extends from the mouth to the anus (figure 10.12).
It consists of the major parts of the digestive system where digestion and
absorption of food take place.
VLZVWOHN\ZMYVTTV\[O
SV^LYW/
VW[PT\TW/
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OPNOLYW/
KPHWOYHNT
SP]LY
9H[LVMYLHJ[PVU
VW[PT\TW/VM
TVZ[LUa`TLZ
PUO\THUJLSSZ
VW[PT\TW/
VMWLWZPU
VW[PT\TW/
VM[Y`WZPU
NHSSISHKKLY
IPSLK\J[
WHUJYLH[PJK\J[
JHYKPHJZWOPUJ[LY
Z[VTHJO
W`SVYPJZWOPUJ[LY
WHUJYLHZ
K\VKLU\T
ZTHSS
PU[LZ[PUL PSL\T
W/
‹WLWZPU^VYRZILZ[H[W/
‹[Y`WZPU^VYRZILZ[H[W/
‹TVZ[LUa`TLZPUJLSSZ^VYR
ILZ[H[W/
Figure 10.11 The effects of pH on an
enzyme-catalysed reaction. The activity of
the enzyme rises sharply near the optimum
pH and falls just as sharply as that pH is
exceeded.
saliva ❯
salivary amylase ❯
JHLJ\T
HWWLUKP_
JVSVU
YLJ[\T
SHYNL
PU[LZ[PUL
HU\Z
Figure 10.12 The human alimentary canal.
The mouth
Digestion begins in the mouth, after food is ingested using the hands, lips and
tongue. The teeth break the food down into smaller pieces. This is done with
the help of saliva, which moistens the food. Saliva is secreted from the salivary
glands and is a mixture of water, mucus and salivary amylase. The water
111
Life Processes and Disease
and mucus soften the food, while the enzyme salivary amylase begins to digest
the starch in the food. The mucus also helps food to move easily along the
alimentary canal.
Salivary amylase breaks down the bonds in starch by hydrolysis, and
so hydrolyses starch into smaller and smaller chains, eventually to glucose
molecules (figure 10.13).
NS\JVZL
starch¶SVUN
JOHPUVMNS\JVZL
HT`SHZL
HKKP[PVUVM^H[LY
O`KYVS`ZPZ
THS[HZL
ZTHSSLYJOHPUZ
IVUK
IYVRLU
IVUK
IYVRLU
IVUK
IYVRLU
L]LU[\HSS`HSS[OLIVUKZ^PSSILIYVRLU
NS\JVZL
Figure 10.13
bolus ❯
Starch is broken down to glucose by the enzyme amylase.
So both physical digestion and chemical digestion occur in the mouth. The
tongue churns food and rolls it into a bolus, or a ball-like structure, which is
then swallowed.
The oesophagus
oesophagus ❯
epiglottis ❯
When food is swallowed it enters the oesophagus. The trachea, or windpipe,
which opens to the lungs lies to the front of the oesophagus. If you press your
hands gently on your throat, you can feel the rings of cartilage of the trachea.
The oesophagus is directly behind this.
When food is swallowed, it is prevented from going into the trachea by a
small, flap-like structure called the epiglottis, which covers the trachea as you
swallow (figure 10.14). It is therefore impossible to swallow and inhale at the
same time. Try it!
MVVK
[VUN\L
LWPNSV[[PZ
HPYJHUNV
PU[V[OL[YHJOLH
[YHJOLH
VLZVWOHN\Z
Figure 10.14 The epiglottis stops food entering the trachea when you swallow.
112
IVS\ZNVLZPU[V
[OLVLZVWOHN\Z
[YHJOLH
JSVZLK
10 • Feeding and Digestion
peristalsis ❯
However, if a person is eating while talking and laughing, the food can
become stuck in the trachea. The Heimlich manoeuvre, which forces air rapidly
out of the lungs, can be applied to remove the stuck food (figure 10.15).
The oesophagus is a muscular tube and food moves down it by peristalsis.
This is a wave of muscle contraction that moves downward and squeezes the
food into the stomach.
IJSHZWIV[OOHUKZ
HYV\UK[OL^HPZ[
HNP]LMP]LZOHYWZSHWZ
IL[^LLU[OL
ZOV\SKLYISHKLZ
JW\SSZOHYWS`\W^HYKZ
HUKILSV^[OLYPIZ
Figure 10.15 The Heimlich manoeuvre.
The stomach
oesophagus
muscles contract, narrowing
the oesophagus and pushing
chyme
the❯bolus down
bolus is pushed
down and into
the oesophagus
gastric juice ❯
The muscular walls of the stomach relax and contract to
churn the food as it arrives. Food is mixed with enzymes,
mucus and hydrochloric acid. This mixture is called chyme.
The stomach walls are dotted with pits leading to gastric
juice glands that secrete gastric juice into the stomach
(figure 10.17).
pits in the stomach
wall secrete gastric juices
stomach
Figure 10.16 A bolus moves down the oesophagus to
the stomach by peristalsis.
part of the
stomach wall
magnified
Figure 10.17
Gastric juice pours out of pits in the stomach wall.
Gastric juice consists of:
• mucus;
• hydrochloric acid;
• pepsin.
rennin ❯
Digestions of proteins begins in the stomach as the long protein chains are
broken down by the enzyme pepsin into amino acids. Hydrochloric acid
provides the acidic medium in which pepsin works most efficiently. It also kills
any pathogens that may have entered the body with the food.
The stomachs of young mammals produce the enzyme rennin, which
curdles or clots the milk that they get from their mother. The milk proteins
113
Life Processes and Disease
pepsin ❯
are then broken down by pepsin into shorter polypeptides and then into
amino acids.
After one or two hours in the stomach, small amounts of chyme pass
into the next region of the alimentary canal, the duodenum as the sphincter
muscles at the bottom of the stomach relax and open.
Peptic ulcers
A peptic ulcer is a hole or ‘sore’ in the stomach lining. It used to be thought
that excessive secretions of hydrochloric acid and pepsin damaged the stomach
wall and caused these ulcers. But recent research has shown them to be caused
by the presence of Helicobacter pylori, a species of bacterium that lives in the gut.
Some patients have been successfully treated with antibiotics; others have been
successfully treated with a drug which suppresses the production of stomach
acid.
The duodenum
duodenum ❯
bile ❯
emulsification ❯
pancreatic juice ❯
ITQ5
Describe, giving examples, the role of
enzymes in digestion.
ITQ6
Copy and complete this table.
Part of alimentary canal
Importance
mouth
stomach
duodenum
The duodenum is the first region of the small intestine. It receives chyme
from the stomach and secretions from the gall bladder and pancreas. Bile,
which is produced by liver cells and stored in the gall bladder, breaks down
large lumps of fat into tiny droplets. This process, called emulsification,
increases the surface area of the fats making it much easier for the enzyme
lipase to digest the fat.
Pancreatic juice is secreted from the pancreas and contains many
enzymes.
• Amylase continues the digestion of starch into maltose before it can be
digested into glucose.
• Lipase digest fats or lipids into fatty acids and glycerol.
• Trypsin is a protease (an enzyme which digests protein) that breaks down
long protein chains into shorter ones (polypeptides) so that they can be
broken down into amino acids by other proteases.
These enzymes work best in a neutral environment, but the chyme, which
came from the stomach is acidic because it contains hydrochloric acid.
Pancreatic juice also contains sodium hydrogencarbonate which neutralises
the hydrochloric acid so the pH of the mixture increases to pH 7–8, which is
optimum for pancreatic enzymes. Bile also contains bile pigments which are
waste products from the liver that need to be excreted.
The food is now fully broken down physically and chemically into the endproducts of digestion.
The ileum
ileum ❯
villi ❯
microvilli ❯
114
The ileum is the second part of the small intestine and is the site of absorption
in the alimentary canal. By the time food reaches the ileum, it has been broken
down into glucose, fatty acids, glycerol, amino acids, vitamins, minerals and
water. These nutrients are small enough to be absorbed and used by the body.
The structure of the ileum has many adaptations which make it good for
absorption.
• It is about 6 metres long and has a large surface area.
• There are folds and ridges that are invaginations in the intestinal walls that
increase the surface area even more for efficient absorption of nutrients
(figure 10.18). They have villi (finger-like projections) on their surfaces.
They are covered with epithelial cells which themselves have microscopic
folds on their surface, called microvilli. These further increase the surface
area for absorption.
10 • Feeding and Digestion
lacteal ❯
• The epithelial cells have large numbers of mitochondria, which provide the
energy for transport of the nutrients from the ileum to the blood.
• Each villus has a good blood supply in the form of a dense network of
capillaries that transport those nutrients that do not diffuse across and
require energy away from the ileum to the liver for processing.
• Each villus contains a lacteal, or lymph capillary, which absorbs fatty acids
from the digestion of fat.
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Figure 10. 19 The arrangement of the
intestines in humans.
Figure 10.18 (a) The wall of the ileum is made up of villi that increase the surface area. (b)
Diagrammatic section through a villus.
The colon
faeces ❯
colon ❯
After most of the nutrients have been absorbed into the blood in the ileum,
the remaining intestinal contents, now called faeces, continue to move
slowly along the colon, or large intestine. The main function of the colon is to
reabsorb water from the faeces into the bloodstream so that water loss from the
body is minimised. The arrangement of the small and large intestine in humans
enables a very long tube to be packed into a very small space (figure 10.19).
The rectum
Faeces consist of undigested cellulose and plant fibre, dead bacteria and
intestinal cells scraped off the gut walls. The faeces are stored temporarily in
115
Life Processes and Disease
rectum ❯
anus ❯
the rectum. As faeces accumulate, pressure increases in the rectum which
results in a desire to defecate, or expel faeces through the anus.
About 24 hours after eating, food has traversed the length of the alimantary
canal, most of the nutrients have been absorbed, and the undigested part is
ready to be expelled.
Constipation
constipation ❯
ITQ7
Describe the route taken by a bolus
from the mouth to the anus.
Constipation results from poor eating habits. A diet lacking fibre can lead to a
blockage of the alimentary canal. Egestion of undigested waste material cannot
then occur normally.
Constipation sometimes results in haemorrhoids, which are protrusions
of tissues through the anus because of forced ‘pushing’. Constipation also
increases the chance of developing colon cancer. Dietary fibre, the undigestible
part of food from plants (mainly cellulose), aids peristalsis and prevents
constipation (figure 10.20).
(a)
(b)
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Figure 10.20 Dietary fibres help to prevent constipation. (a) The faeces containing fibre stay bulky
and soft and are easy to egest. (b) Without fibre, the faeces become hard and solid and are difficult
to get rid of.
Assimilation
assimilation ❯
Assimilation is the process of incorporating and making use of the digested
food into the body. These absorbed food molecules may be stored by the body
for future use, broken down to produce energy or used for growth, repair and
to maintain good health.
Monosaccharides (glucose)
This is taken to the liver, then to the rest of the body where:
• it is used in respiration;
• excess amounts are converted into glycogen in the liver, and stored in liver
and muscle cells;
• excess amounts are converted to fat and stored under the skin or around
organs.
Amino acids
These are taken to the liver and then to the rest of the body where:
• they are used by the body cells for growth and repair;
116
10 • Feeding and Digestion
• they are used to make hormones and enzymes;
• excess amounts are converted to glycogen or fat;
• excess amounts are broken down, or deaminated, in the liver and converted
to urea to be excreted by the kidneys.
Fatty acids
Fat molecules are carried by the lymph to the blood and are:
• stored under the skin and around the organs;
• used to form new membranes in cells and organelles;
• used for respiration in some circumstances.
Functions of the liver
The liver is one of the most important organs in the body as it has many
functions that are essential to keeping the body healthy.
• Carbohydrate metabolism – Excess glucose is stored up as glycogen and
reconverted to glucose when blood sugar levels fall. Excess carbohydrate
may also be converted to fat.
• Lipid metabolism – Excess cholesterol is excreted into the bile and
removed from the body.
• Protein metabolism – Excess amino acids are broken down to form
ammonia and then converted to the less toxic substance, urea. Urea is
transported to the kidneys by the blood and excreted in urine.
• Production of bile – Bile salts are produced and temporarily stored in the
gall bladder. They then travel to the duodenum to aid in digestion.
• Storage of vitamins – A number of vitamins are stored in the liver and
released if the diet is deficient in vitamins.
• Storage of minerals – The liver also acts as a store for some essential
minerals, such as iron and potassium. (This is why liver is a nutritious food.)
They can be released into the body if the diet lacks these minerals.
• Synthesis of plasma proteins – These important proteins are found in
blood plasma. For example, prothrombin and fibrinogen are needed for
blood clotting.
• Detoxification – Toxic materials absorbed from the intestines are stored,
broken down or removed by the liver.
• Breakdown of red blood cells – Red blood cells are broken down (they
live only for three months) and the iron components may be stored, reused
or excreted as bile pigments.
• Production of heat – A lot of metabolic activity occurs in the liver,
which requires a considerable amount of energy. Much of the energy from
respiration is lost as heat, so the liver generates a lot of heat. In mammals
(including humans) and birds, the liver also plays an important role in
keeping the body at the right temperature inside.
Chapter summary
• A balanced diet is important for good health.
• A balanced diet has appropriate proportions of carbohydrate, proteins, lipids, vitamins
and minerals. Water and fibre are also important.
• Plants need minerals for healthy growth.
117
Life Processes and Disease
• Ingestion in humans is the intake of food using the mouth, hands and lips.
• Physical digestion involves breakdown of food by teeth, and chemical digestion is the
breakdown of food by enzymes.
• In humans, there are four different kinds of teeth: incisors, canines, premolars and molars:
– incisors are chisel-shaped and are used for biting food;
– canines are dagger-shaped and are used to tear or rip food;
– premolars and molars are used to chew food into small pieces.
• Enzymes are biological catalysts: they speed up the chemical breakdown of food.
• Food is broken down from insoluble to soluble substances by enzymes.
• Digestion takes place in the mouth, stomach and duodenum.
• Absorption is the movement of the end-products of digestion into blood. It occurs in
the ileum.
• The ileum has several adaptations for absorption, such as a large surface area
provided by the villi.
• The food is assimilated when the body cells make use of it.
• The liver has many important functions relating to the assimilation of food.
Answers to ITQs
ITQ1 (i) Diet is the quantity and quality of food eaten every day by an
individual.
(iii) A balanced diet has the quantity and quality of food needed to maintain
good health.
ITQ2 (i) Organic nutrients can be found in food such as chicken, bread, liver.
(ii) Foods which contain inorganic nutrients include: lettuce, liver, banana.
ITQ3 Your answer needs to include an example from each food group. For
example:
• Staple foods
rice
• Peas and beans
red beans
• Dark green leafy vegetables
salad leaves, spinach
• Food from animals
chicken
• Fruit
orange juice
• Fats
oil for cooking
ITQ4 Physical digestion is the mechanical breakdown of food into small
pieces by the teeth.
Chemical digestion is the breakdown of food by enzymes into soluble
compounds.
ITQ5 Enzymes speed up the breakdown of food molecules into their respective
end-products. Enzymes are not used up themselves. Some examples are:
• amylase, which breaks down starch eventually to glucose;
• pepsin, which breaks down proteins into polypeptides;
• lipase, which converts lipids into fatty acids and glycerol.
ITQ6
Part of alimentary Importance
canal
Mouth
Food is moistened and lubricated. Physical digestion takes place. The
conversion of starch to maltose (chemical digestion) begins.
Stomach
Acid contents kill bacteria in food. Food is churned into chyme. In babies,
curdling of the milk occurs. In adults, protein digestion begins in the
stomach as proteins are converted to polypeptides.
(continued)
118
10 • Feeding and Digestion
Part of alimentary Importance
canal
Duodenum
Chemical digestion takes place here.The enzymes amylase, trypsin, and
lipase are secreted. They break down starch to maltose, which is further
broken down into glucose and fructose. Polypeptides are broken down to
amino acids. Lipids are broken down to fatty acids and glycerol.
ITQ7 mouth oesophagus stomach duodenum ileum colon rectum anus
Examination-style questions
1
(i)
The starch test can be summarised in a series of stages:
1 A leaf is dipped in boiling water for 10 seconds.
2 The leaf is immersed in ethanol which is placed in a water bath at 80 °C.
3 The leaf is then dipped in tap water.
4 The leaf is tested with iodine.
(a) Why was the leaf dipped in boiling water?
(b) What is the role of ethanol?
(c) Describe the iodine test for starch.
(d) A starch test was performed on a leaf and positive results were seen. What
interpretations can be suggested about activities in the leaf?
(ii) Explain the meaning of the term ‘destarched’ as it refers to a leaf. Give details of the
process by which a leaf is destarched.
(iii) Describe an investigation to show that plants need CO2 for photosynthesis.
2
(i)
Five main processes occur in holozoic nutrition. Define each in the order in which they
occur.
(ii) Give two functions of the tongue during eating.
(iii) Describe how food moves down the oesophagus.
(iv) Name the enzyme found in saliva and describe its action.
3
(i)
List the functions of these substances in the stomach:
(a) mucus (b) hydrochloric acid (c) the enzyme pepsin.
(ii) A peptic ulcer is a damage to the stomach wall. It can be very painful and is easily
infected.
(a) How can the stomach all be damaged? Give details of how ulcers are formed.
(b) Why is an ulcer painful?
(c) Why is an ulcer easily infected?
(iii) What are the products of digestion of:
(a) starch? (b) lipids? (c) protein?
4
(i)
Explain how the structure of the wall of the small intestine is adapted for its function
of absorption.
(ii) The table below refers to some enzymes involved in digestion of food in the digestive
system. Copy and complete the table.
Name of enzyme
Site of production
Production of reaction
Fatty acids and glycerol
Salivary gland
Stomach wall
Maltase
119
Life Processes and Disease
(iii) The diagrams below show different types of teeth found in a human mouth. Copy and
complete the table below to show the type and function of each tooth.
(
)
*
Tooth
A
B
C
Type of tooth
Function of tooth
5
(i) List the components of a balanced diet.
(ii) Define (a) obesity (b) malnutrition;
(c) deficiency disease (d) food additive.
(iii) Vitamins and minerals are essential to a healthy life. Explain why:
(a) pregnant women must include calcium and iron in their diet;
(b) it is recommended that we eat an orange a day.
(iv) Nutritional requirements vary with age, sex and activity. Describe the nutritional
requirements of:
(a) a 19-year-old male who plays competitive football;
(b) a 19-year-old female who loves to read.
6
The figure below shows the graphs obtained from an investigation into an enzymecontrolled reaction. Each represents an experiment performed to study the time taken for
the enzyme to break down the substance. Graph 1 shows the time taken under different
temperature conditions with the reaction at a constant pH of 6.7. Graph 2 shows the time
taken under different pH conditions at a constant temperature of 40 °C.
Graph 1
Graph 2
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10 • Feeding and Digestion
Study the graphs and answer the following.
(i)
(a) At what temperature did the reaction occur in the shortest time?
(b) At what pH did the reaction occur in the shortest time?
(ii) In graph 1:
(a) Why did the reaction slow down at higher temperatures?
(b) What effect on the reaction rates is shown by a steady increase from low to
medium temperatures?
7
An experiment was carried out to investigate the effect of temperature on the rate of
an enzyme-controlled reaction. The concentration of enzymes and substrate were kept
constant at all the temperatures investigated. The results are shown in the table below.
Temperature (°C )
Rate of reaction (mg of products per unit time)
5
0.3
10
0.5
15
0.9
20
1.4
25
2
30
2.7
35
3.3
40
3.6
45
3.6
50
2.3
55
0.9
60
0
(i) Plot the results on graph paper.
(ii) Interpret and explain them as fully as you can.
(iii) If the enzyme used was amylase, name the substrate used in the experiment.
(iv) If the enzyme used was amylase, what effect would pH have on its activity?
8
The table below shows the activity on an enzyme in relation to pH.
pH
4.5
5.5
6.5
7.5
Units of enzyme activity
3.1
9.6
14.5
10.1
(i) At which pH is most activity seen?
(ii) At which pH is least activity seen?
(iii) What is the optimum pH for this enzyme?
(iv) Give an example of an enzyme that might give these results.
(v) Give examples of enzymes that would not be expected to give these results.
121
11
By the end of this
chapter, you should
be able to:
Respiration
understand that respiration takes place at the level of the cell
understand the function of ATP
describe the process of aerobic respiration
distinguish between aerobic and anaerobic respiration
describe the uses of anaerobic respiration to humans
understand simple investigations that show the products of respiration
respiration
aerobic
anaerobic
mitochondrion
ADP → ATP
humans
yeast
oxygen
debt
bread
production
alcohol
production
bacteria
yoghurt
production
Aerobic respiration
aerobic respiration ❯
Respiration is the process by which the energy in food is made available for
a cell to do the work necessary to keep it alive (figure 11.1). When oxygen is
used in the reaction, we call it aerobic respiration. The process is catalysed by
enzymes and is also called cellular, internal or tissue respiration.
energy for building up materials
food
cell
respiration
energy for breakdown of materials
energy
energy for secretion
oxygen
energy for contraction
Some of the uses of
the energy made by
a cell.
can move
organism is
alive when all
the cells are
alive and working
can grow
can reproduce
Some of the activities
of an organism.
They all require energy.
can respond
Figure 11.1 An organism is alive when all its cells are respiring.
122
11 • Respiration
How do food and oxygen get to respiring cells?
ITQ1
What is the purpose of respiration?
ITQ2
When do animal cells and plant cells
respire?
A respiring cell needs both food and oxgen (figure 11.2)
• Food – In animals, food eaten is digested and absorbed into the
bloodstream. The end-products of digestion eventually reach all the body
cells. In plants, the food made in photosynthesis in the leaves travels around
in phloem tubes and eventually reaches all body cells.
• Oxygen – In vertebrates, oxygen comes from the air that is inhaled into
the lungs. It diffuses into the bloodstream and is transported to all the body
cells. In plants, some of the oxygen comes from photosynthesis and some
through diffusion in through the leaves and other parts of the plant.
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Figure 11.2 A respiring cell in a mammal is supplied with food and oxygen.
Product and waste product of aerobic respiration
Practical activities
SBA 11.1: Is carbon dioxide produced
during respiration? page 350
SBA 11.2: Is heat produced during
respiration? page 351
SBA 11.3: Is oxygen used up during
respiration? page 352
In both plants and animals, the type of food from which energy is released is
usually glucose. Energy is released when glucose combines with oxygen (the
oxidation of glucose). Carbon dioxide is a waste product of this reaction. In
vertebrates, carbon dioxide diffuses back into the bloodstream, to be taken to
the lungs and exhaled out of the body. In plants, it is used for photosynthesis
during daylight.
How does aerobic respiration occur?
respiration equation ❯
ITQ3
What is the important product of
respiration? What are the waste
products of respiration?
ATP ❯
Respiration or cellular respiration occurs in a series of steps, each of which is
catalysed by enzymes. The overall process can be summarised in words or by
the respiration equation below:
glucose + oxygen energy + carbon dioxide + water
C6H12O6 + 6O2 energy +
6CO2
+ 6H2O
During aerobic respiration, glucose is broken down completely into carbon
dioxide and water.
At each step in the breakdown of glucose, energy is released. This is used
to convert a chemical called adenosine diphosphate (ADP) into adenosine
triphosphate (ATP). Each molecule of ATP acts as a little ‘packet’ of energy.
The energy can be stored and used later when needed.
There are many advantages of storing and using energy in small packets
like this.
• The energy can be released from ATP wherever and whenever it is required
by a cell.
123
Life Processes and Disease
• The energy can be released rapidly.
• Energy is not wasted. A large amount of energy is released by oxidising
one glucose molecule and many ATP molecules are formed. A cell may not
require very much energy at once. By storing the energy in small packets
in ATP molecules, the cell can use small amounts of energy as required
(figure 11.3).
• The energy can be used to drive many different chemical reactions rapidly.
• Energy can be stored as ATP in one part of a cell and transported and used
elsewhere without causing reactions in between.
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Figure 11.3 The oxidation of glucose results in the formation of many molecules at ATP.
Energy production and utilisation are very efficiently and carefully controlled
by the cell.
Where does aerobic respiration occur?
mitochondrion ❯
ITQ4
Where does respiration occur?
Respiration occurs in an organelle called the mitochondrion (figure 11.4).
Mitochondria are present in all cells, animal and plant, and are sometimes
referred to as the ‘power houses’ of the cell.
The energy stored in ATP (adenosine triphosphate) is released when it is
converted back to ADP (adenosine diphosphate).
CO2 transported
to lungs
ITQ5
Give three reasons why it is
advantageous to store energy in
small packets.
O2 transported
to cell
O2
red blood cell
O2
capillary bringing
blood from lungs
P
P
ATP
Figure 11.4
124
CO2
O2
P
energy (
energy used
by the cell
CO2
ATP
Energy
)
+
energy is released
during respiration in
the mitochondrion
P
ADP
P
+
P
+ phosphate
Energy can be released from ATP made during respiration in the mitochondrion.
11 • Respiration
Anaerobic respiration
anaerobic respiration ❯
Respiration can also occur without oxygen and this type of respiration is called
anaerobic respiration. Both anaerobic and aerobic respiration involve the
breakdown of glucose (figure 11.5). However, in anaerobic respiration, it is not
completely broken down.
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energy
glucose
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Figure 11.5
ITQ6
Give two examples of organisms that
respire aerobically, and two that respire
anaerobically.
One mole (mol) of a chemical contains 6 × 1022
molecules of that substance.
Cells can respire anaerobically and aerobically.
Habitats such as stagnant ponds and deep underground have no oxygen.
Organisms living there have adapted to survive without oxygen; they must
respire anaerobically all the time. These organisms include some worms, some
bacteria and some fungi. Parasites that live inside other organisms, such as the
gut parasite tapeworm and bacteria, also live in conditions that lack oxygen.
They must also respire anaerobically.
Many living cells that normally respire aerobically can also respire
anaerobically if oxygen is lacking. Animal and plant cells do this in different
ways (table 11.1).
Aerobic respiration
Anaerobic respiration
uses oxygen
does not use oxygen
• in plants and animals:
C6H12O6 + 6O2 energy + 6H2O + 6CO2
water and carbon dioxide are waste products
• in animal cells:
C6H12O6 energy + 2C3H6O3
lactic acid is the waste product
• in plant cells:
C6H12O6 energy + 2C2H5OH + 2CO2
ethanol and carbon dioxide are waste products
large amounts of energy produced (2880 kJ per small amounts of energy are produced (150 kJ
mole for the breakdown of glucose)
per mole for breakdown of glucose in animals
and 210 kJ per mole in plants)
glucose is broken down completely to inorganic glucose is not broken down completely –
molecules
ethanol and lactic acid are organic molecules
that still contain useful energy
occurs in the mitochondria of the cell
occurs in the cytoplasm of the cell
Table 11.1 Differences between aerobic and anaerobic respiration.
125
Life Processes and Disease
Anaerobic respiration in humans
Human cells respire normally aerobically. However, during strenuous exercise,
muscle cells need much more energy for the extra work that they are doing.
The breathing rate and heart rate increase in an attempt to get more oxygen
to these cells. Sweating occurs to help lose some of the extra energy as heat.
With increased respiration, a lot of heat is produced which is lost from the skin
(chapter 19). After a period of sustained exercise, the oxygen supply becomes
inadequate, even with panting for air and the increased heart rate. The muscle
cells then respire anaerobically.
Energy is still produced when cells respire anaerobically, although it is a
much smaller amount for each molecule of glucose. This means that they can
continue to do work (contract and relax).
CHAPTER 19
anaerobic respiration
Figure 11.6 The build-up of lactic
acid in muscle cells after strenuous
exercise can be painful.
lactic acid + energy
glucose
in muscle cells
lactic acid ❯
fatigue ❯
ITQ7
Humans respire mostly aerobically.
When do humans respire anaerobically?
oxygen debt ❯
Lactic acid is a waste product of this reaction. It builds up in the muscles and
causes them to ache (figure 11.6). This is often called fatigue. After exercise,
the body has to get rid of the lactic acid as quickly as possible. This is done
by using oxygen to change it back to a chemical like glucose so that it can be
broken down completely in aerobic respiration. When anaerobic respiration
occurs in muscles it is in addition to aerobic respiration and not in place of it.
A person continues to ‘breathe hard’ or pant for some time after exercise as
oxygen is needed to get rid of the lactic acid. The oxygen required to get rid of
the lactic acid is called the oxygen debt (figure 11.7).
JLSSK\YPUN
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ITQ8
What is alcoholic fermentation and
what are two of its uses?
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alcoholic fermentation ❯
sugar
fermentation
• sugar from barley seeds
• cane sugar or molasses
ethanol + carbon dioxide
fermentation
fermentation
beer
rum
•ÅV\YHUK`LHZ[KV\NOHM[LYRULHKPUN¶
ÅV\YOHZZ[HYJO^OPJOPZIYVRLUKV^U
to maltose
• yeast uses the maltose as a source of
sugar and fermentation occurs
after some
time
• dough rises as bubbles of CO2
get caught in the dough
•IHRPUNRPSSZ[OL`LHZ[HUK
evaporates the ethanol
Figure 11.8
126
Uses of fermentation.
SHJ[PJHJPK
ZLYPLZVMYLHJ[PVUZ
SLHKPUN[VIYLHRKV^U[V
*6/6
Figure 11. 7 The oxygen debt is the oxygen needed to break down the lactic
acid formed during exercise.
Anaerobic respiration in yeast
During anaerobic respiration in yeast, ethanol and carbon
dioxide are produced as waste products. Ethanol is an alcohol
and the process is known as alcoholic fermentation.
Yeast is very important in the making of alcohol and bread
(figure 11.8). The ethanol can be produced in many ways to
make a wide range of alcoholic drinks, including beer and wine,
which are enjoyed by humans.
The production of carbon dioxide is used in bread-making
to make dough rise. The carbon dioxide produced by the yeast
as it respires accumulates inside the dough in small pockets.
The dough is seen to get bigger or rise as the gas expands with
warmth. Ethanol is also produced but in small quantities – it
evaporates when the bread is baking in the oven.
11 • Respiration
Anaerobic respiration in bacteria
ITQ9
Sometimes bacteria can be found
in canned foods or tins, despite the
fact that the cans and tins are sealed
so that no air can enter. How is this
possible?
Some bacteria also respire anaerobically. Like animal cells, they make lactic
acid as a waste product. We make use of this in the manufacture of yoghurt
and cheese (figure 11.9).
milkJVU[HPUZSHJ[VZL
pasteurisation
OLH[[YLH[TLU[ V *[VRPSSKPZLHZLJH\ZPUNVYNHUPZTZ
inoculation
JVVSLK[V V *HUKHZ[HY[LYJ\S[\YLVMIHJ[LYPHHKKLK
LN3HJ[VIHJPSS\ZI\SNHYPZ
fermentation
PUJ\IH[LKPUSHYNL]H[Z V *MVYHIV\[OV\YZ¶
SHJ[VZLJVU]LY[LK[VSHJ[PJHJPKWYVK\JPUNUH[\YHS`VNO\Y[
cool, add fruits, etc.
package and distribute
H[ V *[OLIHJ[LYPHYLTHPUHSP]LI\[UVTVYL
MLYTLU[H[PVUVJJ\YZH[[OPZ[LTWLYH[\YL
storeH[ V *
Figure 11.9 The manufacture of yoghurt depends on the anaerobic respiration of Lactobacillus
bacteria.
Chapter summary
• All cells respire to release energy to carry out the processes of life.
• Respiration takes place in the mitochondria of cells.
• Food is oxidised during respiration, and carbon dioxide and water are produced as
waste products:
C6H12O6 + 6O2 energy + 6H2O + 6CO2
• Energy is stored in phosphate bonds in ATP (adenosine triphosphate).
• There are many advantages to storing energy as small packets of ATP.
• There are two types of respiration: aerobic and anaerobic.
• Aerobic respiration uses oxygen and releases a lot of energy.
• Anaerobic respiration releases a small amount of energy without the use of oxygen.
• Humans usually respire aerobically but their muscle cells can respire anaerobically
during prolonged exercise.
• Lactic acid is produced during anaerobic respiration in animals and creates an
oxygen debt which has to be repaid.
• Anaerobic respiration in yeast produces ethanol which is used in the alcohol industry
and carbon dioxide which is used in making bread.
• Anaerobic respiration in bacteria is used in the making of yoghurt and cheese.
127
Life Processes and Disease
Answers to ITQs
ITQ1 During respiration, the energy from the food eaten by an organism is
made available. This energy can be used to carry out all the processes of life:
movement, growth, reproduction, and so on.
ITQ2 Animal cells respire all the time because the animal is in constant need
of energy. Plant cells also respire all the time. During the day, while sunlight is
available, plants also photosynthesise, but they never stop respiring.
ITQ3 The important product of respiration is energy, which an organism
needs to carry out the characteristics of life. The waste products of respiration
are carbon dioxide and water.
ITQ4 Respiration occurs in the mitochondria of cells.
ITQ5 Energy is released only when necessary; only as much energy as is
needed is used; energy is released rapidly when it is needed.
ITQ6 Organisms that respire aerobically include humans and birds (there
are many others). Organisms that respire anaerobically include yeast and
tapeworms inside the intestine. Yeast can also respire aerobically if it has access
to oxygen.
ITQ7 Human muscle cells use anaerobic respiration during prolonged
exercise, when oxygen cannot be supplied fast enough for sufficient aerobic
respiration to take place. As a result, energy is produced to do the work
necessary when exercising, although less energy is produced from each glucose
molecule than in aerobic respiration.
ITQ8 Alcoholic fermentation occurs when yeast respires anaerobically
to produce ethanol. This process is important in the bread, beer and wine
industries.
ITQ9 The bacteria that are found in cans and tins respire anaerobically. This
means they do not need oxygen to release energy for all their living processes.
So the fact that there is no air in the can does not affect them; they can live in
that environment.
Examination-style questions
128
1
(i)
Respiration is described as a characteristic of life. What is the importance of
respiration to plants and animals?
(ii) Although respiration occurs in a series of steps, it can be summarised in an equation.
(a) Write the equation.
(b) Describe how energy is made and stored.
(c) Discuss three advantages of storing energy in this way.
(iii) A form 2 student remarked that she had not eaten any food for breakfast or lunch and
that she felt ‘weak’. Explain to her why she is feeling weak and why it is important not
to skip meals.
2
(i)
Using a table, outline the differences and similarities between anaerobic and aerobic
respiration.
(ii) Explain the importance of anaerobic respiration in humans.
(iii) Define:
(a) oxygen debt;
(b) alcoholic fermentation.
(iv) Outline the importance of anaerobic respiration in:
(a) the bread-making industry;
(b) the alcohol industry.
(v) Describe how yoghurt is made.
11 • Respiration
3
Diagrams A and B below show investigations to demonstrate the products of respiration
and photosynthesis.
(
)
(i)
Copy the diagrams and, using annotated labels only, complete diagram:
(a) A to show how carbon dioxide is produced during respiration;
(b) B to show that oxygen is produced during photosynthesis.
(ii) The diagrams below are investigations to show that oxygen is used up during
respiration.
(a) What is the importance of soda lime?
(b) How does the investigation show that oxygen is being used up?
(c) Calculate the rate at which oxygen is being used up.
(d) What would you see if more organisms were put in the flask and what would this
indicate about the total amount of respiration?
H[[OLZ[HY[
JHWPSSHY` VPSKYVW
[\IL
^PYLNH\aL
ZVKHSPTL
HM[LYTPU\[LZ
ZTHSSHUPTHSZ
LN^VVKSPJL
VYTPSSPWLKLZ
129
12
By the end of this
chapter, you should
be able to:
Gaseous Exchange
understand the function and mechanisms of breathing and gaseous exchange
in humans
understand the function and mechanisms of gaseous exchange in plants
identify characteristics common to gaseous exchange surfaces
discuss the effects of cigarette smoking in humans
understand marijuana addiction
gaseous exchange
respiratory system
cigarette
smoking
inhalation
respiratory surface
exhalation
lungs
humans
leaf
plant
gill
fish
Amoeba
cell membrane
characteristics
thin
large surface area
rich blood supply
constantly moving
transport medium
ITQ1
(i) What is gaseous exchange?
(ii) List two places where it occurs in
the human body.
gaseous exchange ❯
130
Importance of gaseous exchange in
humans
Respiring cells need a continuous supply of oxygen. They must also be able to
get rid of the carbon dioxide that is being produced constantly. The blood is the
means by which oxygen and carbon dioxide are transported to and from cells.
At some point, blood has to pick up oxygen and give off carbon dioxide, that is,
exchange these two gases. In humans, gaseous exchange takes place in the
lungs (figure 12.1).
12 • Gaseous Exchange
TV\[OUVZL
HPYPUOHSLK¶OHZ
TVYLV_`NLU
[OHUHPYL_OHSLK
ISVVKYPJOPUoxygenMSV^Z[V[OLIVK`JLSSZ
respiring body cell
\ZLZV_`NLU
WYVK\JLZ
JHYIVUKPV_PKL
HPYL_OHSLK¶
OHZTVYLJHYIVU
KPV_PKL[OHU[OL
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6
lungs
*6
.(:,6<:,?*/(5.,
oxygenUL[KPMM\ZPVU
PU[V[OLISVVK
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carbon dioxideUL[
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KPMM\ZPVUV\[VM[OLISVVK
6
*6
ISVVKYPJOPUcarbon dioxideMSV^Z[V^HYKZ[OLS\UNZ
Figure 12. 1 The role of the lungs in exchanging gases with the environment.
Mechanism of gaseous exchange
in humans
alveoli ❯
The human respiratory system is involved in the exchange of gases in humans.
The lungs are very important and are made up of many tiny air spaces or air
sacs called alveoli (figure 12.2).
nose
mouth
movement of air
trachea opens to mouth and nose
larynx (voice box)
ring of cartilage
trachea
rib
internal intercostal muscle
external intercostal muscle
bronchiole
air sacs/alveoli
left bronchus
left lung
heart
pleural membrane
WSL\YHSÅ\PK
diaphragm
Figure 12.2 The human respiratory system.
131
Life Processes and Disease
trachea ❯
Air enters the nose and/or mouth and moves down the trachea (windpipe).
The trachea is supported by C-shaped rings of cartilage so that it is kept ‘open’ at
all times. The trachea then divides into two bronchi, the right and left. These are
also supported by rings of cartilage. Each bronchus branches into smaller and
smaller tubes called bronchioles. At the end of each bronchiole are the many
tiny sacs called alveoli (figure 12.3). Gaseous exchange occurs in the alveoli.
bronchiole ❯
bronchus ❯
YPUNVMJHY[PSHNLZ\WWVY[Z
[OLZVM[[PZZ\LVM[OL[YHJOLH
HUKRLLWZ[OL[YHJOLHVWLU
ZV[OH[HPYJHUWHZZLHZPS`
IYVUJO\ZIYHUJOLZPU[V
ZTHSSLYHUKZTHSSLYIYHUJOLZ
ITQ2
What is the importance of the rings of
cartilage in the wall of the trachea?
[YHJOLHIYHUJOLZPU[V[^V
IYVUJOPVSL
HS]LVS\ZVYHPYZHJMV\UK
H[[OLLUK
ITQ3
Describe the passage taken by an
oxygen molecule from the air to a
capillary in the lungs.
site of
gaseous exchange
Figure 12.3 The route taken by air into and out of the lungs.
The walls of the alveoli are the gaseous exchange surfaces or the respiratory
surfaces. The smallest blood vessels, capillaries, are closely wrapped around
each alveolus (figure 12.4). Blood is thus brought to and taken away from each
alveolus. Oxygen diffuses across the walls of the alveolus into the capillary and
the blood in the capillary becomes oxygenated. Carbon dioxide diffuses from
the capillary into the alveolus and is exhaled out of the body. The walls of the
alveolus and capillary are very thin (only one cell across) so that diffusion can
occur readily (figure 12.5).
capillary, transporting blood that has
little oxygen (deoxygenated
blood) to and from the alveolus – blood has
high concentration of carbon dioxide
capillary transporting oxygenated
blood from lungs –
blood has low concentration
of carbon dioxide
bronchiole
HPY
KLV_`NLUH[LKISVVK
V\[
V_`NLUH[LKISVVK
PU
JHWPSSHY`
[OL^HSSPZ
VULJLSS[OPJR
air sac or alveolus
JHYIVUKPV_PKL
V_`NLU
HS]LVS\Z[OL^HSSPZ
VULJLSS[OPJR
section of one alveolus
network of capillaries surrounding
alveolus – gases are exchanged here
Figure 12.4 The blood supply to one alveolus.
132
MSV^VMISVVKPUJHWPSSHY`
Figure 12.5 Gaseous exchange between an alveolus and the blood
in a capillary.
12 • Gaseous Exchange
ITQ4
List the difference between the blood in
the capillary coming to, and the blood
leaving, the alveolus in figure 12.5.
cilia ❯
inspiration ❯
expiration ❯
The continuous exchange of gases in the lungs is extremely important. Body
cells can obtain a constant supply of oxygen for respiration and the carbon
dioxide that is constantly being produced is exhaled out of the body.
Gaseous exchange also occurs at the level of the cells. Here oxygen leaves
the blood and diffuses into the cells. Carbon dioxide moves in the opposite
direction (figure 12.1).
The trachea is lined with mucus, a slimy substance which traps and holds
dust and microorganisms. The trachea is also lined with microscopic hair-like
extensions called cilia. These beat in a wave-like manner, moving the mucus
containing dust and microorganisms upwards and out of the lungs.
Pathogens can enter the lungs with air as it is breathed in. The mucus and
cilia afford some protection by trapping and moving them out of the lungs.
If an irritating substance like dust is breathed in, this can stimulate a sneeze
during which the irritant is ejected out of the lungs.
The other parts of the respiratory system, namely the ribs, intercostal
muscles and diaphragm, are also involved in gaseous exchange. They help
to move air in and out of the lungs. Breathing in is called inspiration and
breathing out is called expiration. Table 12.1 compares the constituents
of inspired and expired air. Table 12.2 (overleaf) compares inspiration
with expiration.
Constituent
gases
Inspired Expired air
air
Reason for difference
oxygen
21%
16%
some of the oxygen is used by the cells of the body
during respiration
carbon dioxide 0.04%
4%
carbon dioxide is made by the cells and is transported
by blood to the lungs
nitrogen
78%
not used
78%
water content variable
usually higher the alveolar surface has a thin film of moisture to aid
gaseous exchange, and some of this evaporates
than when
inspired
temperature
usually higher air is warmed by the body heat while within the body
than when
inspired
variable
Table 12.1 A comparison of inspired and expired air.
133
Life Processes and Disease
Inspiration
Expiration
• External intercostal muscles contract (internal intercostals relax) and
the ribcage is raised.
• The muscles of the diaphragm contract and the diaphragm moves
downwards.
• Internal intercostal muscles contract (external intercostals relax) and
the ribcage is lowered.
• The diaphragm muscles relax and the diaphragm moves upwards.
HPYPU
HPYV\[
YPIZTV]L\W
HUKV\[
KPHWOYHNTJVU[YHJ[Z
TV]LZKV^U
]LY[LIYHS
JVS\TU
YPIZTV]LKV^U^HYK
HUKPU
KPHWOYHNTYLJVPSZ
\W^HYKZ
volume increased
volume decreased
HPYPU
HPYV\[
Z[LYU\TTV]LZ
\W^HYKZHUK
MVY^HYKZ
Z[LYU\TTV]LZ
KV^U^HYKZHUK
IHJR^HYKZ
YPIZ
KPHWOYHNTJVU[YHJ[Z
KPHWOYHNTYLJVPSZ
• These two movements increase the volume of the thorax.
• These two movements decrease the volume of the thorax.
• The pressure inside the thorax is lowered to below atmospheric
pressure. This pushes air into the lungs so they expand.
• The pressure inside the thorax increases which squeezed the lungs.
• Air rushes into the lungs through the mouth/nose and trachea.
• Air is pushed out of the lungs. It passes out through the trachea and
the mouth or nose, out of the body.
Table 12.2 A comparison of inspiration and expiration.
134
12 • Gaseous Exchange
Importance and mechanism of gaseous
exchange in plants
CHAPTER 8
ITQ5
(i) What is breathing and why is it
important?
(ii) Which muscles are involved in
breathing in humans?
ITQ6
Which gases leave and enter a leaf at:
(i) 12 noon?
(ii) 12 midnight?
The leaf is the respiratory surface or gaseous exchange surface. There are tiny
pores called stomata on the underside of the leaf through which the gases pass.
From the air space inside the leaf, the gases diffuse into and out of the plant
cell. The gases move down their concentration gradients (chapter 8).
During the day, plants photosynthesise and need carbon dioxide. Oxygen is
a waste product and must be removed. Plants respire all the time but, during
the day, photosynthesis is also being carried out. More oxygen is made in
photosynthesis than is used up in respiration and more carbon dioxide is used
than is made. So there is a net flow of oxygen out of the leaf and a net flow of
carbon dioxide into it.
At night photosynthesis stops because there is no light but respiration
continues. Oxygen moves into the leaf and carbon dioxide moves out
figure 12.6).
Day
Night
‹YLZWPYH[PVUVJJ\YZ
‹WOV[VZ`U[OLZPZPZ[OLmainHJ[P]P[`
‹YLZWPYH[PVUVJJ\YZ
‹UVSPNO[[OLYLMVYLUVWOV[VZ`U[OLZPZ
*6
6
*6
6
6
*6
*6/6*/66
WOV[VZ`U[OLZPZLX\H[PVU
*/66LULYN`/6*6
YLZWPYH[PVULX\H[PVU
Figure 12.6 The net flow of gases diffusing in and out of a leaf during the day and at night is
different.
Characteristics common to gaseous
exchange surfaces
gaseous exchange surface ❯
Gaseous exchange or respiratory surfaces are those surfaces where the
exchange of oxygen and carbon dioxide occur. These surfaces must have
certain characteristics that encourage:
• a lot of gaseous exchange to take place;
• gaseous exchange to take place quickly;
• gaseous exchange to take place continuously.
This means that organisms respiring aerobically can get a constant supply of
oxygen and remove carbon dioxide. Without oxygen, cells die and carbon
dioxide, if allowed to accumulate in cells, could poison and kill them.
135
Life Processes and Disease
Adaptations for efficient gaseous exchange
Large surface area
For gaseous exchange to take place quickly and in large amounts, respiratory
surfaces must have a large surface area or a large area over which the exchange
of gases can occur (figures 12.7, 12.8 and 12.9).
ISVVKIYV\NO[[V[OL
HS]LVS\Z¶P[PZSV^PU6
HUKOPNOPU*6
ISVVK[HRLUH^H`¶
P[PZYPJOPU6
HUKSV^PU*6
[OPU^HSSVMJHWPSSHY`
*6
[OPU^HSSVMHS]LVS\Z
6
*6 6
6 *6
SH`LYVMTVPZ[\YL¶V_`NLUKPZZVS]LZPU
[OPZTVPZ[\YLHUK[OLYLPZHS^H`Z
HOPNOJVUJLU[YH[PVUVMV_`NLU
UL_[[V[OLJHWPSSHY`
ISVVKMSV^ZJVUZ[HU[S`
S\UNZOPNOS`MVSKLK
[VPUJYLHZLZ\YMHJLHYLH
Figure 12.7 Adaptations of the lungs in humans for efficient gaseous exchange.
stiff gill rakers, which filter out food
particles from water as it passes over them;
the food particles are then swallowed
bony gill bar,
supporting the gill
SLHMPZ[OPUHUKMSH[MVY
SHYNLZ\YMHJLHYLH
^PUK
6
*6
6
6
6
6 *6
*6
V_`NLUSLH]PUNSLHMPZ
ISV^UH^H`SLH]PUNH
SV^JVUJLU[YH[PVU
HYV\UK[OLSLH]LZ
6
^PUK
soft, dark red gill lamellae, where
gas exchange takes place –
surface area greatly increased
WHSPZHKL
TLZVWO`SS
gill lamella
blood capillaries – bring blood
to pick up O2 and lose CO2,
take away blood rich in
O2 and low in CO2
water flowing around gills
Figure 12.8 Adaptations of the lamellae on the gill of a fish for
efficient gaseous exchange.
136
SLHMPZHIV\[
¶JLSSZ
[OPJR
*6
*6
ZWVUN`
TLZVWO`SS
6
6
*6
*6
6 HPYZWHJL
6
*6
Figure 12.9 Adaptations of leaves on a plant for efficient
gaseous exchange.
12 • Gaseous Exchange
MSV^VM^H[LYIYPUNZ
TVYL6
[OPUTLTIYHUL
MSV^VM^H[LY[HRLZ
*6H^H`
6
*6
ZTHSS]VS\TLYLSH[P]L[V
SHYNLZ\YMHJLHYLH
In humans, the lungs are made up of thousands of sacs called alveoli,
which, if laid out side by side, would cover a tennis court. In fish, gill lamellae,
which are part of the gills, form the respiratory surface. There are thousands of
lamellae in each gill creating a large surface area for exchange.
In plants, the respiratory surface is the leaf. Leaves are thin and flat to
create the largest area possible for gaseous exchange. On a tree, the thousands
of broad flat leaves show what a large surface area is available for gaseous
exchange.
Protozoans, like Amoeba, are microscopic unicellular organisms. Their
surface area to volume ratio is alreay large. Gaseous exchange occurs across the
cell membrane by diffusion and, because the cell is so small, the entire body of
Amoeba can be supplied with oxygen (figure 12.10).
Thin surface for gaseous exchange
TV]LTLU[VM^H[LY
Figure 12.10 The unicellular Amoeba
needs no special organ for gaseous
exchange. It is so small that the gases can
be exchanged efficiently across its cell
membrane.
ITQ7
What is the respiratory surface for each
of the following organisms: a human, a
fish, a plant, Amoeba?
For gaseous exchange to take place quickly, the respiratory surfaces must be
thin so that diffusion of the gases can take place rapidly.
In humans, the walls of the alveoli and capillaries are just one cell across.
The walls of the alveoli are also moist, so that the gases dissolve in the moisture
before they diffuse. In fish, the lamellae and capillaries are also one cell across
and diffusion can thus readily occur across the gills.
Air spaces inside leaves ensure that the gases can get close to most of the
cells into and out of which they must diffuse.
The cell membranes of protozoans are very thin and diffusion readily
occurs.
Constantly moving transport medium
For gaseous exchange to take place continuously, the medium which brings
the gases to the respiratory surface must be constantly moving. This ensures
that a concentration gradient is always maintained and diffusion will take
place constantly. For example, in humans and in fish there is a rich blood
supply constantly flowing past the respiratory surface. In humans, breathing
continuously refreshes the air in the lungs; in fish, water is continuously forced
across the gills.
In plants, the wind blows or moves the gases away from the leaves ensuring
that a concentration gradient is always maintained and that diffusion occurs
readily.
In unicellular protozoans, the water around the organism constantly takes
away and supplies the gases which dissolve easily in water.
The effects of smoking
Tobacco may be the cause of over 3 million deaths a year, worldwide. Death
from cigarette smoking comes mainly from lung cancer, but heart disease is
also associated with smoking. The products of cigarette smoke, (whether the
smoke is directly from smoking a cigarette or from inhaling another person’s
cigarette smoke) include nicotine, tar and (like car exhaust fumes) carbon
monoxide.
Nicotine
• Makes cigarettes highly addictive.
• Reduces air flow in and out of the lungs.
• Paralyses the cilia lining the trachea, so they cannot remove dirt and
bacteria.
137
Life Processes and Disease
• Raises blood pressure.
• Raises heart rate.
• Increases the risk of osteoporosis.
Osteoporosis is the loss of calcium carbonate from the bones which can happen
in older people. It makes the bones brittle, so they break more easily and are
more difficult to heal.
Tar
•
•
•
•
Sticks to cells in the lungs.
Causes the development of cancer.
Damages lung tissue.
Breaks down the alveoli, thus decreasing the surface area for gaseous
exchange.
• Causes bronchitis or inflammation of the lining of the air passages.
• Causes ‘smokers cough’.
Carbon monoxide
•
•
•
•
•
Combines irreversibly with haemoglobin in the blood.
Causes less oxygen to be transported by blood.
Reduces the smoker’s ability to take strenuous exercise.
Causes breathlessness.
If a pregnant woman smokes, carbon monoxide gets into the blood of the
fetus and combines with the haemoglobin. Less oxygen gets to the growing
tissues, resulting in a baby with a lower birth-weight; this is associated with
greater risk of health problems during and after birth.
Although studies show that there is a connection between cigarette smoking
and lung cancer, millions of people worldwide continue to smoke. A large
percentage of smokers are young people who become addicted very quickly
and continue to smoke throughout their lives. Statistics show that 25% of
smokers die of lung cancer. Figure 12.11 shows the effects of smoking on
human lungs.
(a)
(b)
Figure 12.11 A normal lung (a) and a cancerous lung (b). The cancerous lung came from a heavy
smoker.
138
12 • Gaseous Exchange
Marijuana addiction
This is a green or grey mix of dried shredded flowers and leaves of the hemp
plant Cannabis sativa – also called by many, many other names including pot,
herb, ganja and weed. The active ingredient is THC (tetrahydrocannabinol)
which provides to the ‘high’ that users experience when they smoke the drug.
The short-term effects of marijuana can include:
• problems with memory and learning;
• distorted perception;
• difficulty in thinking;
• difficulty in problem-solving;
• loss of coordination;
• increased heart rate;
• anxiety;
• panic attacks.
Marijuana smoke is unfiltered, users inhale more deeply and hold the smoke
in the lungs. The effects on the lungs are thus greater than those caused by
tobacco smoke because more tar and more carbon monoxide are inhaled.
Chapter summary
• Living cells need a constant supply of oxygen, and need to get rid of carbon dioxide
as respiration takes place.
• Gaseous exchange is the exchange of these gases and takes place at the respiratory
surface.
• In humans, the respiratory surface is the lungs.
• Plants also photosynthesise and thus, during the day, require carbon dioxide and must
get rid of oxygen.
• In plants, the respiratory surface is the leaf.
• Characteristics common to respiratory surfaces are:
– a large surface area;
– thin walls;
– rich blood supply;
– presence of moisture.
• In humans, the lungs are adapted for gaseous exchange.
• The gills of fish are adapted for gaseous exchange.
• In plants, the leaves are adapted for gaseous exchange.
• Gaseous exchange occurs across the cell membrane of Amoeba.
• The components of cigarette smoke include nicotine, tar and carbon monoxide.
• Cigarette smoking causes lung cancer and is dangerous to good health.
139
Life Processes and Disease
Answers to ITQs
ITQ1 (i) Gaseous exchange is the exchange of gases, in particular oxygen
and carbon dioxide.
(ii) In the lungs, where gases are exchanged between the alveoli and blood.
In the tissues, where gases are exchanged between the blood and cells.
ITQ2 These are complete rings which keep the trachea open. They also
support the trachea.
ITQ3 nose trachea bronchus bronchiole alveolus capillary
ITQ4 Blood in the capillary approaching the alveolus contains a higher
concentration of carbon dioxide and lower concentration of oxygen than blood
leaving the alveolus.
ITQ5 (i) Breathing is the process whereby air is pushed into the lungs by
atmospheric pressure and expelled from the lungs by muscular contraction.
It is important because it brings a supply of oxygen, which is needed for
respiration, and it also takes away carbon dioxide, a waste gas.
(ii) The muscles involved in breathing are: the diaphragm muscles and the
external and internal intercostal muscles.
ITQ6 (i) At noon, oxygen is leaving the plant and carbon dioxide is entering
the leaf because photosynthesis is happening at a much faster rate than
respiration can use the oxygen or produce carbon dioxide.
(ii) At midnight, there is no light so photosynthesis cannot take place.
Respiration is the only process that is occurring, so carbon dioxide is leaving
the plant and oxygen is being taken in.
ITQ7 Human respiratory surface is the alveolus;
fish – gill; plant – leaf; Amoeba – cell membrane.
Examination-style questions
1
(i)
Inhalation and exhalation are movements that ventilate the lungs. The diagram below
shows the ribs and diaphragm.
YPIZ
KPHWOYHNT
(a) Copy the diagram and use arrows to show the movements of the diaphragm, the
ribs and air during exhalation.
(b) Explain fully how the volume of the thorax is increased by giving details of
contraction and relaxation of muscles involved in raising of the ribs and lowering
of the diaphragm.
140
12 • Gaseous Exchange
(ii) List three ways inhaled air differs from exhaled air.
(iii) A boa constrictor kills its prey by squeezing it to death. This is termed ‘asphyxiation’.
Explain how asphyxiation results in death.
2
(i)
Define the following terms:
(a) gaseous exchange;
(b) respiratory surface.
(ii) List three characteristics of respiratory surfaces. Describe how the lungs of humans
are adapted in these three ways to increase the rate of gaseous exchange.
(iii) The nicotine found in tobacco smoke can prevent the beating of cilia in the trachea.
Suggest how this contributes to the development of lung diseases.
(iv) List two effects of each of the following products of cigarette smoke:
(a) nicotine (b) tars.
(v) How are plants adapted to exchange gases efficiently by diffusion?
3
The apparatus shown in the diagram below is used to investigate the chemicals in
cigarette smoke.
HPY
V\[SL[
SPNO[LK
JPNHYL[[L
JV[[VU^VVS
(i)
After some time, the cotton wool turns black and appears oily.
(a) What are the black particles trapped in the cotton wool?
(b) What chemical causes the oily appearance?
(ii) Describe and explain the colour change of the litmus paper.
(iii) If a cigarette with a filter is used, what difference in the appearance of the cotton wool
would you expect?
(iv) One of the gases in cigarette smoke is carbon monoxide. What is the effect of carbon
monoxide on the body?
(v) How do chemicals in cigarette affect the cilia in the trachea?
(vi) Name two respiratory diseases that may be caused by prolonged smoking.
141
13
By the end of this
chapter, you should
be able to:
Transport and
Defence in Animals
understand why there is a need for a transport system in multicellular organisms
understand why certain materials must be transported in animals
describe the circulatory system of humans
relate the structure of the heart to its function
relate the structure of blood vessels to their function
describe the composition and functions of blood
understand how and why blood clots
understand blood groups and their importance in blood transfusion
understand the nature and danger of hypertension
describe the role of blood in defending the body against disease
explain how the principles of immunisation are used in the control of
communicable diseases
plasma
red blood cells
haemoglobin
blood
white blood cells
platelets
hypertension
transport system
in humans
clotting
action of
heart
heart
heartbeat
functions
take important
substances
to cells
take waste
products
away from cells
blood vessels
arteries
veins
capillaries
The need for a transport system
Large multicellular animals, like humans, have a large volume in relation to
their surface area. Substances would therefore take a long time to diffuse from
the air into the body and would get to cells deep in the body at a much slower
than the rate at which they are needed by the cells.
Imagine oxygen diffusing into the skin of an organism. It may not be able to
get to all the cells of the skin, far less the cells of organs inside the body. Oxygen
would have to pass through millions of cells to get to the liver. Also, the skin is
142
13 • Transport and Defence in Animals
ITQ1
A unicellular organism like Amoeba
does not have a transport system and
a multicellular organism like a human
cannot live without one. Explain why
this is so.
CHAPTERS 10, 11, 12, 16, 18
ITQ2
Name two substances which must be
transported to a cell and explain why
each substance is needed.
circulatory system ❯
a tough waterproof layer and may also be covered with hairs, fur and feathers.
It is impossible for oxygen to diffuse to cells inside the body of a mammal or
other large organism. In any organism larger than a few cells, any substance
needed by a cell within the bocy must be specially transported to the cell.
A transport system is necessary to get important and needed substances
to every single cell and also to transport waste or toxic substances away from
every cell. Just to stay alive, a multicellular organism requires a constant
supply of substances like oxygen and glucose to all its cells. When active, these
substances are required in even greater amounts.
Table 13.1 shows some of the substances which need to be transported in
animals.
Substance to
Transported from
be transported
Transported to
dissolved food
(chapter 10)
ileum where it is absorbed
cells of the body – to be used for
respiration, stored, converted to other
materials, etc.
nitrogenous
waste
(chapter 16)
cells where produced
kidneys to be excreted
oxygen
(chapter 12)
lungs where it diffuses into the
blood
body cells to be used for respiration
carbon dioxide
(chapter 11)
body cells where it is produced in
respiration
lungs to be excreted
hormones
(chapter 18)
endocrine glands where they are
produced
organs where their effects are needed
white blood
cells including
antibodies
marrow of bones where they are
produced
where there are infections or invasions by
microorganisms
Table 13.1
Some substances which are transported in animals.
The circulatory system of humans
Blood is the means by which substances travel to and from cells. These
substances dissolve in blood, which is mainly water and diffused into the cells
where they are needed. The blood is transported around the body in blood
vessels. The heart helps to push blood around the body.This transport system is
called a circulatory system. Most substances dissolve in the plasma, but the
red blood cells are specialised to transport oxygen.
The circulatory system is made up of three parts:
• the heart, which is a pump;
• the blood, which is the fluid being pumped and contains all the materials to
be transported around the body;
• the blood vessels (like pipes) through which blood flows to get to and from
the cells, these are the arteries, veins and capillaries.
The structure of the heart
The heart pumps blood so that it can get around the body. It pushes blood
forcibly thus causing it to be constantly moving in the blood vessels. The
walls of the heart are made of a special type of muscle, called cardiac muscle.
143
Life Processes and Disease
cardiac muscle ❯
atrium ❯
ventricle ❯
Cardiac muscle contracts and relaxes regularly and constantly throughout
life. It never grows tired. But it may stop working if it is not supplied with the
substances it needs to release energy – oxygen and glucose. These are supplied
via the coronary circulation.
The mammalian heart is divided into a right side and left side (figure 13.1).
Each side has two parts or chambers:
• the atrium, which receives blood;
• the ventricle, which pumps blood away.
(a)
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Figure 13.1 The heart: (a) showing its blood supply and (b) in section.
The action of the heart
Deoxygenated blood, that is blood coming from the body cells where some of
the oxygen has been used in respiration, flows into the right atrium through
the vena cava. This blood is also rich in carbon dioxide made during respiration
in the cells. The blood must now be transported to the lungs where it can load
up with more oxygen and offload the excess carbon dioxide.
144
13 • Transport and Defence in Animals
tricuspid valve ❯
bicuspid valve ❯
ITQ3
The heart beats continuously for years.
How is heart muscle nourished and
supplied with oxygen and glucose?
From the right atrium, blood passes through the tricuspid valve into the
right ventricle. The walls of the ventricle contract and the blood is pushed into
the pulmonary artery and travels to the lungs. There, gases are exchanged.
Excess carbon dioxide leaves the blood and diffuses into the lungs, and oxygen
moves into the blood from the alveoli.
Oxygen-rich blood returns from the lungs via the pulmonary veins and
flows into the left atrium. It passes through the bicuspid valve and flows
into the left ventricle. The thicker muscular walls of the left ventricle contract
strongly and blood is pushed forcefully into the aorta and all the way around
the body. Blood therefore flows through the heart twice in one circuit of the
body (figure 13.2)
OLHKULJR
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ISVVKMYVTOLHK
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Figure 13.2
atrioventricular valves ❯
semi-lunar valves ❯
ITQ4
Describe the route taken by a red blood
cell from the vena cava to the aorta.
systole ❯
diastole ❯
V_`NLUH[LK
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Blood flows through the heart twice in one circulation.
Valves prevent the back-flow of blood in the heart. The bicuspid and
tricuspid valve, known as the atrioventricular valves, ensure that blood
flows in one direction through the heart only. Tendons attached to the walls of
the heart hold them in place. When the ventricles contract, blood pushes back
on these valves, forcing them shut. So the blood can only move forward into
the pulmonary arteries and aorta.
Semi-lunar valves are found at the start of the pulmonary artery and
aorta. They prevent the back-flow of blood into the ventricles when they relax.
Heartbeat
The heart ‘beats’ when the muscles of the heart contract and relax. There
are three phases to a heart beat. The sounds heard – ‘lub dub’ – are the
sounds made by the valves closing and blood hitting the valves. The ‘lub’
sound is made during ventricular systole as blood is forced against the closed
tricuspid and bicuspid valves. The ‘dub’ sound is made during ventricular
diastole when blood impacts on the closed semi-lunar valves in the aorta and
pulmonary artery. The third stage, diastole, when blood flows into the empty
atria and ventricles, makes no sound (figure 13.3).
The rate of heartbeat is controlled by the ‘pacemaker’, which is found in the
muscle between the ventricles. It has its own natural rhythm of stimulating
145
Life Processes and Disease
contractions, which is usually around 70–80 beats per minute. This can be
speeded up by hormones such as adrenalin, and by activity.
MYVTOLHK
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Z`Z[VSL
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JVU[YHJ[HUKW\ZOISVVKV\[VM[OLOLHY[
Figure 13.3 The three phases of a heartbeat.
ITQ5
List the three main stages of the
heartbeat and explain the importance
of each.
artery ❯
capillary ❯
vein ❯
arterioles ❯
venules ❯
146
Blood vessels
Blood flows through blood vessels to get to all parts of the body from the heart
and then from the body back to the heart. There are three kinds of blood
vessel:
• arteries (and arterioles) which carry blood away from the heart;
• capillaries which are tiny vessels that pass close to all body cells;
• veins, (and venules) which carry blood back to the heart.
An artery branches into smaller and smaller vessels called arterioles. These
branch into even smaller and smaller vessels, until the vessels are very small
and the walls are only one cell thick. These tiny vessels are called capillaries.
Capillaries flow in between the cells of the organs and the exchange of
substances food, oxygen, wastes, etc. takes place at this level.
Capillaries then join up to form larger and larger vessels called venules,
which then join to form veins which carry blood back to the heart.
Fiigures 13.4 ansd 13.5 show the relationships between the three types of
blood vessel. Table 13.2 compares arteries, veins and capillaries.
13 • Transport and Defence in Animals
V_`NLUMVVKL[JKPMM\ZL
V\[VM[OLISVVK
^HZ[LWYVK\J[ZKPMM\ZL
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Figure 13.4 The relationship between arterioles, capillaries and venules.
]LPU
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Figure 13.5 A network of capillaries surround all the cells of tissues.
Arteries
MPIYV\ZSH`LY
Capillaries
^HSSJVTWVZLKVMH
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• thick elastic walls to withstand the hgh
pressure of blood and absorb some of
the energy of the pulse
Veins
• walls one cell across – thin enough for
diffusion to take place easily
SHYNLS\TLU
[OPUT\ZJSLHUK
LSHZ[PJSH`LY
• thin elastic walls (do not have to withstand high
pressure)
(continued)
147
Life Processes and Disease
Arteries
Capillaries
Veins
• carry blood away from the heart
• carry blood to the cells of the tissues
and organs
• carry blood towards the heart
• blood pressure is high
• blood pressure decreases along the
length of the capillaries
• blood at low pressure
• blood flows rapidly in pulses created by • blood flow is smooth and slow
contractions of the ventricles (this is the
pulse you can feel most easily at your
wrist)
• smooth and slow flow – the large lumen offers little
resistance
• carry oxygenated blood, except the
pulmonary artery
• as it flows through a capillary network • carry deoxygenated blood, except the pulmonary vein
the blood loses oxygen to body cells and
gains carbon dioxide
• lie deep within the body
• run through the tissues
• lie close to the body surface
• no valves present
• no valves
• valves prevent the back-flow of blood because the
‘push’ of the heart is not felt here
MSV^
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• blood can flow in one direction only
ITQ6
Describe two differences between
blood leaving an arteriole and blood
entering a venule, having passed across
a capillary network.
aorta ❯
vena cava ❯
coronary arteries ❯
CHAPTER 16
Table 13.2 The main differences between arteries, capillaries and veins.
The circulation
Blood leaves the left side of the heart at a high pressure and flows through the
aorta, the largest artery, to all parts of the body. When the capillaries reach
the body cells, the blood gives up food and oxygen and picks up wastes, such
as carbon dioxide and urea. Deoxygenated blood returns to the heart via the
veins which collect into a main vein called the vena cava. From the right side
of the heart, blood flows to the lungs to be oxygenated, then back to the left
side of the heart. This flow is repeated continuously. The tissues of the heart
itself are supplied with oxygen by the coronary arteries.
In its circulation throughout the body, blood picks up food (such as glucose
and amino acids) from the gut, hormones from endocrine glands, and other
vital substances. It also drops off waste products to be excreted, like urea and
carbon dioxide, at sites where the body an get rid of them, that is the kidneys
(chapter 16) and the lungs (figure 13.6).
ITQ7
Why does the aorta have the thickest
walls of all the vessels in the
circulatory system?
148
ITQ8
(i) What is the pulse?
(ii) What is the pulse rate?
13 • Transport and Defence in Animals
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ITQ9
Describe the route taken by a red blood
cell from the renal vein to the hepatic
vein.
ITQ10
Describe the differences in composition
between blood:
(i) in the renal artery and the renal vein
(ii) in the pulmonary artery and
pulmonary vein. blood plasma ❯
red blood cells
white blood cells ❯
platelets ❯
V_`NLUH[LKISVVK
Figure 13.6 The circulatory system in humans.
Blood
Blood is the medium by which substances or materials are transported. It is
made up of about:
• 55% blood plasma;
• 45% blood cells.
The blood plasma is about 90% water and most of the substances which
must be transported are dissolved in it. This includes dissolved food (glucose,
amino acids, fatty acids and glycerol), carbon dioxide (as the bicarbonate ion),
nitrogenous wastes, hormones and mineral salts (as ions such as Na+, K+, Cl+).
The blood cells are of two main types, red and white. There are also
fragments of cells called platelets.
Table 13.3 (overleaf) summarises the structure and function of blood cells.
149
Life Processes and Disease
Blood cell
Function
Red blood cells or erythrocytes
• biconcave disc shape (squeezed in from both sides)
gives large surface area for diffusion
• have no nucleus so only live for 3–4 months
• new cells constantly made in the bone marrow and
destroyed in the liver and spleen
• contain the red pigment haemoglobin which combines
with and releases oxygen readily
• 1 mm3 of blood contains about 5 million of these cells
JVU[HPUZ
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transport oxygen combined with haemoglobin,
from the lungs to tissues where the oxygen is
given up readily
White blood cells or leucocytes
• two main types: phagocytes and lymphocytes
Phagocytes
• move like Amoeba by pseudopodia – can move through
the capillary walls to sites of infection
• formed in bone marrow
engulf disease-causing organisms at sites of
infection
SVILKU\JSL\Z
Lymphocytes
• produce antibodies
• formed in lymph nodes and spleen
produce antibodies that kill pathogens by causing
them to clump together, or neutralise their toxins
YV\UKU\JSL\Z
Platelets or thrombocytes
• cell fragments
• no nucleus
• formed in bone marrow of lone bones
help blood to clot to prevent loss
Table 13.3
ITQ11
Protection of the body is one of the
functions of blood. List two components
of blood concerned with protection and
explain how each works.
ITQ12
Describe the process that leads to
blood clotting after a cut to a blood
vessel.
Structure and function of blood cells.
Carriage of oxygen and carbon dioxide in
the blood
The respiratory gases, oxygen and carbon dioxide, are transported around the
body in the blood. Most of the carbon dioxide is transported in solution in
blood plasma as hydrogen carbonate ions. Oxygen is carried by the molecule
haemoglobin, which is found inside red blood cells. Haemoglobin is a protein
that is combined with iron – this gives it its red colour. Each molecule of
haemoglobin combines reversibly with up to four molecules of oxygen.
haemoglobin + oxygen oxyhaemoglobin
The oxygen is readily given up in the body tissues where oxygen levels are low.
The body cells can then use the oxygen for respiration.
oxyhaemoglobin haemoglobin + oxygen
Red blood cells are so full of haemoglobin that there is no space for a nucleus.
That is why they only survive for 3–4 months, after which they are cleaned out
of the blood by the liver.
150
13 • Transport and Defence in Animals
Blood clotting
blood clot ❯
haemorrhage ❯
When the skin is cut and a small blood vessel is broken, a blood clot forms
to prevent further blood loss (figure 13.7). A series of reactions take place at
the site of the cut vessel which results in the formation of fibrin, an insoluble
fibrous protein which traps blood cells and plugs the gap (figure 13.8).
The clot also prevents the entry of disease-causing organisms. Loss of blood
from a vessel is called a haemorrhage, and losing a lot of blood could result in
death. In this case, a blood transfusion can be given to replace blood and save
the person’s life.
H
KPZLHZLJH\ZPUN
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The cut is sealed
with a blood clot
A cut vessel
I
plateletsL_WVZLK[VHPYPUKHTHNLK[PZZ\L
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]P[HTPU2
thrombin
HJ[P]L
prothrombin
PUHJ[P]LWYV[LPU
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fibrinogen
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fibrinPUZVS\ISLMPIYLZ
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JLSSZHUKMVYTHJSV[
Figure 13.7 The formation of a blood clot.
Blood groups
Figure 13.8
fibrin.
Red blood cells trapped in
donor ❯
recipient ❯
During a blood transfusion, a person is given another person’s blood. Early
attempts at transfusion worked in some cases, but in many they resulted in
death. We now know that for a transfusion to be successful, the two sets of
blood must be compatible – able to mix wirht each other without the red cells
sticking together..
There are four blood groups, known as A, B, AB and O. These groups are
based on proteins, called antigens, that are present on the surface of red blood
cells. For example if antigen A is present on all the red blood cells of a person,
that person is said to have blood group A.
There are also antibodies present in the blood plasma. These are associated
with the antigens. So a person with blood group A, for example, has antigen A
(A) on their red cell and and antibody anti-B (b) in their plasma (table 13.4).
During a transfusion it is important to note:
• the protein (or antigen) on the red blood cell of the donor;
• the type of antibody present in the plasma of the recipient.
If the antibody matches the antigen, the red blood cells stick together and
transfusion will not be successful.
151
Life Processes and Disease
Table 13.4 shows the success of transfusion for all the blood groups. A tick
means that this combination of donor and recipient will make a successful
transfusion; a cross indicates that this combination will lead to a reaction
(potentially fatal) in the recipient.
Donor’s blood type
Recipient’s
blood type
ITQ13
State whether these transfusions
are possible:
• donor AB, recipient O
• donor AB, recipient A
• donor O, recipient A
• donor B, recipient A
• donor B, recipient B
A
B
AB
(A)
(B)
(A) (B)
O
none antigen present
A
(b)
✓
✘
✘
✓
B
(a)
✘
✓
✘
✓
AB
none
✓
✓
✓
✓
O
(a) (b)
✘
✘
✘
✓
K
antibody
present
universal recipient: blood
group AB
K
universal donor: blood group O
Table 13.4 The success or failure of blood transfusions between different blood groups.
Hypertension
hypertension ❯
stroke ❯
ITQ14
(i) What is hypertension?
(ii) What factors in a person’s life may
increase the chances of suffering
from hypertension?
152
High blood pressure is when the pressure caused by the blood pushing against
the inside walls of the main arteries is high. Persistent high blood pressure is
called hypertension.
Capillaries are tiny blood vessels, with walls that are one cell across. Blood
flowing at a high pressure can cause these vessels to burst. If a vessel in the
brain burst, then a portion of the brain becomes damaged from a lack of
oxygen. This is called a stroke and can result in paralysis or even death.
Capillaries in other important organs like the kidneys may burst because
of high blood pressure. This could lead to shutdown of the organ (e.g. kidney
failure) and can have serious consequences on the body.
Hypertension can develop without symptoms or signs and is sometimes
called the ‘silent killer’. It is linked with a number of factors such as:
• high levels of emotional stress;
• lack of exercise;
• obesity;
• tobacco smoking;
• high alcohol intake;
• high blood cholesterol levels.
All these factors are influenced by lifestyle, and can be controlled by changing
lifestyle. A healthy lifestyle, that includes regular exercise, no smoking, low
intake of fat, salt and alcohol, can prevent the development of hypertension.
13 • Transport and Defence in Animals
The role of blood in defending the body
against disease
physical barrier ❯
phagocytes ❯
Microorganisms are all around us. These microscopic organisms (viruses,
bacteria, etc.) are in the air we breathe, in the food we eat, on everything we
touch and all over our bodies.
The skin is the body’s first line of defence (figure 13.9). It acts as a physical
barrier. When there are breaks in this barrier, such as cuts or sores, the
body reacts to produce blood clots and a meshwork of fibrous scar tissue. The
opening is thus blocked, which prevents pathogens (microorganisms that can
cause disease) from entering the body.
Sometimes the white blood cells called phagocytes move out of the blood
and to the infected areas. There they engulf the invading microorganisms,
killing and removing them from the body before they can cause disease. This is
our second line of defence (figure 13.10).
wax in the auditory
canal traps dust
and other particles
tears contain a mild antiseptic
hairs in the nose trap dust
and other particles
trachea lined with mucus
and cilia to move dust
and other particles out of the lungs
skin – a
physical barrier
stomach produces hydrochloric
acid which can kill microorganisms
vagina – mucus moves out constantly
break in the skin – a clot forms
Figure 13.9 The skin is our first line of defence. Any openings in the skin have special means of
expelling dust which carries many disease-causing organisms.
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Figure 13.10 White blood cells
(phagocytes) are our second line of
defence. They leave the blood and
migrate to a site of infection.
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153
Life Processes and Disease
Immune response
antibody ❯
The phagocytes can cope with any small, non-specific invasion by pathogens.
If more dangerous, specific pathogens enter, then an immune response is
activated. In this case, another kind of white blood cell, called lymphocytes,
recognise the specific pathogen and mobilise other lymphocytes to make
antibodies to attack, disarm, destroy and remove these pathogens.
antigen ❯
Antigens and antibodies
lymphocyte ❯
immune response ❯
memory lymphocytes ❯
natural immunity ❯
Anything that is foreign or different and causes antibody formation is called
an antigen. This is our third line of defence against disease.When antigens,
such as the measles virus, enter the body, lymphocytes recognise them and
start to produce specific antibodies on a large scale to destroy the viruses. The
immune response is very specific – only the antibodies for that particular
antigen are made.
To defend the body against disease, antibodies act in a number of ways:
• they cause the antigens to clump together resulting in their death and easy
removal by the phagocytes;
• they neutralise toxins produced by the antigens;
• they prevent the antigen from entering body cells.
Recognition of antigens and production of the specific antibodies against them
takes time. During that time, the antigens will have produced symptoms of
the disease. Once the antibodies are produced, the antigens or the toxins
they produce are destroyed or neutralised and the symptoms disappear. The
antibodies then gradually disappear from the blood, but they leave behind
special memory lymphocytes. If the specific antigen invades a second time,
the memory lymphocytes immediately recognise them, and rapidly make large
amounts of the specific antibody. This time, the antigens are destroyed before
symptoms develop, and the person is said to be immune to that disease. This
happens naturally and is called natural immunity (figure 13.11).
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Figure 13.11
ITQ15
We are surrounded by pathogens. How
is the body protected from infection?
154
Immunity is a rapid large increase of antibodies in the blood.
There are two types of natural immunity.
• Actively acquired immunity – When the body has already experienced
an infection by a pathogen or antigen, as described above, the lymphocytes
produce large quantities of antibodies to fight the disease before symptoms
develop a second time.
• Passively acquired immunity – Antibodies can pass across the placenta
providing a newborn baby with immunity against diseases that the mother’s
13 • Transport and Defence in Animals
body is immune to. Also, antibodies present in breast milk help to protect
the baby against antigens.
Immunisation and the control of communicable
diseases
vaccination ❯
ITQ16
Explain what is meant by a vaccine.
artificial immunity ❯
ITQ17
Copy and complete the table.
Natural
immunity
Artificial
immunity
Active
Passive
Immunisation provides immunity to communicable diseases. This is achieved
by injecting, or administering orally, small amounts of dead or weakened
(attenuated) antigens into the body. This is called vaccination. The body is
stimulated to produce antibodies.
One example is the MMR vaccine given at around 2 years of age or
younger to protect children against measles, mumps and rubella. DTP vaccines,
administered at any age, protect against diphtheria, tetanus and pertussis
(whooping cough)
Smallpox has been eradicated because of immunisation programmes.
Vaccines against tuberculosis (TB) and hepatitis B have also been developed,
but there are still not vaccines against diseases such as cancers, leprosy, malaria
and AIDS, despite much research. The World Health Organization (WHO)
Expanded Program of Immunisation (EPI) aims to extend immunisation to
children all over the world, especially in developing countries so that children
can be immunised at no cost to their parents. Immunisation is known as
artificial immunity.
There are two types of artificial immunity,
• Actively acquired – This is by vaccination at a suitable time in the person’s
life, when they are not infected with the antigen. The vaccine used contains
treated antigens which cannot cause the disease, but which can stimulate
the body to make antibodies. Immunity is obtained because if the real
antigen should enter the body, antibodies are immediately and rapidly
produced to destroy it. This happens before symptoms develop and the
person is said to be immune to that disease.
• Passively acquired – The vaccine contains ready-made antibodies which
provide immediate relief by destroying the antigens. This is given when the
person has been infected with the antigens and has no previous immunity.
The importance of immunisation or vaccination is seen when children are
protected from dangerous diseases like polio, measles, mumps, tetanus
and whooping cough (figure 13.12). This is achieved in a programme of
immunisation where often a second, booster injection is given. This stimulates
a much quicker production of antibodies which is longer lasting and which
protects the child from the disease for a considerable time.
ITQ18
Explain the meaning of the term
‘immunisation’ and give one advantage
of immunisation.
(a)
(b)
(c)
Figure 13.12 (a) Mumps and (b) chickenpox are common childhood diseases. In some children
they can cause long-term damage. (c) Poliomyelitis can cause life-long damage to the body even
after the infection has gone.
155
Life Processes and Disease
Chapter summary
• Large multicellular organisms have a small surface area-to-volume ratio. This means
that they need transport systems to carry substances to and from cells around the
body.
• A cell needs nutrients, oxygen and other substances to stay alive.
• Waste products are produced and need to be removed from cells so they do not
damage them.
• The transport system of humans is composed of a pump (heart) a transport medium
(blood) and vessels (blood vessels) through which blood flows. This is the circulatory
system.
• The structure of the heart is suited to its function as a pump.
• Blood passes through the heart twice for each time it circulates the body; after one of
these passes through the heart blood goes to the lungs for the exchange of gases.
• There are three kinds of blood vessel: arteries, veins and capillaries.
• Blood is composed of plasma, blood cells and platelets.
• Plasma is mainly water with the substances being transported dissolved in it.
• There are two types of blood cell; red blood cells transport oxygen and white blood
cells protect the body against pathogens.
• Platelets help blood to clot; this is important to prevent blood loss.
• For a successful blood transfusion, the donor’s and the recipient’s blood groups
must match because if the antigens and antibodies in their blood react together, the
transfusion will not be successful.
• Hypertension is persistent high blood pressure, which is dangerous to health.
• White blood cells protect the body against pathogens.
• Phagocytes can leave the bloodstream, gather at sites of infection and engulf and kill
pathogens.
• The body has three lines of defence against infection: the skin (and blood clotting),
phagocytes and the immune system.
• In the immune system, lymphocytes form antibodies which are specific for the
pathogen.
• After an infection, memory cells remain in the blood, which recognise the pathogen
again quickly. A second infection does not result in symptoms of the disease because
the production of antibodies is much faster and greater.
• A person is immune to a disease if, on infection with the disease, no symptoms
develop.
156
13 • Transport and Defence in Animals
Answers to ITQs
ITQ1 In the unicellular organism, the surface area to volume ratio is large,
which means that there is a lot of surface area for the volume of the organism.
Diffusion can occur fast enough across the cell membrane and get to all parts of
the cell for all life processes to happen effectively.
In a multicellular organism, for each cell to get a supply of oxygen and
everything else it needs as fast as it needs it, a transport system is necessary
because the surface area is not large enough in proportion to the volume for
diffusion from the external environment to be effective.
ITQ2 (i) Oxygen is used in respiration, which in turn provides the body
with energy.
(ii) Glucose is oxidised during respiration to provide the body with energy.
(You may have chosen other substances.)
ITQ3 Heart muscle has its own set of blood vessels, called the coronary
arteries and coronary veins. The coronary arteries supply it with glucose and
oxygen needed for respiration.
ITQ4 vena cava right auricle right ventricle pulmonary artery lungs pulmonary vein left ventricle aorta
ITQ5 Atrial systole – pushes blood from the atria into the ventricles.
Ventricular systole – pushes blood out of the heart, so that it can be pumped
to the lungs through the pulmonary artery, and through the aorta to all parts
of the body.
Diastole – allows blood from the body to collect in the atria, before it is
forced into the ventricles by contraction of the muscles around the atria.
ITQ6 Blood leaving arteriole:
• has a lot of oxygen in it as the arteriole carries oxygenated blood to body cells;
• is rich in glucose, hormones, water, vitamins, etc., which will be used by the
cells for different purposes.
Blood entering venule:
• has less oxygen, as some has been used by the cells in contact with the
capillary network;
• has less of other substances, as the blood has been depleted of these when
passing through the capillary network
• has more carbon dioxide and other waste products from cells.
ITQ7 The aorta receives blood at the highest pressure from the contraction
of the muscles of the left ventricle. As the blood enters the aorta, its thick
muscular walls are stretched but do not burst.
ITQ8 (i) Each heartbeat results in a surge of blood, which can be felt as
the arterioles stretch to accommodate blood flowing at a high pressure. Each
heartbeat results in one pulse.
(ii) The pulse rate shows the rate at which the heart is beating because each
heartbeat is felt as one pulse.
ITQ9 renal vein vena cava heart lungs heart aorta hepatic
artery liver hepatic vein
Ļ
Ĺ
mesenteric artery gut hepatic portal vein
ITQ10 (i)
Renal artery
Renal vein
rich in oxygen
little oxygen present
little carbon dioxide present
rich in carbon dioxide
rich in glucose
little glucose present
157
Life Processes and Disease
(ii)
Pulmonary artery
Pulmonary vein
rich in carbon dioxide
little carbon dioxide
little oxygen present
rich in oxygen
ITQ11 Platelets are involved in blood clotting, which prevents entry of
pathogens when there is a cut on the skin.
White blood cells destroy foreign bodies that might harm the organism.
ITQ12 On exposure to air, platelets in the blood, in the presence of calcium
ions and vitamin K, cause prothrombin, an inactive blood protein, to be
converted to thrombin. The presence of thrombin causes t he conversion of
fibrinogen, another inactive blood protein, into fibrin. Fibrin is made up of
insoluble fibres that trap red blood cells and form a clot.
ITQ13
• donor AB, recipient O: no
• donor AB, recipient A: no
• donor O, recipient A: yes
• donor B, recipient A: no
• donor B, recipient B: yes
ITQ14 (i) Hypertension is prolonged high blood pressure.
(ii) Factors that contribute to the chances of suffering from hypertension are:
• a genetic predisposition (having a relative who suffers from the disease);
• smoking;
• obesity;
• diet that contains many fatty foods;
• no exercise;
• old age.
ITQ15 The body has three lines of defence against infection:
• the skin is a physical barrier and openings in the skin have special
mechanisms, such as blood clotting, to keep pathogens from entering the body;
• if pathogens get into the body through a wound, phagocytes migrate to this
site and ‘eat’ the invading microorganisms;
• antibodies are produced to seek out antigens (foreign invaders like bacteria)
that have entered the body, and destroy them or neutralise their effects. They
are completely destroyed at this point.
ITQ16 A vaccine is a substance injected into the body. It contains antigens
which cause the immune response, or antibodies which protect the body.
ITQ17
Natural immunity
Artificial immunity
Active
Antigens enter the body naturally
and bring about the immune
response
Antigens are introduced in a vaccine
to the body and the immune
response is generated
Passive
Antibodies enter the body naturally Antibodies are introduced in a
and protect the body against disease vaccine to the body so the body is
protected
ITQ18 Immunisation is a programme of dispensing vaccines for various
diseases. The act of introducing the vaccine to a person is also immunisation.
One advantage is protection against disease.
158
13 • Transport and Defence in Animals
Examination-style questions
1
The functions of blood include transport of substances and protection against blood loss
and infection
(i) Describe the transport of a named substance from the site where it is picked up by
blood to a named body cell.
(ii) Describe how a blood clot forms and how the clot protects the body against blood loss
and infection.
2
(i)
3
(i) Compare the structure of an artery with that of a vein.
(ii) How does the structure of an artery related to its role as a blood vessel?
(iii) Describe how a glucose molecule moves:
(a) from the heart to a capillary next to a body cell;
(b) from the capillary into the cell.
(iv) Compare the composition of blood as it enters and leaves the lungs.
A blood vessel in the brain may burst, resulting in a condition called a stroke. Major
strokes can result in severe paralysis or even death. Minor stokes may occur without
symptoms.
4
(i)
Make a labelled drawing of a transverse section of the heart. Use arrows to show the
movement of blood through the heart.
(ii) Describe the role of the heart in the circulatory system.
(iii) Explain why the muscles of the left ventricle are thicker than those of the right.
(iv) Explain the effects on the heart if the coronary artery becomes blocked.
Suggest a likely consequence of the bursting of a blood vessel in the brain and how
this could result in paralysis or even death.
(ii) Suggest reasons why some strokes occur without symptoms.
159
14
By the end of this
chapter, you should
be able to:
Transport in Plants
describe the movement of substances in plants
understand the structure of xylem vessels, sieve tubes and companion cells
understand how the structure of xylem vessels suits them for their function
describe the movement of water through a plant
describe the processes involved in transpiration
describe the effect of external factors on transpiration
discuss adaptations in plants to conserve water
understand the function of phloem in the transport of substances in plants
explain how the structure of the phloem is suitable for its function
transport in plants
vascular bundle
xylem
phloem
uptake of
water from soil
translocation –
movement of food
water in soil
around root hairs
food
storage
across cells of
root to xylem
perennating
organs
movement
through xylem
movement
through cells of leaf
transpiration
potometer
movement through
stomata to air
The importance of transport in plants
There is much activity going on inside a plant, but it is difficult to imagine that
just by looking at one. The main activity is photosynthesis. During daylight,
160
14 • Transport in Plants
all the leaves of a plant are actively photosynthesising and therefore need all
the substances necessary to carry out this process. Transport in plants is thus
related to photosynthesis as substances are transported to and away from
leaves (figure 14.1).
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Figure 14.1 Transport in plants.
Photosynthesis is summarised by the equation:
light
carbon dioxide + water
glucose + oxygen
chlorophyll
• The gases carbon dioxide and oxygen move between the atmosphere and
the leaf. The leaves are thin, broad and flat and cover a wide surface area so
that diffusion is adequate to transport these gases.
• Light rays from the Sun pass into the leaf and get to all the
photosynthesising cells.
• Chlorophyll is present in the leaf cells.
• Water must be transported from the soil through the roots to the leaf.
Dissolved salts are present in the water.
• Some of the manufactured food is transported away from the leaves to be
used and/or stored in other parts of the plant.
ITQ1
Use a table to show the substances
that are transported in a plant. Indicate
where the substance is transported
from and to, and its importance to the
plant.
xylem vessels ❯
phloem tubes ❯
Transport systems of plants
The transport system of plants is much simpler than that seen in animals.
There is no pump (heart) or specialised transport medium (blood). It is made
up of two types of transport vessel:
• xylem vessels, which carry water and minerals;
• phloem tubes, which carry food materials that the plant has made.
Structure of xylem vessels
lignin ❯
Xylem vessels are long, very narrow, tubes formed from columns of elongated
cells that are joined end to end. The end walls of the cells have disappeared, so
a long, open tube is formed. These cells are all dead and contain no cytoplasm
or nuclei (figure 14.2). The cell walls become thickened with tough lignin.
Lignin is very strong and so xylem vessels help to support the plant by
keeping them upright. Wood is composed almost entirely of lignified xylem.
161
Life Processes and Disease
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transverse section
longitudinal section
Figure 14.2 Transverse and longitudinal sections through xylem tissue.
Structure of phloem tubes
sieve plate ❯
sieve tube element ❯
companion cell ❯
JVTWHUPVU
JLSS
Phloem tubes are also made up of cells joined end to end. However these end
walls do not break down completely, but become perforated with small holes.
These perforated end walls are called sieve plates. Each cell, called a sieve
tube element, contains living cytoplasm, but no nucleus. The cell walls do not
contain lignin. Each sieve tube element has a companion cell next to it. The
companion cell has a nucleus which probably controls both cells (figure 14.3).
;OLLUK^HSSZVM[OL[^VJLSSZHYL
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longitudinal section
(a)
(b)
Figure 14.3 (a) Transverse and longitudinal sections of a sieve tube element and a companion cell. (b) Longitudinal section through a phloem tube
and transverse section across it.
162
14 • Transport in Plants
Vascular bundles
vascular bundles ❯
Vascular bundles are made up of bundles of xylem vessels and phloem tubes
close together. The arrangement of these bundles in roots and shoots is shown
in figures 14.4, 14.5 and 14.6.
phloem
xylem
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phloem is made up
of phloem tubes
which run the length
of the stem, trunk,
branch, etc.
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xylem is made
up of xylem vessels
which run the length
of the stem, trunk,
branch, etc.
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Figure 14.4 Transverse section of a root of a
dicotyledonous plant.
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Figure 14.5 Transverse section of a stem of a
dicotyledonous plant.
Figure 14.6 Diagram relating the
transverse section to the longitudinal section
of a stem.
Movement of water through a plant
ITQ2
(i) How are xylem vessels and phloem
tubes arranged in a vascular
bundle?
(ii) Explain how the structure of the
(a) xylem and (b) the phloem are
suited to their function.
The movement of water through a plant can be broken down into five stages
(Figure 14.7). The numbers in the following text relate to numbers in the
figure).
4
3
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ITQ3
(i) What are the functions of vascular
bundles?
(ii) Draw a sketch to show how they
are arranged in the stem of a
dicotyledonous plant.
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3
1
2
3
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Figure 14.7
Diagram showing the movement of water through a plant.
163
Life Processes and Disease
Practical activity
SBA 14.1: The rate of transpiration,
page 353
1
2
3
4
5
Absorption of water by the root hair cells.
Movement of water across the root cortex to the xylem.
Movement of water up the xylem.
Movement of water across the leaf cells.
Movement of water from the leaves.
Evaporation of water from the leaves
Stomata are found on the underside of leaves (chapter 9). Just inside the
stomata are the leaf cells which contain and are surrounded by water. The
concentration of water molecules inside the cells is higher than in the air space,
and higher there than outside the leaf. So some of the water evaporates from
the cells into the air space and diffuses out of the leaf through the stomata,
down the concentration gradient (figure 14.8).
CHAPTER 9
ITQ4
Describe the route taken by a water
molecule from the soil to the air as it
passes through a plant.
/6
/6
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/6
/6
/6
/6
/6
/6
/6
/6
/6
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/6
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Figure 14.8 Diagram showing how water evaporates from cells around the air space and diffuses
out of the stoma.
transpiration ❯
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MYVT[OL_`SLT
/6
Transpiration is the loss of water by evaporation from the surface of leaves.
This constant loss of water from the leaves creates a ‘pull’ of water through
the plant. During the day, water is also constantly being taken up from the top
of the xylem vessels to supply the photosynthesising cells in the leaves. This
reduces the pressure at the top of the xylem vessels, and water thus flows up
to the top because the pressure below is greater. This constant flow of water
through the plant is known as the transpiration stream.
The conversion of liquid water into
watervapour as it leaves the cells and
enters the air space requires heat. Using
heat to convert water into water vapour
B
2>H[LYTV]LZMYVT
/ 6
helps to cool the plant.
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A
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Movement of water
within the leaf
As water evaporates from the
surfaces of cells near the air spaces,
its concentration in those cells
/
6
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(A in figure 14.9)) is lowered. Its
VM[OLWSHU[
concentration in the adjacent cells (B)
/ 6
is now higher than in the A cells. This
Figure 14.9 Diagram showing the movement of water from the xylem, through the cells of the results in the movement of water from
the B cells to the A cells by osmosis.
leaf, to the air space and then out of the leaf as water vapour.
164
14 • Transport in Plants
ITQ5
Describe how water travels from the
xylem in a leaf to an air space.
adhesion ❯
cohesion ❯
capillarity ❯
ITQ6
List the processes by which water
travels up the xylem.
Water is drawn through all the cells of the leaf by osmosis as it moves
towards the air space and then out of the leaf.
Movement of water up the xylem
Water moves up and through the xylem vessels because of three factors.
• Capillarity – When a thin straw is placed in a glass of water, the water
rises a little up the straw (figure 14.10). This is due to attraction between
the water molecules and the walls of the straw, which is called adhesion.
Water molecules tend to stick together, which is known as cohesion. Thus,
water molecules stick together and to surfaces of narrow tubes and the
water rises up the tube, which is called capillarity. The narrower the straw,
the higher the water will rise. Xylem vessels are extremely narrow and the
attraction between the water molecules and the xylem walls is great.
• Root pressure – Water constantly moves into root cells by osmosis because
the presence of sugars and other dissolved substances in the root means
that the water concentration can never be as high in root cells as in the soil.
Water absorbed into the plant from the soil creates a pressure in the root
xylem. The pressure there is greater than in the leaf xylem because water
is being pulled out of the leaf xylem by transpiration. So water moves from
the high pressure in the roots up the xylem vessels in the stem to the low
pressure in the leaves (figure 14.11).
• Transpiration pull – The flow in the system is mainly by cohesive forces
holding water molecules together and the loss of water by evaporation in
the upper areas of a plant creating a tension that ‘pulls’ water upwards. This
is the transpiration pull.
NSHZZ[\IL
^H[LYTV]LZ
OPNOLY\WH
UHYYV^[\IL
^H[LYSVZ[[V[OLH[TVZWOLYL
HSV[VM^H[LYJVTPUN
MYVT[OLYVV[ZJYLH[LZH
WYLZZ\YLHOPNOWYLZZ\YL
SV^LYWYLZZ\YLPU[OPZ
YLNPVUZPUJL^H[LYPZ
SVZ[H[[OL[VW
_`SLTTHKL\WVM
UHYYV^_`SLT]LZZLSZ
^H[LYTVSLJ\SLZ
HYLH[[YHJ[LK[V
[OL^HSSZVM[OL[\IL
^H[LY
Figure 14.10 Water moves up narrow
tubes.
Figure 14.11
Low pressure at the top, high pressure below and water is pushed up.
Movement of water across the root cortex
Water moves between the cortex cells of the roots by osmosis (figure 14.12,
overleaf). As water enters the xylem, the cells next to the xylem now have a
lower concentration of water. Water then moves into those cells from adjacent
cells. These cells now have a lower concentration of water and water flows
into them by osmosis. In this way, water moves from the root hair cells to
the xylem.
165
Life Processes and Disease
_`SLT]LZZLSZ
JVY[L_JLSSZ
1^H[LYTV]LZPU[V[OL
_`SLTHUKPZW\SSLK\W
)
(
2^H[LYTV]LZMYVT[OLZL
JLSSZPU[V[OL_`SLTHUKZV
[OLZLJLSSZOH]LSLZZ^H[LY
[OHU[OLVULZUL_[[V[OLT
3[OLZLJLSSZ)OH]L
TVYL^H[LY[OHUJLSSZ(
HUKZV^H[LYTV]LZI`
VZTVZPZ[V[OVZLJLSSZ
Figure 14.12 Water moves between all the cells of the root cortex by osmosis.
Absorption of water by the root hair cells
ITQ7
Describe how water travels from the
soil to the xylem vessel in the root.
The soil particles are surrounded by a film of water which contains some
dissolved salts. Inside the root cells, there are sugars and other dissolved
substances at a much higher concentration. So water is continuously moving
into the root cells by osmosis (figure 14.13).
The root hair cells extend into the surrounding soil and the surface area for
absorption is thus increased.
ZVPSWHY[PJSL
MPSTVM^H[LY
JVY[L_JLSSZ
_`SLT]LZZLSZ
YVV[OHPY
^H[LYTV]LZMYVT[OLYVV[OHPYJLSSZ
[V[OLJVY[L_JLSSZHUKZV^H[LYPZ
W\SSLKPU[V[OLYVV[OHPYZ
^H[LYTV]LZHJYVZZ
[OLZLJLSSZ[V[OL_`SLT
Figure 14.13 Water moves into the root hair cells.
Transpiration
Transpiration is the evaporation of water from a plant. It is important beause:
• it pulls water up to the leaves for photosynthesis;
• the moving water carries dissolved mineral salts up to the leaves;
• the evaporation of water cools the plant.
166
14 • Transport in Plants
transpiration rate ❯
• The rate at which a plant takes up water depends on the rate at which
it is lost from the plant, called the transpiration rate. The faster the
transpiration rate, the faster the plant takes up water. Environmental
factors affect the transpiration rate.
• Temperature – With high temperatures, as on a hot day, evaporation
occurs rapidly. Transpiration rate increases as temperature increases.
• Humidity – With high humidity, the air is almost saturated with water
vapour. So the concentration gradient of water between the air spaces and
the outside air is low and the rate of evaporation of water through the
stomata is slow. Transpiration decreases as humidity increases
(figure 14.14).
• Air movement (wind) – In windy conditions, water vapour is carried
rapidly away from the leaves and the rate of transpiration is fast. During still
conditions, the water vapour remains around the leaves and transpiration is
slow. Transpiration increases as wind speed increases (figure 14.15).
• Light intensity – During bright light, the stomata are fully opened. This
may be to supply carbon dioxide for photosynthesis. With stomata fully
open, the rate of transpiration can be high. With dim light, the stomata
almost close and transpiration is slow.
HPYZWHJL
PUZPKLHSLHM
/6
/6
/6
/6
Moist air outside the leaf
ZHTLJVUJLU[YH[PVUVM^H[LY
TVSLJ\SLZV\[ZPKLHZPUZPKL
/6 /6
/6
/ 6
/6
/6
/ 6
/6
;OLYLPZSP[[SLUL[
V\[^HYKKPMM\ZPVUVM
^H[LYTVSLJ\SLZ
Figure 14.14 High humidity means that the
concentration gradient of water molecules inside
compared to outside the plant is low.
ITQ8
(i) What is transpiration pull?
(ii) What is the transpiration stream?
(iii) What is transpiration?
xerophytes ❯
mesophytes ❯
hydrophytes ❯
/6
/6
/6
/6 / 6
/6
^H[LYTVSLJ\SLZ
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^PUKJ\YYLU[Z
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KPMM\ZLV\[[OL`HYL[HRLU
H^H`I`^PUKJ\YYLU[Z
;OLJVUJLU[YH[PVUVM^H[LY
TVSLJ\SLZV\[ZPKL^PSS
HS^H`ZILZTHSSHUKZV[OLYL
^PSSILUL[V\[^HYKKPMM\ZPVU
/6
Figure 14.15 Windy days result in more
rapid transpiration.
When there is very little water in the soil, the stomata almost close. This
reduces the rate of transpiration to conserve water. The plant cells become
flaccid and the plant wilts as the water lost in transpiration cannot be replaced.
Adaptations in plants to conserve water
Plants need water for their existence. Transpiration occurs constantly, so a
supply of water from the environment is vital. Plants that live in places where
water is in short supply are called xerophytes. They show striking adaptations
which:
• reduce the transpiration rate;
• maximise water uptake from the environment.
Other plants are grouped into three categories.
• Mesophytes are plants that live in areas where water is readily available.
• Hydrophytes are plants that live in very wet, freshwater environments
such as ponds, lakes and rivers.
167
Life Processes and Disease
halophytes ❯
• Halophytes are plants that live in water with a high concentration of salt,
such as in salt marshes, swamps or areas close to the sea.
Xerophytes may have any of the following features:
• fine spine-like leaves to reduce the number of stomata and so reduce
transpiration;
• thickened stems or leaves capable of storing large amounts of water;
• an extensive root system to absorb water quickly when it rains;
• a thick epidermis covered with a thick waxy cuticle to reduce water loss and
reflect light and infra-red radiation (so the plant remains cooler);
• the ability to trap carbon dioxide at night so that the stomata can be closed
during the day;
• other features such as sunken stomata, rolled leaves and interlocking hairs.
Table 14.1 compares the adaptations of a xerophyte (e.g. cactus) and a
hydrophyte (e.g. water lily) to the environments in which they live.
Xerophyte
Hydrophyte
Description of
environment
Very hot, sunlight intense as few or no Still, fresh water from 15 cm to 2 m
clouds, no larger shade trees, soil hot deep
and dry
Description of
leaves
Small spikes, not green, usually black Broad, flat, green, lie on surface of
or grey in colour
water, stomata on upper surface
Description of
stems
Thick, green, with thick cuticle
Table 14.1
Colourless, entirely under water and
extending to roots in mud at bottom
Comparison of a xerophyte and a hydrophyte.
Uptake and movement of mineral salts
Mineral salts are absorbed by the root hairs as ions dissolved in soil water. They
are taken up using energy because the concentration inside the root is much
higher than outside. They are then carried throughout the plant in the xylem.
Transport of manufactured food
translocation ❯
168
The soluble product of photosynthesis are sugars (mainly sucrose) and amino
acids. These are transported in the phloem tubes. The transport of organic food
through a plant is called translocation. This manufactured food is transported
from the leaves (called the source) to wherever it is needed (called the sink) for
respiration or storage.
14 • Transport in Plants
Phloem and the movement of food
Plants rely on pressure gradients to move their phloem sap. The pressure–flow
hypothesis, also called the mass flow hypothesis, was proposed by Ernst Munch
in 1930 to explain how the phloem transports food (figure 14.16).
pholem
sieve tube
elements
leaf
sugar is made during
photosynthesis
source
sugar lowers the
water potential
(increases the
concentration)
which draws in
water and raises
the pressure
xylem
vessels
sucrose solution
transported from
high to low pressure
roots
water
sink
sucrose enters the
roots and lowers
the pressure in
pholem as water
is lost
Figure 14.16 The pressure–flow hypothesis.
1
ITQ9
How are sugars transported in
the phloem?
ITQ10
What is translocation?
Sugar made during photosynthesis moves into the sieve tubes at the source
(leaf) which makes it more concentrated there.
2 Water then moves into the sieve tubes since it is now more concentrated.
3 The uptake of water causes the pressure to build up in the sieve tubes at
the source (leaf) which pushes the sap down.
4 Unloading of sugar at the sink (other parts of the plant) relieves the
pressure since water is also lost at the sink.
Sugar is thus translocated from the leaf to the root and other parts of the plant
that are respiring, storing and using the sugar.
Evidence that phloem translocates organic food
Radioisotopes
If a plant is supplied with carbon dioxide containing radioactive carbon it
will make food containing radioactive carbon. If the source of radioactive
carbon is removed then, after a while, the radioactivity is detected only in the
phloem tubes. This means that the food which the leaves have made is being
transported in the phloem.
169
Life Processes and Disease
Ringing
MVVK
HJJ\T\SH[LZ
MVVKJHUUV[NL[
[VSV^LYYLNPVUZ
Figure 14.17 Removing a ring of bark also
removes the phloem.
The phloem tubes in a woody stem are just underneath the bark. If a ring of
bark containing the phloem is removed, sugars accumulate above the ring,
resulting in a slightly swollen appearance (figure 14.17).
Using aphids
Aphids are insects that feed on plant juices by pushing their mouthparts
(stylets) into phloem tubes. If the mouthparts of a feeding insect are cut off,
phloem sap keeps flowing out through the stylets (figure 14.18). The sap can
be analysed and shows the presence of organic material.
H
I
ITQ11
Describe three ways in which the food
manufactured during photosynthesis is
used by a plant.
4V\[OWHY[J\[VMMHUK
SLM[PU[OLWSHU[
Z[`SL[
VVaPUNMVVK
4V\[OWHY[Z[`SL[VMHWOPKWLUL[YH[LZ
[OLWOSVLT0[Z\JRZMVVK"[OPZPZ
OV^P[VI[HPUZMVVKMVYLULYN`
(WOPKZHYLWHYHZP[LZVMWSHU[Z
Figure 14.18
If the mouthparts of (a) a feeding aphid are cut off (b) they continue to ooze sap.
Chapter summary
•
•
•
•
•
•
•
•
•
•
•
All materials needed for photosynthesis must be transported to the leaves of a plant.
Water is transported from the soil.
Carbon dioxide diffuses in through the stomata.
The products of photosynthesis, manufactured food and oxygen, must be transported
from the leaf.
Manufactured food is transported in the phloem to various sites in a plant.
Oxygen diffuses out through the stomata.
The structure of the xylem is suited to its transport and support functions.
The structure of the phloem is suited to its transport function.
Transpiration is the evaporation of water from the leaves of a plant.
The rate of transpiration is influenced by many external factors.
Translocation is the transport of organic food through a plant.
Answers to ITQs
ITQ1
Substance Taken from
Transported to
carbon
dioxide
air surrounding leaves all photosynthesising
cells
water
soil – water forms a
thin layer around soil
particles
all cells
Importance to plant
Means of
transport
needed for
photosynthesis
diffusion
needed for many
purposes including
photosynthesis
xylem
(continued)
170
14 • Transport in Plants
Substance Taken from
Transported to
Importance to plant
Means of
transport
minerals
soil – present as
soluble ions in water
all cells
healthy growth
xylem
organic
food
(glucose)
leaf cells where it was all cells
made
all cells must respire to phloem
have energy for giving
processes
oxygen
leaf cells
oxygen produced during diffusion
photosynthesis and not
needed for respiration
must be removed
outside the leaf
ITQ2 (i) The xylem vessels are positioned together in the vascular bundles
in a region where only xylem vessels are found. Similarly, the phloem tubes
and their accompanying companion cells are positioned together in a part of
the vascular bundle where only these structures are found.
(ii)
(a) Xylem vessels are elongated, tubular and made up of dead cells
thus providing a water-proof vessel for the transport of water and absorbed
minerals.
(b) Phloem is also elongated and tubular but made up of living cells. Energy is
thus available for the transport of manufactured food.
ITQ3 (i) The vascular bundles consist of xylem vessels and phloem tubes.
The xylem vessels transport water containing dissolved minerals from the roots
to the leaves of the cell. The phloem tubes transport organic food from the
leaves where it is produced to all the cells of the plant.
(ii) Your sketch should look like figure 14.5.
ITQ4 soil root hair cell root cortex cells xylem palisade mesophyll
air space stoma air
ITQ5 Water travels by osmosis out of the xylem and then travels across the
cells of the leaf to an air space by osmosis also. The water is moving down a
concentration gradient, from a high concentration of water molecules to a
lower concentration.
ITQ6 Water travels up the xylem by:
• root pressure;
• transpiration pull;
• capillarity, cohesion and adhesion.
ITQ7 Water moves by osmosis into the root hair cell from the soil. The
water then moves across the cells of the root cortex to the xylem. The water is
moving along a concentration gradient. As water travels up the xylem, more
water moves into the root from the soil.
ITQ8 (i) The flow in the xylem is mainly by cohesive forces holding water
molecules together and the loss of water by evaporation in the upper areas of
a plant creating a tension that ‘pulls’ water upwards. This is the transpiration
pull.
(ii) The water moving up the xylem is the transpiration stream.
(iii) Transpiration is the loss of water vapour from the leaves of a plant.
ITQ9 Sugar is transported as sucrose which loads into the phloem from the
leaf (source). This increases the concentration of the solution in the phloem,
which draws in water. This increases the pressure of the solution. The pressure
is lower at the roots and movement thus occurs from the leaf to the root. At
the root (sink), the pressure is lower because the sugar moves into the roots
thereby lowering the concentration of the solution, causing water to move out.
ITQ10 Translocation is the movement of manufactured food from the leaves
to all the cells of a plant.
171
Life Processes and Disease
ITQ11
• It is stored as starch in plant cells.
• It is used for respiration to release energy for use by plant cells.#
• It is used for the production of fruits and seeds during reproduction.
Examination-style questions
1
(i)
State the functions of:
(a) phloem;
(b) xylem.
(ii) Describe how aphids can be used to investigate the function of phloem.
(iii) Explain why it is necessary to water many potted plants at least once a day.
2
(i) Define transpiration.
(ii) State three environmental factors that affect the rate of transpiration.
(iii) Using an annotated diagram only, describe how the photometer can be used to
measure the rate of transpiration.
3
(i) Suggest three ways in which transport in plants differs from transport in animals.
(ii) Suggest two ways a xylem vessel is similar to an artery.
(iii) A plant may be 50 metres in height and does not have a pump to push water to
the leaves at the top. Describe how water travels in the xylem, from the soil to the
uppermost parts of a plant.
4
Two tubes A and B were set up as shown below. Both tubes were left indoors for 50
minutes and then taken outdoors for another 50 minutes. The tubes were weighed every
10 minutes. The table shows the results obtained.
JV[[VU^VVS
^H[LY
;\IL(
;\IL)
Time (min.)
0
10
20
30
40
50
60
70
80
90
100
Tube A (g)
305
294
285
272
262
250
220
206
184
150
120
Tube B (g)
280
280
280
279
279
279
278
278
278
278
278
(i) State the processes by which water was lost form (a) tube B, and (b) tube A
(ii) Explain the role of the cotton wool in the investigation.
(iii) Plot a graph of the results for both tubes A and B on the same page.
(iv) Explain fully, the differences seen between tubes A and B.
(v) Describe the differences seen for tube A between the first and second parts of the
investigation.
(vi) Explain fully the differences seen for tube A between the first and second parts of the
investigation.
172
15
At the end of this
chapter, you should
be able to
Storage in Plants
and Animals
understand the importance of food storage in living organisms
identify some products stored and the sites of storage in plants
draw and annotate stages in germinating seeds
describe the structure of a dicotyledonous seed
describe the processes taking place within a seed during germination
draw buds from plant storage organs
identify some products stored and the sites of storage in animals
food storage
plants
roots
stems
leaves
importance
animals
seeds
liver
fat
deposits
fruits
development
of embryo
vegetative
reproduction
provide for
periods of
scarcity
overcome the
need for
continuous
food intake
Why do organisms store food?
ITQ1
Why do humans store food in the
cupboards in the kitchen? Give three
reasons.
Manufactured food (from photosynthesis using the Sun) is the source of
chemical energy for all living organisms. Glucose, which is the chemical
compound made during photosynthesis, is oxidised to release energy. All living
things depend on this energy for life processes to take place. Some of this food,
however, is stored. Plants and animals store food in their bodies for the same
reasons, some of which are listed below:
• to overcome the need for continuous manufacture; during the night,
photosynthesis stops because there is no light;
• to overcome the need for continuous food intake; animals cannot eat
continuously because other activities are also important;
• to provide for periods of scarcity, like droughts and famines;
• to provide for special functions (muscle cells need their own store of food);
• to produce reproductive structures (fruits, seeds and embryos must store
food).
Food storage in plants
The food made by a plant during photosynthesis may be stored temporarily
as starch in the leaves. For longer periods of time, other parts of the plant
are used, such as roots, stems, fruits and seeds. Table 15.1 (overleaf) lists the
importance of the various sites of food storage in a plant.
173
Life Processes and Disease
Site of storage
Importance of storage
Leaves
The cells of the leaf need to respire and there is a store of glucose as starch in starch grains.
bok choy
Sometimes underground leaves are used to store food.
cabbage
onion
garlic
I\SI
SLHMZ^VSSLU
^P[OMVVK
bulbs, e.g. lily
Stems
Some stems can become swollen with stored food.
Some plants can protect themselves against unfavourable conditions of weather by reserving food in underground stems which
will be used to generate new plants.
harsh environmental
conditions
food stored
in 'good' conditions
plant stores food
plant is still 'alive'
underground
good conditions again
– new growth seen
(continued)
174
15 • Storage in Plants and Animals
Site of storage
Importance of storage
Underground swollen stems are sometimes used to store food.
new shoot
growing
from rhizome
new shoot
stem tuber – the tip
of an underground
stem is swollen
with food
roots
roots growing
from rhizome
rhizome – food stored
in an underground stem
stem tubers, e.g. potato, yam
rhizomes, e.g. ginger
bud – grows into
new shoot
corm – the base of a
vertical stem becomes
swollen with food
roots
corms, e.g. eddo, dasheen
Above-ground swollen stems are sometimes used to store food.
swollen stem
stores food
sugar cane
(continued)
175
Life Processes and Disease
Site of storage
Importance of storage
Roots
Underground swollen roots are sometimes used to store food.
root tuber – tip of
root swollen with food
root tubers, e.g. cassava, sweet potato
tap root – swollen
main root
tap root, e.g. carrot, radish, beetroot
These swollen organs (like underground stems) are called perennating organs. They are filled with food stores built up in the
time of good growing conditions. They lie protected in the soil. The food store enables the plant to grow quickly when good
conditions arrive again.
Fruits
Most fruits are adapted to protect seeds and to help their dispersal. Succulent or juicy fruits store mainly sugars to attract
CHAPTER 21 animals that use the fruits as a food source. The animals help to disperse the seeds that are in the fruit (chapter 21).
fleshy edible
part of fruit
seeds
epicarp
mesocarp
endocarp
pumpkin
seed
grape
Seeds
Seeds contain a store of food for germination. The cotyledon(s) or endosperm of seeds store starch, protein and lipids. This store
is used up during germination as the embryo develops. The seed respires, using the stores to provide energy for growth and
development into a seedling. The stores are needed until the seedling develops leaves and can photosynthesise.
Table 15.1 Sites of food storage in pants.
176
15 • Storage in Plants and Animals
Germination
cotyledon
plumule ❯
radicle ❯
endosperm ❯
testa ❯
Germination is the growth of the seed into a seedling. The seed contains the
embryo which is made up of the plumule (grows into the shoot) and the
radicle (grows into the root). The parent plant sends the embryo out into the
world with a store of food in the cotyledon and/or endosperm. The embryo
is protected by a tough testa (figure 15.1).
testa (seed coat)
ITQ2
(i) What is a perennating organ?
(ii) Distinguish between a stem tuber
and a root tuber
(iii) What is the importance of the food
stored in each of the following – a
fruit, a seed, a leaf and an above
ground stem?
cotyledon (food store)
hilum
micropyle (tiny hole)
micropyle
radicle
embryo
plumule
(a)
(b)
Figure 15.1 A seed (a) in side view, (b) in section.
dormant ❯
ITQ3
(i) What is germination?
(ii) Describe the differences between
epigeal germination and hypogeal
germination.
In its inactive and dehydrated state, a seed can stay a seed for a long time. It
is said to be dormant. When conditions are favourable, germination begins
(figure 15.2). Germination requires three conditions:
• water – moves rapidly into the micropyle and to all the cells. Enzymes are
activated and starch is broken down to glucose for respiration;
• oxygen – needed for respiration;
• warmth – to provide the optimum temperature for enzymes.
The energy demands of a germinating seed are very high. Energy is released
from the stored food by respiration and is used for growth of the radicle and
plumule. The radicle grows down into the soil and the plumule grows upward
to develop into the shoot above ground. When the first leaves develop, the
seedling begins to photosynthesise to make its own food. It continues to grow
and develop more leaves and a root system until it is an adult plant ready to
produce flowers.
Epigeal germination – cotyledons brought above ground
cotyledons open, are
green and photosynthesise
for a while
food store used
up as grows
Hypogeal germination – cotyledons remain below ground
leaves grow and begin
to photosynthesise
leaves grow and
photosynthesise
plumule
radicle
(a)
Figure 15.2
radicle
(b)
Germination.
177
Life Processes and Disease
Food storage in animals
ITQ4
Why would a human being die after 10
minutes without oxygen but could go
for 5 days without food?
ITQ5
How do humans become obese? Why
are wild animals never obese?
Animal cells also store glucose, but not as starch. Animals store glucose
as glycogen in granules. Cells respire continuously and animals breathe
continuously for their supply of oxygen but they do not feed continuously for
their supply of glucose.
Fats
Triglycerides (fats) have a higher proportion of hydrogen than either
carbohydrate or protein. This means fats are a more concentrated source of
energy than either carbohydrate or prootein. One gram of fat yields twice the
amount of energy that a gram of carbohydratecan yield. In mammals, excess
fat is laid down for storage under the skin. When we eat excess food, we
become obese, ‘fat’. Animals that live in cold conditions have a thick layer of
fat (blubber) under the skin which serves both as an energy store and provides
insulation for the extreme cold (figure 15.3).
Glycogen
As blood passes through the liver, the excess glucose (from a meal) is changed
to glycogen and stored. The liver is the main storage organ for glycogen.
Glycogen is also stored in the muscles where it can be quickly accessed for
muscle contraction. The liver also stores minerals (iron and potassium) and
the vitamins A, D and B12. After a meal, vitamins, minerals and other nutrients
from the food pass from the intestine into the blood. This nutrient-rich blood
then passes through the liver where the vitamins and minerals in excess are
stored for times when they are lacking in the blood (figure 15.4).
hepatic vein
Figure 15.3 Penguins and whales need
extra fat stores to stay warm in polar
conditions.
hepatic artery
gall bladder
bile duct
ITQ6
What is the importance of food stored
in:
(i) a seed
(ii) an egg
(iii) the liver
(iv) a fruit
(v) a tap root?
178
Figure 15.4
hepatic portal vein
from ileum
Nutrient-rich blood travels directly to the liver from the intestine.
Eggs
Embryo birds and reptiles (snakes, turtles, alligators, etc.) develop inside a shell
from the time eggs are laid until they hatch (figure 15.5). The egg white is
made up of water and a protein called albumen. The yolk contains protein, fat
and lecithin (a natural emulsifier).
15 • Storage in Plants and Animals
yolk sac
stalk
amniotic cavity
allantoic cavity
chorion
chorionic cavity
allantois
amnion
yolk sac
Figure 15.5 The eggs of birds and reptiles store food for the developing embryo.
Chapter summary
•
•
•
•
•
•
•
•
•
•
Glucose is manufactured by plants during photosynthesis; some of it is stored
Plants and animals store food in their bodies
Plants store food in their leaves, stems, roots, fruits and seeds
Animals store glucose as glycogen for respiration; plants store glucose as starch
One gram of fat yields more energy than one gram of carbohydrate
Germination is the growth of a seed into a seedling
Water, oxygen and warmth are needed for germination
In animals, the liver is an important storage organ
Glycogen is stored in the liver and muscle of animals
Birds and reptiles store food in the eggs for development of the embryo into a
hatchling
Answers to ITQs
ITQ1 So that food is available when needed.
To prevent shopping for every meal..
To have the choice to plan a healthy and balanced diet.
ITQ2 (i) An organ which enables plants to survive adverse conditions, such
as cold, extreme heat, lack of light or drought.
(ii) A stem tuber is an underground stem swollen with food and a root tuber
is an underground swollen root.
(iii) To attract agents of dispersal for the seeds.
To store the energy needed for germination.
To store the energy needed for the reactions that take place in the leaf cells.
To help the plant survive adverse weather conditions.
ITQ3 (i) Germination is the growth of a seed into a seedling.
(ii) In epigeal germination, the cotyledon is brought above the ground; in
hypogeal germination the cotyledon remains underground.
179
Life Processes and Disease
ITQ4 Oxygen is not stored but food is; both are needed at every moment for
respiration.
ITQ5 Humans over-eat; wild animals have no fast-food outlets or groceries.
Wild animals hunt for every meal, humans may have passive lifestyles and
food is readily availableITQ6 (i) Food stored in a seed is used for germination of a seed into a
seedling.
(ii) An egg stores food for the developing embryo.
(iii) The liver stores glycogen, vitamins etc. so that when lacking in the diet,
they are still available for metabolism.
(iv) Food stored in a fruit attracts animals to eat the fruit and so disperse the
seed(s).
(v) After adverse conditions, the tap root
generates a new plant using the stored food.
Examination-style questions
1
(i) List three reasons why living organisms may need to store food.
(ii) Copy and complete this table.
Storage organ in plant
Importance
fruit
seed
root tuber
(iii) Discuss the importance of storing glucose in plant and animal cells.
2
The weight of a germinating seedling was measured over 7 days and recorded in the table
below.
Day
Weight in grams
1
10
2
10
3
6
4
4
5
12
6
15
7
20
(i) Plot a graph of the change in weight of the seedling over the 7 days.
(ii) Describe the changes seen in the graph.
(iii) Explain the changes seen in the graph.
(iv) Distinguish between epigeal and hypogeal germination.
3
180
(i)
Explain the importance of storage in:
(a) an animal cell
(b) the liver.
(ii) Use a diagram to show the importance of the hepatic portal vein.
(iii) What is the importance of food stored in (i) an egg and (ii) a seed?
16
By the end of this
chapter, you should
be able to:
Excretion,
Osmoregulation and
Homeostasis
understand the importance of excretion in living organisms
distinguish between excretion, egestion and secretion
give examples of substances excreted by plants
give examples of substances excreted from animals
understand how substances are excreted from plants
understand how substances are excreted from animals
understand the structure of the excretory system
relate the structure of the kidney to its excretory function
relate the structure of the kidney to its osmoregulatory function
understand homeostasis
understand the control of blood glucose
understand the control of body temperature
understand the control of the amount of carbon dioxide in the body
protein – source of urea
excretion of metabolic
waste (especially
nitrogenous waste)
animals
plants
antidiuretic
hormone (ADH)
excretory
system
kidney
failure – dialysis
variation in
concentration
of urine
kidneys
osmoregulation
homeostasis
control of
• amount of
water in body
• blood glucose
• body temperature
• CO2 concentration
in blood
nephrons
pressure
filtration
selective
reabsorption
urine
Metabolism
metabolism ❯
The sum total of all the chemical reactions going on in cells is known as
metabolism. Chemical reactions must occur in all living cells, and therefore
181
Life Processes and Disease
excretion ❯
defecation ❯
ITQ1
Explain the terms ‘metabolism’,
‘excretion’, ‘egestion’ and ‘secretion’.
all living organisms, to sustain life. These metabolic reactions produce a range
of waste products, called excretory products, which must be eliminated from
the organism. The removal of the excretory products is called excretion. Many
of these excretory products are toxic and slow down metabolic reactions. If
these substances were allowed to accumulate in the body, they could damage
and kill body cells. They need to be continually removed.
Excretion must not be confused with the removal of faeces (defecation)
during egestion in humans (figure 16.1). Defecation or egestion is the removal
of undigested food but excretion is the getting rid of waste products produced
by cells during metabolism. Undigested food simply passes through the
alimentary system and is not absorbed into the cells of the body. It passes out
of the anus as faeces.
Excretion must also not be confused with secretion, which is the release of a
substance, such as a hormone, from cells (figure 16.1).
secretion of female
hormones into blood
by the ovaries
egestion of
faeces containing
undigested food
by the rectum/
anus
rectum
bladder
anus
excretion of urine
containing metabolic
waste by the urethra
bladder
egestion of
faeces containing
undigested food
by the rectum/
anus
rectum
anus
excretion of urine
containing metabolic
waste by the urethra
secretion of male
hormones into
blood by the
testes
Figure 16.1 The difference between egestion, secretion and excretion.
Excretory products in animals
Waste products of respiration
All cells respire to release energy which is used to do the work necessary to
keep the cell, and therefore the organism, alive. During respiration, carbon
dioxide is also produced. Carbon dioxide is dangerous to living tissue because
it increases the acidity of fluids which can affect other reactions. In humans,
carbon dioxide is transported in the blood to the lungs and removed from
182
16 • Excretion, Osmoregulation and Homeostasis
CHAPTER 12
CHAPTERS 10, 19
the lungs during exhalation (chapter 12). Other animals have similar ways of
removing the carbon dioxide.
Some of the energy produced during respiration is converted into heat
energy, the accumulation of which increases the temperature of the body.
At high temperatures, enzymes can be denatured (chapter 10) which means
reactions will stop. Excess heat is lost through the skin (chapter 19).
During exercise, when the rate of respiration is increased, the excretory
products of respiration are being produced at a faster rate. Breathing rate
increases to get rid of the excess carbon dioxide and sweating occurs to get rid
of the excess heat faster.
Waste products from red blood cells
A red blood cell has a short life span of about three months. After this
time, it is destroyed in the liver or spleen. The red blood cell is packed with
haemoglobin, a protein pigment which transports oxygen. The excess protein
portion is broken down into excess amino acids and reused by the body. The
iron is extracted, stored, and may be reused later. The remainder is broken
down into bile pigments and is later excreted by way of bile into the gut and
out with faeces.
Waste products of protein metabolism
Proteins are essential in the diet. However, proteins contain nitrogen and the
breakdown of protein that is not needed by the body produces nitrogenous
waste which is converted to urea (figure 16.2). Urea is removed by the kidneys
during the production of urine (discussed later in this chapter).
Excretory products in plants
Waste products of photosynthesis
CHAPTER 9
Plants photosynthesise or manufacture food from inorganic compounds. This
food can then be used by the plant to make energy. During photosynthesis,
oxygen is produced as a waste product. It is lost from the leaves of the plant
through the stomata (chapter 9). Water is also a product of photosynthesis, but
this is either needed by the cells or lost from them in transpiration.
Ingestion of protein
liver¶[OLHTPUVHJPKNYV\WZ5/
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Figure 16.2
Excretion of urea
Ingestion of protein leads to the excretion of urea.
183
Life Processes and Disease
Other plant wastes
ITQ2
Draw up a table to show how the
following excretory products are
produced and where they are
excreted. In animals: carbon dioxide,
heat, nitrogenous compounds;
in plants: oxygen.
Plants also produce some nitrogenous wastes which are converted into
insoluble substances. Calcium oxalate is another insoluble waste product. These
wastes are stored in leaves, bark, flowers, fruits and seeds. When the plant
sheds these structures, the excretory products are removed. These products can
be poisonous to the plant but may be useful to humans as dyes, oils, perfumes
and for medicinal purposes. These waste products include tannins, resins,
latexes, nicotine, caffeine, morphine and gums. Some waste products are stored
permanently by the plant, such as in the old xylem (hard wood)
The human excretory system
The kidney
kidney ❯
osmoregulation ❯
The human excretory system includes a pair of bean-shaped organs, the
kidneys, which are positioned in the lower back region behind the intestines
(figure 16.3). The kidneys are the major excretory and osmoregulatory organs
of mammals. Osmoregulation is the control of the amount of water in the
blood. Since the blood is constantly in close contact with all cells of the body,
this means that the kidneys control the amount of water in the body.
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Figure 16.3 The excretory system of humans.
urine ❯
bladder ❯
urethra ❯
ITQ3
Describe the function of each of the
following: kidney, bladder, renal vein,
urethra, the bladder sphincter muscle.
184
The renal artery brings blood with nitrogenous and other waste products to
the kidneys to be cleansed. After passing through the kidneys, this cleansed
blood returns to the heart via the renal vein, while the nitrogenous and other
wastes flow down through the ureter as urine to the bladder to be stored. The
bladder stores urine temporarily before it is released into the environment via
the urethra. Sphincter muscles control the release of urine from the bladder.
Sense cells in the bladder walls are stimulated when the bladder fills,
triggering the desire to relax the sphincter muscles and contract the walls of
the bladder. When this happens, urine flows out of the bladder through the
urethra. Trying to hold back the release of urine requires conscious tightening
of the sphincter muscles which can be uncomfortable. Babies are not usually
capable of controlling this muscle before the age of 2–3 years.
When a person is said to be suffering from a ‘weak bladder’, that person
has to urinate frequently. In this case, it is really the sphincter muscles that are
weak and the bladder does not hold as much urine as it normally stores.
16 • Excretion, Osmoregulation and Homeostasis
nephron ❯
ureter ❯
Figure 16.4 illustrates a longitudinal section through a kidney. The three
regions seen are the cortex, medulla and pelvis. Each kidney is made up of
thousands of tiny structures called nephrons. Each nephron spans the cortex
and medulla, the two outer regions. The pelvis, the innermost region, collects
urine from the collecting ducts. The nephrons all end at collecting ducts so that
urine, as it forms, flows through these collecting ducts and then out into the
pelvis. The urine then flows to the bladder through the ureter.
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(a)
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(b)
Figure 16.4 (a) False-colour X-ray showing blood supply to kidney. (b) A simplified diagram of a
longitudinal section through a kidney, showing the position of the nephrons.
Bowman’s capsule ❯
glomerulus ❯
Bowman's capsule
The nephron and urine production
proximal
convoluted tubule
distal convoluted
tubule
arteriole from
renal artery
cortex
medulla
glomerulus
(knot of capillaries)
capillaries
venule to
renal vein
collecting duct
loop of Henlé
The main regions of the human nephron
are shown in figure 16.5. It is basically
made up of a cup-shaped structure called
a Bowman’s capsule and a long tubule
with clearly defined regions. These are
called the proximal convoluted tubule,
the loop of Henlé, and distal convoluted
tubule and the collecting duct. Each has
a very important role in the formation of
urine.
A mass of capillaries, called a
glomerulus, is enclosed by the
Bowman’s capsule. The blood supply to
the glomerulus comes from the renal
artery which brings blood carrying
nitrogenous and other waste products to
be cleansed.
urine
Figure 16.5
Detailed structure of a nephron.
185
Life Processes and Disease
Pressure filtration
filtrate ❯
The afferent arteriole which comes to the capsule has a bigger diameter than
the efferent arteriole leaving it. As a result, pressure builds up in the capillaries
of the glomerulus. As blood flows under this high pressure, the smaller
components of the blood plasma are pushed out into the surrounding cup-like
Bowman’s capsule. This becomes the filtrate which contains water, glucose,
amino acids, vitamins, hormones, salts and urea, which are some of the small
components of blood. Large molecules, such as plasma proteins, and blood cells
(erythrocytes and leucocytes) remain in the blood. The arteriole leaving the
capsule continues to flow through a network of capillaries which surrounds
the rest of the nephron as shown in figure 16.6. The filtrate flows into the
proximal convoluted tubule (figure 16.7).
High blood pressure can cause the capillaries of the glomerulus to burst thus
destroying the nephron which is the basic unit of the kidney. This can lead to
kidney failure.
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Figure 16.6
Bowman’s capsule.
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Figure 16.7 The proximal convoluted tubule
reabsorbs glucose from the filtrate.
Selective reabsorption
proximal convoluted tubule ❯
186
Selective reabsorption is the reabsorption of a substance in preference to others
that are present. This occurs in the region of the nephron called the proximal
convoluted tubule. Glucose is a small molecule, so it is a component of
the filtrate as it moves through the proximal convoluted tubule. Here it is
reabsorbed into the plasma of the capillaries that are wrapped around the
tubules. Glucose is not a waste product – it is needed by the body because it is a
source of energy. It is reabsorbed from the filtrate, which continues on into the
loop of Henlé.
A person who has diabetes mellitus has glucose in the blood at such a high
level that it exceeds that which the kidneys can reabsorb and so glucose is
excreted in the urine. This condition is also known as sugar diabetes. The urine
of non-diabetics does not contain glucose since all is reabsorbed back into the
blood. The urine of a person with diabetes tests positively for reducing sugar
(glucose).
16 • Excretion, Osmoregulation and Homeostasis
Reabsorption of water
loop of Henlé ❯
The filtrate now flows through the loop of Henlé where water is reabsorbed
into the blood capillaries. The longer the loop of Henlé, the more water is
reabsorbed. The filtrate continues to the distal convoluted tubule.
The kangaroo rat, a rodent which lives in deserts, has a very long loop of
Henlé. Most of the water in the filtrate is thus reabsorbed and conserved by the
animal. It is so good at this that the rat rarely has to drink water.
Selective reabsorption
distal convoluted tubule
collecting duct ❯
CHAPTER 18
As the filtrate moves through the distal convoluted tubule and collecting
duct, reabsorption of salts and water occurs (figures 16.8 and 16.9). This
reabsorption, however, is controlled by hormones and depends on the
concentration of solutes in the blood (chapter 18).
Urine
ITQ4
Describe the route taken by a red blood
cell from the renal artery to the renal
vein.
ITQ5
Describe the route taken by the urea
molecule as it travels from the renal
artery to the external environment
(in urine).
The filtrate is now called urine, and contains the water, salts and urea that are
not needed by the body. The urine flows to the pelvis of the kidney from the
thousands of collecting ducts. It then travels, via the ureter, to the bladder to be
stored before urination.
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Figure 16.8 Water is reabsorbed from the
filtrate in the loop of Henlé.
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Figure 16.9 Reabsorption of salts and more
water occurs in the distal convoluted tubule
and collecting duct.
Table 16.1 compares the composition of the renal artery with the renal vein
and shows the effect of the kidneys on ‘cleansing’ the blood.
Renal artery
Renal vein
• contains more water
• contains less water because some is lost with the urine
• contains a high concentration • contains little or no urea because all is filtered and lost as
of urea
urine
Table 16.1 Composition
of blood in the renal artery
and renal vein.
• salt concentration is higher
• salt concentration is lower
• more oxygen and less carbon • more carbon dioxide and less oxygen because kidney cells
dioxide
respire to stay alive and do their work
187
Life Processes and Disease
ITQ6
Describe the route taken by a glucose
molecule as it travels from the renal
artery to the renal vein.
ITQ7
Explain these terms: ‘pressure
filtration’, ‘filtrate’, ‘selective
reabsorption’, ‘urine’.
kidney dialysis ❯
Both the renal artery and the renal vein contain red blood cells and blood
proteins since these are too large to be filtered out into the Bowman’s
capsule. All glucose is reabsorbed apart from that used by the kidney cells for
respiration. The glucose content is thus lower in the renal vein than in the
renal artery.
Kidney failure and kidney transplants
Kidneys sometimes fail as a result of damage or infection. If one kidney is lost,
the remaining one can undertake the work necessary to remove metabolic waste
and keep the body healthy. However, loss of both kidneys is fatal if not treated.
It is possible to transplant a kidney from one person (the donor) into the
patient (the recipient). The tissue of both persons, the donor and recipient,
must match closely since the body rejects anything that it ‘perceives’ to be
foreign or not itself.
A person suffering from kidney failure must have regular treatment on a
kidney machine which carries out dialysis (figure 16.10). Dialysis must take
place for many hours (up to 10 hours) every few days, to ensure the removal
of wastes and prevent the build-up of toxic compounds that could poison and
kill the body cells.
Dialysis is a method of separating particles of different size in blood
by passing the blood through a tube made from a selectively permeable
membrane. This tube is surrounded by a dialysis fluid that has the same
concentration as normal blood. Any substance in excess in the blood, such as
urea and salts, will diffuse out. Dialysis fluid leaving the machine will therefore
be rich in salts and body wastes like urea.
(b)
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Figure 16.10 (a) A dialysis machine can do the job of the kidneys after kidney failure. (b) How the
dialysis machine works.
188
16 • Excretion, Osmoregulation and Homeostasis
Osmoregulation
osmoregulation ❯
ITQ8
(i) Define osmoregulation.
(ii) Reabsorption of water occurs in
the loop of Henlé, and in the distal
convoluted tubule and collecting
duct. How is reabsorption of water
different in the two areas?
The kidneys have a second important function – osmoregulation. They
regulate the concentration of body fluids. The amount of water and salts found
in the blood is never constant. Daily activities such as sweating and eating
cause the concentration to vary. The kidneys regulate the concentration of
blood by controlling the amount of water and salts that are reabsorbed into the
capillaries during selective reabsorption in the distal convoluted tubules and
collecting ducts.
During its normal circulation, blood passes through the hypothalamus in
the brain. The hypothalamus monitors the concentration of the blood and if
the blood is too concentrated – for example, from excessive sweating, ingesting
large amounts of salt or drinking too little water – the hypothalamus sends
a message to the pituitary gland. The pituitary gland is situated next to the
hypothalamus; when it receives the message, it secretes more antidiuretic
hormone (ADH) into the blood. ADH stimulates the walls of the distal
convoluted tubules and collecting ducts to reabsorb most of the water from
the filtrate. As a result, small amounts of concentrated urine are produced
(figure 16.11).
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Figure 16.11 The concentration of urine is controlled by the secretion of ADH by the pituitary.
If the hypothalamus detects that the blood is too dilute – possibly due to
drinking large volumes of water, little sweating or low salt intake – less ADH
is released and little water is reabsorbed. In this case, large amounts of dilute
urine are produced.
Homeostasis
homeostasis ❯
Homeostasis is used to describe all the mechanisms by which a constant
internal environment is maintained. While the external environment outside
the body may change, the internal environment inside the body must remain
189
Life Processes and Disease
fairly constant otherwise all the reactions needed in living cells may be
disrupted. The body must detect any deviation from the normal and make
the necessary adjustments to return it to its normal condition as quickly as
possible. The temperature within the body and the composition of tissue fluid
which bathes the body cells must remain as steady as possible for the chemical
reactions that occur within these cells to proceed normally (figure 16.12).
4Z\IZ[HUJLZ[VIL
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Figure 16.12
Body cells surrounded by tissue fluid and capillaries.
Tissue fluid must:
• be within a small range of pH (acidity);
• contain enough glucose for respiration and activity;
• contain enough oxygen for respiration;
• not contain high levels of carbon dioxide;
• not contain high levels of nitrogenous wastes;
• contain enough, but not too much, water;
• be within a small range of temperature;
• be specific in many other ways for body cells to function normally.
ITQ9
Define homeostasis and explain why it
is important.
Excretion and osmoregulation are examples of homeostasis. A build-up of
waste products could damage and even kill cells. Here are some examples of
how.
• Carbon dioxide causes the pH of the blood and tissue fluid to be lowered,
which then affects the rate at which chemical reactions can occur within
cells.
• Nitrogenous wastes are toxic to cells so they must be cleared from the blood
quickly.
• Too low a temperature makes chemical reactions too slow, and too high a
temperature denatures proteins, including enzymes.
• In extreme amounts, water causes body cells to malfunction.
Feedback
The body can detect changes in these factors in the blood and has mechanisms
to bring the levels back to a normal range. These mechanisms are called
feedback mechanisms because a change in the internal environment causes
a correction to happen which feeds back to the conditions in the internal
190
16 • Excretion, Osmoregulation and Homeostasis
negative feedback ❯
ITQ10
In the regulation of carbon dioxide in
the blood, when does an increase in
carbon dioxide concentration come
about, and how is the concentration
brought back down to a normal level?
environment. Such mechanisms are used to keep the internal environment
constant.
If the internal environment is disturbed, the disturbance sets in motion
a sequence of events which tends to restore the system to its original state.
This is called negative feedback because it removes the effect of the change.
Examples of negative feedback can be seen in figures 16.13, 16.14, 16.15,
16.16 and 16.17.
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Figure 16.13 A typical feedback mechanism.
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Figure 16.14
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Osmoregulation: control of concentration of blood plasma and body fluids.
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Figure 16.15
IYLH[OPUNYH[L
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Control of the amount of carbon dioxide in the body.
191
Life Processes and Disease
• skin produces sweat
• skin capillaries dilate
‹OHPYZSPLÅH[
• respiration slows
• panting occurs
body temperature
o
rises above 37 C
• fever
• exercise
• hot environment
body temperature drops
normal body
o
temperature (37 C)
normal body
o
temperature (37 C)
body temperature rises
• cold environment
• no sweat produced
• skin capillaries constrict
• hairs become erect
• shivering occurs
body temperature
drops below 37 o C
Figure 16.16
Control of body temperature.
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Figure 16.17
WHUJYLHZZLJYL[LZ
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Control of blood glucose.
Chapter summary
•
•
•
•
•
•
•
•
•
•
•
•
•
192
Excretion is the elimination of waste products from organisms.
If these waste products are allowed to accumulate they would damage body cells.
Animals excrete carbon dioxide from the lungs.
Protein in the diet is the source of nitrogenous waste in humans.
Plants excrete oxygen by diffusion from the leaves.
Excretory products of plants are stored in leaves, bark, flowers, fruits and seeds.
Animals excrete nitrogenous waste from the kidneys.
The kidneys are important organs of excretion and osmoregulation.
The structure of the kidney is related to its function of excretion.
Nitrogenous waste is filtered in the kidneys and excreted in urine.
Osmoregulation is maintaining the concentration of body fluids around a certain value.
The distal convoluted tubule and collecting duct are important in osmoregulation.
By controlling the amount and concentration of urine produced, the concentration of
body fluids is regulated.
16 • Excretion, Osmoregulation and Homeostasis
• Homeostasis is the term used to describe all the mechanisms by which a constant
internal environment is maintained.
• Feedback mechanisms are used during homeostasis.
• Feedback mechanisms are used to control blood glucose levels, water, body
temperature and the amount of carbon dioxide in the blood.
Answers to ITQs
ITQ1 •
Metabolism is all the activities of a cell. These require certain
substrates and produce many useful products as well as some waste. The term
‘metabolism’ encompasses all these reactions at any given time in a cell.
• Excretion is the process by which cells and the organisms get rid of
metabolic waste.
• Egestion is the process by which undigested food in the alimentary canal is
leaves the body through the anus.
• Secretion is the process by which a chemical, such as a hormone, leaves a
gland; for example, the salivary gland secretes saliva into the mouth.
ITQ2
Excretory product
How produced
Where excreted
carbon dioxide
during respiration
from the lungs
heat
during respiration
through the skin
nitrogenous compounds
from breakdown of protein
from the kidneys
during photosynthesis
through the stomata
In animals
In plants
oxygen
ITQ3 •
The kidney is the organ of excretion of metabolic waste and excess
water from the body.
• The bladder stores urine temporarily before excretion.
• The renal vein takes cleansed blood away from the kidneys.
• The urethra is the tube through which urine passes from the bladder to the
outside environment.
• The bladder sphincter muscle controls the release of urine from the body.
ITQ4 renal artery afferent arteriole glomerulus efferent arteriole renal capillary renal vein
ITQ5 renal artery afferent arteriole glomerulus Bowman’s capsule
proximal convoluted tubule loop of Henlé distal convoluted tubule collecting duct pelvis ureter bladder urethra
ITQ6 renal artery afferent arteriole glomerulus Bowman’s capsule proximal convoluted tubule renal capillary renal vein
ITQ7 •
Pressure filtration is the filtration of the smaller components of
blood into the Bowman’s capsule. It occurs because of pressure that builds up
as blood flows from a wider vessel into a smaller vessel.
• The filtrate consists of the smaller components of blood (urea, water, salt,
glucose, etc.) that are filtered into the Bowman’s capsule and move along the
tubes of the nephron.
• Selective reabsorption is the reabsorption of a substance in preference to
others that are present. Glucose is selectively reabsorbed back into the blood
while other components of the filtrate continue along the nephron.
• Urine is the substance that collects in the bladder. It is made up of water,
salts and urea.
193
Life Processes and Disease
ITQ8 (i) Omsoregulation is the maintainance of constant osmotic conditions
in the body. The regulation of the water content and solute concentration of
body fluids is important for cells to work efficiently
(ii)
Loop of Henlé
Distal convoluted tubule and collecting duct
reabsorption of water is automatic
reabsorption of water is controlled by
antidiuretic hormone (ADH)
the longer the loop of Henlé, the more water is
reabsorbed
water reabsorbed according to needs of body
ITQ9 Homeostasis is the maintenance of a constant internal environment.
Cells need a constant environment in which to function efficiently.
Homeostasis describes all the mechanisms that come into play to keep
the internal environment constant. For example, enzymes need a specific
temperature and pH to function efficiently.
ITQ10 During exercise, respiration increases and so does the concentration
of carbon dioxide because it is a waste product of respiration. Carbon dioxide
is transported to the lungs to be excreted. When there are increased amounts
of carbon dioxide in the blood, the heart beats faster, allowing blood to flow
faster to the lungs and therefore more carbon dioxide is excreted. The carbon
dioxide concentration is thus brought back to the normal level in the blood.
Examination-style questions
1
(i) Define: (a) excretion
(b) osmoregulation.
(ii) Using an annotated diagram only, describe how urine is formed in a nephron.
(iii) Describe how and why the volume and composition of urine changes:
(a) after strenuous exercise; (b) after drinking large volumes of water.
2
(i) List three excretory products, besides nitrogenous waste, produced by animals.
(ii) List two excretory products produced by plants.
(iii) (a) Describe the production of nitrogenous waste in humans.
(b) Describe the excretion of nitrogenous waste in humans.
3
(i)
(a) Label the parts A, B, C, D, E, F and G in the figure below.
(b) In each case state its role in excretion.
(c) List four differences between A and B.
(
)
*
+
,
.
194
16 • Excretion, Osmoregulation and Homeostasis
(ii) The kidneys are very important organs involved in the removal of toxic substances
which, if allowed to accumulate in the body, could be fatal.
(a) The body offers some physical protection of its internal organs. How are the
kidneys protected?
(b) Suggest two ways the kidneys may be damaged.
(iii) Describe how a dialysis machine works to cleanse blood during kidney failure.
4
(i) Define homeostasis.
(ii) The figure below shows some body cells and their supply of blood.
(a) List some differences between blood at A and B.
(b) Name the process occurring at C.
(c) Name the process occurring at D.
(d) Explain fully how the cell labelled E is supplied with oxygen.
(e) Name the substance found in F.
(f) State three properties of F.
(g) State the importance of the properties listed in (g) above.
(h) Describe the mechanism by which one of these properties is regulated.
)
+
*
,
(
195
17
By the end of this
chapter, you should
be able to:
Movement
understand the importance of movement in animals
understand growth movement in plants
understand how external factors affect plant movement
describe the structure and function of the skeleton of humans
describe the mechanism of movement in a limb of humans
describe the long bones of a fore and hind limb
describe the cervical, thoracic and lumbar vertebrae
movement
animals
plants
skeleton
auxin
limbs
muscle
tendons
vertebral
column + skull
joints
hinge
ligaments
phototropism
geotropism
vertebrae
• cervical
• thoracic
• lumbar
ball-and-socket
The importance of movement in animals
locomotion ❯
ITQ1
Describe two ways in which movement
is important to the survival of an
animal.
ITQ2
Describe two ways in which movement
is important to the survival of a plant.
196
Most animals have to look actively for food and, to do this, they must move
from one place to another. Movement from one place to another is called
locomotion and it involves the expenditure of energy. There are a number of
reasons why animals move from one place to another and these include:
• to find food;
• to escape predators;
• to find a mate;
• to disperse offspring;
• to reduce competition;
• to avoid danger;
• to avoid waste products;
• to avoid extreme environmental conditions.
Animals move in many ways, which include flying, swimming, walking,
running and gliding. Each animal is adapted to move in its own special way.
For example, humans are adapted to walk or run from place to place.
17 • Movement
Movement in plants
tropism ❯
Movement in plants can be demonstrated by a germinating seedling. When
a seed has germinated, it will grow into a seedling if all the conditions for
germination are met. Movement is seen when it grows. The shoot grows
towards a light source and the roots always grow downwards towards gravity
into the soil. This is related to nutrition in the plant as plants need light
and water for photosynthesis. Movement in plants is thus usually growth
movement, and when we study movement in plants we look at factors that
affect their growth. Growth movements are called tropisms.
A few plants show another kind of movement apart from growth
movement. The sensitive plant (Mimosa pudica), for example, can fold its leaves
when touched (figure 17.1). Some plants like the hibiscus can fold their petals
at night. Insectivorous plants like the Venus flytrap can catch small insects by
moving a part of its body, and the pods of pigeon pea can curl and split when
dry to disperse the seeds, as a part of reproduction.
a
b
Figure 17.1 Plant movements. (a) Mimosa pudica ‘wilts’ when touched. (b) The trap of a Venus
flytrap closes when an insect touches the sensitive hairs on the surface.
Growth movement in plants
phototropism ❯
geotropism ❯
auxin ❯
Practical activities
SBA 17.1: Does gravity affect plant
growth? page 354
SBA 17.2: The growth of a radicle,
page 355
SBA 17.3: Does light affect plant
growth? page 356
The most important plant movements are tropisms or growth movements.
Growth in response to the stimulus of light is called phototropism, and
geotropism is growth in response to gravity. Growth in plants is controlled by
the hormone, auxin. Auxin is made in the tips of roots and shoots which are
the growing parts of the plant. It diffuses to the region just behind the tip and
there it causes growth (figure 17.2).
Light and gravity are examples of external factors (factors in the
environment) that affect growth in plants. The shoots of plants respond to light
by growing towards it. When a shoot is lit from one side, auxin breaks down
on the light side and accumulates on the shaded side. This results in more
growth on the shaded side so the shoot bends towards the light (figure 17.3,
overleaf). In a shoot which is not upright, gravity causes the auxin to collect
on the lower side. This has the same effect as before, to make the shoot grow
faster on that side, so it bends away from gravity.
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Figure 17.2 A shoot grows because of the hormone auxin.
197
Life Processes and Disease
more growth
auxin accumulated on
the shaded side
light
less growth
light
shoot grows
towards the light
shoot is
horizontal
less growth
more growth
auxins accumulate on the lower side due to gravity
shoot grows upwards
ITQ3
Agar blocks were placed under cut tips
of shoots. The blocks were then placed
on growing shoots as seen. How will
each shoot grow?
Figure 17.3
Shoots always grow towards light and upwards or against gravity.
However in roots, concentrations of auxins slow down growth. As in the
shoot, auxin accumulates on the lower side because the gravity, but in a root
the upper side will grow faster because it is less affected by the auxin. No matter
how a root is placed in the soil, it will always grow downwards (figure 17.4).
gravity
gravity
more growth
agar block
root placed
horizontally
growing tip
root grows
downwards
roots always grow downwards
or towards gravity
Figure 17.4
less growth
Roots always grow down.
Simple investigations can show the effects of light and gravity on
germinating seedlings as shown in figure 17.5. They use agar blocks containing
auxin because the hormone can easily diffuse through the agar.
and placed on an
agar block
shoot tip cut
agar absorbs
the auxin
agar
shoot tip cut off –
cut shoots are used
in investigation
agar block with auxin
placed on a cut shoot
– shoot will grow
Figure 17.5
198
agar block with auxin
placed to one side
of a cut shoot
– shoot grows
more on that
side and so bends
agar block with no
auxin results in
no growth
Investigations of growth in seedlings. Agar blocks which absorb auxin are used.
17 • Movement
Uses of plant hormones
herbicide ❯
Pesticides are poisonous chemicals which kill pests. Some plants, especially
weeds, are described as being pests and herbicides are used to kill them or
remove them from the environment.
Synthetic plant hormones, for example auxin, can be used as a herbicide.
When present in excessive amounts, much more than produced naturally,
auxin can disrupt plant growth and so kill plants. 2,4-D and 2,3,5-T are
examples of selective herbicides which kill broad-leaved plants. They stimulate
auxin production in the plants. The weed killer causes dicotyledons to grow so
fast that they cannot sustain their own growth and they die.
Some herbicides work because they are translocated throughout a plant.
They are called systemic herbicides. They are translocated from the leaves,
where they were applied to the roots where they interfere with root function.
Because the root is killed, the whole plant dies.
The skeleton of humans
endoskeleton ❯
exoskeleton ❯
axial skeleton ❯
appendicular skeleton ❯
The skeleton of humans is an endoskeleton, which means that it is inside
the body. All vertebrates have the same arrangement of endoskeleton, with
the bones inside and the muscles and other body tissues surrounding it. Some
invertebrates also have an endoskeleton, such as squid and octopus, but many
have an exoskeleton where the hard part is on the outside. For example,
insects have a jointed exoskeleton made of chitin, and many molluscs, like
clams, have a hard calcified shell. Exoskeletons have an advantage in that they
can protect the whole of the body, but they also limit the size to which the
organism can grow.
The body of humans is held upright by a skeleton which is made of bones
arranged as seen in figure 17.6 (overeaf).
The human skeleton can be divided into two parts: the axial skeleton,
which is the skull and vertebral column with the rib cage, and the
appendicular skeleton, which includes all the other bones, the fore and hind
limbs, and the pelvic and pectoral girdles.
Functions of the skeleton in humans
ITQ4
Name the bones found in the lower
limb, from the pelvic girdle to the toes.
ITQ5
What is the importance of the blood
vessels and the marrow in a long bone?
• Protection of organs – The skull protects the brain, the vertebral column
protects the spinal cord and the ribs protect the lungs, heart and much of
the liver. Bones surround these delicate organs, forming cup-like structures
or tube-like structures in which the organs are housed.
• Support of the body – Humans are supported upright more than most
mammals and can stand on two feet. The skeleton acts like a frame
supporting the soft body parts. The limbs are separated by the width of the
girdles and this helps to keep the body stable.
• Movement – The skeleton is made up of a number of bones joined
together. Muscles, and other tissues such as tendons, can cause movement
of a single bone. The coordinated movement of many bones results in
walking, running and all the movement seen in a human.
• Manufacture of red and white blood cells – These are made in the bone
marrow of the pelvis, ribs, sternum and leg bones.
Structure of a long bone
The long bones are the femur, tibia and fibula of the hind limb and the
humerus, ulna and radius of the fore limb. The structure of the long bone is
shown in figure 17.7 (overleaf).
199
Life Processes and Disease
skull
cranium
face
shoulder girdle
(pectoral girdle)
clavicle
scapula
thorax
sternum
ribs
upper limb
humerus
veterbral column
radius
pelvic girdle
ulna
carpals
metacarpals
phalanges
lower limb
femur
patella
tibia
ÄI\SH
the axial skeleton (skull and
vertebral column) is coloured
tarsals
metatarsals
phalanges
Figure 17.6 The human skeleton.
The vertebral column
vertebrae ❯
ITQ6
Describe two functions of the vertebral
column.
200
In humans, the vertebral column extends from the neck to tailbone or coccyx
(figure 17.8). It is made up of 33 bones called vertebrae. All vertebrae have
the same basic structure (figure 17.9). There are 7 neck or cervical vertebrae,
the first of which are the atlas and axis. The cervical vertebrae are followed by
12 thoracic vertebrae, then 5 lumbar vertebrae. The sacrum follows the lumber
vertebrae and is made of several vertebrae fused together. Finally, the tail
vertebrae are fused to form the coccyx.
17 • Movement
neural spine
anterior facet
cervical
vertebrae (7)
epiphysis
transverse process
spongy bone
(contains red
marrow)
Structure of a typical vertebra
thoracic
vertebrae (12)
marrow cavity
shaft
neural arch
neural canal
cartilage
compact bone
posterior facet
spinal cord runs
through the neural canal
blood vessel
lumbar
vertebrae (5)
three vertebrae interlock
to form part of the
vertebral column
sacral
vertebrae (fused)
epiphysis
coccyx or 'tail'
Figure 17.7 The structure of a long bone.
Figure 17.8 The vertebral column in humans.
neural spine
neural canal
facet
transverse process
centrum
vertebrarterial canal (two small
holes in vertebra, one on either side)
Cervical vertebra – has two small holes apart from the large neural canal
neural canal
transverse process (short)
neural spine (long)
rib
facet
neural canal
neural spine (short)
centrum
point of articulation
for rib
Thoracic vertebra – articulate with ribs as well as other vertebrae
facet
transverse process (long)
centrum big and
well developed
Lumbar vertebra – has large centrum and long transverse processes
Figure 17.9 The cervical, thoracic and lumbar vertebrae in humans.
201
Life Processes and Disease
Cervical vertebrae
•
•
•
•
large neural canal because these vertebrae are closest to brain;
vertebraterial canals present;
short neural spine;
short transverse processes.
Thoracic vertebrae
• neural canal smaller than cervical because further from brain;
• very long neural spine for attachment of back muscles;
• short transverse processes to accommodate rib bones on either side.
Lumbar vertebrae
•
•
•
•
centrum big and well developed to support weight of body;
neural canal small;
long, wide neural spine;
long transverse processes for muscle attachment.
Table 17.1 summarises the functions of the various surfaces and projections of
each vertebra.
Part of vertebra
Function
neural canal
protects the spinal cord
neural spine
muscle attachment
transverse process
muscle attachment
facet
articulates with facets of adjacent vertebrae and allows slight movement
centrum
central rigid body of vertebra, discs of cartilage separate adjacent vertebrae
Table 17.1 The functions of the different parts of a human vertebra.
Movement in a limb of humans
Practical activity
SBA 17.4: Compare the movements of
four animals, page 357
spongy
bone
compact
bone
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Movement in a limb is brought about by many tissues, such as muscles,
tendons, ligaments and bones, all working together. Bones are able to move
because of the presence of joints in the skeleton. A typical joint is seen in
figure 17.10.
Bones are attached to each other by ligaments. They cannot move on
their own. Muscles are seen around the bones and move the bones when
they shorten (contract) and lengthen (relax). This is shown in the simplified
diagram in figure 17.11.
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202
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Figure 17.11
Bones are moved by muscles.
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17 • Movement
antagonistic muscles ❯
tendon ❯
The muscles of the arm move the bones of the arm to flex or extend the arm
in the same way as seen in figure 17.12. The bones are attached to each other
by ligaments and attached to muscles by tendons. They have special names
(triceps and biceps) and contract or relax to move the bones. All the bones
of the body need muscles to help them move. Imagine the coordination of
contraction and relaxation of muscles needed to cup the fingers around a
bottle, and then move the bottle to the lips to take a drink of water.
Movement is brought about by the contraction of antagonistic muscles.
Antagonistic muscles are pairs of muscles that always work together: when
one is contracting, the other is relaxing. They move many bones of the human
skeleton. In the joint of the upper arm, the triceps and biceps are antagonistic
muscles. They are attached to the bones by tendons which are non-elastic.
A muscle shortens when it contracts and is lengthened when it relaxes.
Movement of the bone is brought about when the muscles pull on the bones
(figure 17.12).
Flexing the arm
Extending the arm
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flexor muscle ❯
extensor muscle ❯
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Flexing and extending the arm.
When the biceps contracts (and triceps relax), it pulls the bones of the lower
arm upwards so the arm bends or flexes. The biceps is called a flexor muscle.
When the triceps contracts (and biceps relaxes), it pulls the bones of the lower
arm so that the arm straightens or extends. The triceps is called an extensor
muscle.
Types of joint
There are three types of joint:
• immovable joint;
• partially movable joint;
• movable joint.
suture ❯
gliding joint ❯
pivot joint ❯
Immovable joints are also called sutures. The bones are fused together
allowing no movement. Examples are joints of the cranium and pelvic girdle.
Partially movable joints allow some movement. Examples of joints between
the tarsals (ankle) and carpals (wrist). The bones can slide over each other
producing the movements seen in the wrist and ankle. These are also called
gliding joints.
A partially movable joint also exists between the atlas and axis at the top
of the neck allowing some movement of the head in relation to the spine (e.g.
nodding or shaking). This is called a pivot joint.
203
Life Processes and Disease
synovial joint ❯
Moveable joints are also called synovial joints. Synovial fluid in these joints
reduces friction allowing free movement of the bones. There are two types of
synovial joint (figure 17.13).
• Hinge joint – Allows movement in one plane; for example, elbow, knee
and finger joints. Bones of a hinge joint are capable of carrying heavy loads.
• Ball-and-socket joint – Allows movement in all planes; for example, the
shoulder and hip joints.
Hinge joint
Ball-and-socket joint
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Figure 17.13 The hinge and ball-and-socket joints.
ITQ7
(i) What is a joint?
(ii) What is the importance of joints?
ITQ8
Name the structures found around a typical joint, giving a
reason why each is important.
ITQ9
Put these in the order they occur when extending the arm:
(a) the arm is pulled down.
(b) biceps muscle relax.
(c) ligaments stretch arm.
(d) muscles pull on radius and ulna.
(e) triceps muscle contract.
ITQ10
(i) What kind of joints are seen in the fingers?
(ii) What is the advantage of each finger having a number of
hinge joints, rather than one hinge joint?
204
17 • Movement
Chapter summary
• Movement is a characteristic of life.
• There are a number of reasons why animals move.
• Some plants can move some of their parts, but all plants show growth movement or
tropisms.
• During growth, plants respond to light (phototropism) and gravity (geotropism).
• The tips of the growing parts of a plant produce auxin, the hormone responsible for
growth in plants.
• The skeleton of humans has many functions, one of which is movement.
• The skeleton forms a framework inside the body of humans and is made up of the
axial and appendicular skeletons.
• The axial skeleton consists of the cranium and the vertebral column.
• The vertebral column is made up of many bones called vertebrae and includes the
cervical, thoracic and lumbar vertebrae.
• The appendicular skeleton consists of the limbs and rib cage.
• The skeleton is made up of many bones joined together; movement is seen at these
joints.
• Movement is brought about by muscles, tendons and ligaments at these joints.
• There are many kinds of joint: immovable, partially movable and movable joints.
Answers to ITQs
ITQ1 Any two of the examples from the bullet list on page 000 is suitable.
ITQ2 A plant has to move (grow) towards the light because light is
necessary for photosynthesis.
Some plants are able to close special leaves that trap small insects. These
plants need to acquire their protein from insects, because they live in nitratedeficient soil.
ITQ3
ITQ4 Pelvic girdle, femur, tibia and fibula, tarsals, metatarsals, phalanges.
ITQ5 The blood vessels bring nutrients and oxygen to the bone, since it is
alive and must respire. Also, these vessels take away waste produced by the
bone.
The marrow cavity is important for the production of red blood cells. Red
blood cells are constantly produced in the bone marrow.
ITQ6 The vertebral column holds the body upright and protects the spinal
cord.
ITQ7 (i) A joint is where two bones meet. It is lubricated to reduce friction
when the two bones move.
(ii) Joints are important for movement. All movement takes place because
muscles contract and move the appropriate bones. The bones move from
where they are joined to another bone. Without joints, no movement would
not be possible (not movement of the entire body, nor movement of a part of
the body).
205
Life Processes and Disease
ITQ8
Structure
Importance
ligament
joins bone to bone, and can stretch as the bones move away from and towards
each other during movement
muscle
can contract or lengthen – because it is attached to bone, it can pull on (extend)
the bone, so muscles bring about movement
tendon
joins muscle to bone, is non-elastic, so the effect of the muscle contraction or
relaxation can be applied to the bone
synovial fluid
fluid found in the joint which helps to reduce friction when bones move with
respect to one another.
ITQ9 1 – b and e (antagonistic muscles); 2 – d; 3 – c; 4 – a
ITQ10 (i) The fingers have hinge joints.
(ii) Having a number of hinge joints allows fingers to be curled around an
object.
Examination-style questions
206
1
(i) List the main functions of a vertebrate skeleton.
(ii) Make a labelled drawing of:
(a) a typical vertebra;
(b) a vertical section of a typical long bone.
(iii) The human skeleton, as is typical of mammalian skeletons, can be divided into two
components or parts. Name these parts and the bones included.
2
(i) Suggest some differences between movement in plants and animals.
(ii) Define:
(a) phototropism;
(b) geotropism.
(iii) Explain fully how plants respond to light.
(iv) How do plants growth substances differ from animal hormones?
(v) (a) Since the late 1980s, scientists have been conducting experiments on the effects
of space travel on seed germination. Why do you think there is an interest in such
studies?
(b) Experiments conducted on seeds in space yielded growing plants, but these
‘extra-terrestrial’ plants did not grow straight, they grew in all directions. Explain
what might have caused this to happen.
(c) Suggest ways of producing ‘straight plants in space.
3
(i) Make a labelled drawing of a typical joint.
(ii) The exoskeleton of an insect lies outside the muscles that are attached to it. Like the
joints of the endoskeleton, the joints of an exoskeleton, provide an excellent means of
locomotion. The diagrams I and II below show joints seen in an insect and humans.
17 • Movement
I – insect's limb
exoskeleton
extensor muscle
flexor muscle
II – human limb
A
F
E
C
B
D
(a)
(b)
(c)
(d)
Name the parts A, B, C, D, E and F.
Using a diagram, show how the insect can flex or bend its leg.
Using a diagram, show how the human can bend the arm.
What name is given to muscles that work together to move a limb? Give
examples of these muscles as seen in the insect’s limb and human’s limb.
(iii) Describe the state of these muscles when:
(a) the insect’s limb is extended or straightened.
(b) the human’s limb is extended or straightened.
207
18
By the end of this
chapter, you should
be able to:
Irritability, Sensitivity
and Coordination
define the terms ‘stimulus’ and ‘response’
describe responses of green plant and invertebrates to stimuli
understand why responses to stimuli are important for survival of organisms
define the terms ‘receptor’ and ‘effector’
identify the main sense organs and the stimuli to which they respond
describe the main sense organs
describe the nervous system
describe the endocrine system
explain a simple reflex action
distinguish between a cranial and spinal reflex
describe the functions of the main regions of the brain
discuss the physiological, social and economic effects of drug abuse
sense organs
eye
ear
nose
tongue
skin
receptor
stimulus
survival of
organism
nervous system
spinal chord
response
brain
effector
motor
sensory
relay
synapse
208
neurone
movement towards
or away
18 • Irritability, Sensitivity and Coordination
Irritability
CHAPTER 1
In chapter 1, we found that irritability is one of the seven characteristics of
living things. It means that living organisms can respond to changes in their
internal environment and the world around them. These responses usually
increase their chances of survival.
Animals and plants react to changes in the environment, not only drastic
climate changes, but also simple everyday changes. For example, a snake
looking for food will move toward the scent of a rat, and the shoots of a
seedling will grow towards light.
Stimulus
stimulus ❯
response ❯
A stimulus is a change in the environment that an organism reacts or
responds to. It could be light, temperature, a texture, a chemical in the air
or moisture, a response is the change in the organism brought about by the
stimulus (figures 18.1 and 18.2). The response to stimuli is important for the
survival of organisms.
Response of animals
Stimulus
Figure 18.2 This male moth has large
antennae that can sense just a few
molecules of a chemical attractant that a
female several miles away has released.
ITQ1
(i) Define irritability.
(ii) Why is it important for the survival
of an animal?
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Response
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Figure 18.1 A snake responds to the stimulus of food.
Table 18.1 shows some examples of stimuli, the responses and the importance
to the organism of responding in this way.
Stimulus
Possible response
Importance to organism of response
chemical from
an organism
move towards organism
organism may be a potential mate or
potential prey
moisture in soil
move towards moist areas
prevent desiccation or dying, especially
for organisms without a waterproof
outer covering
light
move from light to darker areas
escape from predators since it is
harder to be seen in darker areas
cold
temperatures
move away from cold temperature
organism cannot survive in cold
temperatures, body not adapted
Table 18.1 Responses to some stimuli and the importance of those responses.
209
Life Processes and Disease
Response of green plants
CHAPTER 17
ZLLK[HRLUPU[VH
JH]LI`HIPYKVYIH[
In chapter 17, we saw that seedlings respond to unilateral (one-directional)
stimuli of light and gravity. The roots grew in the direction of gravity and the
shoots grew towards light. Plants need water and minerals from the soil, so
the roots must grow down into the soil to reach them. Green plants, including
seedlings, also need light for photosynthesis. It is therefore important for the
survival of green plants to grow towards light (figure 18.3).
A plant in a room will grow towards the window where there is sunlight. A
seed may be taken into a cave by a bird or bat. It may germinate and then the
seedling will grow towards light and out of the cave’s entrance or any other
opening. Otherwise, the plant will die for lack of food in the darkness.
Invertebrates, like millipedes, earthworms and woodlice, need certain
conditions to survive. They respond to variations in light intensity, temperature
and moisture. The investigation illustrated in figure 18.4 shows that these
invertebrates respond by moving towards a cooler temperature, moist soil and
away from bright light. These responses ensure that they do not dehydrate and
are hidden from predators, that is, the responses help to ensure their survival.
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Figure 18.3 A plant responds to the stimulus of light.
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Figure 18.4
ITQ2
The senses of some animals are said to
be better developed than in a human.
Give two examples of animals like this,
and explain the importance of the sense
to the animal.
sense organs ❯
210
(M[LYH^OPSL[OL`HSSTV]L
[V^HYKZ[OLKHYRTVPZ[HYLHZ
Many small invertebrates respond to the stimuli of light and moisture.
Unlike most humans, animals in the wild have to find food every day and
maybe avoid being food for another organism. They have to be very aware of
stimuli coming from their environment and be able to make the appropriate
response. More often than not, these everyday changes in the environment are
a matter of life or death.
The sense organs of humans
Humans have five senses: hearing, sight, smell, taste and touch. In humans,
the main sense organs are the eyes, ears, nose, tongue and skin. A group of
sense cells and other tissues form a sense organ.
• Eye – At the back of the eye is the retina which is a layer of sensory cells that
respond to light. Impulses are sent from these cells to the brain by the optic
nerve so that changes in shape, colour, brightness and distance are detected.
18 • Irritability, Sensitivity and Coordination
taste buds ❯
Figure 18.5 Taste buds on a human
tongue.
• Ears – Sensitive hairs in the inner ear respond to vibrations in the air (sound
waves). Impulses are sent from these hairs to the brain by the auditory
nerve so that changes in the quality, tone, pitch and loudness are detected.
• Nose – As air flows into the nose during breathing, chemical molecules in
it touch sensitive hairs. These send messages to the brain so that changes in
scent are detected.
• Tongue – Groups of receptor cells, called taste buds, respond to chemicals
in the food (figure 18.5). Different parts of the tongue are sensitive to
different flavours like salt, sweet, bitter and sour. These send messages to
the brain so that changes in flavour of the food are detected.
• Skin – This is the largest organ of the body. Nerves ending as sensory cells
are scattered throughout the skin. These are sensitive to pain, touch, change
in temperature, light pressure and heavy pressure. They send impulses to
the brain so that it can detect what has been touched.
The nervous system
neurone ❯
Practical atctivity
SBA 18.1: Touch receptors in skin,
page 359
The nervous system is made up of neurones or nerve cells. Neurones transmit
electrical impulses to and from the brain. The nervous system is made up of:
• the central nervous system (CNS) which consists of the brain and spinal cord;
• the peripheral nervous system (PNS) which consists of all the nerves outside
the central nervous system (figure 18.6).
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Figure 18.6 The nervous
system.
receptor ❯
effector ❯
sensory neurone ❯
motor neurone ❯
relay neurone ❯
The peripheral nervous system forms a vast communication network linking
the reception of the stimuli to a response. Receptors receive stimuli from the
environment and responses are brought about by effectors.
Sensory neurones conduct impulses from receptors to the central nervous
system. Motor neurones conduct impulses from the central nervous system
to the effectors. Intermediate or relay neurones link sensory and motor
neurones. They are found in the central nervous system (figure 18.7, overleaf).
211
Life Processes and Disease
KLUKYP[L
JLSSIVK`
KLUKYVU¶JHYYPLZPTW\SZLZ
[V^HYKZ[OLJLSSIVK`
intermediate or
relay neurone
motor
neurone
sensory
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Figure 18.7
Motor, relay and sensory neurones.
stimulus
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Figure 18.8 The structures of the sensory, relay and motor neurones can be related to this typical
nervous pathway.
ITQ3
(i) Describe the nervous system of
humans.
(ii) How do you respond to a stimulus?
212
Figure 18.8 is a typical pathway, from the stimulus touching the receptor to the
effector bringing about a response. The numbers in the following paragraphs
refer to figure 18.9.
1 The stimulus is, say, a hot object touching a pain receptor in the skin of the
hand.
2 A signal travels along the sensory neurone to the central nervous system
(CNS).
3 In the CNS, a relay neurone carries the signal through the brain.
4 The relay neurone passes the signal to the motor neurone.
5 The signal travels along the motor neurone to the effector (biceps muscle)
which responds (contracts).
6 The hand is moved away from the hot object.
18 • Irritability, Sensitivity and Coordination
6
1
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Figure 18.9 A typical pathway of receptor to effector.
ITQ4
Describe a typical nervous pathway.
The nervous system is adapted to carry messages quickly between specific
locations in the body, so that quick responses can be made. Sometimes the
effector may be a gland. Endocrine glands are found throughout the body and
they regulate a wide range of activities, including heart rate, metabolism and
reproduction. Together, the nervous system and endocrine system co-ordinate
all of the body’s activities.
The synapse
synapse ❯
Signals travel along neurones as electrical impulses, which are very fast.
However, there are millions of neurones in your body, and where the ends
of two neurones meet there is a small gap called a synapse (Figure 18.10).
Electrical impulses cannot cross these gaps, so they are converted to a chemical
signal in order to cross the synapse. As they reach the other neurone, they are
converted back into electrical impulses so that they can continue quickly on
their way.
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Figure 18.10 A synapse.
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213
Life Processes and Disease
Table 18.2 describes some receptors, effectors and responses in humans.
ITQ5
What is the receptor, the effector and
response of an animal when it sees and
moves towards a mate?
Stimulus
Receptor
Effector
Response
object moving towards retina of the eye
the face
receives and sends a
message to the brain
muscles of the neck
head turns away so the
object cannot hit the
face
very hot object which is nerve endings in
about to be picked up the skin sensitive to
temperature send a
message to the brain
muscles of the arm
hand pulls away from
the hot object
chemicals from food
(smell) reach the nose
Table 18.2
ITQ6
(i) What is a synapse?
(ii) Describe what happens at a
synapse.
chemoreceptors in the salivary gland
nose send a message
to the brain
saliva secreted and
body prepares to digest
food
Receptors, effectors and their responses in humans.
All activity involves the coordination of the brain, spinal cord, sensory and
motor neurones. Stimuli are constantly being received, sent to the brain where
they are analysed and appropriate responses sent back (figure 18.11).
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Figure 18.11 Every day, millions of messages are received by the brain and appropriate
responses made. This involves the coordination of sensory neurones, CNS and motor neurones.
214
18 • Irritability, Sensitivity and Coordination
Reflex actions
reflex action ❯
Practical activity
SBA 18.2: Two reflex actions, page 360
ITQ7
What is the reflex action?
A reflex action is a rapid and automatic response to a stimulus. It does not
require conscious control (you do not think about doing it). Examples of reflex
actions are the knee jerk, sneezing, the pupil reflex and blinking. The pathway
between the receptor and effector is called the reflex arc. There are two kinds
of reflex:
• spinal reflexes are nerve impulses that pass through the spinal cord and do
not go to the brain (e.g. the knee jerk response, figures 18.12 and 18.13);
• cranial reflexes are reflexes in the head region (e.g. blinking and the
response of the pupil in the eye to light, figure 18.12).
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Figure 18.13
Detailed description of the knee jerk spinal reflex.
215
Life Processes and Disease
The brain
ITQ8
(i) A person who suffered brain
damage is now unable to see.
Explain how this could happen.
(ii) What consequences would result
from damage to the cerebellum of
the brain?
The brain is the most important part of the nervous system. It enables humans
to ‘think’ or ‘reason’, a skill which is supposedly lacking in most animals.
The brain has grey matter on the outside and white matter on the inside. It
is surrounded by tough membranes, called meninges, and cerebrospinal fluid
which cushion it from knocks. It is also surrounded by the bones of the skull.
Your brain is very well protected (figure 18.14). Humans can perform complex
mental and physical activities co-ordinated by different areas of the brain
(figure 18.15). They receive stimuli from the environment and the brain brings
about the appropriate response.
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Figure 18.14 The brain. (a) Section through the head. (b) External view of human brain.
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IVK`»ZPU[LYUHSLU]PYVUTLU[
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Figure 18.15
216
Functions of the various parts of the brain.
18 • Irritability, Sensitivity and Coordination
The spinal cord is also composed of grey and white matter, but here the
white matter is on the outside and the grey matter on the inside (figure 18.16)
IHJRVMIVK`
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Figure 18.16
Cross-section of the spinal cord.
Autonomic nervous system
autonomic nervous system ❯
CHAPTER 16
ITQ9
(i) Name six activities that occur
in the body while a person is
sleeping.
(ii) How is it possible for these
activities to take place?
endocrine gland ❯
The autonomic nervous system is the n ame given to all the nerves which
automatically control the normal functioning of internal organs like the heart
without conscious control. For example, your heart keeps beating, peristalsis
occurs, breathing occurs, pupils dilate and blood vessels constrict, without you
having to think about any of these responses – they occur even when you sleep.
The internal environment of the body must be kept stable (chapter 16).
Homeostasis, the maintenance of a constant internal environment, depends
on the autonomic nervous system. All cells, and therefore tissues and organs,
function efficiently in certain conditions of temperature, pH and water. Any
change in these conditions must be remedied: for example, if there is a lack of
water, cells become dehydrated, so the body responds to increase the amount
of water available. Animals and plants respond to internal changes in ways that
lead to stabilising the internal environment.
The endocrine system
In humans, the endocrine system consists of a number of glands called
endocrine glands. The endocrine system controls growth and development.
A gland is a structure which secretes a specific chemical substance. In
humans, there are two types of gland: exocrine glands and endocrine glands
(figure 18.17).
exocrinegland
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endocrinegland
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Figure 18.17
NSHUK
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MSV^Z[V[OL[HYNL[VYNHU
Exocrine and endocrine glands.
217
Life Processes and Disease
exocrine gland ❯
ITQ10
Draw up a table to show the differences
between endocrine and exocrine
glands. Describe how they work and
give examples of the substances that
they secrete.
Exocrine glands transport their secretions by ducts to other parts of the body.
For example, salivary glands in the mouth secrete saliva via ducts into the
mouth; tear glands by the eye secrete fluid which passes through ducts onto
the eye’s surface. Endocrine glands secrete chemicals called hormones directly
into the bloodstream. These glands have a rich blood supply to collect the
hormone and transport it to its target organ.
The pancreas is an organ of the digestive system. It contains two types of
secretory cell. One type produces enzymes that make up pancreatic juice which
is secreted through a duct to the duodenum. The type produces the hormone
insulin that diffuses into blood vessels which pass through the pancreas.
The pancreas is therefore a structure that is made up of both exocrine and
endocrine glands (figure 18.18).
WHUJYLHZ¶ZVTLJLSSZ
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LUa`TLZPU[OLN\[
LUa`TLZWHZZ
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ISVVK^P[OUVOVYTVUL
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¶ISVVKYPJOPUPUZ\SPU
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Figure 18.18 The pancreas contains both endocrine and exocrine glands.
Hormones help to control and coordinate many body activities, including
growth and development. They are produced by endocrine glands positioned in
specific areas of the body (figure 18.19).
O`WV[OHSHT\ZTHUHNLY
WP[\P[HY`THZ[LYNSHUK
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Figure 18.19 The endocrine system in humans.
Hormones are produced in very small amounts and travel through the body
in the bloodstream to target organs. Hormones influence the activities of these
target organs.
218
18 • Irritability, Sensitivity and Coordination
The pituitary and the hypothalamus
pituitary ❯
hypothalamus ❯
ITQ11
(i) What are hormones?
(ii) Name four different hormones
produced in humans.
Most, but not all, endocrine glands work under the influence of a single
master gland – the pituitary, which is situated beneath the brain. The
hypothalamus is situated close to the pituitary. While the hypothalamus is
not an endocrine gland, it regulates the secretion of some of the pituitary gland
hormones. If the pituitary is thought of as the master gland of the endocrine
system, then the hypothalamus can be thought of as the manager. The
hormones produced by the pituitary and their effects are shown in Table 18.3.
Hormone
Functions
pituitary growth hormone
stimulates growth of the entire body: too much
causes gigantism; too little causes dwarfism
antidiuretic hormone (ADH)
stimulates the kidneys to reabsorb more water
from filtrate when the blood plasma becomes
too concentrated
stimulate other glands such as the
other hormones: e.g. follicle stimulating
hormone (FSH), luteinising hormone (LH), thyroid ovaries,thyroid, and testes into activity
stimulating hormone (TSH)
Table 18.3
Functions of the hormones produced by the pituitary.
Drugs and the effects of drug abuse
drug ❯
A drug is any substance or chemical which alters the body’s action, or
interferes with some aspect of the body’s metabolism. It affects chemical
reactions in the body and ultimately, has effects on the brain. A drug can be
administered to the body in many ways:
• by injection;
• orally;
• applied to the skin;
• inhaled.
Medicinal use of drugs (prescription drugs)
Doctors assess the need for these drugs
very carefully, since many have side-effects.
However, some people abuse steroids,
diet pills, tranquillisers and antibiotics for
personal ‘miracles’, ignoring and sometimes
ignorant of the harmful effects. You should
take care to read all instructions on any
medicines that you use, or are prescribed by
your doctor to make sure you are aware of
the risks and side-effects (figure 18.20).
Figure 18.20 The side-effects of one
medicinal drug.
219
Life Processes and Disease
therapeutic drugs ❯
Medicinal drugs are widely used to diagnose, prevent and treat disease. These
include painkillers, antibiotics and vitamins which are described as being
therapeutic drugs.
Penicillin is a common antibiotic, which is used for the treatment of
many bacterial infections. Antibiotics save millions of lives each year, but are
sometimes used unnecessarily and ineffectively to treat viral diseases. This
misuse can increase the risk of resistant strains of bacteria developing and
eventually serious bacterial diseases may become untreatable.
Painkillers (analgesics) are very useful but can be abused. Aspirin is a
common painkiller which also reduces fever and inflammation. It works by
blocking the transmission of pain signals from the receptors to the spinal cord
and brain. However, it can cause irritation to the stomach walls and is not
recommended for children as it may cause fatal brain and liver damage. We
often resort to painkillers too easily, unmindful of the side-effects.
Many people drink coffee every morning, and some continue to consume
coffee all day. Caffeine is the drug found in coffee and is also added to some
soft drinks, including many colas. It is a stimulant, making the user feel more
alert and energetic. However, it is addictive and interferes with the proper
functioning of the central nervous system. Caffeine also prevents calcium
absorption and can thus lead to weak bones and teeth in the older years.
The use of diet pills, laxatives and diuretics is an unhealthy and potentially
very dangerous way to lose weight. Some diet pills contain ephedrine which
increases metabolism and makes the heart beat faster. This can lead to heart
palpitations, cardiac arrest, stroke, and death, even in an otherwise healthy
person.
Steroids, which work similarly to some hormones, are used in the treatment
of asthma. However, they are sometimes abused by athletes and body-builders
to build up muscle, thus increasing strength and speed. The associated risks
include aggression, reduced sex drive and masculinisation of women.
Tranquilisers are sedatives that depress the nervous system, and are used in
the treatment of anxiety and stress. They are valuable drugs but over-use can
make a person unmotivated and unable to cope with daily activities.
Abuse of drugs
psychoactive drugs ❯
drug addiction ❯
CHAPTER 12
withdrawal symptoms ❯
Drug abuse refers to use of substances which may cause a person to become
dependent. These range from mild stimulants like caffeine to powerful
chemicals like narcotic drugs that can alter mood and behaviour. Drugs
that interfere with the nervous system and cause change in mental state
and behaviour are called psychoactive drugs. These include LSD, alcohol,
cocaine, nicotine and heroin. Use of these can lead to drug addiction, which
is the state of psychological dependence on the drug. Marijuana addiction is
discussed in chapter 12. Physical dependence occurs when the body adapts
to a drug and increases its tolerance to the drug’s effects. This leads to using
larger doses of drug to achieve the original effect. Severe physical withdrawal
symptoms occur if the drug is not taken.
Alcohol
alcoholic ❯
220
Alcohol is found in intoxicating beverages and is a depressant of the central
nervous system. In small amounts, its effect is to make the drinker more
sociable, more self-confident and to give a sense of well-being and release from
anxiety. However, people abuse alcohol by repeatedly drinking it in excessive
amounts. People who must drink alcohol every day to cope with life are
alcoholics: this dependence is classed as a disease called alcoholism.
18 • Irritability, Sensitivity and Coordination
Short-term effects of alcohol abuse:
• slurred speech;
• impaired mental function;
• loss of muscular coordination;
• increased excretion leading to dehydration;
• nausea and vomiting;
• possibly violent or aggressive behaviour;
• possible loss of consciousness.
Long-term effects of alcohol abuse:
• physical and psychological dependence;
• severe physical withdrawal symptoms that include nausea, vomiting,
shaking, abdominal cramps and pain, enlarged blood vessels in the face;
• sudden discontinuation can lead to severe shaking, hallucinations and
sometimes fatal convulsions;
• malnutrition and risk of deficiency diseases;
• cirrhosis of the liver (when dead cells in the liver are replaced with fibrous
tissue);
• liver cancer;
• stomach ulcers as alcohol irritates the stomach lining causing it to produce
excess gastric juice;
• coronary heart disease and high blood pressure;
• a range of social, personal and occupational problems.
Drinking during pregnancy can cause low birth weight, poor physical and
mental development in the fetus, even fatal abnormalities; it can also lead to
miscarriage.
Cocaine
Cocaine has long been used by doctors as a local anaesthetic. However, it
is abused as a drug for the sense of euphoria or ‘high’ it produces. It is a
stimulant, which helps in social situations. It can be very addictive, especially
as ‘crack’.
Symptoms of cocaine abuse include:
• strange and violent behaviour;
• hallucinations leading to schizophrenia, mental illness and sometimes
death;
• increased blood pressure and heartbeat;
• lung and nasal damage;
• reduced need for sleep.
Cocaine addiction is a worldwide problem. Many governments try to educate
their people about the dangers associated with cocaine, including the murder
and corruption which often follow its production and sale. An addict’s family
suffers emotionally, financially and socially. But a lot of money is involved in
the illegal drug trade and trafficking and the problem of addiction persists in
many parts of the world.
Social and economic implications of drug abuse
• Loss of working time which reduces the productivity of the economy and
causes loss of earnings for the country and reduced standard of living for its
people.
• Loss of life due to overdose.
221
Life Processes and Disease
• Increased demands on health services for treatment and sometimes
prolonged and expensive care. Research for cures is also very expensive.
• Increased crime and social unrest.
• Family and personal neglect.
Chapter summary
• A stimulus is a change in the environment that an organism reacts to.
• A response is the change in the organism brought about by the stimulus.
• Responding to stimuli is important for the survival of animals; responses may help to
find food, to escape predators and to find a mate.
• Green plants respond to the stimulus of light by growing towards it.
• Sense organs are organs that receive stimuli. In humans, the sense organs are the eyes,
ears, nose, tongue and skin. The eye responds to light; the ears respond to sound; the
nose and tongue respond to chemicals; the skin responds to pressure and temperature.
• The nervous system is responsible for receiving stimuli and coordinating a response.
• The nervous system is made up of the central and peripheral nervous systems.
• The central nervous system is composed of the brain and spinal cord.
• The peripheral nervous system is composed of all the other nerves.
• Sensory nerves carry impulses towards the central nervous system.
• Motor nerves carry impulses away from the central nervous system.
• The junction between two neurones is called a synapse.
• Receptors receive stimuli from the environment.
• An effector brings about a response to a stimulus. It is often a muscle.
• A reflex is an automatic response to a stimulus and does not require conscious control.
• The brain enables humans to perform complex mental and physical activities.
• The autonomic nervous system controls those responses that do not require
conscious control.
• Any substance which changes a body’s action is a drug.
• Prescription drugs and drugs used for medicinal purposes can also be abused.
• Some drugs such as alcohol, cocaine and marijuana can lead to addiction or a state
of physiological dependence on the drug.
• Drug abuse has many social and economic implications for the family, community
and country.
222
18 • Irritability, Sensitivity and Coordination
Answers to ITQs
ITQ1 (i) Irritability is the ability of living organisms to respond the stimuli
coming from the environment.
(ii) Irrirability is important because stimuli from the environment carry
information about food, predators, competitors and so on. An organism may
die quickly if it cannot respond to this information.
ITQ2 Dogs have a very keen sense of smell which they use to hunt for food. A
rabbit has eyes at the sides of its head, which give the rabbit a bigger field of view
to see predators. You may have thought of other examples – there are many.
ITQ3 (i) The nervous system of humans consists of the brain, spinal cord
and a number of nerves that extend throughout the body.
(ii) Sensory nerves detect stimuli from the environment and messages are
sent to the brain, which when processes and stores the information. Messages
are sent back to certain organs in response to the stimuli.
ITQ4 stimulus receptor sensory neurone central nervous system motor neurone effector response
ITQ5
Receptor
Effector
Response
eyes see the potential mate
muscles
animal moves towards the mate, showing courtship
behaviour, etc.
ITQ6 (i) A synapse is the junction between two nerve cells.
(ii) Messages in the form of electrical impulses arrive at the end of one nerve
cell. This causes a chemical to cross the junction between the two nerve cells.
On arrival of the chemical at the other nerve cell, electrical impulses are
generated there and the message is sent on.
ITQ7 A reflex action is a response to a stimulus without conscious
knowledge.
ITQ8 (i) The person must have received damage to the region of the brain
responsible for receiving and processing information coming from the eyes.
Even though the eyes may function perfectly, the brain cannot receive or
process signals from them so there is no perception of seeing.
(ii) The cerebellum is responsible for balance and coordination of muscle
activity. If the cerebellum is damaged, the person will not be able to stand,
walk, eat or perform activities that involve the muscle coordination.
ITQ9 (i) Respiration in all cells; beating of the heart; breathing movements;
movement of food along the alimentary canal; movement of blood throughout
the body; production of urine by the kidneys.
(ii) The autonomic nervous system is responsible for all these activities.
Messages are sent to various organs constantly, ensuring that they continue to
do their jobs, even during sleep.
ITQ10
Endocrine
Exocrine
gland lies next to a blood capillary
gland has a tube that connects to its site of action
hormone diffuses across membranes to the
blood and is transported by the blood to the
site of action (e.g. insulin and adrenalin)
chemical passes through the duct to the site of
action (e.g digestive juices and saliva)
ITQ11 (i) A hormone is a chemical messenger. It travels in the blood and has
its effect at a target site where the chemical brings about a reaction.
(ii) Insulin, adrenalin, testerone, oestrogen
223
Life Processes and Disease
Examination-style questions
1
(i)
Explain the meaning of the terms:
(a) stimulus;
(b) response.
(ii) Describe fully a named response and explain why it may be important to the survival
of the organism.
(iii) Copy and complete the table below.
Sense
organ
Stimulus to which An example of its importance (describe an everyday
it responds
activity that uses the sense organ and explain how
it is used)
eye
ear
nose
tongue
skin
(iv) A person deficient in one sense organ is said to develop another sense to a greater
extent than normal. For example, a blind man is said to have a better sense of hearing.
(a) Suggest an explanation for this phenomenon in nature.
(b) Suggest two ways a person who has lost his/her sense of sight may be affected.
2
(i)
The nervous system is made up of two parts. Name the parts and give a description of
each part.
(ii) Make a labelled drawing of a typical motor neurone.
(iii) List some differences between a motor neurone and a sensory neurone.
(iv) A child touches something hot, and pulls away her hand from the hot object. Describe
the pathway of this response through her nervous system, from the time the hot
object touches the receptors in her skin to the contraction of muscles as she pulls her
hand away.
3
(i)
Copy the figure below which shows a section of the human brain. Name the parts
labelled A, B, C, D and E.
(
)
*
,
+
(ii) All the activities of the body are controlled by the brain. Annotate your copy of the
figure to show the sites of control of these functions:
(a) intelligence
(b) hearing
(c) sight
(d) coordination of muscular activity.
224
19
By the end of this
chapter, you should
be able to:
The Eye, the Ear and
the Skin
relate the structure of the human eye to its function as a sense organ
understand sight defects and their correction
relate the structure of the ear in humans to its function as a sense organ
understand how we hear
understand how the ear is used for balance
explain the terms poikilotherm and homeotherm
understand why temperature regulation is an example of homeostasis
relate the structure of the human skin to its function in temperature regulation
and protection
discuss skin care
sense organs
light
tongue
sight
defects
sound
eye
nose
pupil
ear
light
intensity
retina
homeotherm
middle
ear
conserve heat
inner
ear
skin care
lose heat
distance
optic nerve
to brain
balance
ear sac
behaviour
skin
outer
ear
accommodation
lens
poikilotherm
semicircular
canals
hearing
cochlea
Our survival, indeed our very existence, depends on our reactions to stimuli
coming from the environment. We feel, hear, see, taste and smell our
surroundings every living moment. The main sense organs in humans are the
skin, ears, eyes, tongue and nose (table 19.1).
Sense organ Stimulus to which it responds
tongue
Taste buds on the tongue are composed of sensory cells. Chemicals dissolved in
moisture in the mouth are detected by these cells. The taste buds on the tongue
are sensitive to four different tastes – sweet, sour, salt and bitter. Messages are
sent to the brain to determine the taste.
nose
Sensory cells line the nasal passage. These cells detect chemicals in the air
entering the nose. Messages are sent to the brain to determine the smell.
(continued)
225
Life Processes and Disease
Sense organ Stimulus to which it responds
skin
There are many different nerve endings in the skin. It is thus very sensitive to
many different stimuli – pain, touch, temperature change and pressure. The skin
‘touches’ the environment. The receptors in the skin send messages to the brain
to determine what we have touched. The skin is also a protective barrier against
the environment. The internal organs are protected from the dangers of the
physical environment, such as UV rays and microorganisms (pathogens).
ear
The ear collects and directs sound waves (pressure changes) to the eardrum.
Vibrations of the eardrum eventually cause movement of the sensory hairs in the
cochlea. This causes nerve impulses in the auditory nerve which are interpreted
by the brain as sounds.
eye
Sensitive cells on the retina of the eye detect light reflected from objects. An
image is formed in the brain when we see.
Table 19.1 The main sense organs in humans and the stimuli to which they respond.
The eye
Structure of the human eye
The eye is a light-sensitive organ that enables us to see small variations
of colour, shape, size, brightness and distance. Light rays from objects are
converted to nerve impulses which are sent to the brain. The brain, not the
eye, is where the actual process of seeing is performed.
The eyehas a number of related structures (figure 19.1):
• eyebrow directs sweat moving down the forehead away from the eye;
• eyelids close to protect the eye against dust and bright light;
• tear gland produces tears which wash away dust particles and contain the
enzyme lysozyme which kills bacteria;
• eyelashes keep the front of the eye free from dust and dirt;
• pupil is the hole which allows light to enter the eyeball;
• iris gives colour to the eye and controls the size of the pupil.
• sclera is the white fibrous coat which protects the eyeball.
L`LIYV^
ITQ1
(i) Where are the eyes positioned in
the human body and how is this
different from a herbivore such as
a zebra?
(ii) Give an explanation for the
difference in the position of the
eyes between a carnivore and a
herbivore.
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Figure 19.1 The eye and associated structures.
ITQ2
List three reasons why eyes are
important to a human.
226
The eyeballs are ball-like structure situated in cavities in the skull called orbits.
Figure 19.2 shows a section through the human eye and figure 19.3 describes
the functions of various parts of the eye.
19 • The Eye, the Ear and the Skin
T\ZJSL
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conjunctiva – thin transparent skin
continuous with lining of
eyelids: protects cornea
ciliary body
choroid – contains blood vessels to
supply retina with food and oxygen;
ISHJRWPNTLU[[VWYL]LU[YLÅLJ[PVU
of light inside the eye
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and cones
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of sclera; refracts (bends) light
rays to a focus on the retina
fovea – contains cones only;
most sensitive part of retina;
most light rays are focused here
pupil – hole in centre of iris;
allows light to enter eyeball
blind spot – point where
optic nerve leaves eye;
no light-sensitive cells
lens – transparent, elastic, biconvex
Z[Y\J[\YL"THRLZÄULHKQ\Z[TLU[Z
to focus light on retina
suspensory ligament – attaches
lens to ciliary body
sclera¶[V\NO^OP[LÄIYV\Z
coat; protects eyeball
optic nerve – carries
impulses from retina to brain
ciliary muscle – circular ring
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shape during accommodation
Figure 19.3
Functions of the various parts of the human eye.
How we see
retina ❯
Light rays from the object travel in a straight line to the eyeballs. They pass
through the structures at the front of the eyeball, through the pupil and
are focused on the retina (figure 19.4, overleaf). The light stimulates lightsensitive cells of the retina which send impulses along the optic nerve to the
brain. The brain then forms an image of size, shape, colour and distance away
from the object.
227
Life Processes and Disease
refract ❯
The cornea bends (or refracts) the light towards the retina. The lens,
however, can vary the amount of bending or refraction and thus ensures the
accurate focusing of the image on the retina (figure 19.5). The iris is composed
of circular and radial muscles and controls the size of the pupil which then
varies the amount of light that enters the eye. The iris closes down in bright
light to protect the cells in the retina.
The lens is transparent and biconvex in shape. The amount of refraction
of the light passing through it depends on its shape. It can be flattened (less
convex) or made more rounded (more convex). This adjustment is needed for
focusing on objects that are different distances away (figure 19.6).
SLUZ¶]HYPLZ[OLHTV\U[VMYLMYHJ[PVU
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Figure 19.4 Light rays pass through the cornea, pupil, lens and
humours and are focused on the retina.
H-SH[[LULKSLUZSLZZYLMYHJ[PVUVMSPNO[
Figure 19.5
[VIYHPU
Light refracts as it passes through the cornea and lens.
+L[HPSVMJPSPHY`T\ZJSL
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JPSPHY`T\ZJSL
I9V\UKLKSLUZTVYLYLMYHJ[PVUVMSPNO[
SPNO[YH`Z
SPNO[YH`ZMYVTH
ULHYVIQLJ[
I\SNLKSLUZZOHWL
TVYLYLMYHJ[PVU
;OLJPSPHY`T\ZJSLPZYPUNZOHWLK"VUJVU[YHJ[PVU
[OLYPUNNL[ZZTHSSLY;OPZJH\ZLZ[OLZ\ZWLUZVY`
SPNHTLU[Z[VZSHJRLUHUK[OLSLUZILJVTLZHTVYLI\SNLKZOHWL
Figure 19.6 The lens in the eye changes its shape to ensure that the rays focus on the retina.
228
19 • The Eye, the Ear and the Skin
Accommodation
accommodation ❯
ITQ3
Name in order the parts of the eye
through which a light ray passes on its
way from the conjunctiva to the retina.
The adjustment of the lens for focusing on near and distant objects is called
accommodation. The lens is connected by ligaments to the ring-shaped
ciliary muscle. Contraction and relaxation of the ciliary muscle affects the
tension in the ligaments which changes the shape of the lens.
When focusing on a distant object, the ciliary muscles relax. This pulls the
suspensory ligaments tight which makes the lens flattened (less convex). This
shape refracts the light less and the image is focused sharply on the retina
(figure 19.7).
When focusing on a near object, the ciliary muscles contract. This reduces the
tension on the suspensory ligaments and they slacken. The ligaments pull less
on the lens and it becomes more rounded (more convex). A very curved shape
refracts the light more. The image is again sharp on the retina (figure 19.8).
SLUZMSH[[LULK
SLZZYLMYHJ[PVU
SPNO[YH`ZMVJ\ZLK
VUYL[PUH
SPNO[YH`ZMYVT
KPZ[HU[VIQLJ[
SPNO[YH`ZMYVT
ULHYVIQLJ[
[VIYHPU
Figure 19.7
Figure 19.8
Focusing on a distant object.
Focusing on a near object.
The effect of pupil size
ITQ4
What is meant by the term
‘accommodation’?
If you wear glasses, try making an artificially
small pupil with the tips of three fingers and look
through it (without your glasses on) at something
which looks blurred (figure 19.10). Because of
the increased depth of focus, you should see
it more clearly.
When the pupil is wide open, more light enters the eye than when the pupil
is small. In bright light the pupil contracts to protect the eye from excess light.
In dim light the pupil expands to take advantage of as much light as possible
(figure 19.9).
To expand the pupil the circular muscles in the iris relax and the radial
muscles contract, pulling the iris back and so opening the pupil.
To make the pupil smaller the radial muscles relax and the circular muscles
contract.
Figure 19.9
Figure 19.10 An artificial pupil.
depth of focus ❯
Changing pupil size in different light conditions.
Depth of focus
When the pupil is small (in bright light) the eye has a greater depth of focus.
The shape of the lens does not need to change quite so much to switch from
viewing a distant object to viewing a nearer one. In dim light when the pupil is
wide, the depth of focus is less and the lens must change more.
229
Life Processes and Disease
This is sometimes noticed if a person is slightly long-sighted or slightly
short-sighted. In bright light they will see objects clearly which in dim light
appear a little blurred.
Also, as a person ages, their power of accommodation gets less and the
range of distances over which they can see sharp images is reduced. This is
much more noticeable in dim light than in bright light.
The retina
rod
The retina is a photosensitive layer at the back of the eye. It is made up of two
types of photoreceptor called rods and cones (figure 19.11). The rods are
sensitive to light and dark only; they do not react to colour. They function best
in low light intensities such as when it is getting dark. This is why we see only
in black and white at night. The rods are located around the sides of the retina
away from the fovea.
Rods are desensitised by bright light, which explains why you cannot see
clearly if you move from a bright area to a dim or dark one. After a few minutes
though, the rods recover their sensitivity and you can see more clearly again.
cone ❯
fovea ❯
YL[PUH
)
MV]LH
JVULZVUS`
)
YL[PUH
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(
YVK
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ISPUKZWV[
VW[PJULY]L¶TLZZHNLZMYVT
HSS[OLWOV[VYLJLW[VYZNV[V[OLIYHPU
Figure 19.11 The retina is made up of rods and cones (light-sensitive cells). Light falling on the
cells causes nerve impulses which travel to the brain via the optic nerve.
ITQ5
Describe how we see.
blind spot ❯
The cones are sensitive to colour and function best in high light intensities.
They are located mostly around the centre of the retina. The fovea is composed
entirely of cones and is at the centre of the retina. Light focused on the fovea
produce a clear well-defined image in the brightest colour.
The point of exit of the optic nerve from the eye is called the blind spot
because it lacks photoreceptors and is insensitive to light. Light falling on this
spot does not cause a response in the nerve, so you are ‘blind’ at this point.
Sight defects and their corrections
A sight defect is caused by any condition that prevents proper focusing of light
on the retina. A faulty focusing mechanism may be caused by a number of
factors, such as the shape of the eyeball or hardening of the lens. Some common
sight defects are long-sightedness, near-sightedness, cataract and glaucoma.
230
19 • The Eye, the Ear and the Skin
Long-sightedness
hypermetropia ❯
Long-sightedness, or hypermetropia, is caused by the eyeball being too
short from front to back, or the lens being too flat. As a result, light from
distant objects can focus on the retina, but light from near objects is focused
behind the retina. So distant objects are seen more clearly than near ones.
The condition can be corrected by wearing convex or converging lens
(figure 19.12).
SPNO[YH`ZMYVT
ULHYVIQLJ[
MVJ\ZVMSPNO[YH`Z
MYVTULHYVIQLJ[HM[LY
JVYYLJ[PVU¶VUYL[PUH
MVJ\ZVMSPNO[YH`Z
MYVTULHYVIQLJ[ILMVYL
JVYYLJ[PVU¶ILOPUKYL[PUH
converging lensILUKZ
SPNO[YH`ZPU^HYKZILMVYL
LU[LYPUN[OLL`L
Figure 19.12
Long-sightedness and its correction.
Near-sightedness
myopia ❯
ITQ6
What kind of lens is needed to
correct (i) long-sightedness (ii) nearsightedness?
Near-sightedness (or short-sightedness), or myopia, is caused by the eyeball
being too long from front to back, or the lens being too curved. As a result,
light rays from a distant object are bent more than necessary and focus in front
of the retina. However, light rays from near objects focus on the retina. So near
objects are seen more clearly than distant ones. Wearing concave or diverging
lens helps the person to see far objects clearly (figure 19.13).
SPNO[YH`ZMYVT
KPZ[HU[VIQLJ[
MVJ\ZVMSPNO[YH`Z
MYVTKPZ[HU[VIQLJ[HM[LY
JVYYLJ[PVU¶VUYL[PUH
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MYVTKPZ[HU[VIQLJ[ILMVYL
JVYYLJ[PVU¶PUMYVU[VMYL[PUH
diverging lensILUKZ
SPNO[YH`ZV\[^HYKZILMVYL
LU[LYPUN[OLL`L
Figure 19.13
Near-sightedness and its correction.
Astigmatism
This is caused by the surface of the lens or cornea being curved irregularly.
Specially shaped lenses, which balance out these irregularities, need to be worn
to provide a clear image on the retina.
Cataract
Figure 19.14 A cataract reduces the light
entering the eye.
This occurs when the lens becomes opaque and light cannot pass through, so
the person is unable to see (figure 19.14). The lens can be removed during
surgery. Adjustments to vision can be made with appropriate spectacles or
contact lenses, so that the person can see clearly again. Alternatively, the lens
can be replaced with an intraocular lens.
231
Life Processes and Disease
Glaucoma
ITQ7
(i) What are defects of the eye?
(ii) Name two defects of the eye and
explain what causes them.
This occurs when there is a build-up of pressure in the aqueous humour. This
increased pressure inside the eyeball can damage the optic nerve. The sufferer
experiences painful and inflamed eyes, and a halo is seen around objects.
Vision is poor, and the sufferer may experience sightless areas in the field of
vision. It is associated with an increase in age but may develop at any time
from infancy on.
The risk factors for glaucoma are age, heredity, myopia, and general disease
such as a stroke. In its early stages, glaucoma can be effectively treated with
medication, like eye drops and oral medication. If left untreated, it can cause
vision loss or blindness. In its later stages, surgery may be necessary to ease the
pressure in the eyeballs.
Glaucoma is the most common cause of blindness. Damage to the optic
nerve is irreversible.
The ear
Structure of the human ear
The mammalian ear performs two functions:
• hearing;
• balance.
It is divided into three regions: the outer ear, the middle ear and the inner ear.
Figure 19.15 shows the structure of the human ear.
WPUUH
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Z[HWLZ
ZHJJ\SL
VZZPJSLZ PUJ\Z
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]LZ[PI\SHY
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Figure 19.15
232
TPKKSLLHY
Structure of the human ear.
PUULYLHY
19 • The Eye, the Ear and the Skin
How we hear
pinna ❯
ossicles ❯
cochlea ❯
hair cells ❯
V\[LYLHY
A noise is set of vibrations or sound waves in the air. The sound waves reach
the ear and the pinna (the outer ear) directs them into the auditory canal. The
sound waves travel down the ear canal to the eardrum. The eardrum vibrates
when hit by the sound waves. This causes the ear ossicles, or ear bones, in the
middle ear to vibrate. The inner ear is filled with fluid. The vibrations at the
oval window start up pressure waves in the fluid of the cochlea (figures 19.16
and 19.17).
The inner ear is made up of two parts, the cochlea and the vestibular
apparatus. The cochlea is a long, coiled, three-chambered tube which is
responsible for our sense of hearing. The inner ear is filled with fluid. The
vibrations at the oval window start up pressure waves in the fluid of the cochlea.
The cochlea contains receptors called hair cells which vibrate in response
to the pressure waves in the cochlear fluid. Nerve impulses are generated
which pass along the auditory nerve to
the brain and we hear. The vibrations
then pass away to the round window
and we are ready to hear again.
TPKKSLLHY
PUULYLHYÅ\PKÄSSLK
TLTIYHULJV]LYPUN
V]HS^PUKV^
]PIYH[PVUPZHTWSPÄLK
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HIZVYIZ[OL^H]LZHUKWYLWHYLZ[OL
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JVJOSLH
Figure 19.16 Sound waves are vibrations that travel through air to the outer ear. They are
amplified as they pass through ossicles of the middle ear and then converted to pressure waves
in the cochlea.
Figure 19.17 The three bones (ossicles) of
the middle ear.
tympanic membrane ❯
The eardrum
[`TWHUPJTLTIYHULVYLHYKY\T
RLW[[H\[I`LX\HSWYLZZ\YLVUIV[OZPKLZ
TPKKSLLHY
PUJ\Z
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V\[LYLHY
WYLZZ\YL
Z[HWLZ
THSSL\Z
air
fluid
air
WYLZZ\YL
,\Z[HJOPHU[\ILMYVT[OL
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The eardrum is a thin membrane which is pulled
taut and separates the outer and middle part of the
ear (figure 19.18). It is also called the tympanic
membrane. The vibrations in the sound waves are
converted to movement when they ‘hit’ the eardrum
and are amplified as they pass through the three
ear bones. Pressure on both sides of the tympanic
membrane must be equal so that it stays straight and
taut, and sound messages can be passed on efficiently.
We sometimes feel our ears ‘pop’, such as when
flying in an aeroplane. This happens as the tympanic
membrane moves back into position when the pressure
on both sides equalises (Figure 19.19, overleaf).
ITQ8
When we hear, what is the role of (i) the pinna (ii) the ear
bones (iii) the cochlea?
Figure 19.18 The eardrum separates two air-filled regions of the ear.
233
Life Processes and Disease
TPKKSLLHY
V\[LYLHY
PUULYLHY
,X\HSWYLZZ\YLVU[OLLHYKY\T
^OPSZ[HPYWSHULVU[OLNYV\UK
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(Z[OLHPYWSHULNVLZ\W[OL
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UH[\YHSS`VYI`JOL^PUNN\T
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Figure 19.19 The eardrum can ‘bend’ if the pressures on either side are unequal.
Balance
vestibular apparatus ❯
semicircular canals ❯
utricle
saccule ❯
ampulla ❯
The vestibular apparatus is responsible for our sense of balance and
information about the position and movement of our body. The vestibular
apparatus is made up of:
• the semicircular canals which detect movement of the head;
• the utricle and saccule (ear sac) which detect the position of the head.
Receptors inside these structures are the hair cells that deflect on movement.
This causes an impulse to be sent to the brain.
The semicircular canals are at right-angles to each other, in the three planes,
so that any movement of the head, and therefore the body, is detected. At the
base of each semicircular canal is a swelling called an ampulla. Figure 19.20
shows how the ampulla works.
movement of the body
ampulla
cupula displaced by
movement of endolymph
Figure 19.20 Movement of the body moves
fluid in the ampullae in the opposite direction.
The brain gets impulses from all three ampullae
and interprets the messages as movement.
234
vestibular nerve to the brain,
which interprets the message
relative movement of endolymph
because of movement of the body
resting position of cupula
sensory hairs
sensory hair cells
(stimulated)
19 • The Eye, the Ear and the Skin
ITQ9
Even with the eyes closed, the brain can
detect movement of the body, that is,
the direction and the relative speed of
the movement. How is this possible?
The ear sac is positioned below the semi-circular canals. Information about
the position of the head and therefore the body is detected by receptor cells
and impulses are sent to the brain. The utricle responds to vertical movements
of the head and the saccule responds to lateral or sideways movement of the
head. Figures 19.21 and 19.22 illustrate how they work.
Z[HUK\W
IHSSYLZ[ZVUZLUZVY`OHPYZ
\[YPJSL
SLHUMVY^HYK
IHSSW\SSZVUZLUZVY`OHPYZ
Figure 19.21 Movement of the head vertically, pulls on
sensory hairs in the utricle. Impulses are sent to the brain
which are interpreted as movement of the head.
Figure 19.22 The saccule
responds to lateral or sideways
movement of the head.
The skin
World temperature varies
from –58 °C in the cold polar
regions to around 30 °C in
tropical rainforest and over
60 °C in hot deserts. Some
animals are adapted to live in
extremely cold environments
while others exist where
environmental temperatures
can exceed 60 °C. Despite
the temperature of the
environment, the body
temperature of a human
is always about 37 °C
(figure 19.23).
Figure 19.23 The body temperature of both boys is
about 37 °C, even though one lives in an extremely
hot environment and the other in an extremely cold
environment.
Temperature control
homeotherm ❯
poikilotherm ❯
In the animal kingdom, birds and mammals are able to maintain a fairly
constant body temperature. They are described as being homeothermic (or,
less correctly, as warm-blooded). This fairly constant body temperature is
maintained using physiological mechanisms or processes which occur within
the body, for example respiration which generates heat, and constriction of
blood vessels which reduces blood flow to the skin and therefore heat loss.
All invertebrates, fish, amphibians and reptiles are unable to regulate
their body temperature by physiological means. They are described as
poikilothermic (or, less correctly, as cold-blooded). They rely on heat derived
from the environment to keep the body warm. Control of body temperature
235
Life Processes and Disease
is achieved by behavioural mechanisms, for example moving to a cool place
under a rock and basking in sunshine. The body temperature of poikilotherms
usually depends on their environment (figure 19.24).
ITQ10
Define (i) homeothermy (ii)
poikilothermy, giving examples of each.
Noon – lizard hides from the Sun
Morning – lizard warms up
0[ZIVK`[LTWLYH[\YL^HZSV^
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PU[OL:\U[VPUJYLHZL
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YPZL[VVOPNOPML_WVZLK[V[OL
:\UZVP[OPKLZ\UKLYHYVJR
Figure 19.24 The body temperature of a lizard (poikilotherm) varies with the environment.
ITQ11
Why are humans able to live in
extremely hot and extremely cold
environmental conditions, unlike some
animals?
The graph in figure 19.25 compares the body temperatures of a human and a
lizard for 24 hours of a day. The change in body temperature of the lizard may
be more than 10 °C, while the change in body temperature of a human is less
than 2 °C. The body temperature of the lizard may drop to about 5 °C at night
when there is no solar heat, and be raised to about 20 °C in the heat of the day.
;LTWLYH[\YLV*
HPY[LTWLYH[\YL]HYPLZMYVT
TVYUPUN[VUVVU[VL]LUPUN
HPY
O\THU[LTWLYH[\YL
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O\THU
SPaHYKJVTLZV\[VMOPKPUN
[VIHZRVYTV]LHYV\UK
SVVRPUNMVYMVVK
SPaHYKPUHJVVSWSHJL¶P[Z
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[OHUHPY[LTWLYH[\YL
SPaHYK
UVVU
TPKUPNO[
Figure 19.25 How the body temperature of a lizard and a human, and the air temperature
may vary in one day.
CHAPTER 16
236
A body temperature of about 37 °C is ideal for chemical reactions to take
place as many enzymatic reactions have an optimum temperature of around
37 °C. These reactions are important to sustain life; they occur continuously
in every cell. The regulation and maintenance of constant conditions within
an organism is called homeostasis (chapter 16). Therefore, keeping the
temperature of the tissue fluid surrounding cells fairly constant is an example
of homeostasis.
The temperature range on Earth is very wide and varies with latitude
(from the poles to the equator): conditions range from extreme cold in the
polar regions to extreme heat in the tropics (figure 19.26). Mosquitoes and
flies (invertebrates) are poikilothermic and infest the tropics where the
environmental temperature (28–31 °C) is ideal for them to live and flourish.
Many are vectors of disease, and so the tropics team with disease-carrying
and disease-causing organisms. Diseases like cholera, malaria, dengue fever
and yellow fever are monitored constantly in order to try to keep them under
19 • The Eye, the Ear and the Skin
ITQ12
Why are people who live on and around
the equator more likely to suffer from
certain diseases, such as malaria and
dengue fever?
control. Poikilotherms are restricted
WVSHY
5
YLNPVU
to certain areas in the world because
they become sluggish and even totally
[LTWLYH[L
inactive in low temperatures. Low
YLNPVU
temperatures slow down enzymatic
reactions.
Mammals are able to maintain their
[YVWPJHS
YLNPVUZ
LX\H[VY
body temperature close to optimum
despite changes in the environment.
They can remain active day and night,
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summer and winter, and can inhabit or
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live in any part of the world. However,
WVSHY
they require more food. Maintaining
:
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a body temperature different from the
environment requires a lot of energy. A Figure 19.26 The surface temperature of
mouse, for example, eats about its own the Earth varies with latitude.
body mass of food per day whereas a
cockroach can go for days without a
meal. All animals will die in temperature extremes.
Temperature regulation in humans
Practical activity
SBA 19.1: Heat flow from a warm object,
page 361
Metabolic reactions (especially in the liver) generate heat and this heat is
transported by blood throughout the body to keep it warm at 37 °C. Some heat
is lost to the environment through the skin. The loss of this generated heat is
regulated and controlled; for example, in a cold environment, less is lost and
more is conserved.
Regulation of body temperature is controlled by the hypothalamus of the
brain. The organ which brings about the changes if necessary, to conserve or
lose heat, is the skin (figure 19.27). Temperature receptors in the skin receive
the stimulus of changing external temperature (figure 19.28, overleaf). They
send impulses to the hypothalamus, which monitors these stimuli as well as
internal body temperature. If body temperature is changing, the hypothalamus
responds by sending impulses to effectors in the skin to bring about the
responses shown in table 19.2 (overleaf).
sweat pore
epidermis
JVYUPÄLKSH`LY
(old skin cells)
Malpighian layer
(pigmented)
pigment protects lower
layers from damage by
ultraviolet rays in sunlight
hair erector
muscle
sebaceous gland
produces oil that coats
and protects hair
dermis
capillary network
hair follicle
fatty layer
below skin
sweat gland
Figure 19.27 A section of human skin.
237
Life Processes and Disease
Heat gain
:\U
Heat loss
H[TVZWOLYL^HYTLK
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L]HWVYH[PVUVM^H[LY
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[OLH[TVZWOLYL
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VM[OLLU]PYVUTLU[
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JVUK\J[PVU[V[OL
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JSVZL[V^H[LY
Figure 19.28 The skin of a mammal is important for temperature regulation.
To lose heat
• Sweating increases –
evaporation of the sweat
removes heat from the body
• Vasodilation occurs – capillaries
in the dermis dilate so blood
flow through skin increases, heat
is lost from the blood
• Hair erector muscles relax –
hairs lie flat so moving air can
get closer to skin and remove
heat
To conserve heat
Z^LH[
OLH[PZJHYYPLKH^H`
HZZ^LH[L]HWVYH[LZ
Z^LH[NSHUK
HSV[VMISVVKMSV^
JSVZL[V[OLZRPU¶
OLH[PZSVZ[MYVT
[OLISVVK
• Sweating decreases
SLZZISVVKÅV^
close to the skin
• Vasoconstriction occurs –
capillaries of the dermis
constrict so blood flow to skin
decreases, heat is retained in
blood vessels deeper in the body
• Hair erector muscle contract –
hairs stand up trapping a layer
of warm air next to the skin
(insulation)
arterioles
constrict
SH`LYVM^HYTHPY[YHWWLK^OPJO
RLLWZIVK`^HYT
OHPYLYLJ[VYT\ZJSLZJVU[YHJ[
HY[LYPVSLZ
KPSH[L
OLH[JHUILLHZPS`
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OHPYLYLJ[VYT\ZJSLZYLSH_
Table 19.2 Responses in the skin of a mammal that help it to conserve or lose heat.
238
19 • The Eye, the Ear and the Skin
ITQ13
Name the main organ of temperature
regulation in humans. Describe two
ways it is adapted to perform this
function.
ITQ14
What changes occur to control body
temperature in the body of a human
who is running a race?
Humans can generate an excessive amount of heat during exercise or increased
activity. To maintain a constant temperature we have to lose this excess heat.
Temperature regulation is physiological in humans since we are a mammal.
However, we may change our behaviour to help the process. If you are is hot
because of strenuous exercise, you could:
• remove some clothing;
• have a cold drink;
• move to a cooler place;
• stop activity.
Humans do not have a thick layer of hair and, if the environment is very cold,
they do not have effective insulation. Heat is generated by the liver, but this
may not be enough to keep body temperature at the right level. Muscles start
to shiver involuntarily, to generate more heat. Humans get ‘goose bumps’ as
the hair erector muscles contract. However, we are considered to be ‘naked’
or hairless and can only trap a thin layer of warm air around the skin. We can
help by:
• putting on thick clothing;
• having hot drinks;
• moving to a warmer place;
• moving around to generate more heat.
Temperature regulation in birds
The effect of erector muscles is most marked in birds. In cold weather, the
muscles contact, as in humans, and the birds’ feathers stand out from the skin
(figure 19.29). This traps a great deal of air next to the skin, which acts as a
good insulator. It also stops air flow over the skin, which reduces loss of heat
by convection.
Skin care
One of the most important way to take care of the skin is to protect it from the
Sun. Ultraviolet rays of the Sun can cause wrinkles, age spots and increase the
risk of cancer. To protect skin from the Sun:
• use sunscreen;
• seek shade;
• wear protective clothing.
Figure 19.29 A bird can insulate itself
from the worst of the cold by fluffing up its
feathers.
Smoking may damage collagen and elastin, the fibres that give skin its elasticity
and strength. So, a good skin care regime includes not smoking. Daily cleansing
and shaving can take a toll on the skin so strong soaps should be avoided and a
moisturiser used. A healthy diet and managed stress promote younger looking
and healthy skin.
239
Life Processes and Disease
Chapter summary
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
The main sense organs in humans are the tongue, nose, skin, ear and eye.
The eye enables us to see variations in colour, shape, size, brightness and distance.
We see when light enters the eye.
Light is refracted as it passes through the cornea and lens.
The iris controls the amount of light entering the eye.
The lens controls refraction of light for near and far objects – this is called
accommodation.
Anything that prevents proper focusing of light on the retina is a sight defect.
Astigmatism occurs when the surface of the lens or cornea is irregular.
A cataract occurs when the lens become opaque and light cannot pass through.
A build-up of pressure in the aqueous humour results in glaucoma.
The ear is a sense organ that enables us to hear sounds from the environment.
The ear detects sound waves from the environment.
The ear is made up of three parts: the outer ear, middle ear and inner ear.
Sound waves reach the cochlea from which impulses are sent to the brain.
The ear is also involved with balance.
The semicircular canals detect movement.
The utricle and saccule in the ear sac detect the position of the head.
Surface temperatures on the Earth vary greatly.
Animals can be grouped as poikilotherms and homeotherms depending on their
ability to control body temperature.
In humans, the skin is an organ of temperature regulation, meaning that skin care is
important.
Answers to ITQs
ITQ1 (i) In humans, the eyes are positioned on the upper front side of the
face. The human skull has a pair of holes called eye sockets which ‘cradle’ the
eyes. In this position, the eyes obtain some protection and the optic nerve can
easily connect with the brain. Human eyes are used mainly for movement and
to focus on the task at hand. A zebra’s eyes are positioned on either side of its
head which greatly increases the animal’s field of vision so it can spot predators
easily. Humans do not need to be on the constant lookout for predators.
(ii) A zebra is a large herbivore and is prey to many large cats such as lions
and cheetahs. Having eyes on the sides of its head allow it to have an almost
complete view of its surroundings at any time, even while it is grazing and
feeding. The enables the animal to be on the lookout for predators and aware
of any movement in its surroundings. Carnivores, on the other hand, need to
focus on their prey. Their eyes are positioned in front of their faces. This makes
it possible for them to judge the distance between themselves and their prey.
ITQ2 To aid in movement (avoid obstacles, note distances, etc.); to aid
eating (finding food, ingestion, etc.); to focus on any task (reading, cooking).
Other answers are possible.
ITQ3 Conjunctiva cornea [pupil] aqueous humour lens vitreous humour retina
ITQ4 Accommodation describes the adjustment of the pupil and the lens to
allow a person to see objects at different distances.
ITQ5 Light rays from an object enter the eye, and a message is sent to the
brain, which interprets the message. The light rays pass through many structures
240
19 • The Eye, the Ear and the Skin
in the eye, each performing an important function. The light rays from an object
must focus or meet at a point on the retina, from where the optic nerve sends
a message to the brain. The cornea, aqueous humour and lens are important
because they bend the light rays to focus on the retina. The cornea and aqueous
humour bend light automatically, but the lens can control the degree of bending.
The pupils are ‘holes’ in the eyes, the size of which can be adjusted to allow
controlled amounts of light to enter the eye. The degree of refraction is adjusted
by the lens and the rays focus on the retina. At the retina, light-sensitive cells
send messages to the brain, which interprets the message as sight.
ITQ6 (i) Converging or convex lens.
(ii) Diverging or concave lens.
ITQ7 (i) A defect of the eye is the malfunctioning of any one part of the eye
so that good vision is prevented.
(ii) A cataract occurs when the lens become hardened and cannot adjust
to focus light rays from objects at varying distances. Astigmatism is a defect
which occurs when the cornea does not have a smooth curve; the rays are not
refracted evenly as they enter the eye. There are other defects.
ITQ8 (i) The pinna traps the sound waves and directs them into the
auditory canal.
(ii) The ear bones amplify the sound waves after they have passed through
the outer ear on their way to the inner ear.
(iii) The sound waves cause pressure waves in the fluid of the cochlea.
Depending on the pressure of the wave, specific hair cells in the cochlea are
stimulated and specific messages are sent to the brain. The brain interprets
these messages as sounds that we hear. The cochlea is responsible for our sense
of hearing.
ITQ9 The ears are also concerned with balance, so any movement of the
body can be detected by the ears. The semi-circular canals in the ear are filled
with fluid. Any movement is detected by this fluid. At the base of the semicircular canal are structures called ampullae. The fluid in the ampullae moves
in the opposite direction to the body’s movement, pulling on sensory hair cells
as it does so. Messages are sent to the brain from the sensory cells and are
interpreted as movement.
ITQ10 (i) Homeothermy is the ability of an organism to control its body
temperature and keep it at a certain value; for example, humans maintain
their body temperature at around 37 °C (birds have a slightly higher body
temperature around 39 °C).
(ii) Poikilothermy describes the inability of an organism to control its body
temperature. The organism’s body temperature varies with the environmental
temperature; for example, the body temperatures of reptiles and fish vary with
their environmental temperature.
ITQ11 Humans can live in extremely hot and extremely cold environments
because they help maintain their body temperature at a constant value by
some behavioural processes. For example, they can wear clothes to suit their
needs, live in buildings which protect them and which may be cooled or
heated. Also, humans do not have to go in search of food every day in extreme
temperatures.
ITQ12 Organisms like bacteria and viruses that cause disease can survive in
any temperature, but the vectors that carry the pathogen from host to host
are usually insects (like mosquitos and flies) which are poikilothermic. These
flourish in the steady warm temperatures around the equator.
ITQ13 The skin is the major organ of temperature regulation. It is adapted to
suit this function in several ways:
• it contains a layer of fat, which acts as insulation;
• it can control the flow of blood into the many capillary networks close to the
skin;
241
Life Processes and Disease
• it has hairs which can be raised or lowered to increase or reduce air flow
next to the skin;
• it contains sweat glands which produce sweat that evaporates to cool the
surface of the body. (Any two of tha above)
ITQ14 •
Sweat is produced by the sweat glands.
• Erector muscles relax, causing the hairs to lie flat against the skin.
• Heat is lost from the body as the water produced in sweat uses the heat from
the body to evaporate.
• Blood vessels near the skin open wider, allowing blood to flow through
the many capillaries close to the skin. As a result, heat is brought close to the
surface of the body, and can be lost by radiation, conduction, or evaporation of
sweat.
Examination-style questions
242
1
(i)
Make a diagram of the eye as seen in a vertical section. Label these parts in each
case stating its function:
(e) retina
(a) iris
(f) sclera
(b) cornea
(g) choroid
(c) pupil
(d) lens
(ii) (a) What is the shape of the lens when the eye is focused on a near object?
(b) Describe fully the mechanism that changes the shape of the lens when focusing
on a near object.
(iii) (a) Which structure controls the size of the pupil?
(b) Using annotated diagrams only, explain how the size of the pupil is controlled.
(iv) Suggest why, on first entering a dimly lit room, it is difficult to see objects clearly, but
that they gradually become more clearly visible.
2
(i)
A sight defect is caused by any condition that prevents proper focusing of light on the
retina. Describe fully these common sight defects:
(a) long-sightedness;
(b) short-sightedness;
(c) glaucoma.
(ii) Using annotated diagrams only, describe how these eye defects are corrected:
(a) short-sightedness;
(b) long-sightedness.
3
(i)
The diagram below shows the structure of the human ear. Copy the diagram and label
the parts listed below and in each case state its function.
19 • The Eye, the Ear and the Skin
(a) pinna
(b) tympanium
(c) ear ossicles
(d) vestibular apparatus
(e) cochlea
(f) auditory nerve
(ii) Describe how the ear functions as an organ of balance when
(a) the position of the head changes;
(b) the body moves.
4
(i)
Explain the meaning of the following terms:
(a) homeotherm;
(b) poikilotherm.
(ii) The graph below shows the variations in temperature during the course of one day for
a human, a lizard and the air. Copy the graph and label, appropriately, the three lines
in the graph.
;LTWLYH[\YLV*
UVVU
TPKUPNO[
(iii) Explain fully, the changes seen in the body temperature of the human.
(iv) Explain the changes seen in the air temperature.
(v) Suggest what the lizard might be doing and where it might be found during these
times:
(a) 6.00 a.m.
(b) 12.00 noon
(c) 6.00 p.m.
(d) 12.00 midnight
5
(i)
Draw a diagram of a section of human skin and include the following labels:
(a) epidermis
(b) dermis
(c) receptor
(d) capillary network
(e) sweat gland
(f) sweat pore
(g) hair follicle
(h) hair erector muscle
(ii) Describe fully the possible behavioural activities and physiological mechanisms
that enable the human body to lose excess heat and maintain a fairly constant
temperature.
243
20
By the end of this
chapter, you should
be able to:
Reproduction in
Animals
distinguish between sexual and asexual reproduction
describe the structure and function of the human male reproductive system
describe the structure and function of the human female reproductive system
describe the structure and function of the ovum and spermatozoon
understand the menstrual cycle
understand fertilisation
understand the development of the embryo in humans
understand the role and methods of contraception
discuss the advangates and disadvantages of contraception
discuss the transmission and control of AIDS and gonorrhoea
reproduction
asexual
sexual
advantages and
disadvantages
reproduction in humans
contraception
Reproduction
Reproduction is a characteristic of life. Every living thing must die and,
although individuals die, a species will continue as long as its members are able
to live long enough to reproduce. If the members of a species die before they
can reproduce then that species is in danger of becoming extinct. Reproduction
is therefore important for a species to continue to exist, to be able to colonise
new habitats and to survive changing environmental conditions.
Ther-e are two main types of reproduction:
• asexual;
• sexual.
Asexual reproduction (one parent)
CHAPTER 23
Asexual reproduction happens when one individual produces offspring
without fertilisation. This involves cell division by mitosis only (chapter 23).
These offspring are genetically identical to each other and to their parent. It is
described as being conservative and, in essence, clones are produced.
Advantages of asexual reproduction
• No time or energy is wasted seeking a mate.
• Large numbers of offspring can be produced.
244
20 • Reproduction in Animals
• Offspring can be produced continuously and therefore quickly.
• Offspring can make good use of favourable environmental conditions.
• If the parent is of ‘superior’ quality, all the offspring will also be of ‘superior’
quality.
Disadvantages of asexual reproduction
Figure 20.1 An aphid producing live
young.
CHAPTER 23
CHAPTER 23
• If the environment is changing, the offspring may find it difficult to survive.
• If the parent is of ‘poor’ quality, the offspring will also be only of that ‘poor’
quality.
• Over-crowding and competition may occur as offspring colonise the same
area as the parent.
There are several types of asexual reproduction:
• vegetative propagation (chapter 23);
• cloning (chapter 23) (figure 20.1);
• binary fission seen in unicellular organisms like bacteria and protozoans
such as Amoeba (Figure 20.2).
VULWHYLU[
PKLU[PJHSVMMZWYPUN
Figure 20.2 Asexual reproduction in Amoeba.
Sexual reproduction (two parents)
gamete ❯
CHAPTER 24
Sexual reproduction involves two parents producing special reproductive
cells or gametes. This happens as a result of meiosis (chapter 24). Fusion of
the gametes produces offspring that are different from each other and from
both parents.
Advantages of sexual reproduction
• Genetic variability of the species is increased.
• The species is thus more likely to be able to adapt to a changing
environment.
• The species may be able to colonise new areas successfully.
• If the parents are both of poor quality, the offspring may be of better quality.
ITQ1
Draw up a table to show the differences
between asexual and sexual
reproduction.
ITQ2
What is the importance of
reproduction?
Disadvantages of sexual reproduction
•
•
•
•
A lot of time and energy is spent seeking a mate.
Offspring are not produced continuously and therefore not very quickly.
Few offspring may be produced (as in humans).
Even if the parents are of good quality, the offspring can be of poor quality.
245
Life Processes and Disease
CHAPTER 24
Practical activity
SBA 20.1: Observing the reproductive
cells of a mammal, page 362
spermatozoon ❯
ovum ❯
Reproduction in humans
In humans there are two sexes: male (man) and female (woman). Each sex
produces gametes or reproductive cells, by meiosis (chapter 24). In males the
gametes are called spermatozoa, and in females, ova.
The singular of spermatozoa is spermatozoon, and the singular of ova is
ovum (figure 20.3).
Ovaries make eggs
ITQ3
(i) What type of reproduction do
humans show?
(ii) Describe two advantages of this
type of reproduction.
V]HY`
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Figure 20.3
Details of the ovum and spermatozoon.
The male reproductive system
testis ❯
The plural of testis is testes.
semen ❯
246
The visible parts of the male reproductive system are the penis and scrotum
(figure 20.4). The scrotum contains a pair of testes. Each testis is composed
of coiled tubes called seminiferous tubules, inside of which spermatozoa (or
sperms) are formed. After formation, the sperms are stored in the epididymis.
During sexual intercourse, the sperms are moved out of the epididymis and
pass through the vas deferens on the way to the penis. Fluid is made in the
prostate gland and seminal vesicles which mixes with the sperms to make
semen. This semen, containing 200–500 million sperms, is ejaculated or
released from the erect penis during mating or copulation.
20 • Reproduction in Animals
(a)
ureter
sperm
duct
bladder
seminal
vesicle
spermatic cord
(sperm duct and
blood vessels)
(b)
prostate
gland
erectile tissue: blood
ZPU\ZLZ[OH[JHUÄSS
with blood from the
artery at the base
of the penis
rectum
sperm duct
urethra
urethra
coiled tubes
(epididymis)
penis
foreskin
glans
testis
testis
scrotum
penis
Figure 20.4 The human male reproductive system (a) in section, (b) seen from the front (front
section).
ITQ4
Describe the route taken by a
spermatozoon from its site of
production to ejaculation.
menstruation ❯
ITQ5
How does an ovum travel along the
oviduct?
The female reproductive system
The female reproductive system is positioned in the pelvic region (figure 20.5).
There are two ovaries, each usually releasing a single ovum (or egg) every
other month into the funnel of the oviduct. The ovum is moved along the
oviduct, or Fallopian tube, by the beating of cilia which line the tube. If sperms
are not present, then the ovum moves down the uterus and out of the body
during menstruation. Each month the wall or lining of the uterus is built up
in preparation for a fertilised ovum. The lining is shed if fertilisation does not
take place.
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Figure 20.5 The human female reproductive system (a) in
section, (b) seen from the front (front section).
247
Life Processes and Disease
Hormones of the gonads
puberty ❯
secondary sexual characteristics ❯
The gonads, testes in males and ovaries in females, also secrete hormones that
influence growth and development. Even before birth, while still in the uterus,
the testes of a boy produce the hormone testosterone which influences sexual
development and causes the male sex organs to develop.
At puberty, the ovaries in girls secrete the hormone oestrogen. Boys,
at this time, make larger amounts of testosterone from the testes. These
hormones are secreted in response to signals from the pituitary which is able
to determine that further development into a man or a woman must begin.
This starts between the ages of 10 and 14, as boys and girls begin to develop
the physical features that distinguish male from female. These district physical
and emotional features, or characteristics, are called secondary sexual
characteristics (table 20.1).
Males
Females
• enlargement of reproductive organs, e.g.
penis, testes, etc.
• enlargement of reproductive organs and
breasts
• ejaculation is possible
• menstruation starts
• increased muscle development
• broadening of the hips for child-bearing
• growth of pubic and underarm hair
• growth of pubic and underarm hair
• extra growth of hair on face and chest
• deepening of the voice
Table 20.1
ITQ6
What are the secondary sex
characteristics?
(ii) When do they arise?
(iii) Why are they important?
ITQ7
(i) How many ova are produced by a
normal adult female in a year?
(ii) How many spermatozoa are
produced by a normal adult male
in a year?
ITQ8
When in the menstrual cycle is the
likelihood of fertilisation of an ovum at
its lowest?
ITQ9
Why is the uterine lining built up every
month, only to be shed during each
monthly period?
248
Secondary sexual characteristics of males and females.
These behavioural and physical changes are associated with courtship, mating
and parental concerns.
More importantly, these
hormones also result in the
release of the gametes. At
puberty, girls begin to menstruate,
a sign that the menstrual cycle
has begun. Female gametes or ova
are released and can be fertilised
by spermatozoa as boys also
begin to ejaculate or release male
gametes into the environment.
A child grows and develops
into a sexual individual with
easily recognisable features
that are attractive to a potential
partner, thus ensuring
reproduction and continuation
of the species (figure 20.6).
Production of young is a natural
characteristic of life and these
hormones produced by the
gonads are important, not only
for growth and development of
an organism into a sexual being,
but also for those attractive forces
necessary for the continuation of
Figure 20.6 Typical physical characteristics of adult
the species.
human female and male.
20 • Reproduction in Animals
The menstrual cycle
ovulation ❯
menstrual cycle ❯
menopause ❯
ITQ10
When does ovulation occur in the
menstrual cycle, and which hormone is
responsible for ovulation?
ITQ11
(i) What is the importance of the
corpus luteum?
(ii) Why is progesterone called the
pregnancy hormone?
Figure 20.7 The human menstrual cycle.
On reaching puberty (around 12 years old), a human female will start to
release ova from her ovaries: this is known as ovulation. Ovulation is one
part of her monthly menstrual cycle, which starts at puberty and continues
until menopause (around the age of 45–50 years). Each cycle lasts for
approximately 28 days. The events of the cycle are controlled by hormones
which ensure that, if the ovum is fertilised, the uterus is ready to receive it.
The cycle starts with menstruation (the shedding of the uterus lining) which
lasts for about 5 days (figure 20.7). After a few days the uterus lining starts
to build back up again – by day 14 of the cycle it has thickened considerably
and has an increased blood supply. This is called the follicular phase and is
controlled by the hormone oestrogen. The events are synchronised so that one
ovum is now fully developed in a Graafian follicle in the ovary and ovulation
takes place (the ovulatory phase). The peak in oestrogen level causes ovulation.
After ovulation, the Graafian follicle develops into the corpus luteum. The
hormone progesterone is secreted by the corpus luteum and is responsible for
maintaining the built-up uterus lining. This is the luteal phase of the cycle.
If the ovum is not fertilised by a sperm, it passes through the uterus and
vagina during menstruation. The corpus luteum degenerates and the level
of progesterone decreases. This causes the built-up uterus lining to start to
disintegrate and peel away from the uterus wall. It passes out of the vagina in
menstruation or the monthly period. And the cycle starts again.
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249
Life Processes and Disease
Fertilisation
pregnancy ❯
gestation period ❯
courtship ❯
copulation ❯
fertilisation ❯
ITQ12
What do you understand by the terms
(i) courtship behaviour (ii) copulation?
ITQ13
(i) What is fertilisation and where
does it occur?
(ii) Describe the route taken by a
sperm after ejaculation in the
vagina until it fertilises an ovum.
zygote ❯
embryo ❯
implantation ❯
If fertilisation takes place and the zygote successfully implants itself into
the built-up uterus lining the female is said to be pregnant. The uterus
lining must now stay built-up to nourish the embryo, so there is no more
menstruation (‘periods’). This lasts for the entire pregnancy (or gestation
period) which is usually 9 months in humans. This means that the woman’s
progesterone level must remain high to maintain the built-up uterus lining.
Also oestrogen levels must remain low so that no more ovulation can
take place.
The pregnant woman may experience ‘morning sickness’ (nausea) for the
first three months or so as she gets used to the high level of progesterone and
its effects on her body.
Mating, for humans (and other mammals), is usually preceded by
courtship behaviour. Courtship establishes a bond between the partners that
may keep them together while the young are brought up. A male and female
are attracted to each other and a successful courtship leads to copulation
and fertilisation.
The act of copulation, or mating, brings the gametes close together. The
penis becomes erect during sexual arousal as the erectile tissue fills with blood.
In the female, sexual arousal results in the lubrication of the vagina. The penis
is then inserted into the vagina, bringing the gametes closer together. The
sperms are usually ejaculated just below the cervix, and then ‘swim’ across the
uterus and up the oviduct. Close to 500 million sperms are released, but only
one will fuse with the ovum. This is fertilisation.
Development of the embryo, fetus and
placenta
The nuclei of the sperm and fertilised egg fuse to form the zygote (figure 20.8).
The zygote divides as it moves slowly to the uterus. After several hours, it is
a ball of cells called an embryo, and on reaching the uterus, it sinks into the
thick spongy lining (figure 20.9). This is called implantation. Here it obtains
protection and nutrients until the placenta develops.
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ovum.
Figure 20.9
250
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Events that occur in the oviduct leading to implantation.
20 • Reproduction in Animals
fetus ❯
placenta ❯
ITQ14
Describe implantation and explain
its importance.
The embryo develops tissues and organs and by 8 weeks it is clearly human. It
is now a fetus (figure 20.10).
As the embryo grows, it develops a placenta which connects it very closely
with the wall of the uterus (figure 20.11).
uterine wall
oviduct
uterus
villi of placenta
umbilical cord
placenta
fetus
HTUPV[PJÅ\PK
amnion
mother's blood
fetus' blood
to
fetus
Figure 20.10 The embryo
develops into a fetus and lives for
nine months in the uterus.
diffusion occurs sending
nutrients to fetus' blood
and waste products to
the mother's blood
mother’s blood
vessels
from
fetus
uterus wall
space filled with
mother's blood
umbilical blood
vessels
umbilical cord
placenta
umbilical cord
Figure 20.11
cervix
Structure of the placenta.
251
Life Processes and Disease
The structure of the placenta is shown in figure 20.11. The placenta has various
functions which include the following.
• It allows exchange of materials between the mother and the fetus, by
bringing their blood systems very close together without the two bloods
mixing. Oxygen, water, amino acids, glucose and essential minerals diffuse
through the placenta to the blood of the fetus. Carbon dioxide, urea and
other wastes diffuse from the fetus into the mother’s blood.
• It protects the embryo by preventing many pathogens and chemicals from
crossing the placenta. However, there are some exceptions, like the German
measles virus, the HIV virus, nicotine, alcohol and heroin, which are all able
to cross the placenta.
• It protects the fetus and the mother since it allows their two blood systems
to operate at different pressures (the mother’s body needs a higher pressure
to get blood round a larger system).
• It produces the hormones important for a successful pregnancy.
Effects of drug abuse in pregnancy
ITQ15
The placenta is described as the lungs,
kidneys and alimentary canal of the
embryo. Why is it so described?
Nutrients diffuse from the mother’s blood to the placenta, and then travel to
the fetus during gestation. Harmful substances may also diffuse across to the
fetus if they are present in the mother’s bloodstream.
• Carbon monoxide and nicotine from cigarette smoke – Problems
associated with cigarette smoking include premature birth, reduced birth
weight and the risk of miscarriage.
• Alcohol – There are serious consequences of alcohol abuse during
pregnancy. Alcohol crosses the placenta easily and causes symptoms in
the baby including poor mental dvelopment, small head and brain size,
hyperactivity, poor concentration and reduced growth rate.
• Drugs like heroin and cocaine – Babies may become addicted to these
drugs while inside the mother’s womb.
• Pharmaceutical products – These are carefully tested for harmful effects,
but it is a wise precaution not to use any drugs (even for headaches or
nausea) during pregnancy unless prescribed by a doctor.
Birth
amniotic fluid ❯
parturition
birth ❯
labour ❯
ITQ16
Give a brief explanation of (i) gestation
(ii) parturition (iii) prenatal care (iv)
postnatal care.
252
The fetus is surrounded by a strong membrane called the amnion. Inside is
a liquid called amniotic fluid which helps to keep a constant environment
around the fetus. The amniotic fluid also helps to support and protect the fetus
from harm.
After 40 weeks in the mother’s uterus (also called the womb), the baby is
sent out into the world. Parturition is the act of giving birth and is controlled
by hormones. The hormone oxytocin causes contractions of the uterus which
can be very painful. This is known as ‘labour’ or ‘labour pains’.
During these powerful contractions, the amniotic sac bursts. The amniotic
fluid pours out of the uterus and the baby is then pushed out. The umbilical
cord is cut, separating the baby from its mother. After a few minutes, the
placenta separates from the uterus wall and passes out of the body. This is
sometimes called the after-birth.
Prenatal (antenatal) care ensures good health of the baby and mother
during the pregnancy. The mother should, for example, eat a balanced diet,
should not smoke or drink alcohol and should avoid drugs.
Postnatal care describes care of the child from birth to teens. It involves
the physical, emotional and mental care essential for healthy growth
and development.
20 • Reproduction in Animals
Breast-feeding
prolactin
colostrum ❯
Mammals suckle their young. After birth of a baby, milk is produced by the
breasts or mammary glands as a result of the effects of many hormones, in
particular, prolactin. The first secretion of the breast is called colostrum. It
is rich in antibodies and protects the new-born from some pathogens it may
encounter on the first days of its life out of the uterus.
Human breast milk contains the appropriate proportions of sugar, fat and
protein suitable for a young human baby. If she is breastfeeding, the mother’s
diet should be rich in foods that will provide the energy, proteins, vitamins and
minerals necessary for healthy growth and development of the infant.
‘Formula’ milk, which is often bottle-fed to infants, attempts to recreate this
balance. It consists of dried milk made to a special formula and is mixed with
water and fed to the baby in a bottle.
The role of contraception
The world’s population is doubling every 44 years or so. It may soon be difficult
to supply all the needs of all of its people. A solution to the over-population
problem lies in contraception (also known as birth control). Table 20.2
summarises some common methods of contraception and figure 20.12
(overleaf) shows the sites of action of some contraceptive methods.
Method
How it works
Effectiveness
Advantages
Disadvantages
sterilisation
male (vasectomy) – the vas
deferens are cut and tied off
100%
no drugs or artificial device used, irreversible
no further costs
100%
very reliable if taken as
prescribed
possible nausea, breast
tenderness, and water retention
leading to an increase in weight;
may increase risk of cervical
cancer, but decreases risk of
breast cancer
female (tubal ligation) – the
oviducts are cut and tied off
contraceptive pill contains progesterone which
prevents fertilisation, some also
contain oestrogen which prevents
ovulation
intra-uterine
device (IUD)
(loop, coil)
device inserted into the womb by a
doctor – prevents implantation
99–100%
reliable
possible menstrual discomfort
spermicide
cream, jelly or foam inserted in
vagina before copulation
not reliable alone
simple to use
may reduce the sensitivity of the
penis
mechanical
barriers
male (condom) – sheath of latex
unrolled onto the erect penis
reliable especially
when used with
spermicide
available for use by all men and may reduce the sensitivity of the
penis
women, and the condom gives
some protection against sexually
transmitted diseases
not very reliable
no devices or drugs used
female (diaphragm, cap) – domeshaped sheet of thin rubber inserted
over the cervix before copulation
rhythm method
refraining from sexual intercourse
during those times in menstrual
cycle when fertilisation is likely
not really reliable because
women can have irregular
menstrual cycles
(continued)
253
Life Processes and Disease
Method
How it works
Effectiveness
Advantages
Disadvantages
injectable
hormone
prevents release of ova and
thickens the mucus in a woman’s
cervix
very reliable
no need to remember
medication, no device used
injection must be repeated by a
doctor every 13 weeks
abstinence
no traditional sexual intercourse (i.e. almost 100%
penis / sperm entering vagina)
Table 20.2
Some methods of contraception, their effectiveness, advantages and disadvantages.
0U[OLMLTHSL
ITQ17
Match these forms of contraceptive
with mode of action A or B:
• contraceptive pill
• IUD
• spermicide
• condom
• rhythm method
• tubal ligation
• vasectomy
Mode of action:
A prevents implantation
B prevents fertilisation.
protects against sexually transmitted diseases if there is no transfer
of fluids
[\IHSSPNH[PVU
PU[YH\[LYPUL
KL]PJL0<+
PUQLJ[PVUPTWSHU[JVU[YHJLW[P]LWPSS
0U[OLTHSL
IHYYPLY[LJOUPX\LZ
‹KPHWOYHNTJHW
‹JVU[YHJLW[P]LZWVUNL
‹ZWLYTPJPKLZ
‹MLTHSLJVUKVT
JVUKVT
]HZLJ[VT`
Figure 20.12 The sites of action of different contraception methods.
HIV/AIDS and other STDs
HIV ❯
opportunistic infections ❯
254
STDs are sexually transmitted diseases; this means they are diseases that
are transferred from one person to another during sexual intercourse. AIDS
(acquired immune deficiency syndrome) is thought to have originated in
Central Africa and has already killed over 3 million people worldwide. AIDS is
caused by the human immunodeficiency virus (HIV), which can only survive
in body fluids. HIV can be transferred in other ways as well as by sexual
intercourse because is transmitted when the blood or semen of an infected
person mixes with the body fluids of another person. This can happen during
sexual intercourse, blood transfusion or when sharing a hypodermic needle.
An infected pregnant woman can also pass HIV to her baby through the
placenta or by later breast-feeding. Close contact between people with open
wounds has also been known to pass on the virus.
Infection with HIV weakens the body’s natural defence system (the
immune system) because the virus attacks particular white blood cells, called
T-lymphocytes (figure 20.13). This means the body is vulnerable to other
infections (known as opportunistic infections) like common viral, bacterial
and fungal infections.
Table 20.3 compares two STDs: HIV/AIDS and gonorrhoea.
20 • Reproduction in Animals
Figure 20.13 False-colour scanning
electron micrograph of a T-lymphocyte white
blood cell infected with HIV.
Disease
Causative
agent
Symptoms
Control
AIDS
(acquired
immune
deficiency
syndrome)
Virus (HIV)
• Persistent cough, fever. Skin
rashes, swollen lymph glands,
diarrhoea, wasting away of
body, weakness.
• Secondary (opportunistic)
infections – pneumonia,
tuberculosis (TB), candidiasis
(fungal), cancers.
• Keep to one sexual partner
(or to partners who have been
safely screened for STDs)
• Do not inject drugs
• Use condom during sex
• Education about the spread /
prevention of disease
• A vaccine is being sought
Gonorrhoea Bacterium
Table 20.3
ITQ18
What kind of disease is an STD and why
is it so called?
• Yellowish discharge from urethra, • Keep to one sexual partner
(or to partners who have been
pain when urinating. Often
safely screened for STDs)
not noticed in females. If left
untreated, causes inflammation • Treatment by anti-biotics, e.g.
penicillin, streptomycin
of Fallopian tubes and sperm
• No known vaccine
ducts leading to sterility.
• Arthritis, weakened heart,
blindness.
Information on AIDS and gonorrhoea.
Prevalence (%) by WHO region
Western pacific: 0.1 [0.1-0.1]
Eastern Meditteranean: [0.1-0.3]
South-East Asia: 0.3 [0.2-0.4]
Europe: 0.4 [0.4-0.5]
Americas: 0.5 [0.4-0.6]
Africa: 4.6 [4.4-4.8]
Figure 20.14
Global prevalence: 0.8% [0.7-0.8]
Estimate of the numbers of people (15–49 years) living with HIV (2011).
Social and economic implications of STDs –
especially HIV/AIDS
• The cost of treating and caring for those affected is high, especially in
countries where a high percentage of the population is infected.
• There is a reduction in the workforce and loss of valuable working hours.
• The family of an infected person suffers emotionally and financially.
• Millions of dollars are spent worldwide on research for a possible cure for
HIV infection.
• People with AIDS (including children) may be scorned and alienated from
society.
• STDs are easily spread by sexual intercourse.
• Millions of children worldwide are living with the effects of HIV/AIDS;
many are orphans.
255
Life Processes and Disease
Chapter summary
• Reproduction is necessary for the propagation of life on Earth.
• Asexual reproduction involves only one parent and no fertilisation. The offspring are
genetically identical to the parent and each other.
• Sexual reproduction involves two parents and fertilisation. The offspring are different
from each other and their parents.
• Variation in offspring resulting from sexual reproduction is important when there are
changes in the environment.
• In humans there are two sexes: female and male.
• The female gamete is the ovum and the male gamete is the spermatozoon.
• The menstrual cycle starts at puberty and is usually a 28-day cycle in human females.
• Ovulation, the build-up of the uterine walls and menstruation are processes which
are part of the menstrual cycle. They are controlled by the hormones oestrogen and
progesterone.
• If fertilisation of the female gamete by the male gamete occurs in the oviduct, a
zygote is formed.
• The zygote implants itself in the wall of the uterus.
• The developing embryo is protected by the amniotic fluid and is nourished by the
developing placenta.
• Drug and alcohol abuse are very harmful to a developing fetus.
• Contraception methods prevent pregnancy from occurring.
Answers to ITQs
ITQ1
Asexual reproduction
Sexual reproduction
single parent involved
two parents involved
offspring identical to parent
offspring different from parents
offspring identical to each other (i.e. no variation offspring different from each other (i.e. variation
between individuals)
is seen)
less likely to survive a changing environment
(none may be able to survive because no
variation in offspring)
more likely to survive a changing environment
(some offspring may be able to survive as a
result of variation in offspring)
evolution of the species less likely (only through evolution of the species can occur more readily
mutation
because of variation
type of cell division is only mitosis
type of cell division involves meiosis
ITQ2 Reproduction is the production of offspring and it ensures the
continuation of the species. If most individuals in a population die before they
reproduce, then that population could become extinct.
ITQ3 (i) Sexual reproduction.
(ii) Any of the advantages mentioned on page 000 could be mentioned.
ITQ4 testes epididymis sperm duct urethra
ITQ5 An ovum is pushed along the oviduct on release from the ovary. It
is ‘sucked’ into the oviduct and is pushed along by a current produced by the
256
20 • Reproduction in Animals
beating of the cilia that line the oviduct. Also, contractions of the oviduct walls
help to move the ovum along.
ITQ6 (i) Secondary sexual characteristics are those special features that
make a male organism look different from a female organism (e.g. broad hips,
deep voice).
(ii) They start at puberty.
(iii) They are important for attraction to the opposite sex and courtship.
ITQ7 (i) A female usually produces one ovum a month, that is a total of
twelve ova in a year.
(ii) A male can produce over 1 million spermatozoa in one ejaculation. There
is no set rate at which he ejaculates, as it depends on how often he has sexual
intercourse or engages in some sort of sexual activity. A male can produce
billions of spermatozoa in a year.
ITQ8 An average menstrual cycle is taken to be about 28 days. During
the first 10 days, no ovum is present to be fertilised. Spermatozoa can live
inside the female for 2–3 days after ejaculation, so intercourse 2–3 days before
ovulation may result in fertilisation. The ovum may live for 3–4 days, so sexual
intercourse up to 5 days after ovulation may result in fertilisation. Ovulation
usually occurs around day 14. So, in an average cycle, intercourse is least likely
to result in fertilisation during days 1–10, and 20–28.
ITQ9 Every month, ovulation occurs, so that fertilisation can occur. Thus,
every month, the uterus has to be prepared for implantation. If implantation
does not occur, that month’s lining is shed.
ITQ10 Ovulation occurs in the middle of the cycle, around day 14 in a 28-day
cycle. Oestrogen is the hormone responsible for ovulation. It is secreted by the
Graafian follicle as it develops in the ovary. When the oestrogen concentration
in the blood reaches a certain level, ovulation occurs.
ITQ11 (i) The corpus luteum produces the hormone progesterone, which
maintains the lining of the uterus for a few days after ovulation. This prepares
the body for implantation, if fertilisation occurs.
(ii) Progesterone is called the pregnancy hormone because its level stays high
during pregnancy. This hormone causes the uterine lining to stay thick and rich
with blood vessels, so that the developing offspring can obtain the nutrients it
needs.
ITQ12 (i) Courtship behaviour is used to attract a mate and, hopefully,
results in mating and production of offspring. It includes special body
movements, calls and dances.
(ii) Copulation is the sex act, the insertion of the penis into the vagina. On
ejaculation, spermatozoa are released at the base of the cervix. Copulation
results in the transfer of male gametes to the female where fertilisation with
the female gamete is possible.
ITQ13 (i) Fertilisation is the fusion of the male nucleus, carried by the
spermatozoon, with the female nucleus that is in the ovum. It occurs in the
oviduct or Fallopian tube.
(ii) vagina cervix uterus oviduct ovum
ITQ14 The zygote or fertilised egg travels down the oviduct to the uterus. It
implants itself in the wall of the thickened uterus. A placenta then develops
from the embryo and beings to obtain nutrients and oxygen from the mother’s
blood.
ITQ15 The placenta is the site of exchange of materials between mother and
fetus. By diffusion across the placenta, the fetus obtains oxygen and nutrients
and gets rid of its waste products. These are the functions that the lungs,
kidneys and alimentary canal will carry out after birth.
ITQ16 (i) Gestation is the period of development from implantation to birth.
In humans it is about nine months.
(ii) Parturition is birth. It is the expulsion of the baby from the uterus.
257
Life Processes and Disease
(iii) Prenatal care describes the care of pregnant woman takes during
pregnancy to ensure the birth of a healthy baby. It includes a proper diet and
abstinence from drugs and alcohol.
(iv) The newly born baby is totally helpless and dependent on its mother to
satisfy all its needs. Postnatal care is care of the baby after it is born.
ITQ17 An IUD has mode of action A; all the other forms of contraception
have mode of action B.
ITQ 18 An STD is a sexually transmitted disease.
This means it can be passed on by sexual intercourse.
Examination-style questions
1
(i) Make a labelled drawing of the human female reproductive system.
(ii) Indicate on your drawing with:
(a) an X, where fertilisation normally occurs;
(b) a Y, where spermatozoa are deposited during copulation;
(c) a Z, where implantation can occur.
(iii) List three advantages of sexual reproduction.
(iv) Is it possible for a woman to have 30 children? Explain fully.
(v) Suggest reasons why you think it is disadvantageous to have many children.
(vi) List four methods of contraception.
2
(i)
Define the following terms:
(a) implantation;
(b) fertilisation;
(c) gestation period;
(d) contraception;
(e) asexual reproduction.
(ii) Illustrate, using large, clearly labelled diagrams, to show the differences in size, shape
and activity of male and female gametes.
(iii) Give full and accurate accounts of how:
(a) the zygote develops and moves to be implanted, from the time right after
fertilisation to implantation;
(b) the embryo is nourished and protected as it develops in the uterus:
(c) the baby is nourished and protected right after birth.
3
The events of the menstrual cycle are divided into three phases: the follicular, ovulatory
and the luteal.
(i) Copy and complete the table below to show the activities in the uterus and ovary
during these phases.
Events that occur in the ovary
Events that occur in the uterus
Follicular phase
Ovulatory phase
Luteal phase
(ii) In human females, the menstrual cycle lasts approximately 28 days. What significant
events happen during these parts of the cycle?
(a) days 0–5
(b) days 5–10
(c) days 13–15
(d) days 15-25
(iii) Describe and explain the changes that take place in the menstrual cycle after
fertilisation.
258
21
By the end of this
chapter, you should
be able to:
Reproduction in
Plants
understand the life cycle of a plant
describe the structure of a flower and relate the structures to their functions
understand the differences between wind-pollinatd and insect-pollinated
flowers
understand fertilisation in a flowering plant and the development of fruit and
seed
describe the structure of a fruit and adaptations for dispersal
understand why dispersal is necessary and how it can be brought about
plant
flower
pollen grain –
male gamete
ovule – female
gamete
wind
cross
pollination
insect
self
other animal
fertilisation
development
of seed/fruit
dispersal
wind
water
animal
germination
exploding
new plant
Life cycle of a plant
Reproduction is important for the continuation of life. It is the process by
which new organisms are produced. Flowering plants reproduce sexually
(fusion of male and female gametes). Sexual reproduction in humans involves
two sexes: the male produces the male gamete, and the female produces
259
Life Processes and Disease
the female gamete. However, in plants, the reproductive organ, which is the
flower, usually produces both male and female gametes (figure 21.1).
:L_\HSYLWYVK\J[PVUPUHUPTHSZ
PU]VS]LZ[^VZL_LZ!
THSL
MLTHSL
WYVK\JLZTHSL
NHTL[L
WYVK\JLZMLTHSL
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YLWYVK\J[P]LVYNHU
WYVK\JLZ[OL
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:L_\HSYLWYVK\J[PVUPUMSV^LYPUNWSHU[Z
\Z\HSS`PU]VS]LZVULMSV^LY[OH[WYVK\JLZ
IV[OTHSLHUKMLTHSLNHTL[LZ
YLWYVK\J[P]LVYNHU
WYVK\JLZ[OL
THSLNHTL[L
Figure 21.1
Parents in sexual reproduction of animals and plants.
The life cycle of a typical flowering plant is seen in figure 21.2.
ITQ1
Why is the flower described as a
reproductive organ?
ITQ2
Put these in the correct sequence of the
plant life cycle, starting with (a):
(a) development of flowers
(b) germination
(c) fertilisation
(d) dispersal of seeds
(e) pollination
(f) formation of seeds
(g) growth of plant
MSV^LY
WVSSPUH[PVUMVSSV^LK
I`MLY[PSPZH[PVU
VJJ\YZ
MY\P[JVU[HPUPUNZLLK
ZLLKZHYL
KPZWLYZLK
TH[\YLWSHU[
WYVK\JLZMSV^LYZ
NLYTPUH[PVU
WSHU[NYV^Z
Figure 21.2
pollination ❯
fertilisation fruit ❯
seed ❯
dispersal ❯
260
Life cycle of a typical flowering plant.
The plant grows until it is mature and produces flowers. These flowers are
the organs of reproduction. After pollination (bringing the gametes closer
together) and fertilisation (fusion of the gametes), fruits are formed which
contain seeds. The seed contains the embryos or developing plants which
are usually dispersed. Dispersal take them to new places where the seeds
germinate, if possible, into seedlings or young plants. The seedling then grows
and mature into an adult plant and the cycle repeats itself.
21 • Reproduction in Plants
Structure of a flower
ovule
pollen grain ❯
Flowers are the reproductive organs of a plant. This means that the flowers,
regardless of the colour, size or shape, produce and contain the gametes or
sex cells. The female gamete is the ovule and the male gamete is the pollen
grain. A flower is structured to protect, house and bring together the male and
female gametes.
A typical flower has five main parts. The numbers in the paragraphs below
refer to figure 21.3
stigma
1
Gynaecium carpels (pistils) style
ovary ovule (female gamete);
filament
2
3
4
5
Androecium stamens anther pollen grains (male gametes);
Corolla petals;
Calyx sepals;
Receptacle.
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2androeciumPZ
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Figure 21.3
Table 21.1 shows the importance of thefive main flower parts.
ITQ3
Label the parts of the flower below.
Part of flower Importance of function
(
(
)
*
,
+
,
-
gynaecium
produces and contains the female gamete
androecium
produces and contains the male gamete
corolla
attracts pollinators, such as insects, to the flower
calyx
protects the flower in the bud stage
receptacle
holdings the flower and then the fruit/seed
)
*
+
Parts of a flower.
Table 21.1 The roles of different parts of a flower.
261
Life Processes and Disease
Pollination
self-pollination ❯
cross-pollination ❯
Pollination is the transfer of the pollen grain from the anther to the stigma of
other flowers of the same species (figure 21.4). Self-pollination is the transfer
of pollen to the same flower or flowers on the same plant. Cross-pollination
is the transfer of pollen to flowers on another plant of the same species. Most
plants undergo cross-pollination, which increases the variety in the offspring
produced. Self-pollination generally happens when cross-pollination cannot be
achieved.
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MSV^LY
JYVZZWVSSPUH[PVU
WSHU[ZVM[OLZHTLZWLJPLZ
Figure 21.4
Figure 21.5 Flowers are pollinated by
different agents, such as insects and birds.
ITQ4
(i) What is pollination?
(ii) Why are agents of pollination
necessary?
(iii) Name three agents of pollination.
Self-pollination and cross-pollination.
In plants, the male and female gametes (pollen grains and ovules) are brought
closer together usually by wind or by animals, most commonly insects and
birds. The plant is dependent on these agents to help bring their gametes
together. Flowers of these plants have evolved over millions of years into
organs that are highly specialised to the type of pollinating agent (figure 21.5).
For example, if insects are to transfer pollen, then they must be sufficiently
attracted to the flowers to approach them. This is achieved by insect-pollinated
flowers having nectar, a sugary liquid which is a food source for insects. The
insects must go the flowers for food. Flowers also attract insects with bright
colours and strong scents. And so, an insect visiting flower after flower as it
feeds, picks up the pollen grains (male gametes) from one flower and transfers
them to the female gamete of other flowers.
A wind-pollinated flower has a different type of flower. These flowers can
be inconspicuous and small because they do not need to attract insects or birds.
They are specialised in a way that allows their pollen to be picked up by wind
currents. Insect-polinated flowers and wind-pollinated flowers are compared in
figure 21.6 and table 21.2.
JVSV\YM\S
WL[HS
ITQ5
Define (i) cross-pollination (ii) selfpollination.
MLH[OLY`
Z[PNTH
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Z[HTLUPUZPKL
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Figure 21.6
262
Z[HTLUOHUNZ
V\[VMMSV^LY
Flowers are adapted for either insect-pollination or wind-pollination.
21 • Reproduction in Plants
ITQ6
How is the flower below pollinated?
Give reasons to support your answer.
Insect-pollinated flower
Wind-pollinated flower
Examples
Pride of Barbados, pea (Crotalaria)
corn (Zea mays), grass, sugar cane
Flower
large and brightly coloured
small and inconspicuous
Petal
large, brightly coloured scented nectaries small, green or brown in colour, no scent
at the base of petal
and no nectaries
Stamen
short, with anthers firmly attached inside long filaments, with anthers that hang
the flower
outside the flower
Stigma
sticky and situated inside the flower
large, branched and feathery
Pollen grain large, sticky or spiky – small quantities
produced
ITQ7
Describe what happens in the flower
after pollination, leading to successful
fertilisation.
MSV^LY
(M[LYMLY[PSPZH[PVU[OLV]\SLZ
KL]LSVWPU[VZLLKZHUK[OL
V]HY`PU[VHMY\P[
MY\P[
Table 21.2
small, smooth and light – large
quantities are produced
Comparison of insect-pollinated and wind-pollinated flowers.
Fertilisation and development of seed
During pollination the gametes are brought closer together. Now they must
fuse. This fusion of the male and female gates is fertilisation. Figure 21.7 shows
what happens between pollination and fertilisation. Once the male nuclei have
reached the ovule, fertilisation can take place.
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Z[PNTH
PMP[PZ[OLZHTL
ZWLJPLZP[NLYTPUH[LZ
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Z[`SL
[OLZ[`SL[V[OLV]HY`
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[OLWL[HSZILNPU[VKYVWVMM
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KV^U[OLWVSSLU[\IL
4WVSSLU[\ILLU[LYZTPJYVW`SL
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[VYLHJOMLTHSLU\JSL\Z
TPJYVW`SL
Figure 21.7 The male nucleus is brought close to the female nucleus for their fusion (fertilisation).
;OLMY\P[JVU[HPUPUN[OLZLLKZ
JVU[PU\LZ[VTH[\YL
Figure 21.8
Development of a fruit.
After fertilisation, the ovule develops into a seed containing the embryo. The
ovary grows into the fruit as the petals shrivel and drop off. The stigma, style
and stamens also drop off. The sepals may remain (figure 21.8).
263
Life Processes and Disease
The structure of the fruit and seed of a dicotyledonous plant are related to
the structure of the flower. A fruit, which contains one or more seeds, develops
from the ovary. Its shape and the position of the seeds in it relate directly to the
shape of the ovary and the position of the ovules inside (table 21.3).
Ovary with ovule(s)
Fruit with seed(s)
long ovary containing four ovules
in a row
long pod-like fruit containing four
seeds in a row
ovary containing one ovule
oval-shaped fruit containing one
seed
round ovary with rows of ovules
round-shaped fruit containing
rows of seeds.
Table 21.3 After fertilisation, a fruit develops which relates to the shape of the ovary and the
number and position of the ovules.
Dispersal
Practical activity
SBA 21.1: Dispersal of fruits, page 363
Practical activity
SBA 21.2: Seeds and food storage,
page 364
The fruits containing the seeds are firmly attached to the plant as they develop,
grow and mature. When mature, or ripe, they are dispersed or sent away from
the parent plant. Most plants depend on the help of agents like wind, water
and animals to disperse their seeds. Each fruit is thus highly specialised in
structure, size, shape and composition for its type of dispersal.
Spreading the seeds away from the parent plant is important to:
• prevent overcrowding and therefore competition for light, space, water and
minerals;
• allow colonisation of new areas or habitats.
Dispersal by animals
ITQ8
Why does a fruit have two scars?
The wall of the fruit is called the pericarp and may be composed of three layers
– the epicarp, mesocarp and endocarp. In some fruits, these layers are fleshy
and succulent and animals are attracted to them for food (figure 21.9).
LWPJHYW
TLZVJHYW
ZLLK
LUKVJHYW
Z^LL[ZVM[
Z\JJ\SLU[
JVSV\YM\SZJLU[LKMY\P[
Z\JJ\SLU[MSLZO`PUZPKL
Figure 21.9 A succulent fruit, like a passion fruit or orange, is colourful and scented.
264
21 • Reproduction in Plants
OVVRZVU[OLMY\P[
Fruits like mangoes, tomatoes, oranges, and watermelons contain stored food,
and are colourful and scented to attract animals. The fruit may be green,
unscented and inconspicuous when young but, as it ripens and is ready for
dispersal, it develops bright colours like red and orange, and becomes scented,
so that animals are attracted to the plant for food. As they eat the fruits, they
may move away and so disperse the seeds. If the seeds are large, they are spat
out or discarded in a new place away from the parent plant. If the seeds are
small, they may be swallowed and then egested (figure 21.10). Some, like
tomato seeds, can pass through the digestive system unharmed.
IPYK[OLUMSPLZH^H`
ZLLKZPU[OLMHLJLZ
VM[OLIPYKMHSSMHYH^H`
MYVT[OLWHYLU[WSHU[
MY\P[OVVRZVU[VHUPTHSZM\Y
VYJSV[OPUNHZ[OL`WHZZ[OYV\NO
IPYKLH[ZMY\P[
Z^HSSV^PUN[OLZLLK
Figure 21.10
birds.
Figure 21.11
structures.
Some fruits have hook-like
Berries are succulent fruits. They can be taken far away from the parent plant by
Fruits can also be dispersed by animals in a different way. These kinds of fruit
do not attract animals for food because they are dry. They have hooks or hairs
or spikes, and become attached to the animal instead (figure 21.11). When the
animal is walking through the environment, these fruits, like sweetheart and
burr grass, stick or hook on to the animal’s legs or body and get dispersed as
the animal moves away from the parent plant.
Dispersal by water
Fruits dispersed by water must be buoyant so that they can float away, for
example, coconut trees are usually found on coastlines. Coconuts can be
taken by ocean currents to other coasts, islands or countries. The epicarp is
waterproof and the mesocarp fibrous and light – adaptations for dispersal by
water (figure 21.12).
[OLTLZVJHYWPZÄIYV\ZHUK[YHWZHPY
^OPJOTHRLZ[OLSHYNLMY\P[I\V`HU[
JVJVU\[ÅVH[ZVU^H[LYHUKPZ
[HRLUH^H`MYVT[OLWHYLU[WSHU[
Figure 21.12
coconut sprouting on a new beach
Some fruits such as coconuts are dispersed by water.
Dispersal by wind
Some fruits are carried by the gentlest of wind currents and so must be light
and particular adaptations (figure 21.13, overleaf). Dandelion and silk cotton
265
Life Processes and Disease
seeds have radiating threads that form a parachute; mahogany seeds have
wing-like structures which allow them to be carried away from their parent.
ITQ9
What is dispersal?
^PUNSPRLZ[Y\J[\YL
;OLMY\P[ZWPUZPU[OL^PUK
HUKJHUIL[HRLUMHYH^H`
WHYHJO\[LZOHWLJHU
[HRLMY\P[MHYH^H`
Figure 21.13
Some fruits are adapted to ‘fly’ in wind currents.
Dispersal by explosive devices
When explosive fruits dry, they split and curl suddenly to flick out the seeds
(figure 21.14). These are fruits with pods like garden pea (Crotalaria), thorn
apple and Pride of Barbados. This is also called self-dispersal or mechanical
dispersal. In this case, the help of some other agent is not needed – the drying
out of the pod causes it to split along its line of weakness along the side.
ITQ10
Draw a table with named examples
of plants which show each of the
following methods of dispersal:
animals, water, wind, self-explosive.
Describe how their seeds are adapted
for dispersal in this way and make a
simple sketch in each case.
266
Figure 21.14 Some fruits such as thorn apple (Datura stramonium) ‘explode’ and release their
seeds. The pod splits when the walls curl back as they dry out. The seeds are flicked out.
21 • Reproduction in Plants
If the seeds eventually land on fertile soil, away from the parent, they have
a good chance of germinating into a seedling which can then grow into new
plant. Life continues and the cycle continues.
Chapter summary
• The life cycle of a plant involves events which enable an adult plant to produce
flowers, gametes, seeds and fruits and to disperse the seeds to a new place suitable
for germination.
• The flower is the reproductive organ of a plant, producing both male and female
gametes.
• The flower is made up of five main parts: gynaecium, androecium, corolla, calyx and
receptacle.
• The male gamete is the pollen grain, and is taken from the anther to the stigma of a
flower of the same species during pollination.
• Fusion of the male and female nuclei in the ovary is fertilisation.
• After fertilisation, the ovule develops into a seed and the ovary develops into a fruit.
• These fruits and seeds are then dispersed from the parent plant.
• Agents of dispersal are animals (especially insects and birds), water and wind.
• The fruits of some plants dry and curl and the seeds are flicked from the parent plant.
• If the dispersed seed lands on soil with the right conditions, it will begin to germinate.
Answers to ITQs
ITQ1 The flower produces the reproductive cells or gametes which, on
fusion, produce offspring.
ITQ2 a, e, c, f, d, b, g
ITQ3
(WL[HS
)HU[OLY
*V]\SL
MPSHTLU[+
ZLWHS,
YLJLW[HJSL-
ITQ4 (i) Pollination is the transfer of pollen grains from the anther to a
stigma of a flower of the same species.
(ii) Pollen grains cannot move by themselves. They require agents to move
them from the anther to the stigma.
(iii) Insects (e.g. bee); birds (e.g. hummingbird); wind.
ITQ5 (i) Cross-pollination is the transfer of pollen grains from the anther of
one plant to the stigma of another plant of the same species.
(ii) Self-pollination is the transfer of pollen grains from the anther of one
flower to the stigma of the same flower, or to other flowers on the same plant.
267
Life Processes and Disease
ITQ6 This flower is insect-pollinated because:
• it is brightly coloured;
• the stigma is inside the flower;
• it has large petals;
• the anthers are found inside the flower.
ITQ7 During pollination, the pollen grains land on the stigma of a flower.
If the pollen and the stigma are of the same species, the pollen grain then
germinates and develops a long pollen tube which grows down inside the
style. The pollen tube contains two male nuclei and it continues to grow until
it reaches the ovary and then the ovules. It grows through the micropyle and
enters the ovule. A male nucleus in the pollen tube then fuses with the female
nucleus in the ovule. This is fertilisation
ITQ8 When a fruit develops from an ovary, it has a scar where it was
attached to the receptacle or stem and a scar which used to be the style.
ITQ9 Dispersal is the process which describes how the offspring of a plant
move away from the parent plant. Neither the fruits containing the offspring
nor the seed (offspring) can move from place to place, so they depend on
agents of dispersal like water, wind and animals.
ITQ10
268
Fruit
Agent of
dispersal
Adaptations for dispersal in this way
Cherry
Animals
The fruit is brightly coloured and juicy. Animals are
attracted to it for food, and they disperse the seeds
when they move away from the parent plant with
the fruit.
Coconut
Water
The fruit is dull and very large; it is also buoyant and
light. It can stay afloat for long periods of time.
Purple petria
Wind
Wing-like structures are present. Seeds are light
and small and can be picked up and carried by wind
currents for miles.
Thorn apple
Self-explosive
It is dull-coloured and has four parts. They lose
water and become harder and harder, placing strain
where they join together. Eventually the parts ‘burst’
and scatter the seeds which were inside the fruit.
21 • Reproduction in Plants
Examination-style questions
1
(i)
Define:
(a) pollination
(c) germination
(b) fertilisation
(d) dispersal
(ii) Examine the diagrams of the two fruits I and II below and describe fully how dispersal
occurs in each.
I
II
(iii) List two advantages of dispersal.
(iv) List three conditions necessary for germination.
2
(i)
Name the parts of the flower in the diagrams I and II below.
0
( )
* +
-
00
,
/
0
.
1
2
269
Life Processes and Disease
(ii) State some differences between the stamens of the wind-pollinated flower and
insect-pollinated flower.
(iii) List two characteristics of the pollen grains of insect-pollinated plants.
(iv) What is the main advantage of cross-pollination?
(v) Pollination can be described as an example of symbiosis. Describe fully the
relationship between bees and flowers.
3
(i)
Copy and complete the diagram below which shows the reproductive cycle of a
flowering plant.
HK\S[WSHU[
^P[O
flowers
fertilisation
M\ZPVUVMNHTL[LZ
dispersal
(ii) Name A, B, C, D, E, F and G in the diagram below.
(
.
)
+
*
,
(iii) Distinguish between pollination and fertilisation.
(iv) Pollen grains from many different species may land on a stigma. However, the seeds
produced belong only to the same species as the flower. Explain why this happens.
270
22
By the end of this
chapter, you should
be able to:
Disease and Humans
understand what is meant by pathogenic, deficiency, hereditary and
physiological diseases
distinguish among the methods used to treat and control the four main groups
of diseases
understand the role of vectors in the transmission of disease
understand the importance of knowing the life history of a vector in relstion to
control
understand the social and economic implications of disease in plants and
animals
disease
deficiency
pathogenic
hereditary
physiological
vector
AIDS
role of blood
control of
disease
immunity
drugs
• alcohol
• caffeine
• cocaine
• heroin
social and economic
implications
life cycle
natural
artificial
vaccination
Health and disease
Health has been defined as ‘complete physical, mental and social well-being’.
It is more than just the absence of disease; it includes the mental and social
dimensions of life. A disease is a condition in which the health of an organism
is impaired.
Note that a proper diet and adequate exercise are important to good health.
They help to prevent the onset of, and even help to treat, diseases. Eating foods
that make up a balanced diet increases the body’s resistance to infection. A
programme of exercise strengthens all the organ systems and leads to overall
good health – physical, mental and social.
Types and control of disease
Diseases can be divided into four main types – pathogenic, deficiency,
hereditary and physiological.
271
Life Processes and Disease
Table 22.1 distinguishes between these types of disease, describes one
example of each type and discusses the methods used to treat and control these
types of diseases.
Type of disease
Named
example
Cause
Symptoms
Pathogenic Caused
by parasitic organisms
(pathogens) like viruses,
bacteria, fungi, protozoa
and worms.
Examples: malaria, TB,
cholera, influenza
Influenza
Virus (pathogen) invades Headache, sore throat,
muscular pains, fever
the body by contact
with infected person. It
is airborne or dropletborne.
Deficiency Caused by a
shortage of a nutrient (e.g.
vitamin, mineral) in diet.
Examples: kwashiorkor,
night-blindness, irondeficiency anaemia
Weakness, fatigue,
Iron-deficiency Deficiency of iron
shortness of breath,
anaemia
causes a reduction in
the number of red blood increased heartbeat,
pale appearance
cells which reduces
the oxygen-carrying
capacity of the blood.
This is because iron is
an integral part of the
structure of haemoglobin
in red blood cells.
Hereditary Caused by
genes passed on from one
generation to the next.
Examples: haemophilia,
cystic fibrosis, sickle cell
anaemia
Sickle cell
anaemia
Gene for the disease
is passed to the
offspring. The gene
causes the red blood
cells to be sickle shaped
which reduces oxygencarrying ability.
Physiological Caused by
a malfunction of body’s
organ.
Examples: asthma,
hypertension, diabetes,
glaucoma, stroke
Diabetes
Inability of the islet of Tiredness, continual
Langerhans to produce thirst, weight loss,
increased urination,
insulin. Body cells are
unable to absorb glucose coma
which stays in the blood.
Weakness, tiredness,
weight loss, May lead
to kidney failure, heart
failure
Treatment
Control
Rest and treatment for Prevent overthe symptoms. Vaccine crowding and
exposure to the
for specific strains of
virus. Prevent droplet
the virus.
infection through
coughs, sneezes, etc.
Eat iron-rich foods
(e.g. red meat, green
leafy vegetables). Take
iron tablets.
Education about a
balanced diet, food
groups, etc.
Avoid situations where Genetic counselling
oxygen supply is
reduced. No treatment
or cure available.
Insulin injection/tablet
Low carbohydrate
diet, exercise.
Education on the
importance of diet and
exercise.
Table 22.1 Some diseases in humans.
CHAPTER 13
ITQ1
What do you understand by the terms:
(i) pathogenic disease
(ii) hereditary disease
(iii) physiological disease
(iv) deficiency disease?
272
Some diseases are more commonly found in certain parts of the world than
in others. For example, in developing countries a greater proportion of
deaths occur as a result of infectious diseases, like dengue fever, cholera and
tuberculosis. In developed countries, a smaller proportion of people die from
infectious diseases, and more deaths are due to physiological diseases like
cancer and heart disease. These kinds of disease are influenced by factors such
as diet, life style, genetic predisposition and exposure to harmful conditions.
For example, hypertension (chapter 13) results from a stressful life, filled with
worry, anger, nervous fatigue, no rest or relaxation, and unhealthy eating
habits (e.g. too much fatty, salty fast-food). Hypertension can to some extent
be controlled by good diet and exercise.
This difference in distribution is referred to as the global distribution of
disease. It often reflects the wealth and standards of medical care in the
different countries. Thus, the occurrence of disease in developing countries is
often influenced by factors such as overcrowding, lack of clean water, lack of
preventative medicines and lack of proper nutrition.
22 • Disease and Humans
Pathogenic diseases and vectors
pathogen ❯
A pathogen, or disease-causing organism, lives on or inside an organism, the
host, causing it to be diseased or sick. Pathogens can move from one host to
another, or infect another organism in a number of ways including by:
• water;
• food;
• airborne droplets;
• direct contact;
• dust particles;
• contact with faeces;
• animals (mainly insects), called vectors.
Vectors
vector ❯
Vectors spread disease by carrying the pathogen from host to host. Examples
of vectors include flies, mosquitoes and rats (figures 22.1,22.2 and 22.3).
Table 22.2 gives sosme examples..
Vector
Examples of disease(s) spread
mosquitoes
yellow fever, malaria, dengue fever,
flies
gastroenteritis
rat flea
plague
rat
leptospirosis
Table 22.2 Some vectors and the disease they spread.
]LJ[VYºIP[LZ»PUMLJ[LKOVZ[
HUKWPJRZ\W[OLWH[OVNLU
]LJ[VY^P[O
[OLWH[OVNLU
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UL^OVZ[
[YHUZMLYYPUN
[OLWH[OVNLU
PUMLJ[LKOVZ[
Figure 22.1
Figure 22.2 Flies feed on the food we then
eat and so spread disease.
UL^OVZ[PUMLJ[LK
I`H]LJ[VY
Mosquitoes are vectors for malaria and many other diseases in humans.
ITQ2
(i) What is a vector?
(ii) Discuss why a fly can be considered to be a vector.
273
Life Processes and Disease
Controlling mosquitoes
If the vector can be controlled, then the spread of the disease will also be
controlled, since the chance of being in contact with the vector and so getting the
infection will be reduced. Thus, it is important to study the vector’s life cycle to
find out how to prevent them laying eggs, or how to prevent their development
into adults, or how to destroy the adults. A good example of this is the attempt to
control the mosquitoes that act as vectors for malaria (figure 22.3).
ZWPYHJSLZMVYIYLH[OPUN
SHY]H SP]LZPU^H[LY
YLZWPYH[VY`[\IL
HPYMSVH[
LNNSHPKVU^H[LY
W\WHSP]LZPU^H[LY
HK\S[MLTHSLMLLKZVUISVVK
Figure 22.3 The life cycle of a mosquito.
The eradication of a disease spread by mosquitoes would be possible if a
concerted effort were made by the general public in the following areas.
• Drain stagnant water around the home and workplaces – This would
drastically reduce the number of places for female mosquitoes to lay eggs as
well as reduce the number of eggs and pupae surviving to develop into adults.
• Spread a thin layer of oil over water which must be kept – This
would prevent larvae and pupae in the water from breathing and so kill
them.
• Kill the adults with insecticide.
• Use mosquito nets – This would reduce the possibility of being bitten
when mosquitoes are around.
• Keep the area around the house clear of bush where adult mosquitoes rest.
In the same way, knowing where flies lay their eggs, how they develop and
what they feed on, can help to reduce the number of flies and thus reduce the
incidence of diseases spread by flies.
Pathogens
STDs ❯
274
Pathogens are usually microscopic organisms (like viruses, bacteria and
protozoans) that live in the blood and tissues of their host. Some are larger, like
fungi and worms, which are easier to get rid of and control.
Some pathogens spread by direct contact or close interaction between
the infected host and a new host. These include the sexually transmitted
diseases (STDs) like herpes, AIDS, gonorrhoea and syphilis. AIDS is of most
importance because it has reached epidemic status in the world and can lead to
22 • Disease and Humans
death because it compromises the body’s immune system leaving the patient
defenceless against secondary opportunistic infections. Expensive drug regimes
can prolong the life of someone living with HIV, but there is no means of
elimating the virus from the body and no cure for AIDS. Herpes is also incurable
but not fatal. The other STDs can be treated and controlled if diagnosed early.
Social and economic implications of
disease
The loss of life and loss of working hours to disease are important social
and economic factors. Treatments for pathogenic diseases such as AIDS and
degenerative diseases such as cancer place increasing demands on health
services. Lifestyle diseases related to smoking, lack of exercise and over-eating
are increasingly important economically in developed countries, again because
of the cost of treatment and their effects on social and economic life.
Implications for humans of disease in plants and
animals
Humans are also affected economically by the health of the crops and animal
stocks grown for food. Loss of livestock (cows, pigs, chickens, etc.) and
agricultural crops (rice, wheat, potatoes, etc.) due to disease can have serious
economic implications. A disease can greatly reduce or wipe out the livestock
or food crop of any area in a short space of time; for example, mealy bug
infestation in the Caribbean, and foot-and-mouth disease in Europe. This
results in loss of income for the farmers and reduction in food availability.
Food, in the form of livestock and agricultural produce, moves all over the
world in ships and airplanes on a daily basis. Disease control is therefore very
difficult. Quarantine procedures at ports and airports help but do not prevent
the spread of diseases. Many pathogens are microorganisms so are not seen;
many can exist as spores for long periods of time.
Chapter summary
• A healthy person is physically, socially and mentally well.
• A disease impairs good health.
• There are four main classes of disease: pathogenic, deficiency, hereditary and
physiological diseases.
• A pathogenic disease is caused by parasitic (and often microscopic) organisms like
viruses, bacteria, fungi, protozoans and worms.
• A deficiency disease results when there is a deficiency of a nutrient in the diet.
• A hereditary disease is passed on by genes from a person to their offspring.
• A physiological disease is caused by a malfunction of an organ in the body.
• A vector transports pathogens from one host to another.
• Vectors are usually insects, such as mosquitoes and flies.
• Understanding the life cycle of a vector can help to control or eradicate a disease
spread by that vector.
• The social, environmental and economic implications of disease include loss of life,
loss of working hours, loss of money. Disease in crops and livestock can lead to
famine. Research into cures for disease is expensive.
275
Life Processes and Disease
Answers to ITQs
ITQ1 (i) Pathogenic disease – symptoms of disease are seen because of the
presence of another organism (a pathogen) in the body.
(ii) Hereditary disease – symptoms of disease seen because of the presence of
a ‘disease-carrying’ gene which was passed to an organism from its parents.
(iii) Physiological disease – symptoms of disease seen because an organ or part
of the body is not working.
(iv) Deficiency disease – symptoms of disease seen when a nutrient or
nutrients are lacking in the diet of the organism.
ITQ2 (i) A vector carries a pathogen from host to host. It is able to pick up
the pathogen in or on its body when it feeds and then transfers the pathogen
when it moves to another host.
(ii) Flies pick up microorganisms when they feed. They feed on any
organic matter, especially dead and rotting organic matter. Their bodies
are hairy and can easily carry pathogens. They also regurgitate or vomit
previous food when they eat. If they land to feed on any substance
that is going to be food or drink to another animal, they can pass
the pathogen to a new host. Flies are thus considered to be vectors.
Examination-style questions
1
(i)
Explain what is meant by the following types of disease and give one example of each:
(a) hereditary;
(c) deficiency;
(b) physiological;
(d) infectious.
(ii) Describe the causes and symptoms of:
(a) gonorrhoea
(b) diabetes.
(iii) Explain how diseases like malaria and sickle-cell anaemia are spread and describe
the importance of these diseases worldwide.
(iv) Describe the social and economic implications of AIDS. List some ways the spread of
AIDS can be prevented and controlled.
(v) Describe and compare the global pattern of distribution of yellow fever and coronary
heart disease.
2
(i) Describe how phagocytes protect the body against infection.
(ii) List four ways the skin and openings on the skin are adapted to control the entry of
pathogens into the body.
(iii) (a) Distinguish between active natural immunity and active artificial immunity.
(b) Give one example of artificial passive immunity and one example of natural
passive immunity.
(iv) It is estimated that a human can synthesise 10 million different types of antibodies.
Describe three ways antibodies defend the body against disease.
(v) Copy and complete the table.
Drug
Two effects on the body
Social or economic implications
Alcohol
Cocaine
Caffeine
3
276
(i)
Explain the meaning of the term ‘drug’. Using named examples discuss the use and
abuse of drugs.
(ii) Describe the immediate and long-term consequences of alcohol consumption.
(iii) Discuss the social consequences of excessive alcohol use with particular reference to
drink driving, aggressive behaviour, family breakdown and petty crime.
Section C:
Continuity and
Variation
23
By the end of this
chapter, you should
be able to:
Mitosis
understand the importance of maintaining species chromosome number
describe the process of mitosis
understand the role of mitosis in growth
explain the role of mitosis in asexual reproduction
explain why asexual reproduction gives rise to genetically identical offspring
cell
cell division
meiosis
prophase
metaphase
anaphase
telophase
mitosis
cloning of
animals
tissue culture
in plants
asexual
reproduction
growth
Dolly
Chromosome number
chromosome number ❯
Chromosomes are present in the nuclei of cells. They contain genetic
information in the form of genes. Each species has a specific number of
chromosomes in its body cell – this is called the chromosome number for
that species (figure 23.1 and table 23.1).
ITQ1
(i) What is meant by the ‘chromosome
number’ of an organism?
(ii) What is the chromosome number
for (a) humans and (b) an onion?
Figure 23.1
278
Each species has its own chromosome number.
23 • Mitosis
Species
Chromosome number
onion
16
tomato
24
locust
24
corn
40
mouse
40
human
46
potato
48
The chromosome number for humans is 46. This means that in every body
cell of every human there are 46 chromosomes. A chromosome is made up
of genes. While the chromosome number is the same for all humans, the
combination of genes is different (figure 23.2). The 46 chromosomes are
different for every human except identical twins.
Table 23.1 Chromosome numbers for
seven different species.
ITQ2
(i) List three differences between the
people in figure 23.2.
(ii) List three similarities.
(iii) Why can these differences be
seen?
There are 46 chromosomes in each of his
body cells, but the combination of genes is
special to this individual. This produces outward
characteristics that are special to this individual.
She also has 46 chromosomes in each of her
cells but with her own combination of genes on
the 46 chromosomes. Each individual is unique
and special.
Figure 23.2 Members of the same species have the same chromosome number, but the
combination of genes is different for each.
The cell cycle
cell cycle ❯
mitosis ❯
interphase ❯
JLSSKP]PZPVU
J`[VRPULZPZ
The cell cycle is the sequence of events that occurs between the start of one
cell division (mitosis) and the start of the next (figure 23.3). The longest event
in the cell cycle is called interphase during which the cell grows and carries
out its functions. At the end of interphase cell division begins.
Mitosis is divided into four stages: prophase, metaphase, anaphase and
telophase. At the end of telophase, two nuclei have been formed but the
cytoplasm is not fully divided between two cells. This happens next and the
process is called cytokenisis (figure 23.4).
T
L[HW
OHZL
H
UHW
OHZ
L
[LS
VW
OH
ZL
PU[LYWOHZL
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WYVWOHZL
TP[VZPZ
J`[VRPULZPZ
PU[LYW O H Z L
Figure 23.3 The cell cycle.
Figure 23.4
Mitosis has four stages and is a part of the cell cycle.
279
PU[LYWOHZLILMVYL
TP[VZPZ
Continuity and Variation
U\JSLHYLU]LSVWL
JLU[YPVSLZ
The processes of mitosis and cytokinesis
are shown in figure 23.5. The numbers in
the text on page 281 relate to figure23.5.
U\JSLVS\Z
JOYVTVZVTL
JOYVTVZVTL
JOYVTH[PKZ
JLU[YVTLYL
TP[VZPZ
1WYVWOHZL
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3HUHWOHZL
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Figure 23.5
280
PKLU[PJHSJLSSZ
Mitosis and cytokinesis.
23 • Mitosis
Interphase before mitosis
• The cell is prepared for division.
• The chromosomes become shorter and fatter (and are easily seen).
• Each chromosome makes an exact copy of itself, forming two chromatids
joined at a centromere.
Prophase (1)
•
•
•
•
•
Chromosomes (made of two chromatids) are visible.
The nucleolus shrinks and disappears.
The centrioles move to opposite sides of the cell.
The spindle fibres form.
The nuclear envelope breaks down.
The centrioles play an important part in cell division: the spindle fibres
originate from them. The spindle fibres attach to chromosomes and pull them
to either side of the cell.
Metaphase (2)
• The chromosomes line up along the ‘equator’ of the cell.
• NB Each chromosome is still made up of two chromatids joined at the
centromere.
Anaphase (3)
• The chromatids separate and move to opposite sides of the cell.
• The chromatids reach opposite sides of the cell.
• Exact copies of chromosomes are at both sides of the cell.
Telophase (4)
• The chromosomes lengthen as they unravel.
• The nuclear envelope forms around each group of chromosomes to make
two nuclei.
• New nucleoli form in each nucleus.
• Two identical nuclei are formed.
Cytokinesis (5)
• The cell membrane develops down the middle of the cell to divide it into
two (cleavage).
• Two identical cells are produced.
ITQ3
(i) What is mitosis?
(ii) Why does mitosis occur?
ITQ4
Why is it important to maintain the
species chromosome number?
Importance of maintaining species
chromosome number
As you know, each species has its chromosome number and its own set of
characteristics that make it unique and set it apart from other species.
Rapid and repeated cell division must occur after fertilisation so that an
organism can grow and develop from one cell (the zygote). It is important that
after cell division, the chromosome number remains the same and all the cells
in a multicellular organism retain the correct number of chromosomes and the
characteristics of the species.
281
Continuity and Variation
The process of mitosis
diploid ❯
CHAPTER 20
The only variation that can happen in cells produced by mitosis is mutation,
when a gene is copied incorrectly. Even minor faults in copying can result
in major changes. An example occurs in sickle cell anaemia where a major
change in the structure of haemoglobin arises from just a single fault in
copying of the gene at some time in the past (chapter 26).
Every cell of an organism has, locked within the nucleus, the ‘blueprint’ or
complete set of instructions needed for that organism to develop. Remember that
after the male gamete fuses with the female gamete at fertilisation, a single cell is
formed which contains the complete set of information needed for development
of the organism. Mitosis then occurs over and over, producing thousands of
identical cells that differentiate to perform different functions. This leads to the
formation of a multicellular organism in which every cell has maintained the
chromosome number of the species with its unique combination of genes.
CHAPTER 26
ITQ5
Put these in order as they would occur
during mitosis.
(
)
Mitosis is cell division that occurs in all body cells except in gamete formation.
It results in the formation of two genetically identical cells, each containing the
same number of chromosomes and the same combination of genes. A cell is
described as being diploid or 2n when it has the full chromosome number.
Mitosis is essential for cell repair and for growth from the zygote to the
multicellular organism as described in chapter 20. It is important that all
body cells have the full chromosome number and thus carry all the genetic
information to allow that cell to develop its role within the body.
Mitosis is also the method by which organisms reproduce asexually forming
offspring identical to the parent. It ensures that:
• the species chromosome number is maintained;
• each daughter cell receives an identical combination of genes.
*
Figure 23.6 Replication of
a DNA molecule results in two
identical copies.
Replication of chromosomes
Replication is the process during interphase by which a chromosome is able
to copy itself exactly. A chromosome carries genetic material in the form of
deoxyribonucleic acid (DNA). Watson and Crick published the first description
of DNA in 1953. The structure is a double helix, made of two chains. By the
process of replication, a cell is able to produce two identical cells when it
divides by mitosis (figure 23.6).
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Mitosis and asexual reproduction
Asexual reproduction requires only a single parent. In essence, the parent
divides into two, or a part of the parent separates and then develops into a new
individual. It is important to note that the offspring are genetically identical
to the parent. This means that the physical and behavioural characteristics
are also identical to the parent except for variation due to the environment.
282
23 • Mitosis
Binary fission, vegetative propagation and cloning in animals are examples of
asexual reproduction. Mitosis results in the formation of identical cells. When
organisms divide asexually, they divide by mitosis.
Binary fission
binary fission ❯
This occurs in simple, unicellular (one-celled) organisms, like bacteria and
protozoans such as Amoeba (figure 23.7). The organism divides into two parts,
each of which develops into a new organism. This is known as binary fission.
In Amoeba/protozoa the chromosomes replicate first, then the nucleus divides
into two, followed by the cytoplasm. Two identical organisms are formed.
ITQ6
An Amoeba seen today is identical to
one that existed 100 years ago. How
can this be so?
the cytoplasm begins
to divide into two
the nucleus begins
to divide into two
activities stop as
the Amoeba
prepares for division
Figure 23.7 Binary
fission in Amoeba.
nuclear division
is complete
two identical Amoeba move
away from each other
cytokinesis
continues
Vegetative propagation
vegetative propagation ❯
CHAPTER 15
runner ❯
stolon ❯
H
Vegetative reproduction or vegetative propagation is a common form of
asexual reproduction in plants. In some plants, a bud grows and develops into
a new plant, and then becomes detached from the parent plant. Bulbs, corms,
rhizomes, tubers, tap roots (chapter 15), runners, stolons and tillers can all give
rise to new plants by vegetative reproduction.
In a runner, such as those on a strawberry plant, a number of stems grow
out from the parent plant. The runner touches the ground, adventitious roots
develop and a new plant forms. The runner connecting it to the parent plant
decays and the new plant becomes established (figure 23.8 (a)).
A stolon is simply a runner formed underground. This method of
reproduction can be very effective; nut grass, for example, is very difficult to
eradicate once it is established.
A Bryophyllum leaf (‘leaf of life’) will generate new plants around its
edges. After a while these plantlets become detached from the parent leaf
(figure 23.8 (b)).
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Figure 23.8 (a) New plants are established from a side branch or runner. (b) Many new plants can
be propagated from a single Bryophyllum leaf.
283
Continuity and Variation
Artificial propagation
Horticulturists and agriculturists have extended asexual propagation to
include cuttings, budding, layering and grafting. This is termed artificial
propagation. These techniques are commonly used in gardening and the
commercial growing of plants.
artificial propagation ❯
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Cuttings
A stem is cut near a node and pushed into the soil (figure 23.9 (a)). New roots
grow out from the submerging part of the stem, Particularly if treated with a
plant growth substance (e.g. rooting powder). Examples include sugar cane,
geranium, African violet, chrysanthemum.
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Figure 23.9 Artificial propagation of plants
by (a) cutting, and (b) grafting.
tissue culture ❯
Grafting
A cutting, called the scion, which is to be propagated is inserted into a slit in
the stem of another plant (the stock), and the joint is bound up to seat it. The
stock already has a root system so the scion is able to grow into a new plant
(figure 23.9 (b)).
Tissue culture
Tissue culture is a form of vegetative propagation used to make large
numbers of identical plants (figure 23.10). Like binary fission, it also results
from mitosis. Using tissue culture propagation or cloning, whole plants can be
made from very small pieces cut from the parent plant. This depends on the
fact that the majority of plant cells have the potential to form a whole plant.
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284
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23 • Mitosis
Advantages of tissue culture
ITQ7
(i) Define the term ‘tissue culture’?
(ii) Describe how tissue culture is
used to generate many identical
plants.
• Large numbers of identical plants can be produced relatively quickly from
‘superior’ individuals. This can make them much cheaper.
• Tissue culture can be used to propagate plant species which do not
develop naturally through sexual reproduction easily, such as orchids. The
propagation of orchids for sale on a massive scale is now possible.
Disadvantage of tissue culture
• Variety within a plant species is being replaced with similarity because it
is cheaper. This is risky because if that one kind becomes susceptible to a
particular disease or pest, the whole crop may be lost.
Cloning of animals
clone
identical twins ❯
ITQ8
(i) Explain what is meant by
‘cloning’?
(ii) Describe a natural occurrence of
cloning?
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A clone is an exact copy of an organism. Identical twins are, in essence,
clones, since after the first cell division of the zygote, two identical cells are
formed. These two identical cells somehow separate from each other and
then grow and develop into separate beings that are identical to each other
(figure 23.11). The environment confers subtle differences as they grow
and develop.
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Figure 23.11 The development of identical twins.
CHAPTER 26
Scientists can now easily separate the first four cells of a zygote and use these
to create clones of the organism (figure 23.12, overleaf). This is practised
mainly in the livestock and dairy industries. It is financially advantageous to
make clones of a ‘superior’ animal, such as one which produces large amounts
of a high-quality milk or high-protein meat. It is also used to ‘copy’ individuals
which have been genetically engineered (chapter 26). An example here would
be livestock with genes to produce human hormones in the animal’s milk.
Cloning may be used to produce an animal with some special characteristic
(such as speed in a racehorse) as that could be financially beneficial.
Clones need a surrogate mother in which to develop. This is a female who is
not the genetic mother but in whose womb the fertilised ovum is implanted so
that it can develop as a fetus until birth.
285
Continuity and Variation
organism
growth and development in mother’s womb
zygote
2-cell stage
4-cell stage
organism
organism
clones
(exact copies)
organism
zygote of a
‘superior’ organism
Clones would all have
the same ‘superior’
characteristics.
2-cell stage
4-cell stage
each of the four
cells are separated
organism
Each cell then continues to grow and develop into an organism. They are exact
copies of each other. Each can be implanted in a female’s womb (surrogate mother).
Figure 23.12
ITQ9
Describe one way that scientists can
make copies or clones of a ‘superior’
animal?
Cloning of fertilised eggs.
A second way to create a clone is to take the nucleus of a body cell from the
‘superior’ individual and use it to replace the nucleus of an unfertilised ovum.
The cell can be made to divide as it would have done if it was a fertilised ovum
and implanted in the womb of a surrogate mother, but all the cells it makes
now have the chromosomes from the ‘superior’ animal. The first example of
this kind of cloning was the sheep called Dolly(figure 23.13).
normal
development
unfertilised
egg
how Dolly
was cloned
unfertilised
egg
body cell from
a sheep
egg fertilised by
sperm forms
a zygote
The nucleus is removed and replaces the
nucleus of the ovum. Development continues
in the surrogate mother. The dividing nucleus contains
Dolly’s chromosomes. A surrogate mother is one
in which the embryo is implanted and she
‘carries’ a baby that is not hers.
starts to
divide
Surrogate mother
gives birth to Dolly who is genetically
identical (a clone) with the original sheep.
ITQ10
How was Dolly cloned?
Figure 23.13
286
How Dolly the sheep was cloned.
23 • Mitosis
Advantages of animal cloning
CHAPTER 24
• Superior traits can be passed on to offspring without the risk of losing them
through genetic exchange during meiosis (chapter 24).
• The use of surrogate mothers means that more ‘superior’ offspring can be
created than could be carried by just the genetic mother.
Disadvantages of animal cloning
• The effects of using a body cell, as in creating Dolly, are still being studied. It
is possible that using what is in effect an ‘old’ nucleus may cause problems
in the cloned individual.
• The technique used to create Dolly could be used to clone humans. Many
countries now have legislation to prevent this because it is considered
unethical. For example, it might be done for purely selfish reasons.
Chapter summary
• Each species has its own chromosome number, that is the number of chromosomes
found in each nucleus in the cells of the individual.
• The chromosome number of humans is 46. This means that there are 46
chromosomes in every nucleus in every body cell of a human.
• Although each individual of a species has the same chromosome number, the
combination of genes in the chromosomes varies and so members of a species differ.
• Cells divide by mitosis to produce two genetically identical cells.
• Mitosis is important for growth, repair and asexual reproduction.
• Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase.
• During replication each chromosome makes an exact copy of itself. This occurs in
interphase, just before prophase.
• These two copies of a chromosome are called chromatids – they lie side by side and
are joined at the centromere.
• During prophase, the chromosomes become shorter and fatter and are easily stained
and seen.
• During metaphase, the chromosomes line up along the middle of the cell.
• During anaphase, the chromatids are pulled apart to opposite sides of the cell.
• During telophase, two identical nuclei are formed.
• In cytokinesis, a new cell membrane develops to divide the cell in two identical cells.
• Binary fission in Amoeba is an example of cloning.
• Cloning is the production of identical copies of an individual.
• Animals and plants can be cloned using several techniques.
• Tissue culture is one way of cloning plants.
Answers to ITQs
ITQ1 (i) The chromosome number is the number of chromosomes found in
a typical cell of an individual of the species. It is a fixed and specific number to
each species.
(ii)
(a) 46 (b) 16
ITQ2 (i) The eyebrows are shaped differently and of different thickness.
The size and shape of their lips are different.
The shapes of their faces are different.
(There are other differences that you may have seen.)
287
Continuity and Variation
(ii) Both individuals have two eyes.
In both individuals, the nose is in the middle of the face.
The position of both their lips is the same.
(There may be other similarities that you have mentioned, relating to
characteristics that are general to being of the same species.)
(iii) Differences can be seen because, although they both have 46
chromosomes in each cell, the composition of the chromosomes is different.
Their genes code for different characteristics.
ITQ3 (i) Mitosis is cell division that results in the formation of two identical
daughter cells.
(ii) Mitosis is important for growth and repair.
ITQ4 A species has special characteristics that separate it from other species.
The number of chromosomes is very important. A change in chromosome
number may change the species-specific characteristics.
ITQ5 1 C, 2 B, 3 A.
ITQ6 Amoeba divides by mitosis producing genetically identical offspring.
Over the years, when it divided, identical offspring were produced, so that one
seen today would be genetically identical to one which existed 100 years ago.
ITQ7 (i) Tissue culture uses a piece of tissue from a ‘parent’ plant to make
many plants that are identical to the parent plant.
(ii) A piece of tissue is taken from a parent plant. It is placed in a medium
containing nutrients and growth hormones. It is kept under sterile conditions
to prevent microorganisms from entering the medium. In the nutrient
medium, the piece of tissue divides rapidly by mitosis, forming a structure
called a callus, which is a ball of cells. The callus may be divided and placed in
many jars containing the nutrient medium. Each piece develops into a plantlet
which is cared for carefully. The many plantlets are all identical to the parent
plant.
ITQ8 (i) Cloning means making exact copies. An exact copy of an individual
is made when it is cloned.
(ii) Cloning may occur naturally in the formation of identical twins. After
the zygote is formed it divides into two cells. Usually the two cells stay stuck
together and continue to divide to make one individual. In identical twins,
these first two cells separate and develop into individual organisms. They are
genetically identical.
ITQ9 Scientists allow a zygote of a ‘superior’ animal to divide naturally
twice, producing four identical nuclei. These are then separated and implanted
in the uterus of other animals (surrogate mothers) and allowed to develop.
Four clones of the superior animal are thus made.
ITQ10 Dolly was cloned by extracting the nucleus from one of the cells of
a ewe. This nucleus contained all the information needed for the formation
of Dolly. The nucleus of an ovum was also removed and replaced with
Dolly’s nucleus. The ovum containing Dolly’s nucleus was made to implant
in a surrogate mother and it developed into an individual which was Dolly.
Examination-style questions
1
288
(i) List the stages of mitosis.
(ii) Explain fully the importance of interphase just before mitosis begins.
(iii) Explain the meaning of the term ‘diploid’.
(iv) (a) Label the parts A to G in the diagram on the next page.
(b) Identify each stage.
(c) Describe what happens in each stage of mitosis.
23 • Mitosis
(
)
.
+
,
*
2
(i)
Explain the following terms:
(a) asexual reproduction;
(b) binary fission.
(ii) List two advantages of asexual reproduction.
(iii) One major disadvantage of asexual reproduction is that the offspring vary only rarely.
Many species use only asexual reproduction but their offspring are not all clones.
Suggest how variation comes about in these asexually reproducing species.
(iv) What is a clone?
(v) Suggest an argument:
(a) for animal cloning;
(b) against human cloning.
(vi) Give a brief description of tissue culture. Discuss some advantages and disadvantages
of the use of tissue culture in agriculture.
289
24
By the end of this
chapter, you should
be able to:
Meiosis
understand the importance of halving of the chromosome number in the
formation of gametes
describe the process of meiosis
distinguish between mitosis and meiosis
understand the role of meiosis in the transmission of inheritable genetic
characteristics
cell
division
mitosis
meiosis
meiosis
I and II
variation
evolution
The importance of meiosis
meiosis ❯
haploid ❯
290
Body cells divide for growth and repair, and it is important that the new cells
are identical to the existing ones. This is the significance of mitosis. However,
cells of the reproductive organs must also divide, but in this case, to form the
gametes or reproductive cells.
Two gametes, one from the male and one from the female, fuse to form the
zygote which develops into the new organism. These gametes must therefore
contain half the chromosome number of chromosomes. If they did not, the
new organism would have twice the species chromosome number.
Meiosis is the cell division which occurs only in the reproductive organs
during gamete formation, and results in the formation of cells containing
half the number of chromosomes as the parent cell. Half the number of
chromosomes is the haploid or n number.
For example, a human body cell has 46 chromosomes. When body cells
divide by mitosis for growth and repair, cells containing 46 chromosomes
(diploid or 2n number) are always produced. However, cells of the reproductive
organs must divide by meiosis to make gametes. The gametes must contain 23
chromosomes (haploid or n number) so that, after fusion with another gamete,
the original number of 46 chromosomes is restored (figure 24.1).
24 • Meiosis
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ITQ1
Where does meiosis occur (i) in females
(ii) in males?
KPWSVPKn
ITQ2
List the differences between a diploid
cell and a haploid cell. Give an example
of where each can be found in the
human body.
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maintaining the chromosome number.
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The process of meiosis
Meiosis ensures that:
• each daughter cell has the haploid number of chromosomes so that the
diploid number can be restored after fertilisation;
• each daughter cell has a different combination of genes which leads to
variation among the offspring.
homologous pair ❯
A human cell has 46 chromosomes: 23 came from the mother and 23 came
from the father. Each chromosome from the set from the mother pairs up with
a corresponding chromosome from the father. These are called homologous
pairs. The chromosome in homologous pairs in humans are the same size and
shape apart from the sex chromosome (figure 24.2).
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Figure 24.2 The homologous pairs
of chromosomes in a cell with four
chromosomes
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291
Continuity and Variation
crossing over ❯
ITQ3
Why is meiosis important in
gamete cells?
ITQ4
Explain the terms (i) homologous pairs
(ii) chromatid.
At the beginning of meiosis, each chromosome forms two chromatids
joined by a centromere, as in mitosis. The homologous chromosomes then
come together, so there are now four chromatids close together. Genetic
information is exchanged randomly between the chromatids. This is known as
crossing over.
In metaphase I, the homologous chromosomes align randomly across the
equator of the cell, and then the members of homologous pairs separate and
move to opposite sides of the cell. The cell then splits to form two cells. The
division repeats with the chromosomes again lining up randomly along the
equator of the cell, only the second time around the chromatids separate,
resulting in four daughter cells, each with different genetic information
(figure 24.3 and table 24.1).
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292
Meiosis of a cell with four chromosomes.
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24 • Meiosis
Mitosis
Meiosis
occurs in body cells or somatic cells
either occurs in reproductive cells only or occurs
in formation of gametes only
number of chromosomes remains the same in
the daughter cells
number of chromosomes is halved in the
daughter cells
daughter cells are identical to parent cells and
each other
daughter cells are genetically different to parent
cell and each other
two daughter cells are formed
four daughter cells are formed
homologous chromosomes do not come
together
homologous chromosomes come together
no exchange of genetic material between
chromosomes
exchange of genetic material between
chromosomes
Table 24.1 The differences between mitosis and meiosis.
Variation of gametes
A single human male can produce over 100 million spermatozoa or male
gametes in one ejaculation. These gametes are all different. This variation of
the gametes comes about when the cell divides by meiosis. Variation results
from the following processes.
• Crossing over between homologous pairs of chromosomes in the early
stages of meiosis is random. There are no limits to how this happens. Every
homologous pair of chromosomes exchanges genetic material differently.
Imagine the various ways a cell with 23 homologous pairs can exchange
genetic material.
• During metaphase I, the pairs of chromosomes align themselves long
the equator of the cell randomly. Imagine the various ways 23 pairs of
chromosomes can be aligned along the equator. The pattern of alignment
determines which chromosomes are grouped together.
• During metaphase II, the chromosomes (now formed of two chromatids)
align randomly along the equator of the cell. This also determines how the
chromosomes are grouped in the gamete.
Significance of meiosis
Figure 24.4 These people all belong to the same family and so share
some of the same genes.
At the end of meiosis, four genetically different cells are
produced from each original cell. This means that the
gametes from each individual are all different. When
these fuse with gametes from another individual, there
will be even more variation in the genetic information
of the offspring (figure 24.4).
The gametes carry genetic information from
the parents. When they fuse to form an offspring,
genetic information is transmitted from the parents
to the offspring. The offspring are all different from
each other, since the gametes are all different. They
are also different from their parents, though some
characteristics will clearly come from the mother and
some from the father. Some features may appear that
are unlike either parent.
293
Continuity and Variation
ITQ5
The daughter cells of meiosis are all
different from each other. List three
ways in which this variation is brought
about.
Conditions in the environment are not constant. They may change,
sometimes abruptly. The survival of a species depends on the ability of the
individuals in that species to adapt to changes in the environment. When
there is variation among offspring, some will be able to withstand the changes
of the environment and survive to reproduce. The survival of the species is
thus ensured.
Darwin’s theory of evolution
Darwin’s theory of evolution ❯
ITQ6
Variation obtained from meiosis
ensures that the gametes are all
different. Give one advantage and one
disadvantage of this variation.
Darwin’s theory of evolution through natural selection is based on the
fact that among the variety of offspring produced, some will be better able to
withstand changes in living conditions than others. That is, some are better
adapted or ‘fitter’ to survive in the struggle for existence. These offspring will
then produce offspring that are similar (not identical) to themselves, passing on
the advantageous characteristics. Through these gradual changes, over many
generations, the evolution of new species is possible.
Chapter summary
• Each human cell contains 46 chromosomes. When two gametes fuse the resulting
zygote must contain 46 chromosomes.
• When a human cell divides by meiosis, four cells, each containing 23 chromosomes,
are formed.
• Meiosis occurs in the reproductive organs, the testes and ovaries, where gametes are
produced.
• On fusion of the two haploid gametes, the diploid or original number of chromosomes
is restored.
• A cell with the full species chromosome number is called a diploid cell.
• A haploid cell has half the species chromosome number.
• Gametes, male and female, are formed when cells in the reproductive organs divide
by meiosis. They are haploid cells.
• Gametes are all different from each other and the parent cells.
• During meiosis, homologous chromosomes come together and crossing over occurs,
whereby genetic information is exchanged.
• Each gamete of the millions produced is unique and so each organism produced by
their fusion is unique.
• Inheritable genetic characteristics are transmitted from the parents to the offspring by
the gametes.
• The resulting offspring are all different to or vary from each other and to their parents.
• This variation can be important if the environment changed, as those organism better
adapted will survive.
• This variation can lead to evolution.
294
24 • Meiosis
Answers to ITQs
ITQ1 (i) In females, meiosis occurs in the ovaries.
(ii) In males, meiosis occurs in the testes.
ITQ2
Haploid cell
Diploid cell
half the number of chromosomes in the nucleus the full number of chromosomes in the nucleus
found only as gametes in the reproductive
organs – ovaries and testes
found all over the body
occur as individual cells as gametes, some are
able to move (e.g. sperm)
most are fixed and occur together, forming
tissues
ITQ3 Meiosis is important for the formation of haploid gametes so that,
when two gametes fuse during fertilisation, a diploid zygote with the original
number of chromosomes is obtained.
ITQ4 (i) The 46 chromosomes of a human cell are made up of 23
homologous, or corresponding, pairs. One chromosome of each homologous
pair came from the father and one from the mother.
(ii) In the early stages of cell division each chromosome replicates to form
two identical copies of itself that are joined by a centromere. Each copy is
called a chromatid.
ITQ5 •
Crossing over – the exchange of genetic information between
chromosomes.
• Random alignment of the homologous pairs of chromosomes along the
equator before separation of the chromosomes.
• Random alignment of the chromosomes along the equator before separation
of the chromatids.
ITQ6 One advantage is that all the offspring have different characteristics,
so some may be able to survive a change in an environmental condition. The
propagation of the species is more likely to be ensured.
One disadvantage is that all the organisms may be
different from the parents and not as adapted to the
environment as the parents. All the offspring may die easily.
Examination-style questions
1
2
(i)
Explain these terms and state and importance of each:
(a) mitosis;
(b) meiosis.
(ii) List four differences between mitosis and meiosis.
(iii) Explain the following terms, giving an example of each:
(a) diploid number;
(b) chromosome number.
(iv) Explain the importance of crossing over which occurs during meiosis.
Explain the importance of meiosis in making evolution possible.
295
25
By the end of this
chapter, you should
be able to:
Heredity and
Genetics
understand the terms gene, allele, dominant, recessive, genotype and
phenotype
explain the meaning of the terms codominance, homozygous and heterozygous
use a genetic diagram to explain the inheritance of a single pair of characters
explain the inheritance of traits using sickle cell anaemia and albinism
predict the results of crosses involving one pair of alleles
understand the inheritance of sex in humans
understand crosses involving sex-linked characters
variation
continuous
discontinuous
genes on
chromosomes/DNA
dominant
phenotype
genotype
alleles
recessive
back cross
incomplete
dominance
co-dominance
inheritance
of characters
blood groups
pedigree charts
sickle cell anaemia
sex determination
genotype ❯
296
sex-linked characters
The Earth is home to billions of organisms, every one of which is unique.
Millions of species can be found on the land, and in the water and air of the
Earth’s surface. Different species may differ greatly from each other and may
be easy to distinguish. For example, birds differ greatly from fish. However, the
members of the same species may differ in only small ways.
These differences are the result of the genotype and the environment.
The genotype of organism is its genetic make-up. The environment is the
25 • Heredity and Genetics
surrounding of the organism. Identical twins have the same genetic make-up
but their environments are different (such as the food they eat, their activities,
relationships and experiences) and so subtle differences develop between them
(figure 25.1).
Genes
Figure 25.1 The differences between
identical twins are due to the environment,
as they have the same genes.
genes ❯
allele ❯
homozygous ❯
heterozygous ❯
OVTVSVNV\Z
Genetic information is passed on
JOYVTVZVTLZ
from parents to offspring in the
chromosomes. Chromosomes occur in
pairs in body cells. In a human body
cell, there are 23 pairs of chromosomes:
23 individual chromosomes are
paternal (from the father) and 23 are
maternal (from the mother). Pairs
are called homologous chromosomes
(figure 25.2).
Each chromosome is made up of
genes, or units of inheritance. These
control specific characteristics in the
OVTVSVNV\Z
organism. Each chromosome of a
JOYVTVZVTLZ
homologous pair carries the same set of
genes, therefore each body cell has two Figure 25.2 A diploid cell with four
chromosomes has two pairs of homologous
copies of each gene. However a gene
that are the same as each other, or two chromosomes.
alleles for a gene that are different. If
the alleles of a gene are the same, we say the organism is homozygous for
that gene or character. If the alleles are different, the organisms is said to be
heterozygous for that gene or character (figure 25.3).
gene for eye colour
gene for hair colour
gene for hair texture
Chromosomes exist in homologous pairs –
the genes are the same but the
form the gene can take may be
different. These are called alleles.
gene for shape of nose
blue
gene for size of lip
brown
red
brown
gene for length of finger
curly
straight
gene for length of arm
pointed
A chromosome is made up
of genes. This is a very simplified
diagram of a chromosome.
gene for
hair texture
thin
short
Figure 25.3
gene for
hair colour
pointed
full
long
gene for
eye colour
long
long
In the gene for hair colour, there are many alleles
for hair colour, producing many different hair colours.
In this case the two alleles are for red and brown.
The alleles present determine what the individual will
look like: the outward characteristics. Alleles exist for
every feature of every organism and each organism has
its own combination of alleles which make it unique.
Homologous chromosomes.
297
Continuity and Variation
Dominance
dominant allele ❯
recessive allele ❯
If the alleles are different, one may mask the expression of the other. The
one that is expressed (visible in the organism) is called the dominant allele,
and the one that is masked is the recessive allele. We use capital and lowercase letters to represent the different alleles. For example, in the gene for hair
colour, B represent the allele for black hair, and b represents the allele for red
hair. Black hair, B is dominant to red hair, b; and red hair, b, is recessive to
black hair, B. The dominant allele is expressed in the homozygous (BB) or
heterozygous (Bb) genotype, whereas the recessive allele is expressed only in
the homozygous (bb) genotype (figure 25.4).
OVTVSVNV\ZJOYVTVZVTLZ
NLULMVYOHPYJVSV\Y
HSSLSLMVYISHJROHPY
Z`TIVSBKVTPUHU[
HSSLSLMVYYLKOHPY
Z`TIVSbYLJLZZP]L
genotypePZ[OLNLUL[PJTHRL\W[OLHSSLSLZBHUKb
phenotypePZ[OLV\[^HYKJOHYHJ[LYPZ[PJISHJROHPY
B
B
bb
b
b
heterozygous genotypeOHZKPMMLYLU[HSSLSLZLNBb
B
b
homozygous genotypeOHZZHTLHSSLSLZLNBB
Figure 25.4 The allele for black hair, B, will mask the expression of the allele for red hair, b. The
heterozygous individual will have black hair.
genotype
phenotype ❯
The composition of genes, or genetic make-up, within the cells of an organism
is its genotype. The phenotype is the observable characteristics of the
organism. These observable characteristics are the result of the genotype and
the environment interacting (table 25.1).
genotype
phenotype
BB (homozygous)
black hair
Bb (heterozygous)
black hair: B is dominant to b
bb (homozygous)
red hair
Table 25.1 Phenotype is determined by the genotype.
ITQ1
Define the following: (i) chromosome
(ii) gene (iii) allele.
ITQ2
Define the following: (i) genotype (ii)
phenotype.
298
Genetic diagrams
A genetic diagram shows the cross between two genotypes. It shows the
phenotypes and genotypes of the parents and the possible genotypes and
phenotypes of the offspring. For example, a cross between a homozygous
dominant genotype (BB) and homozygous recessive genotype (bb) is shown in
figure 22.5.
25 • Heredity and Genetics
ISHJROHPY_YLKOHPY
BB
_
bb
WOLUV[`WLVMWHYLU[Z
NLUV[`WL
NHTL[LZ
B
b
ZLNYLNH[PVU
VMMZWYPUNNLUV[`WL
Bb
VMMZWYPUNWOLUV[`WL
ISHJROHPY
(SSVMMZWYPUNOL[LYVa`NV\Z^P[OISHJROHPY
7YVIHIPSP[`VMHYLKOHPYLKVMMZWYPUNPZ
9H[PVISHJROHPY!YLKOHPY
Figure 25.5 A genetic cross of a homozygous black-haired parent and a red-haired parent.
Note in the diagrams that the offspring from a single cross are called the F1
(which means first filial) generation. We then use the term ‘F2 generation’ to
refer to offspring of a cross between individuals of the F1 generation.
If both parents are heterozygous, the cross is as shown in figure 25.6.
ISHJROHPY_ISHJROHPY
Bb
Bb
WOLUV[`WLVMWHYLU[Z
NLUV[`WL
NHTL[LZ
B
b
B
b
BB
Bb
Bb
bb
ZLNYLNH[PVU
VMMZWYPUNNLUV[`WL
VMMZWYPUNWOLUV[`WL
ISHJR
ISHJR
ISHJR
YLK
OHPY
OHPY
OHPY
OHPY
OVTVa`NV\Z OL[LYVa`NV\Z OL[LYVa`NV\Z OVTVa`NV\Z
9H[PVVMYLKOHPY!ISHJROHPY
ITQ3
Albinism (absence of pigmentation)
in humans is caused by a recessive
gene which is transmitted in a normal
fashion. A phenotypically normal (nonalbino) couple have four children: the
first three are normal and the fourth is
albino.
(i) What can you say about the
genotype of the parents?
(ii) What is the possibility that their
next child will be albino?
(iii) One of the normal children
eventually marries a normal
woman. What predictions can be
made of their first child?
(iv) The albino child eventually marries
a normal woman. What predictions
can be made of their first child?
Where there are several possibilities,
state them all.
WYVIHIPSP[`VMHUVMMZWYPUNOH]PUNYLKOHPY
Figure 25.6 A genetic cross showing how two black-haired parents can have a red-haired child.
A cross can also be represented in another way. A cross between a
heterozygous parent and a homozygous recessive parent can be drawn as in
figure 25.7.
ISHJROHPY_YLKOHPY
Bb
bb
WOLUV[`WLVMWHYLU[Z
NLUV[`WL
NHTL[LZ
B
b
b
NHTL[LZ
b
b
B
Bb
ISHJROHPY
bb
YLKOHPY
Bb
ISHJROHPY
bb
YLKOHPY
b
b
9H[PVISHJROHPY!YLKOHPY
VMMZWYPUNHYLOL[LYVa`NV\Z^P[OISHJROHPY
VMMZWYPUNHYLOVTVa`NV\Z^P[OYLKOHPY
*OHUJLZVMHUVMMZWYPUNOH]PUNYLKOHPYPZ
Figure 25.7 A genetic cross using a table.
299
Continuity and Variation
Test cross or back cross
test cross ❯
ITQ4
A breed of dogs has long hair dominant
over short hair. A long-haired bitch
was first mated with a short-haired
dog and produced three long-haired
and three short-haired puppies. Her
second mating, with a long-haired dog,
produced a litter with all the puppies
long-haired. Use the symbol L to
represent the allele for long hair and l
to represent the allele for short hair.
(i) What was the genotype of the
long-haired bitch?
(ii) How could it be determined which
of the long-haired puppies of the
second mating were homozygous?
Homozygous dominant and heterozygous individuals have the same phenotype
– you cannot tell which is which just by looking at them. A test cross (back
cross) is used to determine the genotype of individuals which have the same
phenotype. In a test cross, the individual is crossed with a homozygous
recessive and the offspring examined, as shown in figure 25.8.
WOLUV[`WLVMPUKP]PK\HSISHJROHPY
WVZZPISLNLUV[`WLZBBHUKBb
0UKP]PK\HSPZJYVZZLK^P[ObbHUKVMMZWYPUNL_HTPULK
NHTL[LZ
b
b
NHTL[LZ
b
b
B
Bb
Bb
B
Bb
Bb
B
Bb
Bb
b
bb
bb
0MHSS[OLVMMZWYPUNHYLISHJROHPYLK
[OLU[OLNLUV[`WLPZBB
0M[OLYLHYLYLKOHPYLKPUKP]PK\HSZ
PU[OLVMMZWYPUN[OLU[OLNLUV[`WLPZBb
Figure 25.8 A test cross.
Incomplete dominance
incomplete dominance ❯
ITQ5
What offspring will you expect, and in
what proportions, if two pink-flowered
plants are crossed?
ITQ6
The figure shows the result from a
cross between a red-flowered plant
and a white-flowered plant, and what
happens when the offspring produced
are crossed with red-flowered plants.
Flower colour in some plants, such as Impatiens, shows incomplete
dominance of the alleles. This means that there is a blending or combination
of expression of both alleles in the heterozygous condition. If allele CR
produces red flowers (genotype CRR) and the allele CW produces white flowers
(genotype CWW), then the genotype CRW produces pink flowers (figure 25.9). A
blending of red and white will produce pink.
WOLUV[`WLVMWHYLU[Z
NLUV[`WL
CRR
NHTL[LZ
R
_
(
YLK
^OP[L
_
W
)
_
*
VMMZWYPUNNLUV[`WL
HSSCRW
+
VMMZWYPUNWOLUV[`WL
,
-
(i)
Using the alleles CR and CW,
give the genotypes of the plants
labelled A–F.
(ii) If plant D had been white, what
would the result of the cross
between C and D have been?
300
CWW
Figure 25.9
Incomplete dominance as seen in Impatiens.
HSSWPUR
25 • Heredity and Genetics
Co-dominance
co-dominance ❯
In co-dominance, there is expression of both alleles in the heterozygous
genotype. In this case, the genotype CRW produces flowers that have patches of
red and white colour. There is no blending, each allele is expressed as shown in
figure 25.10.
WOLUV[`WLVMWHYLU[Z
YLK
NLUV[`WL
CRR
NHTL[LZ
R
^OP[L
CWW
_
W
HSS CRW
VMMZWYPUNNLUV[`WL
VMMZWYPUNWOLUV[`WL
Figure 25.10
Genotype
Phenotype
Co-dominance in Impatiens.
Another example of co-dominance is found in ABO blood groups in
humans. Your blood group is controlled by three different alleles: IA, IB and
IO. IA and IB are equally or co-dominant to each other and both are dominant
to IO. Only two alleles can be present in any cell, one on each homologous
chromosome that carries the gene for blood group. This gives four possible
phenotypes for blood group (table 25.2).
AA
blood group A
IAO
blood group A
IBB
blood group B
IBO
blood group B
IAB
blood group AB
Worked example
IOO
blood group O
1
I
Table 25.2 Genotypes and phenotypes of
blood group in humans.
ITQ7
What offspring will you expect, and in
what proportion, if two F1 generation
plants from figure 25.10 were crossed?
ITQ8
A baby has blood type B, his mother
had blood type A. His paternal
grandfather has blood type A and his
paternal grandmother has blood type B.
Determine (i) the genotype of the baby,
and (ii) the possible genotypes of the
baby’s father.
HSSYLKHUK^OP[L
What are the possible blood groups of children whose parents are blood
group A (heterozygous) and B (homozygous)?
The heterozygous genotype for blood group A is IAO.
The homozygous genotype for blood group B is IBB.
NLUV[`WL
VMWHYLU[Z
IAO
IBB
_
NHTL[LZ
A
O
B
B
VMMZWYPUN
NLUV[`WLZ
IAB
IAB
IBO
IBO
Possible blood groups of children are AB and B.
301
Continuity and Variation
Worked example
2
What if both parents had heterozygous genotypes?
The heterozygous genotype for blood group A is IAO.
The heterozygous genotype for blood group B is IBO.
IAO
genotype of parents
gametes
IAO
x
A
O
A
O
gametes
A
O
B
IAB
IBO
O
IAO
IOO
Possible blood groups of children are A, B, AB and O.
Examples of genetic effects
Sex determination
Practical activity
SBA 25.1: How the sex of an offspring is
determined, page 365
Of the 23 pairs of chromosomes in any human cell, one pair determines the
sex of the organism. There are two sex chromosomes, X and Y. The genotype
XX is female and the genotype XY is male in humans (figure 25.11).
??
O\THUJLSS
(SS[OLJLSSZVMHMLTHSLOH]L
[^V?ZOHWLKJOYVTVZVTLZ
U\JSL\ZOHZWHPYZVM
OVTVSVNV\ZJOYVTVZVTLZ
??
?@
VULWHPYKL[LYTPULZ[OL
ZL_VM[OLPUKP]PK\HS
Figure 25.11
(SS[OLJLSSZVMHTHSLOH]LVUL?ZOHWLK
JOYVTVZVTLHUKVUL@ZOHWLKJOYVTVZVTL
;OL@JOYVTVZVTLPZHU?JOYVTVZVTL
^P[OHTPZZPUNWPLJL
How sex is determined in humans.
Figure 25.12 shows the inheritance of sex in humans. Each time a couple has a
child, there is a 50% possibility it could be a boy and a 50% possibility it could
be a girl.
phenotype of parents
male
genotype
XY
x
female
XX
gametes
X
Y
X
X
offspring genotype
XX
XX
XY
XY
offspring phenotype
female
female
male
male
Ratio 1 male : 1 female
Figure 25.12
302
How sex is inherited in humans.
25 • Heredity and Genetics
Sex-linked characteristics
The sex chromosomes also carry genes other than those which determine sex.
The characteristic of those genes are said to be sex-linked, and they are carried
on the X chromosome.
Haemophilia or bleeder’s disease
sex-linked characteristics
haemophilia ❯
carrier ❯
Sex-linked characteristics include haemophilia and colour-blindness.
Table 25.3 shows sex-linkage in haemophilia.
Genotype
Phenotype
XHXH
female, normal clotting of blood
XHXh
female, normal clotting of blood; she is a carrier since she carries the recessive
allele but it is not expressed.
XhXh
female, a haemophiliac
XHY
male, normal clotting of blood
Table 25.3 The genotypes and phenotypes in haemophilia.
The dominant allele, H, causes blood to clot normally. The recessive allele, h,
causes haemophilia, a condition in which blood does not clot and any small
cut will bleed for a long time. The inheritance of haemophilia is shown in
figure 25.13.
ITQ9
(i) What is mean by the term ‘sex
linkage’?
(ii) A normal man marred a normal
woman and all the female
offspring were normal, but half of
the male offspring were colourblind and the other half were
normal. How do you account for
this?
Worked example
3
A carrier female marred a normal male. What is the possibility of their
having a haemophiliac child?
_
JHYYPLYMLTHSL
UVYTHSTHSL
XH Xh
XH Y
XH
Xh
XH
Y
XH XH
XH Y
XH Xh
Xh Y
UVYTHSMLTHSL
UVYTHSTHSL
UVYTHSMLTHSL
JHYYPLY
OHLTVWOPSPHJ
THSL
Figure 25.13
Haemophilia inheritance.
The mother transfers the haemophiliac gene to her son. There is a 25%
possibility of having a haemophiliac son.
Other genetic disorders
Sickle cell anaemia
sickle cell anaemia ❯
CHAPTER 26
In sickle cell anaemia (chapter 26), the red blood cell can take a sickle shape
instead of the normal biconcave shape. Allele HbN produces normal red blood
cells and the allele HbS produces sickle-shaped red blood cells; the possible
genotypes and phenotypes are shown in table 25.4 (overleaf). The inheritance
of sickle cell anaemia is shown in figure 25.14 (overleaf).
303
Continuity and Variation
Genotype
Phenotype
HbNN
all red blood cells are normal, the person is normal
HbSS
all red blood cells take the sickle shape, the person has sickle cell anaemia
HbNS
30–40% of the red blood cells are sickle shaped, the person has sickle cell trait
Table 25.4
Genotypes and phenotypes in sickle cell anaemia.
Worked example
4
If two people with the sickle cell trait were to marry, what are the
possible genotypes and phenotypes of their offspring?
WHYLU[HSWOLUV[`WLZ
[YHP[
WHYLU[HSNLUV[`WLZ
HbNS
_
[YHP[
HbNS
NHTL[LZ
N
S
N
S
VMMZWYPUNNLUV[`WL
HbNN
normal
HbNS
HbNS
HbSS
trait
trait
ZPJRSLJLSS
HUHLTPH
VMMZWYPUNWOV[`WL
Figure 25.14
Sickle cell anaemia inheritance.
The possibility of having a child who suffers sickle cell anaemia is 25%.
The possibility of having a normal child is 25%.
Ratio is 1 normal : 2 trait : 1 anaemia.
Pedigree charts
A pedigree chart shows the occurrence of a particular characteristic in a family
tree (figure 25.15). The chart can be used to show the possible genotypes of
individuals in the chart, which can be important in genetic counselling.
304
25 • Heredity and Genetics
female with black hair
1
2
male with black hair
female with red hair
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
male with red hair
The allele for black hair B is dominant to the allele for red hair b.
What are the genotypes of all the individuals?
The males and females with red hair would have to be bb.
bb
1
bb
3
B?
4
B?
2
B?
6
Bb
4
bb
B?
5
To have a bb offspring, individual 2 would have to be Bb.
All their black-haired children are Bb.
B?
9
Bb
2
bb
1
bb
3
bb
7
Bb
6
bb
7
bb
bb
8
Bb
9
bb Bb
bb
To have a bb offspring, individual 5 would have to be Bb.
bb
Figure 25.15 A pedigree chart showing the inheritance of hair colour in members of a family.
ITQ10
The family tree below shows how coat colour in mice is passed on from generation
to generation.
both parents are homozygous for coat colour
P
F1
1
2
3
4
F2
5
brown coat colour
white coat colour
305
Continuity and Variation
(i) Explain the term ‘homozygous’.
(ii) What do the symbols P, F1 and F2 stand for?
(iii) Which generation of mice is heterozygous for coat colour?
(iv) What is the percentage of brown and white mice in the F2 generation?
(v) Which allele for coat colour is recessive?
(vi) What might be the percentage of white coat mice if breeding pairs were set up
between:
(a) 1 and 4
(b) 1 and 5?
Chapter summary
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Each chromosome is made up of genes, or units of inheritance.
An allele is the form of gene taken.
The genetic make-up of an organism is its genotype.
The observable characteristics of an organism make up its phenotype.
If the alleles of a gene are the same on the homologous chromosomes, the genotype
is described as being homozygous.
If the alleles of the gene are different, the genotype is described as being
heterozygous.
In a heterozygous individual, the allele which is expressed in the phenotype is
described as being dominant.
Recessive alleles are only expressed in the phenotype if present in the homozygous
form. Their effect is masked by the presence of a dominant allele.
A test cross is used to determine the genotype of an individual.
A genetic diagram shows the cross between two organisms for a characteristic.
Incomplete dominance occurs when no one allele is completely dominant over the
other. As a result, the expressions of the alleles blend.
Equally dominant alleles are described as being codominant. Both alleles are
expressed in the phenotype.
The sex of an organism is determined by the sex chromosomes. In humans XX codes
for female and XY codes for male.
Characteristics carried on the X chromosome are said to be sex-linked. Examples are
haemophilia and colour-blindness in humans.
A pedigree chart shows the occurrence of particular characteristics in a family tree.
Answers to ITQs
ITQ1 (i) Chromosome – In humans there are 46 chromosomes inside the
nucleus of each cell. Each chromosome is a separate structure which came
from a parent and is made up of a stand of DNA or deoxyribonucleic acid. Each
chromosome has its homologous partner which came from the other parent.
(ii) A gene is a small part of a chromosome. It has the code to make a specific
protein which may lead to a specific physical characteristic.
(iii) An allele is the form a gene can take. It is the actual code of the gene.
ITQ2 (i) The genotype of an organism is the total combination of all the
alleles that make up that organism.
(ii) The phenotype describes the specific physical characteristics that can be
seen and result from the genotype and the effect of the environment.
ITQ3 Using N for the dominant allele of normal skin colouring, and n for
the recessive allele of albino colouring. Parents are normal; children are 3
normal : 1 albino.
306
25 • Heredity and Genetics
(i) The albino child’s genotype is nn. The parents’ genotypes can only be Nn
and Nn (heterozygous) since they are both normal and the albino child must
get a n allele from each parent.
N
n
N
NN
Nn
N
Nn
nn
(ii) The probability of the next child, or any child of these parents, being
albino is 1 in 4 or 25%. Whenever this couple have a child, the probability of
having an albino child will be 25%. It would be quite possible for them to have
4 albino children and no normal children.
(iii) The normal child can have either of two genotypes, Nn or NN. The
normal woman that he marries could also have either of the same two
genotypes. There are therefore three possible crosses:
• NN = NN offspring all normal
• NN = Nn offspring all normal
• Nn = Nn
offspring 3 normal : 1 albino
First child has a 75% to 100% of being normal.
(iv) The albino child (nn) marries a normal woman (Nn or NN). Two possible
crosses are:
offspring are all normal
• nn = NN
• nn = Nn
offspring 1 normal : 1 albino
Their first child has a 50% to 100% chance of being normal.
ITQ4 (i) To produce short-haired puppies, the long-haired bitch must have
the genotype Ll. The short-haired puppies, being ll, got one of the l alleles from
their mother.
(ii) The long-haired puppies from the second mating can have two possible
genotypes, Ll (heterozygous) or ll (homozygous). To determine which is
homozygous, the breeder must do a test cross, that is cross each puppy (when
mature) with a short-haired dog and examine the offspring of each mating. If
there is at least one short-haired puppy in the litter, then the genotype is Ll. If
all the offspring are long-haired, then the genotype is LL.
ITQ5 Crossing two pink-flowered plants (CRW) would give the following
result: offspring 1 CRR (red) : 2 CRW (pink) : 1 CWW (white)
CR
CW
CR
CRR
CRW
CW
CRW
CWW
ITQ6 (i) A: CRR, B: CWW, C: CRW, D: CRR, E: CRW, F: CRR
(ii) Genotypes of offspring: CRW and CWW. Phenotypes of offspring: pink
and white.
ITQ7 The F1 generation have the genotype CRW. If two of these are crossed,
the offspring would be produced as follows:
CR
CW
CR
CRR
CRW
CW
CRW
CWW
This would give a proportion of phenotypes of 1 red (CRR) : 2 red and white
(CRW) : 1 white (CWW).
307
Continuity and Variation
ITQ8
grandparents
parents
A
B
mother
A
father
baby B
(i) Mother has blood group A, possible genotypes are IAA or IAO. The baby
has blood group B, possible genotypes are IBB or IBO. The baby must get one
allele from the mother, but has no IA allele. So the mother’s genotype must be
IAO and the baby’s genotype must be IBO.
(ii) The father must have given the baby the other allele, IB. The father’s
mother had blood group B and so had genotype IBB or IBO, and the father’s
father had blood group A and so had genotype IAA or IAO. The father got the IB
allele from his mother and could have inherited either the IA or the IO allele
from his father. The possible genotypes for the baby’s father are IAB or IBO.
ITQ9 (i) Characteristics that are carried on the X sex chromosome are
described as being sex-linked.
_
UVYTHS THU
UVYTHS MLTHSL
N
X N Xn
X Y
WHYLU[Z
NHTL[LZ
XN
Y
XN
Xn
VMMZWYPUN
XN XN
XN Xn
XN Y
Xn Y
UVYTHS
MLTHSL
UVYTHS
MLTHSL
UVYTHS
THSL
JVSV\YISPUK
THSL
MLTHSLVMMZWYPUN
HSSUVYTHS
THSLVMMZWYPUN¶
HIV\[OHSMUVYTHSHUK[OL
V[OLYOHSMJVSV\YISPUK
(ii) The gene for colour-blindness is recessive and sex-linked. The normal
female parent is heterozygous XNXn and passes the recessive allele to some of
her sons.
ITQ10 (i) Homozygous describes the genotype with two similar alleles, such
as BB or bb.
(ii) P stands for parents (genotypes and phenotypes). F1 is the first (filial)
generation, and F2 is the second (filial) generation.
(iii) The F1 generation.
(iv) 75% are brown and 25% are white.
(v) The allele for white coat colour is recessive.
(vi) (a) Probability of cross Bb × Bb producing white mice is 25%.
(b) Probability of cross Bb × bb producing white mice is 50%.
308
25 • Heredity and Genetics
Examination-style questions
1
2
3
4
(i)
Define the following:
(a) homozygous;
(b) heterozygous;
(c) dominant;
(d) recessive;
(e) allele.
(ii) Distinguish between genotype and phenotype.
(iii) Explain how mutation contributes to variation.
(iv) Cystic fibrosis is an inherited condition caused by a single recessive allele. The normal
gene is dominant and masks the recessive allele. What is the probability of two
healthy people being parents to a child born with cystic fibrosis? Show all the working
and use a genetic diagram.
(v) (a) Explain what is meant when a person is said to be a ‘carrier’ for cystic fibrosis.
(b) What is the probability of two healthy people (where one is a ‘carrier’) being
parents to a child born with cystic fibrosis?
(vi) Why are most lethal genes (genes which cause mortality) recessive?
(i) Distinguish between continuous and discontinuous variation.
(ii) List all the possible genotypes of a person belonging to blood group B.
(iii) A woman with blood group A has a child who is also of blood group A. What are all the
possible genotypes of the father? Explain fully, using genetic diagrams.
(i) Explain, using genetic diagrams, how sex is determined in humans.
(ii) Relating to the probability of having a boy or girl in part (i), suggest why:
(a) at birth there are about 106 boy babies for every 100 girl babies;
(b) at puberty the proportions of males and females are about equal;
(c) In old age, females outnumber males.
(iv) Define sex linkage and describe how haemophilia is inherited.
(v) What is the probability of a haemophilic father and a mother carrying the allele for
haemophilia having a haemophilic daughter?
(i) In a test cross, the genotype of an organism showing the dominant characteristic
(which can be homozygous or heterozygous) is determined. Using the symbols T for
tall plant and t for short plant, show the results of a test cross on a tall plant which
turned out to be homozygous for height. Draw a genetic diagram to illustrate your
answer.
(ii) Construct a pedigree chart of the following information.
Female
Male
parents
brown hair
red hair
children
one – red hair
two – both brown hair
grandchildren (from daughter who
married brown-haired man)
one – red hair
one – brown hair
(iii) Explain the following using an example for each:
(a) co-dominance;
(b) incomplete dominance.
309
26
By the end of this
chapter. you should
be able to:
Variation and
Evolution
distinguish between genetic and environmental variation
understand why genetic variation is important
distinguish between continuous and discontinuous variation
define a species
describe how new species are formed
understand the process of natural selection in evolution
distinguish between natural and artificial selection
understand the causes and effects of mutation
understand what is meant by genetic engineering and how it can be used
discuss the advantages and disadvantages of genetic engineeering
discontinuous variation
genotype
environment
continuous variation
phenotype
variation in phenotype
selection
selective
breeding
genetic
engineering
natural
selection
mutation
sickle cell
anaemia
Down’s
syndrome
selection
pressure
evolution
species
Genetic variation
genetic variation ❯
Each organism is unique. This uniqueness is a result of genetic differences
and influences of the environment, and is expressed in the phenotype. Each
organism is born with its own genetic make-up inherited from its parents.
The genetic make-up of every organism is different except for clones. Genetic
variation is inherited and the differences may be small or large (figure 26.1).
ITQ1
What is genetic variation?
310
26 • Variation and Evolution
Figure 26.1 Variation is seen among and between species.
Variation due to the environment
environmental variation ❯
The environment also plays a very important role in determining the
phenotype of an organism. The variation seen because of the influence
of the environment is not inherited but occurs because of differences in
the surroundings of the organism. For example, ten genetically identical
plants grown from cuttings taken from a single parent should be identical in
appearance since the genes for all the characteristics are identical. Suppose
these plants are divided into two groups of five, and each group is grown in
two very different environments:
• good soil, watered regularly;
• poor soil, not watered,
After a while, the phenotypes of the two groups will vary greatly. Variation in
appearance will be seen between plants that have the same genetic make-up
and therefore should look the same (figure 26.2).
cutting grown in rich
soil, watered regularly
cutting from the same plant
grown in poor soil, watered little
genetically identical plants in
different environments (soil and water)
ITQ2
(i) What is the phenotype of an
organism?
(ii) How is the phenotype determined?
ITQ3
List three differences which may be
seen in the phenotypes of identical
twins even though they have identical
genotypes.
Figure 26.2 The environment plays a very important role in determining the phenotype of an
organism. Genetically identical plans are very different if grown in different environments.
Genetically identical twins, as they grow and develop, acquire subtle
differences. These differences occur because their environments are different,
even if they live in the same house. They eat different foods at different times
and in different amounts. Their daily activities and interests are different, and
their interactions with people, even their parents, are different. The differences
in their ‘environments’ may be subtle, but enough to produce differences in
their physical appearance. Identical twins also have different fingerprints.
311
Continuity and Variation
Importance of genetic variation
Genetic variation among a species ensures survival of that species if the
environment changed drastically. This can be seen in the following example.
A population of wolves living the wild vary with respect to body hair
length. A few have very long hair (5–6 cm) and a few have very short body
hair (1–2 cm). Most have medium hair length (3–4 cm), which is well suited to
the temperature of the environment (figure 26.3).
Practical activity
most individuals in the population of
wolves will have body hair length 3–4 cm
Number of individuals in
the population of wolves
SBA 26.1: Continuous variation,
page 366
1
Figure 26.3
ITQ4
What do you think would happen
to the population of wolves shown
in figure 26.3 if the environmental
temperature got warmer?
2
3
4
5
6
Body hair length (cm)
7
Graph showing how body hair length varies in a population of wolves.
Suppose the temperature changed drastically, say it got much colder, then
the wolves with short hair would be more likely to die, but those with long
hair would be more likely to live. Because of the variation which existed
naturally, the wolves with very long body hair would be able to survive the
cold temperatures better than those with short body hair. Those with long
body hair, therefore, would be more likely to reproduce. Survival of the species
is thus ensured because of natural variation. Figure 26.4 shows what would
happen to wolf body hair length in such circumstances.
most individuals now have body
hair length 5–6 cm to survive
the colder environment
Number of individuals in
the population
of wolves
Figure 26.4 Graph showing a change in
the occurrence of body hair length as the
environment changed.
continuous variation ❯
discontinuous variation ❯
8
1
2
3
4
5
6
Body hair length (cm)
7
8
Variation is thus the result of the genetic make-up and the influence of the
environment. There are two types of variation:
• continuous variation;
• discontinuous variation.
In continuous variation, the differences are slight and merge or grade into each
other to produce a smooth bell-shaped curve (Figure 26.5): for example, height
in humans, human foot length, human skin colour, leaf size and pod size in
legumes, and body hair length in wolves.
Number of 15-yr
olds in class
Figure 26.5 The height of a class of
15-year-olds shows contiuous variation.
312
most individuals in the population
are between 160 and 170 cm
145
150
155 160 165 170 175
Height of individuals in cm
180
185
26 • Variation and Evolution
In discontinuous variation, the differences are separate and clear cut; they do
not merge or grade into each other (figure 26.6). Examples are tongue-rolling
in humans, blood groups in humans and horns in cattle.
Number of
individuals
Figure 26.6 Graph showing discontinuous
variation in human blood groups.
A
B
AB
Blood groups
O
DNA testing and forensic science
Figure 26.7 Every individual has a unique
DNA pattern.
DNA testing or ‘genetic fingerprinting’ is a technique pioneered by Dr. Alec
Jeffreys. He found short DNA sequences from the non-coding part of the
DNA that were repeated several times and were unique to each individual.
Dr. Jeffreys developed a genetic probe to look for these sequences and was
able to use electrophoresis and autoradiography to produce a DNA image
(figure 26.7).
Each dark band in the autoradiograph shows as area where the DNA probe
attached to a similar sequence in the subject’s genome. Each person’s genetic
fingerprint is unique – this means that each individual can be identified by
their DNA, perhaps from a strand of hair or a scrape of skin. This application
can be used in paternity and maternity tests and in as forensic evidence in rape
and murder trials.
Natural selection
natural selection ❯
Practical activity
SBA 26.2: Natural selection, page 367
Charles Darwin, an English naturalist, first spoke about natural selection. He
observed organisms that lived on the Galapagos Islands in the Pacific Ocean.
From his observations, Darwin concluded that within a population, although
many offspring are produced, many individuals do not survive becuase they:
• compete for limited food and resources;
• try to avoid predators;
• struggle to avoid disease;
• try to tolerate changes in the environment.
There is a constant struggle for existence, and those individuals that are best
adapted to their environment have an advantage. That is, they are more likely
to survive and produce offspring. Their offspring will inherit the advantageous
characteristics and the population will
remain well adapted to its habitat. Selection
by the environment is known as natural
selection. It favours those that have the best
adaptations for the environment in which
they live (figure 26.8). These organisms
5H[\YLºZLSLJ[LK»IPYKZ^P[OZ[YVUNILHRZ
Birds with pointed beaks were able
are said to have a selective advantage.
;OL`^LYLHISL[VJYHJR[OLOHYKZOLSS
[VMLLKVUÅ`PUNPUZLJ[Z5H[\YLHSZV
Sometimes we describe these individuals
VMU\[ZHUKMLLKVU[OLU\[Z
ºZLSLJ[LK»[OLZLIPYKZILJH\ZLVM
as being the fittest for the environment.
;OL`[O\ZVI[HPULKMVVKHUKSP]LK[V
[OLZOHWLVM[OLPYILHRZ
YLWYVK\JL"WYVK\JLVMMZWYPUN^P[OZ[YVUNILHRZ
Because of this, natural selection has
become known as ‘survival of the fittest’.
Figure 26.8 Some individuals are better adapted to the environment.
selective advantage ❯
313
Continuity and Variation
Natural selection and evolution
Natural section provides the mechanism for one species to change into
another. The change is very slow and is called evolution as one species evolves
into another.
Long necks of giraffes
Figure 26.9 Long necks are an advantage
when feeding from tall trees.
antibiotic resistance ❯
insecticide resistance ❯
ITQ5
How do bacteria evolve to acquire
resistance to antibiotics?
The long neck of the giraffe is thought to have evolved when food was in
short supply and only the tallest individuals could reach enough food to
survive. The ‘tallness’ genes were passed on to the next generation so they
were, on average, taller than their parent generation. As selection for long
necks continued, the giraffes which produced most offspring were the tallest
individuals. After many generations of selection, the long-necked species of
giraffe have evolved (figure 26.9).
Antibiotic resistance in bacteria
Antibiotic resistance of bacteria is a serious problem. When antibiotics are
used on a population of bacteria, any bacteria with genes to resist the drug
will survive and most of the rest will be killed. The resistant bacteria then
multiply, producing populations of antibiotic-resistant bacteria. Because of the
widespread use of antibiotics like penicillin, many kinds of bacteria are now
resistant to these drugs.
Insects that are resistant to insecticides have evolved in much the same
way as antibiotic-resistant bacteria, due to the widespread use of insecticides
(figure 26.10). For example, many populations of mosquitoes are now resistant
to DDT which was widely used last century to attempt to control them and the
spread of malaria.
R
R
R
R
original
gene pool
R
R
R
selection
pressure
R
R
R
All the genes present in this population of mosquitoes
make up the gene pool. The resistance gene R is also
present. There are some individuals with resistance to
insecticide.
The application of insecticide to the population of mosquitoes
puts a pressure on the gene pool (selection pressure).
Those individuals with the resistance gene are seen to be
Ä[[LYPLHISL[VZ\Y]P]L4VZ[VM[OLV[OLYZKPL:VTLTH`
Z\Y]P]LILJH\ZL[OL`H]VPKLK[OLPUZLJ[PJPKL
;OLTVZX\P[VLZ^P[O[OLYLZPZ[HUJLNLULSP]L[VYLWYVK\JL
passing the resistance gene to their offspring.
R
R
R R R
R R
R R
Figure 26.10
R
R
R
R
gene pool
after
the use of
insecticide
4VZ[PUKP]PK\HSZHYLUV^YLZPZ[HU[[V[OLPUZLJ[PJPKL;OL
population is said to have developed resistance to the
insecticide. A stronger dose or a new insecticide must
now be used.
New populations evolve under pressure.
Dark form of the peppered moth
camouflage ❯
314
The peppered moth (Biston betularia) is found in many parts of England
(figure 26.11). It was originally found as a pale form, well concealed by
camouflage on the lichen-covered trees on which they rested. Predators
found the moth difficult to spot. Then in the early 19th to mid-20th century,
pollution in industrial areas, blackened the trunks of the trees with soot. The
26 • Variation and Evolution
pale moths were now easily seen by predators, but the rare black form was
better camouflaged. So the frequency (numbers) of the dark form increased as
they were better suited to the environment. In the unpolluted areas, the pale
form still predominated. Today, as pollution from industry gets less, the pale
form is again becoming more common than the dark form.
(a)
(b)
dark form
light form
light form
predominates
dark form
predominates
selection pressure
(industrialisation)
Figure 26.11
(a) Pale and dark forms of Biston betularia moth. (b) Populations change with a change in the environment.
Geographical isolation and speciation
ITQ6
Describe briefly the following terms:
(i) natural selection (ii) selective
advantage (iii) survival of the fittest
(iv) evolution.
A population that is geographically isolated from another may experience
different environmental conditions and so evolve differently due to natural
selection. Over time, the isolated population would become more and
more different from the original population to fill a new and different
ecological niche.
During his visit to the Galapagos Islands, Darwin observed several different
species of finch (now called Darwin’s finches). This group of islands is in the
Pacific Ocean, about 600 miles from the South American mainland. Darwin
concluded that the islands must have been colonised by a few individuals
from a species of finch found on the mainland. These individuals then evolved
independently to fill the different ecological niches on each island. Today 14
different species of finch are found on the Galapagos Islands, differing greatly
in size and other features, including beak size and shape (figure 26.12).
^HYISLYMPUJO
ILHRSVUNHUK[OPU
MLLKZVUPUZLJ[Z
]LNL[HYPHU[YLLMPUJO
MLLKZVUI\KZ
SLH]LZHUKMY\P[
.HSHWHNVZ0ZSHUKZ
Figure 26.12 Four
of the 14 species of
finches that Darwin
observed on the
Galapagos Islands.
WYVIHISLJVTTVU
HUJLZ[VYSPRLS`[VOH]L
ILLUHZLLKHUK
PUZLJ[MLLKLY
^VVKWLJRLYMPUJO
VM[LUOVSKZHZTHSS
[^PNHUK\ZLZP[
HZH[VVS
JHJ[\ZMPUJO
ILHRSVUNHUK
ZSPNO[S`J\Y]LK
4HPUSHUK
315
Continuity and Variation
The islands of the Caribbean are a group of volcanic islands in the Atlantic
Ocean. This arc of islands sweeps north and west from the South American
mainland and is called the West Indies. It has been suggested that the Anolis
lizards of the West Indies have evolved in much the same way as Darwin’s
finches. A few individuals may have drifted (for example on a log) along a
water current from South America and reached the banks of Grenada (the
most likely arrival point for a rafting colonist). The original species may still
exist in Guyana, Venezuela or north-western Brazil.
The colonisation of other close islands followed, such as the islands of the
southern Lesser Antilles, including St Vincent, St Lucia, Martinique, Barbados,
and La Blanquilla and Bonaire far to the west of the main chain. Each
population would have been subjected to different environmental conditions.
The vegetation, insect population, air temperature and weather patterns all
differ from island to island. By natural selection, each population would have
evolved independently adapting to each new ecological niche (figure 26.13).
Over time, the populations would have become different from each other,
evolving into nine different species (table 26.1 and figure 26.14).
Locality
Species
Martinique
A. roquet
Barbados
A. extremis
Grenada
A. aeneus A. richardii
St Vincent
A. trinitatus A. griseus
Martinique
St Lucia
St Vincent
St Lucia
A. luciae
La Blanquilla
A. blanqillenus
Bonaire
A. bonairensis
Barbados
La Blanquilla
Grenada
Bonaire
Table 26.1 Species of Anolis lizards in the
Lesser Antilles.
Naturally occurring
current which
brought the Anolis
lizard to Grenada
VENEZUELA
S o u t h
A m e r i c a n
M a i n l a n d
SURINAM
G U YA N A
Figure 26.13 Possible colonisation sequence of the Anolis lizards of the southern part of the
Lesser Antilles.
(a)
(b)
(c)
Figure 26.14 Anolis lizards of the Lesser Antilles. (a) Anolis roquet, (b) Anolis trinitatis, (c) Anolis nitens tandae – a rare species found in Peru.
It is also suggested that two independent landings on St. Vincent separated
by sufficient time could have resulted in a second colonisation. Two
reproductively isolated species are found there, the giant Anolis griseus and the
smaller A. trinitatus.
316
26 • Variation and Evolution
Ecological speciation and
behavioural speciation
predators
present
no predator
region
A
region
B
fish here do not
breed with fish
from region B
fish here do not
interbreed with fish
from region A
Figure 26.15
some interbreeding
may occur between
fish from region A and
fish from region B
Ecological speciation.
backyard feeding in
the UK means food
is readily available for
birds in winter
UK
GERMANY
birds spend
summer here
Changes in wing
shape, beak size, etc.
develop in birds that
choose different areas
to overwinter. Eventually,
they will not interbreed.
Ecological speciation is the evolution
of barriers to gene flow resulting
from ecologically based divergent
selection. For example, there are
two populations of the Bahamas
mosquitofish (Gambusia hubbsi),
one has larger and more powerful
caudal (tail) region than the other.
The fish with the larger tail regions
are in environments where there are
predatory fish that will feed on the
mosquitofish. The mosquito fish with
the smaller tail regions live in areas
without predators. Modern research
suggests that the more powerful tail
regions are more powerful swimmers
than the mosquito fish with the
smaller tail regions. The bigger fish can
therefore escape from their predators
more easily. Speciation is resulting
because each fish chooses the same
type to mate with (figure 26.15).
Behavioral speciation is seen when
species engage in distinct courtship
and mating rituals. For example, the
birds called blackcaps from Germany.
These birds generally fly to Spain
or north Africa to overwinter but
some have adapted to ‘backyard bird
feeding’ areas in the UK where there
is a ready supply of food waiting
for them. The change in migration
behaviour has led to a shift in mate
availability and populations are
becoming increasingly reproductively
isolated as they choose birds from
the same population to mate with
(figure 26.16).
Artificial selection
SPAIN
normal winter
feeding grounds
Figure 26.16
Behavioural speciation.
artificial selection ❯
Artificial selection is the process by
which plants and animals used by
humans in agriculture, horticulture,
transport, companionship and
leisure have been obtained from wild
organisms. In natural selection, nature
selects the fittest individuals but, in
artificial selection, humans select
individuals with characteristics they
see as useful. Only those individuals
selected by humans are allowed to
produce offspring.
317
Continuity and Variation
selective breeding ❯
Due to the constant removal of those with unwanted features and breeding
of only chosen individuals, the genetic composition of the population changes.
This is called selective breeding and continues today by a combination of
inbreeding (between closely related individuals) and outbreeding (between
genetically distinct individuals).
The aim of artificial selection is to produce animals and plants with
characteristics humans find desirable. These include:
• high yield;
• improved quality;
• reduced production costs;
• faster growth rates;
• greater resistance to disease.
Drugs like growth hormones and steroids are sometimes used to enhance or
quicken growth and development in animals that are used for food, such as
poultry. These can have negative effects on humans and increase the risk of
populations of antibiotic-resistant bacteria evolving.
A comparison of natural and artificial selection is made in figure 26.17.
ARTIFICIAL SELECTION
NATURAL SELECTION
gene pool of
a population
selection pressure
selection pressure
The environment may
change, e.g. get hotter, colder,
drier, etc. Those that can
survive in the ‘new’
environment live to reproduce,
passing on those genes.
Humans allow some individuals
to live to reproduce, passing
on those genes advantageous,
e.g. greater yield, speed (horses),
size, etc.
New population
New population
• able to survive in the wild
• is a natural part of the environment
• may not be advantageous to humans
• retains most genes
• may not be able to survive in the wild
• is advantageous to humans
• may have lost other advantageous
genes
Figure 26.17
Similarities and differences between natural and artificial selection.
Examples of artificial selection in the Caribbean
Very productive (meat and milk) breeds of cattle have been developed in
temperate countries, such as the UK and the US. Beef cattle, like Hereford and
Angus, and dairy cattle, like Friesian and Jersey, do not thrive well in tropical
conditions because:
• they suffer from heat stress;
• tropical grasses are generally less nutritious than temperate species;
• diseases like tick fever, foot rot and mastitis are serious problems that these
breeds suffer from.
318
26 • Variation and Evolution
Thus, cattle farmers in the Caribbean have developed new breeds.
• In Jamaica, cross-breeding Indian and European breeds with local Creole
cattle has led to beef and dairy herds such as the Jamaica Red Poll and
the Jamaica Hope. These can cope with heat stress and poor pasture, and
are disease-resistant, while producing much more milk and meat than
traditional Caribbean breeds.
• In Trinidad, a new breed, the Buffalypso, has been selectively bred from the
water buffalo brought from India in 1903 to pull cars and help in ploughing.
The mature animal produces high grade meat, which is marketed as beef.
The calves are sold for breeding to various countries, including Guyana,
Cuba and other Latin American countries, and the US.
Captive breeding
Plants and animals are kept by scientists in order to introduce particular genes
into the population. The genome is improved by manipulated crossing of
parent organisms or breeding. For example, corn originally grown from wild
seeds would gradually change as breeding introduced into the population
genes for resistance to disease, large cobs and grains rich in nutrients. Similarly,
animals can be kept in breeding programmes to maintain and improve
the genome. Captive breeding programmes are important for preventing
extinction and for improving on the diversity of the population of the
organism concerned.
Mutation
mutation ❯
mutagenic ❯
A mutation is a change in the amount or number of chromosomes, or a
change in the structure of the chromosome or DNA of an organism. It results
in a change in the genotype of an organism. An example of a gene mutation is
sickle cell anaemia, and an example of a chromosome mutation which changes
the number of chromosomes is Down’s syndrome.
Mutation occurs randomly – you cannot predict exactly where or when a
change will happen.
The causes of mutation are:
• exposure to high-energy electromagnetic radiation like X-rays, ultraviolet
light and gamma (g) rays;
• exposure to certain chemicals like mustard gas, caffeine, formaldehyde,
colchicine, tar in tobacco, an increasing number of drugs, food preservatives
and pesticides.
Any substance or process that increases the frequency of mutation is described
as mutagenic.
If a mutation happens in a body cell, it will not be inherited or passed on to
offspring and is lost when the organism dies. However, if it occurs in a gamete
cell, it can be inherited. This can add variation to the population. The offspring
of sexual reproduction show variation naturally, because of crossing over and
random alignment of the chromosomes on the equator of the cell, before
anaphase. A mutation which can be inherited, can add new variation. The
change in the chromosome because of the mutation is new information. It may
result in an advantageous or disadvantageous characteristic in the organism.
Most major mutations are disadvantageous.
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Continuity and Variation
Sickle cell anaemia
Figure 26.18
blood cells.
Sickle and normal red
sickle cell disease ❯
sickle cell trait ❯
Sickle cell anaemia is a good example of how a mutation of a part of a
chromosome can have drastic effects. It also shows the role of natural selection
in controlling the occurrence of mutated genes.
In sickle cell anaemia, the gene or part of the chromosome that determines
the shape of the haemoglobin in red blood cells has mutated or changed. This
new form of allele of this gene causes the red blood cell to take a sickle shape
instead of the normal biconcave disc shape (figure 26.18). The sickle-shaped red
blood cell cannot transport oxygen efficiently which makes it a disadvantageous
characteristic. However, the presence of sickle-shaped red blood cells in the
body makes the person far less susceptible to infection by the malarial parasite
than a person without sickle-shaped red blood cells. This is an advantageous
characteristic since malaria is a leading cause of death in areas where it occurs.
Since every person carries two alleles for this gene, one on each
homologous chromosome, there are three possible genotypes:
• homozygous for normal haemoglobin;
• heterozygous with one allele for normal and one allele for sickle cell
haemoglobin;
• homozygous for sickle cell haemoglobin.
Those people who are homozygous for sickle cell have sickle cell disease.
They experience severe pain in the joints, anaemia, kidney failure, poor
growth and development, are prone to infections and are likely to die young.
In those who are heterozygous, only about half the red blood cells change
to sickle shape. These people are unaffected by the condition except at low
oxygen concentrations, such as when flying in an airplane or going to high
altitudes. This conditions is known as sickle cell trait.
The sickle cell gene was selected for in those regions of the world where
malaria is seen (parts of Africa, the Middle East, India and southern Europe).
People here who are heterozygous for the gene are at a selective advantage, as
they are less likely to die from malaria than those who do not have the sickle
cell allele, and less likely to die than those who have two sickle cell alleles. By
natural selection, the gene continues to be passed on to offspring, since these
people survive malaria.
However, the sickle cell allele is at a selective disadvantage in areas where
there is no malaria. People who originally came from malarial areas, such
as Africa, but now live in areas where there is no malaria, such as America,
still carry the allele. About 1 in 400 black people in America have sickle cell
anaemia, and the disease causes about 100 000 deaths per year worldwide.
Down’s syndrome
Down’s syndrome is a change in the number of chromosomes in a cell.
It occurs in all races and a correlation with the age of the mother is seen.
Incidence of the disease rises with the mother’s age, especially after 40 years.
This may be due to the fact that a woman is born with all her eggs and they
age with her. Men, on the other hand, constantly produce new sperms.
The cells of a person with Down’s syndrome all have 47 chromosomes
instead of 46. People with the condition show typical facial features (flat and
rounded). Other symptoms include:
• learning difficulties;
• short stature;
• heart defects;
• increased risk of infection;
• intestinal problems.
320
26 • Variation and Evolution
People with Down’s syndrome are generally very friendly and cheerful, and
greatly enjoy music.
Genetic engineering
Biotechnology is the science which involves the harnessing and exploitation of
biological processes, systems and organisms (particularly microorganisms) in
manufacturing industries. The most powerful tool available to biotechnologists
genetic engineering ❯ is genetic engineering. The benefits of genetic engineering include the
development of high-performance food crops that grow quickly with less use
of fertiliser. This could ease the pressure on food supplies from the growing
human population. Another important area of development is diseaseresistance in crop plants, which would reduce the need for use of pesticides.
An organism that has genes added to it from another species by genetic
transgenic ❯ engineering is known as a transgenic organism.
Some examples of genetic engineering in food production include:
• resistance to pathogenic fungi in maize and potato;
ITQ7
• resistance to insect pests in many crop plants;
Describe brieflythe following terms: (i)
genetic engineering (ii) a transgenic
• increased growth rates in salmon and chicken;
organism.
• production of meat with less fat in
pork and beef animals;
human cells
• production of higher quality dairy
bacterium
products (e.g. milk with more
protein);
• increase in the proportion of
plasmid (circular DNA
human DNA
found in bacteria)
protein in seeds such as soya;
(insulin gene)
DNA extracted from
plasmid cut
• long shelf-life of fruits such as
human pancreatic cells
using enzymes
tomato and bananas;
human DNA inserted
• tastier and more nutritious foods
recombinant DNA – DNA from
into bacterial plasmid
two different species
like tomato;
and joined back up again
• increase in size, and therefore in
bacterial DNA
yield, of many crop plants and
cattle and dairy animals;
plasmid (recombinant DNA)
• production and subtropical
introduced into a bacterium
crops so they are able to grow
in temperate climates (e.g. sugar
cane and millet);
• production of cows and sheep
The plasmids are mass produced
As bacteria multiply, the genes
from temperate areas so that they
are expressed to make their
as the bacteria multiply. The insulin
gene is also being mass produced
different parts. The insulin gene
can grow well in tropical regions;
which was inserted will also
and insulin is produced when the
• grain crops that can fix
be expressed.
gene is expressed.
atmospheric nitrogen (e.g. wheat
and maize).
O\THUPUZ\SPUPZZLWHYH[LKHUKW\YPÄLK
insulin used in diabetic patients
Figure 26.19
Using genetic engineering to make insulin from bacteria.
Human insulin is now manufactured
in bacteria as a result of genetic
engineering (figure 26.19). Insulin
was previously obtained from cows
or pigs and caused many side-effects
in people with diabetes who needed
it. It is now produced by inserting the
human gene that codes for insulin
into bacteria and allowing them to
321
Continuity and Variation
grow and multiply. As they do so, they produce insulin. The insulin is then
separated, purified and packaged. Production of human insulin this way is now
a large-scale enterprise and the product is used by thousands of people with
diabetes.
Genetic engineering is also being used to help treat some hereditary diseases
in humans. Cystic fibrosis is a disease which affects around one in every 2500
babies. It is caused by a recessive allele which makes the mucus in the lungs
thick and sticky. Bacteria get trapped in the mucus and cause infections which
can lead to early death. Traditionally, the only treatment for cystic fibrosis
is daily physiotherapy to clear the mucus in the lungs. Current research is
studying treatment using a viral vector to transmit the normal allele into the
lungs. If the vector is taken up by the cells, they would than be able to make
normal mucus. Treatment would have to be continuous because the cells lining
the lungs are shed frequently and replaced with new ones.
Implications of genetic engineering
Are there risks to human health?
Some people argue that there may be long-term risks from genetic engineering.
There is much discussion on the effects of genetically modified organisms
(GMOs) used in food production. An example is bovine somatotrophin (BST).
This hormone is produced artificially by bacteria and injected into cows to
stimulate growth and increase milk production. The health of humans drinking
the milk or eating the meat appears to be unaffected by the hormone. But are
there long-term effects on human health? As yet, we do not know. Should
genetically modified food be labelled as such? What would you prefer?
Feeding the world
Many crop plants are modified for disease resistance and increased yield.
Plants can be engineered to incorporate the characters of a number of different
species (e.g. starchy potatoes with beta-carotene from green vegetables and
vitamins from citrus fruits). Millions of people are starving in the world – why
not use such foods to ease the problem of starvation?
Scientists have also been able to insert two genes from daffodil and one
gene from a bacterium into rice so that it can now contain vitamin A and its
precursor beta-carotene. This gives the rice a yellow colour – hence it is widely
known as golden rice. It is hoped that it will help to combat malnutrition in
less developed nations, especially those where a lack of beta-carotene in the
diet leads to blindness.
Economic and diversity problems
Figure 26.20 Genetically modified soya
bean could replace conventional crops.
322
There is also fear that small farmers would no longer find it economical
to cultivate local varieties of crop plants when they have to compete with
imported, economically superior varieties (figure 26.20). Could this lead to a
serious loss of genetic diversity among cultivated crop plants? This would make
the dwindling genetic diversity problem worse. Are there dangers in relying on
just a few varieties of crop plants? A new strain of disease could then wipe out
a major crop. Gene banks of many varieties of seeds and plants have been set
up in many countries to conserve diversity, for example cocoa seeds and plants
are stored in Trinidad.
Some people fear that the genetically engineered trait could get transferred
into wild relatives of A engineered crop plant (it has been shown that pollen
from crops such as oil-seed rape can spread for at least 100 m from the GM
plants). Might this produce pest species which could spread uncontrollably
26 • Variation and Evolution
and eliminate other plants, upsetting the ecological balance? What are the
implications of genetically engineered traits transferring into other species?
Treating disease
Advances in genetic engineering will undoubtedly eventually lead to the
control of genetic diseases, such as cystic fibrosis, by replacing defective
genes with healthy ones. This could be wonderful for those living with these
conditions. There are many advantages of this kind of technology. Might this
be taken further? Would genes for low intelligence by replaced by those for
higher intelligence? Would this be good or bad? Are some human characters
superior to others? Who would decide?
The future
ITQ8
Describe two benefits and two hazards
of genetic engineering.
Should humans be allowed to genetically manipulate animals and plants
to serve the needs of humans rather than the environment as a whole? Is
exploitation of living organisms, whether for commercial gain or to reduce
suffering, the height of misuse of the environment, or is it another example of
humans’s triumph over adversity?
Chapter summary
• Every organism inhabiting the plant Earth is unique. This variation is a result of the
organism’s genotype and environment.
• There are two kinds of variation: continuous and discontinuous.
• In continuous variation the differences are slight and merge into each other.
• In discontinuous variation the differences are clear cut.
• The phenotype of an organism is its outward characteristics.
• The genotype is its genetic make-up.
• The phenotype of an organism is influenced by both its genetic make-up and its
environment.
• Genetic variation in a population is important if the environment changes as some
individuals may be more likely to survive to reproduce, and survival of the species is
thus ensured.
• Speciation can occur as a result of geographical isolation and as a result of ecological
ad behavioural differences;
• The theory of natural selection is based on genetic variation among a population. It is
selection of the fittest organisms by nature.
• In artificial selection, humans select individuals that are allowed to reproduce and
produce offspring. We select characteristics advantageous to us like high yield and
reduced production costs.
• Mutation can occur that change the genotype of an organism.
• A mutation may be a change in the structure of the chromosome, such as sickle cell
anaemia.
• A mutation may change the number of chromosomes in a cell, as in Down’s syndrome.
• Genetic engineering is the deliberate changing of the genotype of an organism by
humans.
• The production of human insulin by bacteria is an example of genetic engineering;
• There is much discussion around the possible advantages and disadvantages of
genetic engineering.
323
Continuity and Variation
Answers to ITQs
ITQ1 Each organism has its own genotype which is different from every
other genotype (except for identical twins and individuals produced by asexual
reproduction). Genetic variation is variation in the genotype that helps to
determine differences in the phenotype. Genetic variation explains why every
organism is unique.
ITQ2 (i) The phenotype is the physical appearance of an organism. It
describes all its physical characteristics.
(ii) An organism develops its physical characteristics from a combination of
its genotype and its environment. The genotype confers on the organism the
possibility of developing certain characteristics. The environment guides the
development of these characteristics.
ITQ3 Height – one may be taller; complexion – one may be darker; body size
– one may be fatter (there are many other examples).
ITQ4 If the environmental temperature got warmer, all might survive, but
the ones with the shorter hair length would be at an advantage. The wolves
with long hair stand a chance of over-heating because of the insulation
provided by the thick coat of hair. They are at a disadvantage. The wolves with
the advantage for that new environment would be selected by nature (i.e. they
would be more able to live and reproduce). Eventually a population of shorthaired wolves would be seen.
ITQ5 The use of an antibiotic on a population of bacteria results in an
increase in occurrence or frequency of those with the gene that gives resistance
to that antibiotic. Over time, and with constant use of many different
antibiotics, a population of bacteria could evolve that is resistant to many
different antibiotics.
ITQ6 (i) Natural selection is a theory first put forward by Charles Darwin.
He explained how the environment could select for characteristics in a
population showing variation. He concluded that new species could come into
being by slow and gradual changes, called evolution, as a result of the process
of natural selection.
(ii) A characteristic that suits an organism to its environment has selective
advantage because organism with that characteristic stands a better chance of
surviving and reproducing than those which do not have it.
(iii) The process of natural selection is also known as ‘survival of the fittest’
because nature selects those individuals best ‘fitted’ (adapted) to the environment.
(iv) Evolution describes the change which takes place in a species over time
and which leads to the formation of a new species.
ITQ7 (i) Genetic engineering is the technology in which genes from one
organism are transferred to another organism, often a different species.
(ii) A transgenic organism is one which has had gene(s) transferred to it from
another species. The transgenic organisms can live and reproduce normally
although it has been changed.
ITQ8 Any of the benefits and hazards mentioned in the text could be
mentioned, or you might have researched some more. This new area of
knowledge is constantly changing and new developments are frequently
reported in the media.
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26 • Variation and Evolution
Examination-style questions
1
(i)
Explain these terms:
(a) evolution;
(b) mutation;
(c) artificial selection;
(d) selection pressure.
(ii) Explain what is meant by ‘selective advantage; using antibiotic resistance as an
example.
(iii) Describe, using an example, how the environment may affect the phenotype.
(iv) Explain, using the sickle cell gene, how mutation may affect the phenotype.
2
(i)
3
(i)
The aim of artificial selection is to produce animals and plants with characteristics
desirable to humans. Suggest four characteristics of animals and plants that may be
chosen.
(ii) The peppered moth exists as two main types, a pale form and a dark form.
(a) What is the importance of the colour of the moth?
(b) What effect did industrialisation and the production of pollution have on both
forms?
(c) Why do you think heavy-metal tolerant plants are rare in unpolluted areas?
4
(i) Outline the general process of genetic engineering.
(ii) Give two uses of genetic engineering in (a) agriculture, and (b) medicine.
(iii) Discuss the possible risks of genetic engineering. How can these risks be reduced?
(iv) Many people are against the practice of genetic engineering. Suggest some reasons
for this.
Describe four examples of artificial selection and the characteristics that are being
selected for by humans.
(ii) Explain, using examples, how environmental factors like temperature, act as forces of
natural selection.
(iii) Using a table, list five differences between natural and artificial selection.
(iv) If two offspring (not identical) are brought up in different environments, suggest why
there may be difference in the development of the following characteristics:
(a) body weight;
(b) intelligence.
Compare this with two identical offspring, brought up in the same environment.
325
326
Section D:
School-Based
Assessment
27
School-Based
Assessment
Practical work in Biology
The present CSEC Biology syllabus (2013) makes clear that assessed practical
work – the School-Based Assessment (SBA) – is an integral part of a student’s
studies. This aspect of the course gives the chance to personalise the
curriculum to meet students’ particular needs and assess the development of
his or her skills.
Specified topics
In biology, assessment in at least 18 exercises (spread across 7 specified topics)
is needed to satisfy the CXC requirements. The specified topics are:
1. Ecological study
2. Movement at molecular level (diffusion, osmosis)
3. Photosynthesis/respiration
4. Food tests
5. Germination
6. Nutrition and diseases
7. Genetics
This chapter includes outlines of 31 activities (in addition to those mentioned in
the syllabus itself), which are suitable, after proper development, for use in SBA
practical investigations. There are both qualitative and quantitative investigations.
The chapter contains at least one practical exercise associated with each of the
specified content areas, as well as other topics encountered in this course.
Sufficient detail is given to make possible the practical conduct of each
experiment, and each one can be developed to illustrate material in the
text. Each gives students the opportunity to develop their experimental and
reasoning skills and also their ability to present results in the clear, appropriate
way detailed in the syllabus.
Assessment of skills
If you are doing an experiment in class as part of your week’s work, your
teacher may have done some or all of the planning for you, collected all the
materials that you need, and give you instructions how to do the work, or
even a written worksheet to follow. But you still have to show that you can
follow the instructions, do the experiment and present your results well. The
skills which will be tested are:
Experimental skills (X/S)
•
•
•
•
328
Manipulation and measurement (M/M)
Observation, recording and reporting (O/R/R)
Planning and designing (P/D)
Drawing (D)
27 • School-Based Assessment
• Use of knowledge (UK)
• Analysis and interpretation (A/I)
A three-step approach in preparing for your SBA
When planning and presenting your project, your SBA has three parts:
1. Planning and designing the experiment
2. Doing the laboratory work
3. Presenting a lab report
1
Planning and designing the experiment
In any experiment, you are trying to find an answer: it might be a relationship
or a value. You will need to devise and follow a logical series of steps to find
out that information. Therefore, whatever your hypothesis, you must have
a plan.
In some cases, the proposed activities already in this section contain an
outline plan. However, you would still need to design your experiment bearing
in mind the equipment and facilities available in your lab. In other activities,
you are given a problem to solve, and here you would have to Plan and Design
your investigation from the beginning. In this latter case that your work would
most likely be assessed under Planning and Designing [PD].
Part of your planning is specifying, in detail, how to carry out your
experiment. You need to plan the experiment in such specific detail that
someone else reading your design would be able to do exactly the same as you
did and get the same results.
Think about:
• What apparatus will you need? (e.g. ‘a 250 cm3 beaker’ – not just ‘a
beaker’)
• What chemicals will you need? (e.g. ‘3 or 4 potassium manganate(VII)
crystals about 2 mm long’)
• What will you do? (e.g. ‘stir the mixture gently with a sturdy plastic
drinking straw just before taking each temperature’ – not just ‘stir the
mixture and take the temperature’)
• What could go wrong?
• What are some possible hazards? What safety precautions should be taken?
You will need to put your plan in writing. It is always a good thing to have
your teacher, as well as colleagues, check your plan before attempting to carry
out your investigation.
Often, no matter how carefully you have planned an experiment, it doesn’t
go as you thought it would. However, it has still told you something – an
experiment never ‘fails’. If it didn’t produce the effect you needed, find out
why. If the experiment ‘worked’ but didn’t give the result you expected, then
you‘ve found out something new.
2
Doing the laboratory work
Here you carry out your plan. Always consult with your teacher if you are not
sure exactly how to use the equipment, and how to use it safely. CSEC expects
you to have practised using equipment before being formally assessed in its use.
3
Presenting a lab report
Labs should be written up using the following format. The questions included
in each activity are a guide to the content of the discussion.
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School-Based Assessment
Writing in the correct tense
Remember that your research is already finished. Use the past tense when talking about
the experiment: ‘The objective of the experiment was ...’ or ‘The mixture was added to
the beaker.’
Your report, the theory you are testing and your equipment still exist; therefore, these get
the present tense: ‘The purpose of this report is ...’
Date and title
The title should be brief and describe the main point of the experiment or investigation.
Aim
Keeping it simple and achievable, describe the purpose of the experiment.
Discussion
Background information to relate the experiment to something you have learnt or seen,
or to a problem you have faced.
Hypothesis
Should be clear and be in the form of a question that you want to find the answer to, for
example: ‘Does vitamin C in orange juice oxidise over time when exposed to the air?’
(From the data you obtain from your experiment you should be able to say if the
hypothesis has been either supported or not in your conclusion.)
Procedure
Use the past tense (see box above). In a bulleted list or in separate paragraphs, state,
in order, what you did. Include clear, quantifiable detail (e.g. quantities stated and
apparatus specified). Your teacher will suggest that you use one of these two forms of
words:
‘I washed a 250 cm3 beaker.’ or
‘A 250 cm3 beaker was washed.’
If you have written a good account then someone else, having read it, should be able to
repeat the experiment exactly as you did it without any other help.
Diagram
Draw the apparatus as neatly as you can.
Results
• Table – title stated, neatly drawn with accurate data (times, volumes, masses, colours
…) and proper units for quantities.
• Graph (if necessary) – title stated, axes labelled with proper units, points accurately
plotted. Use a line or a bar graph as required. Remember that you should choose the
correct type of graph for the data you are presenting.
• Include any calculations you used.
Explain the results in detail, using values found in your results.
Answer the questions given in paragraph style.
Limitations
Explain any condition or factor that is out of your control and affects the results obtained.
Conclusion
One short paragraph to summarise results. Make it related to the aim. Review your data
and state your opinions and arguments of what the results show, for example, ‘as graph
3 shows, there is a marked difference between group A and group B which allows the
conclusion that …’ .
330
27 • School-Based Assessment
State if your hypothesis has been supported or not. If the data you have obtained is
not sufficient to support or reject the hypothesis, state why and say what further work
could be done that would allow you to draw a stronger conclusion.
If you are undertaking a complete project (which will be the case in the second
year when you carry out your investigation) then more will be expected of
you, and you can see from page 45 of the syllabus how marks for the project
will be awarded. Your report will need to be more comprehensive than for a
class experiment. It will be assessed for Planning and Design and for Analysis
and Interpretation. Planning and Design has twice the marks of Analysis and
Interpretation.
Safety first
There are several sources of danger that you need to address as you develop
your SBA activities. There are dangers to yourself, your colleagues, the
equipment and even the school building itself. Here are a few safety symbols
concerning situations you should bear in mind.
Electrical hazard
Hot surface
Laser light
No food and drink
allowed
Biohazard
Wear safety goggles
Radioactive
No pointed objects
allowed
Corrosive substance Flammable materials
Toxic materials
331
School-Based Assessment
School-Based Assessment contents
332
Photocopiable
1.1 To observe visible characteristics of plants and animals
333
2.1 A simple ecological study
334
2.2 To compare the water-holding capacity of three types of soil
338
2.3 To estimate the percentage of water in a soil sample
339
2.4 To estimate the percentage of air in a soil sample
340
8.1 To observe diffusion in a solution
341
8.2 To observe some effects of osmosis
342
9.1 To investigate the presence of starch in a green leaf
343
9.2 To see if light is needed for photosynthesis
344
9.3 To see if chlorophyll is needed for photosynthesis
345
9.4 To see if carbon dioxide is needed for photosynthesis
346
9.5 To see whether oxygen is produced during photosynthesis
347
10.1 To investigate the action of an enzyme
348
10.2 To investigate which food groups are present in a food sample
349
11.1 To discover whether carbon dioxide is produced during
respiration
350
11.2 To observe whether heat is produced during respiration
351
11.3 To discover whether oxygen is used up during respiration
352
14.1 To investigate the rate of transpiration using a photometer
353
17.1 To discover how gravity can affect plant growth
354
17.2 To investigate the growth of a radicle
355
17.3 To discover how light can affect plant growth
356
17.4 To compare the movement of four animals
357
18.1 To find whether the skin of the back of the hand, the palm or
the back of the neck contains the most touch receptors
359
18.2 To investigate two reflex reactions
360
19.1 To investigate heat flow from a warm object
361
20.1 Observing the reproductive cells of a mammal
362
21.1 Dispersal of fruits
363
21.2 Seeds and food storage
364
25.1 To investigate how the sex of an offspring is determined
365
26.1 To investigate continuous variation
366
26.2 To investigate natural selection
367
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014.
27 • School-Based Assessment
1.1 To observe visible characteristics of animals and plants
Chapter 1 The Variety of Living Organisms
Syllabus skills: O/R/R
Procedure: animals
1. Visit a backyard garden, a nearby cocoa estate, a nature centre, foothills of forest (anywhere a range of
organisms can be seen).
2. Copy the table below into your lab book and observe five animals (include three insects). Describe what
each animal was seen doing e.g. sucking nectar from a flower, sitting on the bark of a plant. Make a
simple drawing of each animal.
Animal
3.
4.
5.
6.
What it was seen doing
Simple drawing
For the three insects, list visible characteristics that they share.
Name the phylum and class they belong to.
List two ways one insect is different to the other two.
Draw a simple classification table to include the five animals.
Procedure: plants
1. Visit a backyard garden, a nearby cocoa estate, a nature centre, foothills of forest (anywhere a range of
organisms can be seen).
2. Copy the table below into your lab book and observe five plants. Make a simple drawing of a leaf from
each plant.
Plant
Drawing of a leaf (show parallel or branched veins)
3. List all the dicotyledonous and monocotyledonous leaves.
4. Choose two leaves and list three differences you observe.
5. Choose three leaves and list similarities you observe.
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
333
School-Based Assessment
2.1 A simple ecological study
Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms
Syllabus skills: O/R/R; M/M
The area to be studied should be small, such as a tree, a small pond,
a small area in the foothills, a small area in a cocoa estate or a small
garden. The aim is to study the biotic and abiotic factors of the area.
The area being studied can be marked by funning string and the area
calculated.
The biotic factors
A list of all the animals and plants seen in the area should be made. This
can be done by walking quietly and slowly through the area (if it is on land)
A representative sample of any study area
and observing the organisms. Organisms may be found in and on the soil,
can be taken.
under leaf litter and stones, on the stems and leaves of small plants, flying
in the area, on and under the bark of a tree, on the branches of a tree or just visiting the area for a short time.
Food chains and a simple food web can then be constructed using the organisms (plants and animals)
on the list. Interrelationships between the organisms may be noted as examples of the parasitism,
commensalism and mutualism. Other interrelationships like competition (for light, space, etc.), camouflage,
pollination and protection should also be noted. A reader of the study should have a good idea of the
organisms seen there and what they are doing.
An ecological study may also involve collecting data about the abundance and distribution of organisms.
The population size of an organism in the area may be difficult to obtain since it means counting every
individual in the area. However, the population density may be calculated from a smaller area, as the number
of organisms present per square meter (m3). Then the population size of the whole area can be calculated if
the area is known.
To do this, representative samples of the area must be taken. These are usually chosen at random to
avoid bias. Sampling methods include line transects, belt transects and the use of quadrats and sweeping
nets. The most appropriate sampling method for a particular study depends on the area being studied.
Sampling methods
Quadrats
These can be used if the area is fairly uniform and flat. A quadrat is a square frame (meal, plastic or wooden)
of a known area, usually 0.25 m2 or 1 m2. It is placed randomly at several places within the study area
and the number of individuals counted. This method is suitable for plants and slow-moving animals like
millipedes and some insets. The results can be tabulated as shown.
Quadrat
Number of individuals*
1
7
2
14
3
3
4
23
5
5
*number of individuals of one population e.g. millipedes or nutgrass.
The mean number of individuals per quadrat is then calculated and used to find the population density or
population size of the species counted in the whole study area.
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27 • School-Based Assessment
For example:
7 + 14 + 3 + 23 + 5
= 52
= 10.4
5
5
There are on average 10.4 individuals in every quadrat.
If the 1 m2 quadrat was used then: population density = 10.4 individuals/m2
If the size of the area being studied is known, for example 25.6 m2, then the population size can be
calculated:
If in 1 m2 there are 10.4 individuals, then in 25.6 m2 there are:
10.4 = 25.6 = 266.24 individuals
So the population size for the area studied is 266 individuals (millipedes or nutgrass or whatever was being
estimated).
Line transects
A line transect is a better sampling method if one type of habitat changes into another or the area is sloping,
such as a rocky or swampy shore. A string is pulled in a straight line across the area being studied. All the
animals (slow-moving) and plants actually touching the line are considered to be a representative sample of
the animals and plants there. Measuring the height of the line at regular points can describe the slope of the
area.
A
B
C
D
E
A transect line
Position along line
Distance between
soil and string
Description of soil
(water present)
Plant and animals
observed
A
B
C
D
E
Worksheet for the transect line
Transect line across the edge of a pond, and a recording sheet.
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335
School-Based Assessment
Sweep nets
a
These are used for sampling insects, especially flying insects. At
randomly set places within the area a net is swept through the plants
(like grass) a fixed number of times and the individuals caught are
counted. The mean represents a sample of the insects found there.
Sweep nets can be used with quadrats or line transects.
The abiotic factors
The distribution and abundance of organisms relate to abiotic factors.
Commonly measured and described factors are temperature, pH, light
intensity and wind. The soil is a very important abiotic factor since it
directly influences the distribution of plants and therefore the animals
that feed on them.
b
Temperature
Temperature can be measured using a thermometer. The temperature
range over a period of time like a day may be more important than
a reading at any particular moment. Standard maximum/minimum
thermometers can be used.
pH
pH is a measure of the alkalinity or acidity. To determine the pH of the
soil, about 1 cm3 of soil can be missed with 10 cm3 of distilled water.
After shaking, the mixture is allowed to settle and the pH determined
with the use of universal indicator or pH paper.
8
14
7
13
6
12
5
11
4
10
3
9
2
8
1
7
Using various sampling techniques in an
ecological study. (a) Pond dipping. (b) Colleting
insects in a net.
pH strip
This one is pH 6
A pH strip is placed into the solution being tested, then compared to these standards in order to determine the pH of the solution.
Light intensity
Light can be measured at any
time using light meters (like
those used by photographers).
However, it is the light received
over a long period of time that
affects plant growth.
(a)
arms spin around at a
speed proportionate to
the wind speed
(b)
card
pin upon which
vane is pivoted
funnel
scale
card vane
wooden
arm
Wind
Wind speed and direction
affect those animals and plants
exposed to the elements
of nature. Wind speed is
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supporting pole
upon which wooden
arm pivots
wind
direction
Two simple wind gauges.
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27 • School-Based Assessment
measured by an instrument called an anemometer and a simple wind vane can measure the direction.
Simple, but effective gauges can be improvised which may not give the exact speed values but give
comparative readings.
Water flow
Water speed can be determined by measuring the time taken by a floating object to travel a measured
distance of the stream or river. The speed per hour can then be determined.
Soil
Factors of the soil which affect plant growth include the pH, water content, air content, humus content,
water-holding capacity and soil type (composition and distribution of inorganic soil particles). Investigations
to measure the water content, air content and water-holding capacity follow.
Humus content
A sample of soil is heated at 100 °C (to remove all the water) and weighed to give weight X. The dry
sample is then heated again until red hot; this mean that the humus is burnt off. It is then reweighed to give
weight Y.
The percentage of humus in the soil = X X– Y = 100%
Soil type
The distribution and composition of the rock particles can be determined using the sedimentation test.
A sample of the soil is taken and mixed with excess water in a measuring cylinder. The mixture is shaken
vigorously and left to settle. The largest and heaviest particles will settle first, the smallest last, the particles
will settle in layers. The thickness of each layer can be recorded to indicate soil type.
bits of twigs, leaves, etc.
very small particles (clay)
small particles (silt)
large particles (sand)
stones and gravel
Results of a sedimentation test.
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337
School-Based Assessment
2.2 To compare the water-holding capacity of three types of
soil
Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms
Syllabus skills: O/R/R; M/M
Procedure
You need samples of a sandy soil, a clay soil and a loamy soil.
funnel
ZVPS
ZHTWSL(
ÄS[LYWHWLY
TLHZ\YPUN
J`SPUKLY
1.
2.
3.
4.
Set up three sets of apparatus as shown in the diagram. Use 100 g samples of soils A, B and C.
Draw up a table like the one shown below.
Pour 100 cm3 of tap water through each sample.
Wait until no more water is passing through the samples. (This may take some time!) Record the volume
of water which has passed through each.
Soil sample A
Soil sample B
Soil sample C
Amount of water drained through (cm3)
Amount of water retained in soil (cm3)
Questions
1.
2.
3.
4.
338
Through which soil did the water flow (i) most quickly (ii) most slowly?
Which soil retained (i) least water (ii) most water?
From this data, which do you think is the sandy soil? Explain your reasoning.
From this data, which do you think is the clay soil? Explain your reasoning.
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2.3 To estimate the percentage of water in a soil sample
Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms
Syllabus skills: M/M
Share the samples A, B and C from investigation 2.2 among class members.
Procedure
1. Weigh a suitable sized sample of soil.
2. Heat the sample of soil in a dish until it seems dry. Do not heat the soil strongly enough to decompose
organic matter – a temperature of about 90 °C is ideal.
3. Let the soil cool and reweigh it.
4. Reheat the soil for several minutes.
5. Repeat steps 3 and 4 until there is no further loss of weight.
6. Calculate the percentage of water in the soil from the formula:
soil – mass of dry soil × 100
% = mass of wet
mass of wet soil
Questions
1. Which type of soil contained the highest percentage of water? (Your answer may be different from
season to season!)
2. Explain the necessity for step 3 in the experiment.
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339
School-Based Assessment
2.4 To estimate the percentage of air in a soil sample
Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms
Syllabus skills: O/R/R; M/M
Procedure
1. Choose a small tin with a volume of about 200 cm3. Punch several holes in the base.
2. Press the tin down into the soil which you are going to test. (Take care! Some tins have very sharp edges.)
A
tin
tin of soil collected,
the holes at the bottom
are plugged with plasticine
tin pressed into
the soil
volume
and 30
of water
full of w
3.
4.
5.
6.
Plug the holes in the base of the tin with plasticine.
Pull out the tin without losing any of the soil inside it.
Add 300 cm3 of water to a large (1000 cm3 or larger) measuring cylinder.
Pour the soil from the tin into the water in the cylinder, swirl or stir the mixture and allow it to settle. Note
the new volume. Call this X cm3.
B
1500
1400
1300
1200
1100
1000
900
800
700
600
volume of soil
and 300 cm3
Y
of water and tin
full of water
volume of the tin
500
400
300
200
100
X
volume of soil
and 300 cm3
of water
7. Fill the tin with water to the brim and pour the water into the cylinder. Again note the new volume. Call
this Y cm3.
Calculation
Volume of tin = Y – X cm3. This is the volume of (soil + air).
X = 300 + total volume of the tin – volume of air in the soil (the air is lost as bubbles)
X = 300 + (Y – X) – volume of air.
Therefore volume of air = 300 + Y – 2X.
– 2X) × 100
% of air = 300(Y+ (Y
– X)
Questions
What is the importance of air in the soil?
340
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8.1 To observe diffusion in a solution
Chapter 8 Cells
Syllabus skills: O/R/R; M/M
Data
Potassium manganite(VII) is soluble in water giving an intensely purple solution.
Procedure
1. On a sheet of white paper draw five circles all with the same centre. Make their radii 1, 2, 3, 4 and 5 cm.
2. Place a large beaker over the circles and fill it to three-quarters with water. Put the beaker aside, out of
direct sunlight, for five minutes so that the water can become quite still.
3. Choose a single crystal of potassium manganite(VII) (potassium permanganate) and drop it through the
water so that it lands near the centre of the rings you have drawn.
crystals
placed
in water
4. Time how long it takes for the pool of dark purple solution to spread out through each of the rings. Put
your results in a table.
Questions
1.
2.
3.
4.
Why was it important to keep the beaker of water out of the sunlight?
Why did the colour move through the water?
What is the mean speed of diffusion of the purple coloration through the water?
In a vacuum the coloured particles would move very quickly. Why did they move so much more slowly in
your solution?
Extension
It is not easy to get the potassium manganite(VII) crystal to fall where you want it. Can you devise a better
way of placing it in the water in the beaker? Remember that the water must remain still.
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341
School-Based Assessment
8.2 To observe some effects of osmosis
Chapter 8 Cells
Syllabus skills: A/ I; O/R/R; M/M
Procedure
1. Cut two potato strips roughly 1 cm square and 3 cm long.
2. Measure the length of each as accurately as you can.
3. Rub the potato strips between your fingers to assess their texture.
4. Put each potato strip into a petri dish. Cover one with clean water and the other with a strong solution of
sodium chloride (common salt).
ZHS[
^H[LY
^H[LY
WL[YP
KPZO
WV[H[VZ[YPW
5.
6.
7.
8.
Leave the potato strips in their dishes for 15 minutes.
Remove the potato strips, dry them, and measure the length of each as accurately as you can.
Note the texture of the potato strips.
Record your observations in a table like the one below.
First texture
Final texture
First length
Final length
Change in length %±
water
salt solution
Questions
1. In terms of the cells forming the potato strips, why have the lengths of the strips changed in the way
they have?
2. Do the changes in texture of the strips fit in with your explanation? Explain.
3. The cells of the potato contained water to start with. Why did more water move one way than the other
across each cell wall?
Extension
Design an experiment to investigate the effect of using different concentrations of sodium chloride to
surround the potato strips. What result would you expect to find in your experiment?
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9.1 To investigate the presence of starch in a green leaf
Chapter 9 Photosynthesis
Syllabus skills: M/M
Caution: ethanol is flammable.
Do not heat the tube directly
with a Bunsen flame
Data
Starch reacts with iodine to give a blue-black coloration.
Procedure
1. Take a small fresh green leaf from a suitable
dicotyledonous plant.
2. Dip the leaf into boiling water for about 10 seconds.
3. Put the leaf into a test-tube no more than one-third full
of ethanol (alcohol).
4. Place the test-tube into the beaker of boiling water.
5. When the leaf appears colourless, remove the leaf and
rinse it in water.
6. Lay the leaf in a petri dish and pour a little iodine
solution over it. Leave the leaf for several minutes.
7. Pour the iodine solution back into the beaker provided.
8. Rinse the leaf in water. Observe the colour of the leaf.
Questions
1. What effect did the boiling water have on the leaf?
2. What happened when the leaf was boiled in alcohol?
What did the alcohol remove from the leaf?
3. Why was the leaf then rinsed in water?
4. What was the colour of the leaf at the end of the
experiment?
5. What do you conclude about the original green leaf?
The leaf is dipped in
boiling water
for about 10 seconds
beaker
The leaf is placed in a
test tube of alcohol that
is in boiling water.
5)HSJVOVSPZ]LY`PUÅHTTHISL
HUKT\Z[UV[ILOLH[LK
KPYLJ[S`V]LYHI\UZLUÅHTL
test
tube
alcohol
leaf
boiling
water
The leaf is
dipped in water
The leaf is placed in a petri
dish and covered with iodine
solution. Iodine turns blueblack in the presence of starch.
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343
School-Based Assessment
9.2 To see if light is needed for photosynthesis
Chapter 9 Photosynthesis
Syllabus skills: A/ I; O/R/R; M/M
Procedure
1. Choose a small potted plant (such as Impatiens or a geranium). De-starch the whole plant by putting it
in darkness for at least 24 hours.
2. Cover a part of one leaf on the plant with foil or black polythene held in place with paper-clips. (Leave
the leaf on the plant.)
3. Put the plant in the sunshine for at least 3 hours.
4. Remove the test leaf, remove the covering and at once test the leaf for starch.
5. Make a drawing to show your results.
Questions
1.
2.
3.
4.
5.
344
Why was the plant de-starched?
What was used as a control in the experiment?
Which part of the leaf contained starch before the foil cover was added?
Which part of the leaf contained starch at the end of the experiment?
What do you conclude from your results? Explain fully why you reach this conclusion.
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27 • School-Based Assessment
9.3 To see if chlorophyll is needed for photosynthesis
Chapter 9 Photosynthesis
Syllabus skills: A/ I; O/R/R; M/M
Procedure
1. Choose a small potted plant with strongly variegated leaves. Some portions of the leaves should be as
nearly white as possible.
2. De-starch the whole plant but putting it in darkness for at least 24 hours.
variegated leaf
chlorophyll
absent
chlorophyll
present
3.
4.
5.
6.
Choose a boldly marked leaf and make a careful drawing of it to show the green and white areas.
Place the plant in the sunshine for at least three hours.
Carry out a starch test on the leaf that you sketched.
Make a drawing of the leaf showing the brown and the blue-black areas.
Questions
1.
2.
3.
4.
Why is a variegated leaf used for this experiment?
Why was a drawing of the leaf made before the experiment began?
What do the results of the starch test show?
Is there anything in common between the blue-black areas of the starch test and the green areas of the
original leaf?
5. What conclusions can you draw from your experiment?
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
345
School-Based Assessment
9.4 To see if carbon dioxide is needed for photosynthesis
Chapter 9 Photosynthesis
Syllabus skills: A/ I; O/R/R; M/M
bell jar
X
KOH
potted plant
distilled water
potassium
hydroxide
solution
glass sheets smeared
with Vaseline
Data
Potassium hydroxide, sodium hydroxide and soda-lime all combine with carbon dioxide.
Procedure
1. Set up the apparatus shown in the diagram. If potassium hydroxide is not available, sodium hydroxide
or soda-lime can be used. Leave the apparatus for some hours.
2. Place a thoroughly de-starched plant under each bell jar. Do this quickly so that the bell jar is not
removed from the glass sheet for any length of time.
3. Leave the plants for two days.
4. Test one leaf from each plant for starch.
Questions
1.
2.
3.
4.
5.
6.
7.
346
What does the potassium hydroxide do in this experiment?
What is another name for potassium hydroxide?
Which bell jar contains the control plant?
Why were the glass sheets smeared with Vaseline?
Which plant contained starch at the end of the experiment?
How could you test the air in the bell jars for carbon dioxide?
Why would it be better to test more than one leaf from each plant?
Photocopiable
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27 • School-Based Assessment
9.5 To discover whether oxygen is produced during
photosynthesis
Chapter 9 Photosynthesis
Syllabus skills: A/ I; O/R/R; M/M
Procedure
1. Obtain a fresh sample of a water plant such as Elodea or Ceratorphyllum.
2. Set up the apparatus shown in the diagram, making sure that the test-tube is full of water to begin with.
3. Leave the apparatus in sunlight until the tube is nearly full of gas. This may take hours or several days
depending on the conditions.
4. Light a wooden splint then blow out the flame. The tip should continue to glow.
5. Remove the test-tube from the apparatus and put the glowing splint half-way into the tube.
6. Record what happens.
gas
ILHRLYÄSSLK
with water
glowing
splint
inverted funnel
water plant
photosynthesising
test tube
Questions
1.
2.
3.
4.
5.
What happened to the glowing splint?
What gas was present in the test-tube?
How do you know that the gas in the test-tube was not just ordinary air?
Where did the plant obtain the carbon dioxide needed for photosynthesis?
What can you deduce from this experiment?
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
347
School-Based Assessment
10.1 To investigate which food groups are present in a food
sample
Chapter 10 Feeding and Digestion
Syllabus skills: M/M
Procedure
1. A potato is peeled and a small piece (about 1 cm3) is crushed and placed in a test-tube.
2. The test-tube is half-filled with water.
3. 2 cm3 samples are removed from the test-tube and tested for the presence of reducing sugar, nonreducing sugar, starch, protein and fat.
4. Use a table like the one below to show the tests, the results of the tests and deductions.
5. The albumen (white) of an egg is collected in a test-tube.
6. Repeat steps 3 and 4.
Food tested
Details of test carried
out
Results or observations Deduction (presence or
absence of food group)
potato
Questions
1.
2.
3.
4.
5.
6.
348
Which food groups are present in the potato?
Which food groups are present in the egg albumen?
Why was the potato crushed before being tested?
Describe another method for testing for a reducing sugar.
What is the importance of protein in egg albumen?
Why is potato rich in starch?
Photocopiable
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27 • School-Based Assessment
10.2 To investigate the action of an enzyme
Chapter 10 Feeding and Digestion
Syllabus skills: A/ I
Data
Catalase is an enzyme which catalyses the breakdown of hydrogen peroxide (the substrate) into water and
oxygen gas (the products).
Hydrogen peroxide is available as a dilute solution in water.
Procedure
1. Cut four pieces of fresh liver, each roughly 1 cm2 square.
2. Draw up a table like the following and record your observations as you go along.
Effect of whole tissue Effect of boiled tissue Effect of crushed tissue Effect of whole tissue with acid
liver
potato
Set up four test-tubes each containing about 2 cm3 of hydrogen peroxide solution.
Put one piece of liver into the first tube.
Boil one piece of liver in water for 2 minutes, cool the liver and add it to the second tube.
Crush one piece of liver and add it to the third tube.
Put a glowing splint into the mouth of this test-tube.
Put 2 cm3 of hydrogen peroxide solution and 1 cm3 of concentrated hydrochloric acid into a test-tube.
Add one piece of liver.
9. Repeat steps 3-8 using 1 cm3 pieces of potato.
3.
4.
5.
6.
7.
8.
Questions
1.
2.
3.
4.
5.
6.
Which gas was produced in the reaction? How do you know?
Which tissue, liver or potato, showed more reaction? Suggest why.
Why was the result using boiled tissue different from that using whole tissue?
Why was the result using crushed tissue different from that using whole tissue?
Why was the result of using whole tissue and acid different from that using whole tissue?
What general statement about the conditions necessary for enzyme-catalysed reactions could you write
as a result of these experiments?
Extension
The conditions used in some of these experiments were extreme (boiling; concentrated acid).
1. Devise experiments to investigate the activity of a state enzyme in conditions that vary less sharply.
2. Find out the optimum conditions for the action of a named enzyme.
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
349
School-Based Assessment
11.1 To discover whether carbon dioxide is produced during
respiration
Chapter 11 Respiration
Syllabus skills: A/ I; O/R/R
ÄS[LYW\TW
air drawn out
air in
X
A
potasium hydroxide
solution
(removes carbon
dioxide from the air
by absorbing it)
B
limewater or
hydrogencarbonate indicator
(tests for the presence
of carbon dioxide)
respiring mouse
(produces carbon dioxide)
C
limewater or
hydrogencarbonate indicator
(tests for the presence
of carbon dioxide)
Procedure
1. You need a healthy live mouse! (Do not try to use any wild rodent.)
2. Set up the apparatus as shown in the diagram. A, B and C can be flasks instead of jars. Put the mouse
gently into the other jar.
3. At once turn on the pump to draw air through the apparatus at a rate of about one or two bubbles per
second.
4. Wait until there is a definite change in the liquid in jar C.
5. Note the appearance of the liquid in jar C.
6. Release the mouse gently back into its usual living space.
Questions
1. What is the function of flask A?
2. What did you observe happening in flask B? (For a good answer you must say what the liquid was like
at the start as well as what it was like at the end.)
3. What does this change tell you about the air going into the jar containing the mouse?
4. What can you deduce from the change you saw happening in jar C?
5. What can you deduce from this experiment?
6. Why might it have been better to have a second flask containing limewater between flask B and the jar
containing the mouse?
350
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11.2 To observe whether heat is produced during respiration
Chapter 11 Respiration
Syllabus skills: A/ I; M/M
]HJ\\TÅHZR
(e.g. a Thermos)
dead peas
washed in
disinfectant
germinating
peas
washed in
disinfectant
cotton wool
thermometer
A
B
Procedure
1. Soak some pea seeds for 24 hours. Divide the seeds into two sets of roughly equal number.
2. Kill one set by putting the seeds in boiling water for 5 minutes. Rinse the peas with a mild disinfectant
solution.
3. Rinse the live peas with the same disinfectant.
4. Put the two sets of peas into separate ‘Thermos’ (or similar) flasks. Wedge a thermometer into each
flask with cotton wool and then carefully invert the flasks, as in the diagram.
5. Arrange the thermometers so that you can read the temperature on each.
6. Leave the flasks side-by-side for three days.
7. Read the two thermometers at the end of this time.
Questions
1.
2.
3.
4.
What was the purpose of the disinfectant solution?
Has the temperature indicated by the thermometer in the dead seeds changed? Account for this.
Has the temperature of the live seeds gone up or down? Account for this change.
Would you expect to find a difference between the reading shown by either thermometer in the morning
and in the evening? Why?
5. Suggest one way in which you could make the experiment more reliable.
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
351
School-Based Assessment
11.3 To discover whether oxygen is used up during
respiration
Chapter 11 Respiration
Syllabus skills: A/ I; O/R/R
Procedure
1. Set up the two sets of apparatus shown in the diagram. The capillary tubes must be gas-tight in the
bungs, which must be gas-tight in the flasks.
2. Once the apparatus is gas-tight you can adjust the position of the oil drop by very gently pressing the
bung into the flask or releasing it very slightly.
3. Note the position of the right-hand edge of each oil drop. The drops should be near zero to start the
experiment. Put the flasks out of direct sunlight so that their temperatures do not change.
4. Draw a table recording the positions of the oil drops every 5 minutes.
5. Stop the experiment when you have enough readings to see the pattern of the results. A good method is
to draw a graph of position (y-axis) against time (x-axis) as you go along.
6. Release the animals gently back to where you found them.
9
8
7
6 5 4
capillary
tube
3 2 1
oil drop
0
wire gauze
soda lime (absorbs carbon dioxide)
A
9
8
7
6
5
4
3
2
1
0
small animals, e.g.woodlice
or millipedes
soda lime (absorbs carbon dioxide)
B
Questions
1. Why was it important to keep the flasks at a constant temperature?
2. Why did the oil drop in A move in the way it did, quickly at first and then hardly at all? What was the
purpose of this part of the experiment?
3. What happened to the oil drop in B? How was it different from the behaviour of the oil drop in A?
4. Why did the oil drop in B behave in this different way? Explain what the experiment tells us about the
necessity for oxygen in respiration.
352
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27 • School-Based Assessment
14.1 To investigate the rate of transpiration using a
photometer
Chapter 14 Transport in Plants
Syllabus skills: A/ I; O/R/R; M/M
plant stem, e.g. geranium
Achillea or Impatiens
zero point
water containing
a few drops of ink
clip
10 9
8
7
6
5
4
3
2
1
0
meniscus
Procedure
1. Select a plant with stems which will fit into the rubber tubing on your potometer.
2. Put the stem under water (keep the leaves out if you can) and cut the stem with a sharp knife. Leave the
stem under water to stop air getting into the xylem vessels.
3. Still under water, attach the stem into the photometer which has been filled with coloured water.
(Coloured water is easier to see in the capillary tube.)
4. Take the apparatus to your bench. Open the clip slowly until the meniscus moves to a point near to zero.
Start timing form this point.
5. Record the position of the meniscus every two minutes until it reaches the end of the graduations.
6. Repeat steps 4 and 5 but:
7. (i) with a fan blowing air across the stem
8. (ii) with the lab closed up and lights out.
Questions
1. Draw three graphs (on the same set of axes) showing photometer reading (vertically) against time
(horizontally). Which conditions produced the highest transpiration rate? Which conditions produced the
lowest transpiration rate.
2. How do you deduce these answers from the graphs?
3. Why did the fan cause the transpiration rate to change as it did?
4. Why was the transpiration rate in the closed, darkened room different from that in the open, sunny lab?
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
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School-Based Assessment
17.1 To discover how gravity can affect plant growth
Chapter 17 Movement
Syllabus skills: A/ I; O/R/R
(See also Investigation 17.2.)
Procedure
1. Place three kidney beans or red beans on some tissue paper soaked in water. Leave them for a day.
2. Place several layers of tissue paper along the inside of a glass and soak the tissue with water. The wet
tissue will stick to the glass.
3. Gently place the germinating beans between the glass and the tissue in the position shown in the
diagram. Leave the beans there for one day. Make sure that the tissue is always moist.
4. At the end of one day make drawings of the seedlings.
5. Record your observations of the growth of the seedlings, particularly the growth of the radicle.
6. Turn the glass upside down and leave it for another day.
7. Make further drawings of the seedlings.
layers of moist
tissue paper
against the glass
germinating seedling
placed between the
tisue paper and
the glass
glass turned
upside down
seedling, note in
particular the
growth of the radicle
Questions
1.
2.
3.
4.
Why is the tissue paper always kept moist?
Why did the radicle and the shoot now grow in the same direction?
From the point of view of the seedling, what effect does turning the glass upside down have?
What was the effect on the growth of the shoot and the growth of the radicle of turning the glass upside
down?
5. Why is this response important to a plant?
6. Suggest two ways in which the experiment might be improved.
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27 • School-Based Assessment
17.2 To investigate the growth of a radicle
Chapter 17 Movement
Syllabus skills: D
Procedure
1. Soak three bean seeds in water for 24 hours.
2. Line the wall of a gas-jar or tall beaker with a piece of thick, wet blotting paper or several layers of wet
tissue paper.
3. Place the seeds between the blotting paper and the wall of the gas-jar.
beaker
wet tissue
bean seed
marked
radicle
water
4. Put about 1 cm depth of water in the gas-jar to keep the paper moist.
5. When the radicles have reached a length of about 1 cm remove the seeds carefully and make marks
along each radicle at roughly 2 mm intervals using a fine cotton thread soaked in Indian or other
waterproof ink.
6. Return the seeds to the gas-jar making sure that the weight of the seedling is supported by the testa
and not the radicle.
7. For the next six days, make a sketch of each bean seedling showing how the ink marks have separated.
Make measurements of the gaps between the marked lines if you can do so. Arrange your sketches in a
table like the one below.
Questions
1.
2.
3.
4.
5.
6.
Why did you use more than one seed?
Did all three seeds behave in the same way?
Did the radicles grow uniformly, mostly at their base, or mostly at their tip?
What would be the effect on growth if the extreme tip of the radicle were cut off after three days?
What happens at the tip of the radicle to cause the effect you have observed?
Suggest why flowering plants continue to flower if dead flowers are removed.
0
1
2
3
4
5
6
seed 1
seed 2
seed 3
A possible way of arranging your sketches (after King, Soper and Tyrell Smith; Macmillan, 2nd ed 1991 page 213).
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School-Based Assessment
17.3 To discover how light can affect plant growth
Chapter 17 Movement
Syllabus skills: A/ I; O/R/R
Procedure
1. Place three kidney beans or red beans in a small container lined with wet tissue paper. Leave them for
one day.
2. Cut a small hole in the short side of a box such as a shoe box.
3. Put the terminating beans, in their container, in the box as far away from the hole as possible. Keep the
tissue paper wet. Leave the experiment for two days in sunlight.
4. After this time make drawings of the seedlings and measure the length of each.
5. Replace the box lid and leave the seedlings for a further two days. Again, keep the tissue paper moist.
6. Draw up a table showing the lengths of the seedlings and their mean lengths after two days and after
four days.
7. Write a general statement describing the appearance of the seedlings at each stage.
seedlings
shoe box
length of seedling
hole
Questions
1.
2.
3.
4.
Why is it important that the tissue paper is kept moist?
Did the seedlings grow more in the first two days or the second two days? Why?
What external factor made the seedlings grow in the way they have?
What part does the plant hormone auxin play in this growth? Explain carefully how the effect is
important to the plant.
5. What is etiolation?
Extension
Use six beans. Allow the shoots (coleoptiles) to reach a length of about 2 cm. Cover the tips of two shoots
with a small piece of aluminium foil. Cut off the top of two of the shoots. Do nothing to the remaining two
shoots.
Place all six beans in the growth box, making sure that the tissue is damp. After two days, examine the
shoots and explain why they are different from each other.
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27 • School-Based Assessment
17.4 To compare the movements of four animals
Chapter 17 Movement
Syllabus skills: O/R/R
In this investigation you will observe the movements of an earthworm, a fish, a frog and a human. Record
your observations in your notebook as you work. Where explanations are asked for, write them afterwards,
giving as much detail as you can. (You may have to look up material in your library.) Finally, answer the
questions set at the end of this set of experiments.
Procedure: earthworm
1. Place the earthworm (alive) on a sheet of white tile. Use your index finger to touch the outside of the
earthworm’s body.
2. Is the skin hard or soft? The skin should feel moist. How does this moisture on the skin of the worm help
it to move?
3. Is the body of the worm segmented or unsegmented?
4. Does the external structure of the worm suit its ability to burrow in the soil? Explain your answer.
5. Allow the earthworm to crawl on the sheet of paper. Listen carefully as it moves. Do you hear a
scratching noise? What causes it?
6. Turn over the earthworm and, with the aid of a hand lens, observe each segment. What do you observe?
What are these stiff bristles used for?
7. Describe the movement of the earthworm over the paper. Explain how the earthworm bring about the
changes in its shape in order to move.
8. Place the earthworm on a white tile. Does the earthworm move as quickly on the tile as it did on the
paper? Explain your answer.
Procedure: ¿VK
1.
2.
3.
4.
5.
Watch a fish swim in an aquarium.
Describe the motion of the body, fins and tail as the fish moves from one place to the next.
Which structure moves the fish in a forward motion? Explain how this occurs.
How does a fish turn around in the water to change direction of motion, whether to the left or right?
Did you notice the fish stopping in the water (not swimming around)? Explain how the fish is able to do
this.
6. Use your hand to disturb the water by swaying it side to side. Did the fish sway with the water currents
created? If not, explain how it was able to remain steady in the water.
7. Observe the shape of the fish’s body. Explain how the shape of the fish is suited for its movement in the
water.
Procedure: frog
1. Hold the frog firmly in your hand and observe the length of the front and back legs. Which set of legs are
longer and more muscular?
2. Place the frog in a cardboard box (keep the top open) and watch how it moves.
3. Describe the hopping or jumping motion of the frog.
4. Which set of legs (front or hind) played the more important role in its hopping movement? Explain your
answer.
5. Observe the structure of the feet of the frog.
6. Put the frog in a tank of water and observe it swimming. How did the frog move its legs in order to
obtain a forward motion?
7. Explain how the structure of the feet enables the frog to swim in water.
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
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School-Based Assessment
Procedure: KXPDQ
For this experiment you will work with a partner. Your teacher will put on display in the front of the class a
human skeleton like the one shown here.
1.
2.
3.
4.
5.
6.
7.
8.
9.
With you and your partner taking turns, walk up and down a short distance around your work area.
Describe the motion of your partner while walking.
Did your partner walk upright on two legs?
Look at the skeleton and identify the structure that is responsible for supporting the body in the upright
posture.
How did your partner move their legs while walking? Did they bend the legs or keep them straight?
Look at the skeleton and identify the structures used in the motion of the legs which you have
described. Make a list of the structures, stating the function of each.
Did your partner move their hands while walking? Can you give a reason why?
Are bones able to move on their own? If not, state the structures that are responsible for moving bones.
Explain how the muscles move the legs while walking.
Questions
1. How are the movements of the earthworm different from those of the frog and the fish?
2. How is this difference related to the fact that a frog has a bony skeleton while an earthworm does not?
3. Why can a human perform a wider range of movements than a frog, but cannot jump or swim so well?
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27 • School-Based Assessment
18.1 To find whether the skin of the back of the hand,
the palm or the back of the neck contains the most touch
receptors
Chapter 18 Irritability, Sensitivity and Coordination
Syllabus skills: A/ I; O/R/R
Procedure
1. Attach two pins to a ruler, 2 cm apart, as shown I the diagram.
2. Draw up a table like that shown for your results.
3. Make sure that your partner understands what is going to happen in
the experiment. He or she then closes their eyes.
4. Touch your partner gently on the back of the hand with one or both
pins on the ruler.
5. Ask how many pins they think are touching their hand. Record a tick if
they are right and a cross if they are wrong.
6. Repeat step 4 nine more times making ten in all.
7. Adjust the pins to be 1 cm apart and repeat steps 3, 4 and 5.
8. Adjust the pins to be 0.05 cm apart and repeat steps 3, 4 and 5.
9. Adjust the pins to be 0.2 cm apart and repeat steps 3, 4 and 5.
10. Present your results as a histogram like the one shown below.
2
Number correct
back
palm
neck
10
Number of correct responses
back of hand
9
palm of hand
8
back of neck
7
6
5
4
3
2
1
0
2 cm
1 cm
0.5 cm
0.1 cm
Distance apart of pins
Questions
1.
2.
3.
4.
Which of the three parts tested do you think has the most receptors? Give reasons.
Why do you think that this part of the body needs to be so sensitive?
Why is it important to have touch receptors in the skin all over the body?
What are some sources of error in the experiment? How can the experiment be improved?
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359
School-Based Assessment
18.2 To investigate two reflex actions
Chapter 18 Irritability, Sensitivity and Coordination
Syllabus skills: O/R/R
Procedure: effects of light on the pupil of the eye
1. Prop up a small mirror in front of your face. Look into the mirror and draw a diagram of one of your eyes.
Label the pupil and the iris.
2. Look again into the mirror. Close both your eyes and cover them with the palms of your hands. After
20 seconds remove one hand and at the same moment open that eye. What happens to the pupil
immediately after you uncover your eye? How long is it before there is no further change?
3. Look into the mirror once more. Shine a small torchlight into one eye and observe what happens.
4. Draw diagrams showing the appearance of your eye (a) in dim light, and (b) in bright light.
Questions
1. Explain what caused the pupil of your eye to change size in 2 and 3. Draw a diagram to show the
changes.
2. Did your pupil change size quickly or very slowly? Suggest why this is important to your body.
Procedure: the knee-jerk reflex
1. Sit on the edge of a table with both feet hanging loosely.
2. Use your fingertips to locate the base of one knee-cap.
3. Tap the front of your leg firmly, just underneath your knee-cap, with the side of your hand or the edge of
a ruler.
4. Describe what happens when you do so.
Questions
1. How is this reflex different from the reflex change in pupil size which you studied?
2. Draw diagrams to illustrate this difference.
thigh
knee-cap
hand tapping leg
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27 • School-Based Assessment
19.1 To investigate heat flow from a warm object
Chapter 19 The Eye, the Ear and the Skin
Syllabus skills: A/ I; O/R/R; M/M
Procedure
1. Set up three thin plastic cups as shown in the diagram.
• A is wrapped in several layers of tissue paper held in
place with elastic bands. The paper is soaked in cold
water.
• B is not wrapped.
• C is wrapped with corrugated cardboard (from an old
box) held in place with elastic bands.
2. Draw up a table with four columns and 18 rows for the
measurements.
3. Heat a supply of water in a beaker or a kettle until it is hotter
than 70 °C.
4. Half-fill each cup with the hot water. (Take care!)
5. At once take the temperature of the water in each cup.
6. Take the temperature of the water in each cup every
30 seconds for the next 8 minutes. Record your results in
the table.
thermometer
elastic band
insulation
A
Results
1. On the same axes, plot three graphs (one for each cup) of
temperature against time. Plot temperature vertically and
time horizontally.
2. Complete the graphs by drawing smooth curves through
the points.
3. Use the graphs to explain which cup lost heat most quickly
and which cooled most slowly.
Questions
1. Why did wrapping the cups with the wet tissue and the
cardboard have these effects?
2. Why is it helpful to humans to sweat in hot weather and put
on more clothes when the weather turns colder?
B
elastic band
insulation
C
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School-Based Assessment
20.1 Observing the reproductive cells of a mammal
Chapter 20 Reproduction in Animals
Syllabus skills: O/R/R; D
Procedure: observing the reproductive cells of a mammal
1. Your teacher will provide you with pre-prepared slides of mature ova and of the sperm of a mammal.
2. Observe the slide of the male gamete under the microscope, first under low power and then high power.
Make a note of the magnification at each power.
3. What structure is contained in the head of the sperm?
4. In your laboratory report book make an accurately labelled diagram of the sperm.
5. Write a description of the sperm.
6. Then remove the slide and replace it with the slide of the ova.
7. Which are the mature ova? How can you tell?
8. Locate the cell membrane with a jelly coat, nucleus containing chromosomes and cytoplasm.
9. Make an accurately labelled drawing of one mature ovum.
10. Write a description of the mature ovum in your laboratory report book.
Questions
1. Which are larger, sperm of ova? Make an estimate of their relative sizes.
2. Sate one advantage of sexual reproduction over asexual reproduction.
Procedure: observing the budding of yeast
1.
2.
3.
4.
5.
6.
7.
8.
Make a mixture of yeast, water and a little glucose.
Place a drop of the yeast mixture on a slide and stain it with methylene blue.
Cover with a cover slip.
Observe the slide under a microscope, first at low power then at high power.
Make a sketch in your notebook of a yeast cell.
How many budding cells can you see?
How long does it take for a bud to form and separate from its parent?
Make sketches to illustrate the stages in budding in yeast.
Questions
1. Each new cell must contain a nucleus. What must happen in the parent cell before the bud finally
detaches from it.
2. The family of yeasts are the saccharomycetes – meaning the ‘sugar fungi’. How does this relate to one
important use of yeast?
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27 • School-Based Assessment
21.1 Dispersal of fruits
Chapter 21 Reproduction in Plants
Syllabus skills: O/R/R; D
Procedure
1. Collect examples of fruit whose seeds are dispersed:
(i) by animals eating the fruit,
(ii) by animals passing by the plant,
(iii) by mechanical means,
(iv) by water,
(v) by the wind.
Example
(i)
Tomato
(ii)
Sweetheart
(iii)
Pride of Barbados
(iv)
Coconut
(v)
Dandelion
2. Make a sketch of each and, by each sketch, state which features of the seed are important for that
method of dispersal.
Questions
1. Most plants, in the course of their evolution, have developed an efficient method of seed dispersal. What
would be the consequences for a plant which had no dispersal mechanism? Explain your reasoning
2. Oranges are brightly coloured and have an attractive scent. How do these factors help dispersal of the
seeds?
3. List four ways in which animals (including humans) help in the dispersal of fruits and seeds.
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
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School-Based Assessment
21.2 Seeds and food storage
Chapter 21 Reproduction in Plants
Syllabus skills: O/R/R; M/M; D
Procedure
1.
2.
3.
4.
Remove the seeds from an orange.
Peel the seeds and crush them.
Collect some juice from the fleshy part of the orange.
Test the crushed seeds and the orange juice for sugars, starch, protein and lip (fat) using the tests given
in Chapter 10.
Questions
1. What food groups are stored in the fruit?
2. Why do fruits store these food groups?
3. What food groups are stored in the seeds?
4. Why do seeds store these food groups?
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27 • School-Based Assessment
25.1 To investigate how the sex of an offspring is
determined
Chapter 25 Heredity and Genetics
Syllabus skills: A/ I; O/R/R
Number of individuals
Procedure
1. Fifty black beads are placed in a container. In
another container, 25 black beads and 25 white
beads are mixed thoroughly together. The beakers
are placed side by side with two empty beakers
clearly labelled A and B.
2. Close your eyes. Pick one bead from each of the
first two beakers. If both beads are black, put them
into beaker A. If one is black and the other white,
put them into beaker B. Record the result in a table
like the one shown, but putting a tick to show the
combination of beads produced each time.
Selection number Both black
Height (cm)
(in 2 cm groups)
Black and white
1
2
3
4
5
6
7
8
9
10
3. Do this nine more times, making ten in all.
Questions
1.
2.
3.
4.
5.
What does each black bead represent?
What does each white bead represent?
What does the beaker represent?
Why did you have to close your eyes when taking beads?
Use a genetic diagram to predict the expected ratio of male to female offspring in humans. How does
this relate to the experiment you have just done?
6. How many pairs of black beads did you select?
7. How many pairs containing both black and white beads did you select?
8. What ratio of ‘boys’ to ‘girls’ did you find in the ten offspring of this experiment?
9. Relate your obtained ratio to the prediction you made in 5.
10. In what ways does the experiment differ from what really happens?
© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014. Photocopiable
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School-Based Assessment
26.1 To investigate continuous variation
Chapter 26 Variation and Evolution
Syllabus skills: A/ I; O/R/R
Procedure
1. Measure the height of each member of your class.
2. From the list, draw up a frequency table like that shown in the diagram. Make sure that you have enough
groups to take in all the measurements. (It is quite possible that there are may be a group with no
individual results in it.)
Height group (cm)
No. of individuals
150–152
153–155
156–158
159–161
etc.
3. Use the frequency table to draw a histogram showing how height varies among your classmates.
A
B
Questions
1. What is the height range (i.e. shortest to tallest) in your class?
2. Describe the overall shape of your histogram?
3. Imagine that you had measured the height of a very large number of people, but grouped those heights
in 0.5 cm groups. Make a sketch of what you think that the histogram would look like.
4. What kind of variation is seen in the height of humans?
5. State three other examples of this type of variation in humans.
6. Name, and give one example of, a different type of variation.
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27 • School-Based Assessment
26.2 To investigate natural selection
Chapter 26 Variation and Evolution
Syllabus skills: A/ I; M/M
Procedure
1. You need 240 matchsticks (or toothpicks) and some coloured marker pens.
2. Colour 60 matchsticks blue, 60 brown, 60 yellow and 60 green.
3. Scatter a mixture of 30 matchsticks of each colour onto a large surface such as the floor, a lab table or a
lawn. These are the prey.
4. One student, the predator, has 10 seconds to ‘catch’ as many prey as possible by picking up one
matchstick at a time and putting it into a beaker. The remaining matchsticks are the ‘survivors’. The
number of survivors of each colour is counted.
5. Each survivor is given one offspring of the same colour.
6. Repeat steps 4 and 5.
7. Record the results in a table like the one below.
Prey population
Number of prey caught
Number of survivors
New prey population (after each
survivor is given one offspring)
30 blue
15
15
30
30 brown
5
25
50
30 yellow
20
10
20
30 green
1
29
58
30 blue
20
10
20
50 brown
7
43
86
20 yellow
15
5
10
58 green
0
58
116
Sample results from a background of lawn grass.
Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
What does each matchstick represent?
What does each colour represent?
How did the number of sticks of each colour change over time?
Which colour survived best? Why?
Which colour survived worst? Why?
How do these results relate to the process of natural selection?
Explain what is meant by camouflage.
In this experiment, which characteristic is being pressured and selected?
Predict what would have happened in your experiment if the survivors had been given two offspring
instead of one.
10. Describe some sources of error in the experiment.
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Index
Index
abiotic factors 15, 16, 17
and species distribution 17–21
studying 333–4
ABO blood groups 301
absorption
food 108, 111–16
reabsorption 186, 187
water 166
abstinence 254
accommodation (eye) 229–30
acid rain 49, 68
actively acquired immunity 154,
155
adaptations
environmental 313
for gaseous exchange 136–7
to light 20
for photosynthesis 93–5
in plants to conserve water
167–8
to temperature 20
to water 18–19
addiction 220, 221
adhesion 165
ADP 123–4
aerobic respiration 122–4, 125
agar 198
AIDS see HIV/AIDS
air/atmosphere
carbon dioxide in 43, 44, 45–6,
133
inspired and expired 133, 134
pollution 68
in soil 337
albinism 299
alcohol 220–1
ethanol test 104
fermentation 126
during pregnancy 221, 252
algae 5, 24, 67, 68
alimentary canal 111–16
alleles 297
co-dominance 301–2
dominant and recessive 298
incomplete dominance 300
sickle cell anaemia 320
alveoli 131, 132, 137
amino acids 102, 110–11
assimilation 116–17
ammonium compounds 48
amniotic fluid 252
amoeba 5, 81, 83
adaptation to water 18
binary fission 245, 283
gaseous exchange 137
amphibians 9
adaptation to water 18
movement 354
ampulla 234
368
amylase 112, 114
anaemia 104, 272
see also sickle cell anaemia
anaerobic respiration 125–7
anaphase 279–81
anaphase I/II 292
androecium 261
angiosperms 7
see also flowers
animal cells 79–80
osmosis in 87
in respiration 125
animals 3, 7–9
adaptation to light 20
adaptation to temperature 20
adaptation to water 18
in the carbon cycle 43, 44
cloning of 285–7
as consumers 24–5, 26
dispersal by 264–5
excretory products 182–3
food storage in 178
food supply from plants 91–2
foods from 106
gain and loss of energy 34–5
movement in 196, 354–5
in the nitrogen cycle 46–7
predators and prey 26–7
response to stimuli 209
selective breeding 318
temperature control 235–7
tissues, organs and systems 83
see also humans; living
organisms; mammals
annelids 8
Anolis lizards 316
anorexia 107
antagonistic muscles 203
anthers 261, 262
antibiotics 6, 220
resistance to 313
antibodies 150, 151–2, 154
in vaccination 155
antidiuretic hormone (ADH) 189,
219
antigens 151–2, 154
in vaccination 155
anus 116
aorta 148
aphids, using 170
appendicular skeleton 199
aquatic food chains 26
aquatic food webs 27, 31
aqueous humour 227, 232
arachnids 8
arms, flexing and extending 203
arteries 146, 147–8
arterioles 146, 147
afferent and efferent 186
arthropods 8
artificial classification 9
artificial immunity 155
artificial propagation 284–5
artificial selection 317–18
in the Caribbean 318–19
asexual reproduction 244–5
and mitosis 282–8
aspirin 220
assimilation 108, 116–17
astigmatism 231
atherosclerosis 108
atmosphere see air/atmosphere
ATP 123–4
atria 144–6
atrioventricular valves 145
auditory canal 233
autonomic nervous system 217
autotrophs 91, 92
auxin 197–8, 199
axial skeleton 199
back cross 300
bacteria 4, 80
anaerobic respiration 127
antibiotic resistance 313
in the carbon cycle 43
in decomposition 28
hot water vents 24
insulin production 321
mutualism 29, 47
nitrifying and denitrifying 48
nitrogen-fixing 47
bacteriophages 4
balance 234–5
balanced diet 101, 106–7, 271
ball-and-socket joint 204
bauxite 56, 110
behavioural speciation 317
Benedict’s solution 102, 103
beriberi 103
beta-carotene 322
bicuspid valve 145
bidens 7
bile 114, 183
production 117
binary fission 245, 283
binomial system 10–12
bioaccumulation 38–9
biochemical oxygen demand 68
biodegradable materials 57
biodiversity 64
biogas 68
biogeochemical cycles 42–3
biological control 67
biomass 36
pyramids of 38
biotechnology 321
biotic factors 15, 16, 17, 331
birds
body temperature 239
characteristics 9
Darwin’s finches 315
dispersal by 265
migration 317
birth 252
birth rates 55
Biston betularia 314
Biuret test 104
blackcaps 317
bladder 184
blind spot 227, 230
blindness 232
blood 143, 149–50
circulation 148–9
clotting 151, 153
and defence against disease
153–5
flow through the heart 145
osmoregulation 189
oxygen/carbon dioxide
transport 150–1
oxygen diffusion 85
oxygenated and deoxygenated
132, 145, 149
pressure filtration 186
blood cells 149–50
see also red blood cells; white
blood cells
blood glucose levels 192
blood groups 151–2, 301
discontinuous variation 312
blood plasma 149
antibodies 151–2
concentration 191
blood transfusion 151–2, 254
blood vessels 143, 146–8
see also capillaries
blubber 20, 178
body temperature
animals 235–7
birds 239
control of 192
humans 235, 236, 237–9
bolus 112, 113
bones, movement 202–3
bovine somatotrophin (BST) 322
Bowman’s capsule 185–6
brain 216–17
response to stimuli 214
breast-feeding 253, 254
bronchi 132
bronchiole 132
Bryophyllum 283
budding, yeast 359
Buffalypso 319
Index
caffeine 220
calcium 104
calyx 261
camouflage 314
canine teeth 109
Cannabis sativa 139, 221
capillaries 146, 147–8
bursting of 152
gaseous exchange 132
capillarity 165
car exhaust 68
carbohydrates 101–2
metabolism 117
storage 80
see also glucose; starch
carbon cycle 43–4
human effect on 44–5
carbon dioxide
atmospheric 43, 44, 45–6, 133
in the body 191
diffusion 85
from fermentation 126
in photosynthesis 92–3, 95,
343
concentration 97
pollution from 68
from respiration 123, 182, 347
transport in the body 143,
150–1
see also gaseous exchange
carbon monoxide 68, 138, 252
cardiac muscle 144
cardiovascular disease 108, 137
Caribbean
Anolis lizards 315
artificial selection in the
318–19
endangered species 65
human activity effects on 71–2
carnivores 25, 26, 35
carrying capacity 53
catalase 345
catalysts 110
cataracts 231
cattle farming 318–19
cell cycle 279–80
cell membrane 79, 80
permeability 84, 86
cell wall 79
cells
capillaries surrounding 147
gaseous exchange 133
homeostasis 190, 217
movement in and out of 83
respiration 123
size 78, 79
specialisation 81–3
see also animal cells; human
cells; plant cells
cellulose 102
central nervous system (CNS) 211
response to stimuli 214
centrioles 281
centrum 202
cerebrospinal fluid 216
cerebrum 216
cervical vertebrae 200–2
chemical digestion 110
chemical energy 34
chlorophyll 92, 100, 342
chloroplasts 5, 79, 80, 92
in the palisade layer 93
in photosynthesis 95–6
chordates 8–9
choroid 227
chromatids 281, 292
chromosome number 278–9, 281,
282
and meiosis 290–1
chromosomes 79
Down’s syndrome 320
homologous pairs 291–2, 293,
297
in mitosis 281
mutation 319
replication 282
see also gametes
chyme 113, 114
cilia 133
ciliary muscle 227, 228, 229
circulatory system, humans 143,
149
cirrhosis of the liver 221
class 11, 12
classification
artificial and natural 9
binomial system 10–12
dichotomous keys 10
organisms 2
cloning 245
of animals 285–7
cnidaria 8
co-dominance 301–2
cocaine 221, 252
coccyx 200, 201
cochlea 233
coconuts 265
cohesion 165
collecting duct 185, 187
colon 115
colon cancer 116
colonisation 315–16
colostrum 253
commensalism 29–30
communicable diseases 155
community 16
companion cells 162
competitive species 65
compost 57, 59
concentration gradient 84, 85, 137
condoms 253, 254
cones 230
conjunctiva 227
conservation
environmental 72
resources 72–3
soil 73
water in plants 167–8
constipation 116
consumers 24–5, 26, 35, 92
continuous variation 312, 363
contraception 253–4
contraceptive pill 253, 254
converging lens 231
copper 20–1
copper sulfate 103
coral reefs destruction 71, 72
corms 175
cornea 227
corolla 261
coronary arteries 148
corpus luteum 249
cortex (kidney) 185
cortex (root) 165–6
cotyledons 176, 177
courtship 250
cows 29
cranial reflexes 215
cretinism 105
crop plants
diseases in 275
genetically engineered 321,
322
greenhouse 97
cross-pollination 262
crossing over 292, 293
crustaceans 8
cuticle 93, 94
cuttings 284
cystic fibrosis 322, 323
cytokinesis 279–81
cytoplasm 79, 80
Darwin, Charles 313, 315
Darwin’s finches 315
Darwin’s theory of evolution 294
daughter cells 291, 292
DDT 38–9, 314
death rates 55
decomposers 24–5, 28–9, 35
in the carbon cycle 43–4
in the nitrogen cycle 48
defecation 182
deficiency diseases 272
deforestation 46, 65, 69–70
and water shortage 66
degenerative diseases 55
denitrification 47
deoxygenated blood 132, 145, 149
depth of focus 229
desalination plants 66
detergents 67
detoxification 117
detritivores 28–9, 35
diabetes 108, 186, 272
dialysis 188
diaphragm (contraception) 253,
254
diaphragm (thorax) 134
diastole 145–6
dichotomous keys 10
dicotyledons 7
diet 101–6
balanced 101, 106–7, 271
see also food; nutrition
diet pills 220
diffusion 84–6
gaseous exchange 137
oxygen 85, 142–3
in the placenta 251–2
in a solution 338
digestion
along the alimentary canal
111–16
definition 108
enzymes in 110–11
teeth and 108–10
diploid number 282, 291, 297
disaccharides 101–2, 103, 110–11
discontinuous variation 312
diseases
blood’s role defending against
153–5
communicable 155
hereditary 272, 303–4
pathogenic 272, 273
in plants and animals 275
and population growth 54, 55
from protozoa 81
social and economic
implications 275
types and control of 271–2
vectors 236
from viruses and bacteria 80, 272
dispersal 260, 264
by animals 264–5
by explosive devices 266
by fruit 264–6, 360
by water 265
by wind 266
distal convoluted tubule 185, 187
diverging lens 231
DNA
and evolution 10
function of 46
replication 282
Dolly the sheep 286–7
domestic sewage treatment 69
domestic waste 57
control of 67
recycling 58
dominant allele 298
donors 151–2, 188
Down’s syndrome 319, 320
drug abuse 220–2
and HIV/AIDS 254
in pregnancy 252
drugs
definition 219
prescription 219–20, 252
DTP vaccine 155
duodenum 114
dyes 84, 105
ear sac 234–5
eardrum 233–4
ears 211
hearing 233
role in balance 234–5
and stimuli response 226
structure 232
earthworms 354
ecology 15–16, 23
definition 15
speciation 317
study of 331–4
economic implications
diseases 275
369
Index
drug abuse 222
HIV/AIDS 255
ecosystems 16–17
productivity 36
ectoparasites 30
edaphic factors 16
effectors 211–13, 214
egestion 108, 182
eggs 178
egrets 29
electrical impulses 213
electron microscopes 78
embryos 250–1
emulsification 114
endangered species 64, 65
protection 73
endemic species 64, 65
endocrine glands 213, 217–19
endocrine system 217–19
endoparasites 30
endoskeleton 199
endosperm 176, 177
energy
gain and loss in animals 34–5
gain and loss in plants 34, 35
nutritional requirements 106
pyramids of 36–7
resources 56
from respiration 123–4
solar 33–4
environment
adaptation to 313
carrying capacity 53
conservation and restoration 72
and genetic variation 311
and genotype 296–7
and humans 63–4
marine and wetland 70, 71
and shopping 59
waste products and 57–9, 64, 67
environmental factors 15–16, 17
enzymes
action of 345
in digestion 110–11
optimum temperature 236
in photosynthesis 96
in the small intestine 114
epigeal germination 177
epiglottis 112
Eryngium foetidum 11
Escherichia coli 4
ethanol test 104
etiolation 97
eukaryotes 3
eutrophication 67, 69
evaporation 164
evolution
Darwin’s theory of 294
and DNA 10
and natural selection 313–17
excretion 3, 182
excretory products
in animals 182–3
in plants 183–4
excretory system, human 184–8
exercise
and health 271
370
and heat generation 239
and respiration 125–6, 183
exocrine glands 217–18
exoskeleton 199
expiration 133, 134
exponential growth phase 52
extensor muscles 203
extinction of species 64–6
and deforestation 70
eyes 210
and stimuli response 226
structure 226–7
eyesight 227–8
accommodation 228–9
defects and corrections 230–2
and pupil size 229–30, 357
facet 202
faeces 115–16, 182
Fallopian tubes (oviducts) 247,
250
family (classification) 11, 12
family trees 305
fats 106
as food store 178
see also lipids
fatty acids 102, 110–11
assimilation 117
feedback mechanisms 190–2
Fehling’s solution 103
female
nutritional requirements 106–7
reproductive system 247
fermentation 126, 127
fertilisation
in humans 250
in plants 260, 263–4
fertilisers 48, 67
fetus 251–2
fibre 101, 116
fibrin 151
firming agents 106
fish 9
adaptation to water 18
gaseous exchange in 136, 137
movement 354
fishing 65, 71
flaccid cells 86, 87, 94–5
flavourings 105
flexor muscles 203
flies 273
flooding 70
flowers
colour 300, 301
gametes in 260, 261, 263
pollination 262–3
structure 261
fluorides 110
focusing 227–9, 231
food
absorption 108, 111–16
additives 105–6
from animals 106
diffusion in the body 85
fruit as 264–5
genetic engineering 321
plants as 91–2
in respiring cells 123
as stimulus 209
tests 103–4, 346
transport in the body 143, 148
transport through plants
168–70
see also diet; nutrition
food chains 24, 25–6
bioaccumulation 38–9
energy movement through
35–6
predator/prey relationships 27
pyramids of energy 37
pyramids of numbers 37–8
food storage
in animals 178
carbohydrates 80
in fruits 176, 361
minerals and vitamins 117, 178
in plants 173–8
food webs 27, 31
formula milk 253
fossil fuels 56
and acid rain 49
combustion 43, 44–5, 68
fovea 227, 230
fresh water 17–18
fruits 106
development 263–4
dispersal 264–6, 360
food storage 176, 361
formation 260
fungi 3, 5–6
in the carbon cycle 43
in decomposition 28
Galapagos Islands 313, 315
Gambusia hubbsi 317
gametes 245, 246, 248, 302–3
in flowers 260, 261, 263
formation 290–1
variation of 293
see also ovum; spermatozoa
gaseous exchange
adaptations for 136–7
in the heart 145
in humans 130–4
in plants 135, 136, 137
surfaces 135–6
gastric juice 113
gene banks 322
gene pool 314
genes 279, 297
see also alleles
genetic diagrams 298–9
genetic disorders 303–4
genetic engineering 285, 321–2
implications of 322–3
genetic variation 310
and the environment 311
importance of 311–12
loss of 322
and mutation 319
in offspring 293–4
genetically modified organisms
(GMOs) 322
genotype 296–7, 298–9
co-dominance 301
haemophilia 303
incomplete dominance 300
sickle cell anaemia 304
test cross 300
genus 11, 12
geographical isolation 315
geotropism 197–8, 351
germination 177
gestation period 250
giraffes 313
glaucoma 232
gliding joints 203
global
distribution of disease 272
distribution of HIV/AIDS 255
temperature range 235, 236,
237
global warming 45–6
glomerulus 185–6
glucose 34, 46
in aerobic respiration 123–4
in anaerobic respiration 125, 126
assimilation 116
ball-and-stick model 101
blood glucose levels 192
from hydrolysis 112
manufacture in plants 92–3, 96
selective reabsorption 186
tests 103
glycerol 102, 110–11
glycogen 102, 178
goitre 105
golden rice 322
gonads see ovaries; testes
gonorrhoea 255
Graafian follicle 249
grafting 284
gravity see geotropism
grease spot test 104
greenhouse effect 45–6
greenhouse gases 45, 72
see also carbon dioxide
greenhouse plants 97
grey matter 216
ground water, depletion of 66
growth 3
hormones 219, 318
movements 197–8
see also population growth
guard cells 94
Guyana, bauxite industry 56
gynaecium 261
habitat destruction 65
see also deforestation
habitats 16
haemoglobin 150–1, 183, 319–20
haemophilia 303
haemorrhage 151
haemorrhoids 116
hair cells 233, 234
hair colour 297–9, 305
hair erector muscles 238, 239
halophytes 168
haploid number 290–1
health 271
risks to 322
Index
healthy lifestyle 152, 271
hearing 233
heart
action of the 144–6
blood supply 144
section through 144
structure of 143–4
heart disease 108
and smoking 137
heartbeat 145–6
heat
conservation and loss 238
flow from a warm object 358
production 117, 183, 239, 348
see also temperature
heavy metals 20–1, 68
Heimlich manoeuvre 113
Helicobacter pylori 114
herbicides 199
herbivores 25, 26, 35
hereditary diseases 272, 303–4
heroin 252
heterotrophs 91, 92
heterozygous 297–9
co-dominance 301–2
incomplete dominance 300
for sickle cell anaemia 320
hinge joints 204
HIV/AIDS 254–5
pathogens and 274–5
holozoic nutrition 108
homeostasis 189–90, 217, 236
homeothermic animals 20, 235
Homo species 12
homologous pairs 291–2, 293, 297
homozygous 297–9
for sickle cell anaemia 320
hormones
antidiuretic 189, 219
gonads 248
growth 219, 318
injectable 254
during menstrual cycle 249
plant 199
secretion 182, 218, 219
transport 143, 148
hosts 30, 273, 274
human activities
and the carbon cycle 44–5
effects in the Caribbean 71–2
and marine/wetland
environments 70, 71
and resource consumption
57–9
and species extinction 64–6, 70
and water shortage 66
see also deforestation;
industrialisation; pollutants;
pollution
human cells 291, 297, 302
humans
alimentary canal 111, 115
anaerobic respiration 125–6
body temperature 235, 236,
237–9
chromosome number 279
circulatory system 143, 149
classification 11–12
diffusion in 84–6
ears 232–5
efficiency of food chains 36
endocrine system 217–19
and the environment 63–4
excretory system 184–8
eyes/eyesight 226–32
fertilisation 250
gaseous exchange 130–4
gaseous exchange in lungs
136–7
minerals needed by 104–5
movement in 355
nervous system 211–17
nutritional requirements 106–7
and plant/animal diseases 275
population growth 54–5, 64
reproduction in 246
respiratory system 131
sense organs 210–11
skeleton 199–204
skin section 237
teeth 109
vitamins needed by 103
humidity, and transpiration rate
167
humus 28, 337
hunting 65
hydrochloric acid 113, 114
hydrogen peroxide 345
hydrophytes 167, 168
hypermetropia 231
hypertension 108, 152, 272
hypertonic solution 86, 87
hyphae 5
hypogeal germination 177
hypothalamus 189, 216, 219, 237
hypotonic solution 86, 87
identical twins 279, 285
differences between 297
variation 311
ileum 114–15
immovable joints 203
immune response 154
immune system 254
immunisation 155
Impatiens 300, 301
implantation 250
inbreeding 318
incisors 109
incomplete dominance 300
individuals
adaptation to the environment
313
in a population 331–2
Industrial Revolution 45
industrial sewage treatment 69
industrialisation 64, 70
and pollution 68
and water shortage 66
infectious diseases 55
influenza 272
ingestion 108
inner ear 232–3
inorganic nutrients 101, 104–5
insecticides, resistance to 314
insects 8
aphids 170
pollination 262–3
inspiration 133, 134
insulin 218, 321
intercostal muscles 134
International Union for the
Conservation of Nature (IUCN)
64
interphase 279–81, 292
intestine see large intestine; small
intestine
intra-uterine device 253, 254
invasive species 53
invertebrates
response to stimuli 210
see also insects
iodine 105
iris (eye) 226, 227–8
iron 104, 150, 183
deficiency 272
irritability 3, 209
isotonic solution 86
Jamaica, mineral resources 56
Jamaica Hope 319
Jamaica Red Poll 319
joints 202
types of 203–4
kidneys 184–8
failure 188
longitudinal section 185
osmoregulation 184, 189
pressure filtration 186
selective reabsorption 186
transplants 188
kingdoms 3–4, 11, 12
knee jerk reflex 215, 357
kwashiorkor 107
lab write-up 328–9
labour 252
lactation 106–7
lacteal capillary 115
lactic acid 126
lamellae 136, 137
land pollution 67
large intestine 115
leaching 48, 70
lead 20–1
leaves
adaptations for photosynthesis
93–5
cells and tissues 82
chloroplasts in photosynthesis
95–6
evaporation of water from 164
food storage 174
gaseous exchange 135, 136,
137
movement of water within 164
section of a 94
starch in 340
legumes 106
leguminous plants 29, 47
lens 227–8
accommodation 228–9
converging 231
diverging 231
Lesser Antilles 315–16
life, characteristics of 2, 3
lifestyle, and hypertension 152,
272
ligaments 202–3
light
abiotic factor 333
in photosynthesis 92–3, 96,
97, 341
phototropism 197–8, 210, 353
and species distribution 20
and transpiration rate 167
light microscopes 78
light rays, focusing 227–9, 231
lightning 47
lignin 161
limbs, movement 202–3
limiting factors 96–7
line transects 332
Linnaeus, Carl 10–11
lipase 114
lipids 101, 102
digestion 110–11, 114, 115
metabolism 117
tests 104
see also fats
liver
cirrhosis 221
food storage 178
functions of 117
living organisms
binomial system 10–12
cell specialisation 81–3
classification of 2
decomposition 28–9
ecology and environment 15–
16, 23, 331–4
major groups 3–9
organic compounds in 42–3
respiration 122
transgenic 321
variety of 2–14
visible characteristics 9–10, 330
see also animals; plants
lizards 236
locomotion 196
long bone 199, 201
long-sightedness 231
loop of Henlé 185, 186, 187
lumbar vertebrae 200–2
lung cancer 137, 138
lungs
diffusion in 85
gaseous exchange 131–3, 136
lymphocytes 150, 154, 254, 255
magnesium 92, 100, 104
magnification 78, 79
malaria 5, 30, 273–4
susceptibility to 320
male
nutritional requirements 106–7
reproductive system 246–7
371
Index
malnutrition 107
malpighian layer 237
mammals
characteristics 9
reproductive cells 359
respiring cells 123
temperature regulation 238
mangroves 6
destruction of 71
zones of vegetation 19
marasmus 107
marijuana 139, 220
mechanical dispersal 266
medulla 185
medulla oblongata 216
meiosis 245, 246
importance of 290–1
process of 291–2
significance of 293–4
vs mitosis 292
memory lymphocytes 154
meninges 216
menopause 249
menstrual cycle 249
menstruation 247, 248
mercury 20–1
mesophyll cells 94, 95
mesophytes 167
metabolism 117, 181–2
proteins 117, 183
metaphase 279–81
metaphase I/II 292, 293
mice, coat colour 305
microhabitats 16
micronutrients 20
microorganisms 80
see also bacteria; viruses
microscopes 78
microvilli 114
middle ear 232–3
migration 317
milk teeth 109
mined areas 73
minerals (dietary) 101, 104–5
requirements 107
storage 117, 178
minerals (resources) 56
minerals (soil) 168
mitochondria 79, 124
mitosis 244, 279–81
and asexual reproduction
282–8
process of 282
vs meiosis 292
MMR vaccine 155
molars 109
molluscs 8
monocotyledons 7
monosaccharides 101–2, 103,
110–11
monosodium glutamate 105
mosquitoes 273
controlling 274
DDT-resistant 314
mosquitofish 317
motor neurones 211–13
response to stimuli 214
372
mouth 111–12
movement 3
in animals 196, 354–5
and balance 234–5
by diffusion 84–6
energy through food chains
35–6
joints 204
limbs 202–3
mineral salts in plants 168
by osmosis 86–7
in plants 163–6, 197–9
skeleton and 199
substances in cells 83
water through a plant 163–6
see also transport system
muscle cells 82
muscles
movement 202–3
see also individual muscles
mushrooms 6
mutagens 319
mutation 282, 319–20
mutualism 29, 47
myopia 231
natural classification 9
natural immunity 154
natural selection 294, 313
and evolution 313–17
investigating 364
vs artificial selection 318
near-sightedness 231
nectar 262
negative feedback 191–2
nematodes 8
nephrons 185–6
nervous pathway 212, 213
nervous system 211–17
neural canal 202
neural spine 202
neurones 82, 211–13
niches 16
nicotine 137–8, 252
nitrates 47, 48
nitrification 48
nitrogen 47, 92, 100–1, 104
cycle 46–8
fixation 47–8
nitrogen oxides 47, 49
nitrogenous waste 143, 183, 184
in the kidneys 184
non-biodegradable materials 57, 67
non-renewable resources 55
nose 211, 225
nucleus 79
in cloning 286
formation 281
numbers, pyramids of 37–8
nutrition 3
holozoic 108
human requirements 106–7
malnutrition 107
see also diet; food
obesity 55, 108, 178
ocelot 12
oesophagus 112–13
oestrogen 248, 249, 250, 253
offspring
genotype and phenotype 299,
300
sex determination 302, 362
variation among 293–4
oil pollution 67
omnivores 25
opportunistic infections 254, 255
optic nerve 227
damage to 232
order 11, 12
organelles 79
organic nutrients 101–4
organs 82–3
protection of 199
sense organs 210–11, 225–6
osmoregulation 184, 189
negative feedback 191
osmosis 86–7, 95–6, 164, 165–6
effects of 339
ossicles 233
osteoporosis 138
outbreeding 318
outer ear 232–3
oval window 233
ovaries 246, 247
hormones 248
in plants 264
over-fishing 65, 71
ovulation 249
ovules 261, 262, 264
ovum 246, 247, 359
cloning of fertilised 286
fertilisation 250
release of 248, 249
oxygen
in air 133
biochemical demand 68
diffusion 85, 142–3
from photosynthesis 92–3, 96,
344
in respiration 123, 349
transport in blood 150–1
transport in the body 143, 148,
150
see also gaseous exchange
oxygen debt 126
oxygenated blood 132, 145, 149
oxyhaemoglobin 151
oxytocin 252
‘pacemaker’ 146
painkillers 220
palisade layer 93, 94
pancreas 218
pancreatic juice 114, 218
parasitism 30
partially movable joints 203
parturition 252
passively acquired immunity 154,
155
pathogenic diseases 272, 273
pathogens 153–4, 272, 273, 274–5
pedigree charts 304–5
pelvis 185
penicillin 6, 220, 313
penis 246, 250
peppered moth 314
pepsin 113
peptic ulcers 114
perennating organs 176
pericarp 264
peripheral nervous system (PNS)
211
peristalsis 113
permanent teeth 110
permanent vacuole 79, 80
permeable membrane 84
pesticides 199
bioaccumulation 38
DDT 38–9, 314
effects and control of 67
petals 261, 262, 263
pH 49, 333
optimum 111
phagocytes 150, 153
phenotype 298–9
co-dominance 301
and the environment 311
haemophilia 303
incomplete dominance 300
sickle cell anaemia 304
test cross 300
phloem 93, 94
and movement of food 169–70
structure of 162
in vascular bundles 163
phosphorus 104, 105
photometer 350
photosynthesis 24, 161
and carbon dioxide 92–3, 95,
97, 343
and chlorophyll 342
chloroplast involvement in 95–6
equation 33, 92
leaf adaptations for 93–5
and light 92–3, 96, 97, 341
limiting factors in 96–7
oxygen from 92–3, 96, 344
waste products 183
phototropism 197–8, 210, 353
phylum 7–8, 11, 12
physical digestion 108
physiological diseases 272
phytoplankton 24
pinna 233
pituitary gland 189, 216, 219
pituitary growth hormone 219
pivot joints 203
placenta 251–2
plant cells 79–80
osmosis in 86–7
in respiration 125
plant growth
and gravity 351
and light 353
plant hormones 199
plants 3, 6–7
and acid rain 49
adaptation to light 20
adaptation to water 18–19,
167–8
Index
in the carbon cycle 43, 44
excretory products 183–4
fertilisation in 260, 263–4
food manufacture 91–2, 100
food storage in 173–8
gain and loss of energy 34, 35
gaseous exchange 135, 136,
137
heavy metal tolerance 21
leguminous 29, 47
life cycle 259–60
minerals and 104, 168
movement in 163–6, 197–9
in the nitrogen cycle 46–7
photosynthesis see
photosynthesis
as producers 24–5, 26
response to stimuli 210
selective breeding 318
tissues, organs and systems 83
transport system 161–3
transport through 160–1,
168–70
variation 311
see also crop plants; flowers;
leaves; living organisms;
roots; stems
plasma proteins 117
platelets 149, 150
platyhelminthes 8
plumules 177
poikilothermic animals 20, 235–7
pollen grains 261, 262, 263
pollination 260, 262–3
pollutants
acid rain 49
fluorides as 110
heavy metals 20
origin, effects and control 67–8
pollution 66–9
from human activities 20, 64
mangrove swamps 71
reduction of 72
and water shortage 66
polysaccharides 102, 103, 110–11
pond ecosystem 16–17
population 16
geographical isolation 315
in natural and artificial
selection 318
size 331–2
population growth 52–3
factors reducing 53–4
human 54–5, 64
postnatal care 252
potassium 104
potassium manganite 338
predators 26–7
and population growth 54
pregnancy 250
alcohol during 221, 252
drug abuse in 252
fetus 251–2
and HIV/AIDS 254
nutritional requirements 106–7
premolars 109
prenatal care 252
prescription drugs 219–20, 252
preservatives 105
pressure filtration 186
pressure–flow hypothesis 169
prey 26–7
primary consumers 26, 35
producers 24–5, 26, 35–6, 92
productivity 35–6
products (in digestion) 111
progesterone 249, 250, 253
prokaryotes 3, 4–5
see also bacteria
prolactin 253
propagation see vegetative
propagation
propellants 105
prophase 279–81
prophase I/II 292
proteins 101, 102
animal and plant 46–7
digestion 110–11, 113, 114
metabolism 117, 183
requirements 107
tests 104
see also enzymes
protoctists 3, 5
protozoa 5, 81
see also amoeba
proximal convoluted tubule 185,
186
psychoactive drugs 220
puberty 248
pulmonary circulation 149
pupil (eye) 226, 227–8
effect of size 229–30, 357
reflex 215
pyramids
biomass 38
energy 36–7
numbers 37–8
quadrats 334–5
radicles 177, 352
radioisotopes 169
reabsorption 186, 187
receptacles 261
receptors 211–13, 214
touch 356
recessive allele 298
recipients 151–2, 188
rectum 115–16
recycling 58, 59
red blood cells 149–50
antigens 151–2
breakdown 117
manufacture of 199
oxygen transport 151
sickle shaped 319–20
waste products from 183
reduction of waste 57, 59
reflex actions 215, 357
reflex arc 215
refraction 227–8
relay neurones 212
renal artery 184, 187–8
renal vein 184, 187–8
sedimentation test 334
seedlings 177
growth in 198
movement in 197
seeds 176–7, 260
development 263–4
dispersal 264–5
see also fruits
selection pressure 314, 318
selective advantage 313
selective breeding 318
selective reabsorption 186, 187
selectively permeable membrane
86, 188
self-dispersal 266
self-pollination 262
semen 246
semi-lunar valves 145
semicircular canals 234
seminiferous tubules 246
sense organs 210–11
stimuli responding to 225–6
sensory neurones 211–13
response to stimuli 214
sepals 261
sewage 67
treatment 68–9
sex chromosomes see gametes
sex determination 302, 362
sex-linked characteristics 303
sexual intercourse 246, 250
sexual reproduction 245
in plants 259–60
shoots see stems
shopping, environment and 59
sickle cell anaemia 272, 282,
303–4
and mutation 319–20
sickle cell disease 320
sickle cell trait 320
side effects 219
sieve tubes 162, 169
sight see eyesight
sigmoid growth curve 53
skeleton 199–204
functions of 199
skin 211, 235
care of 239
physical barrier 153
section through 237
and stimuli response 226
temperature control in animals
235–7
temperature control in birds 239
saccule 234, 235
temperature control in humans
sacrum 200, 201
237–9
saliva 112
touch receptors 356
salivary glands 217
small intestine 114–15
salt water 17–18, 66
cells and tissues 82
sampling methods 331–3
smoke 68
saprophytes 29, 91, 92
smoking 55, 137–8
sclera 226, 227
in pregnancy 252
scurvy 103
and skin damage 239
sea levels 46
social implications
sebaceous glands 237
diseases 275
secondary consumers 26, 35
drug abuse 222
secondary sexual characteristics 248
HIV/AIDS 255
secretion 83, 182, 218, 219
renewable resources 55, 56
rennin 113
replication 282
reproduction 3, 244
in humans 246
see also asexual reproduction;
sexual reproduction
reproductive cells 290–1, 359
reproductive system
female 247
male 246–7
reptiles 9
resources
conservation of 72–3
destruction of 70
energy 56
limits of 55
mineral 56
reducing consumption 56–9
respiration 3, 34
aerobic 122–4, 125
anaerobic 125–7
carbon dioxide from 123, 182,
347
equation 43
gases involved in 85
heat production from 348
oxygen in 123, 349
waste products 123, 182–3
respiratory system, human 131
response to stimuli 209–10, 214
in sense organs 225–6
retina 210, 227–8, 230
reuse of waste 57
Rhizobium 29, 47
rhizomes 175
rhythm method 253
ribs 134
rickets 103
ringing 170
rods 230
root hair cells 166
root pressure 165
root tubers 176
roots
development 177
food storage 176
movement of water across
165–6
tropism 198
vascular bundles 163
runners 283
373
Index
soil
abiotic factor 334
conservation 73
degradation 70
erosion 69–70
moisture in 209
percentage of air in 337
percentage of water in 336
and species distribution 21
water-holding capacity 335
solar energy 33–4
sound waves 233–4
species 11, 12
Anolis lizards 316
chromosome number 278, 279,
281
Darwin’s finches 315
distribution of 17–21
and ecology 317
endangered and vulnerable 64,
65, 73
extinction 64–6, 70
genetic variation 310, 311
genotype 296
invasive 53
survival of 294, 311
spermatozoa 246, 248, 359
release of 250
spermicide 253, 254
sphincter muscles 184
spinal cord 216, 217
spinal reflexes 215
spores 6
stamens 261, 262, 263
staple foods 106
starch 96, 102
hydrolysis 112
in a leaf 340
storage in plants 174
tests 103
starvation 107, 321
STDs (sexually transmitted
diseases) 254–5
pathogens and 274–5
protection against 253
stems
development 177
food storage 174–5
phototropism 197–8
vascular bundles 163
sterilisation 253
steroids 220, 318
stigma 261, 262, 263
stimulus 209–10
in the nervous pathway 212,
213
receptors, effectors and
response 214
reflex actions 215
response to sense organs 225–6
stolons 283
stomach 113–14
stomata 93, 94–5, 135
in evaporation 164
374
stroke 108, 152
sub-phylum 11, 12
substrate 111
sucrose 96, 102
tests 103
transport 169
sulfur 104
sulfur dioxide 49, 68
surrogate mother 285–7
survival of a species 294, 311
suspensory ligaments 227, 228,
229
sustainability 72
sutures 203
sweating 238
sweep nets 333
sweeteners 105
symbiosis 29–30
synapse 213
synovial joints 204
systemic circulation 149
systemic herbicides 199
systems 82–3
systole 145–6
T-lymphocytes 254, 255
tap roots 176
tar 138
tartrazine 105
taste buds 211, 225
tear glands 217, 226
teeth 108–10
telophase 279–81
telophase I/II 292
temperature
abiotic factor 333
and enzyme catalysis 111
global range 235, 236, 237
global warming 45–6
and photosynthesis 96
and species distribution 20
and transpiration rate 167
see also body temperature; heat
tendons 203
terrestrial food chains 26
terrestrial food webs 27, 31
tertiary consumers 26, 35
test cross 300
testa 177
testes 246
hormones 248
testosterone 248
thoracic vertebrae 200–2
thorax 134
tissue culture 284–5
tissue fluid 190
concentration 191
tissues 82–3
tongue 211, 225
touch receptors 356
toxic chemicals 20, 67, 68
waste products 182
trace elements 104
trachea 112, 132, 133
tranquillisers 220
transgenic organisms 321
translocation 168–70, 199
transpiration 164, 166–7, 350
pull 165
rate 167
transport system
in animals 142–3
food through plants 168–70
in plants 161–3
see also blood; heart;
movement
transverse process 202
trees, commensalism 29
tricuspid valve 145
Trinidad and Tobago, energy
resources 56
trophic levels 26
bioaccumulation 39
pyramids of energy 37
pyramids of numbers 37, 38
tropisms 197–8
trypsin 114
tubal ligation 253, 254
tubers 175, 176
turgid cells 86, 94–5
tympanic membrane 233–4
under-eating 107
underground stems 174–5
unicellular organisms 80–1
urea 183
ureter 185
urethra 184
urine 184, 187
concentration 189
production 185
uterus 247
fetal development 251
lining 249, 250
utricle 234, 235
vaccination 155
vagina 250
vascular bundles 163
vasectomy 253, 254
vasoconstriction 238
vasodilation 238
vectors 236, 273–4, 322
vegetables 106
vegetative propagation 245, 283
artificial 284–5
veins 146, 147–8
vena cava 148
ventricles 144–6
venules 146, 147
vertebrae 200–2
vertebral column 200–2
vertebrates 8–9
vesicles 83
vestibular apparatus 233, 234
villi 114–15
viruses 4, 80, 272
vectors 322
visible characteristics 9–10, 330
vitamins 101, 103
requirements 107
storage 117, 178
vulnerable species 64
waste products
build-up of 190
cellular 83, 148
environmental 57–9, 64, 67
of photosynthesis 183
respiration 123, 182–3
see also excretory products;
nitrogenous waste
water
conservation in plants 167–8
in the diet 101
and disease 274
dispersal by 265
flow 334
movement through a plant
163–6
in photosynthesis 92–3, 95–6,
97
reabsorption 187
shortage of 66
in soil 335–6
and species distribution 17–19
water pollution 67–8
mangrove swamps 71
white blood cells 143, 149–50
defence against disease 153,
154
manufacture of 199
white matter 216
WHO Expanded Program of
Immunisation 155
wind
dispersal 266
pollination 262–3
speed 333–4
and transpiration rate 167
wisdom teeth 110
withdrawal symptoms 220, 221
wolves, body hair 311
xerophytes 18, 167, 168
xylem 93, 94, 95
structure of 161–2
in vascular bundles 163
in water movement 164, 165
yeast 6
anaerobic respiration 126
budding 359
yoghurt manufacture 127
zones of vegetation 19
zygote 250
division 285, 286
Biology
3rd Edition
for the 2013 syllabus
for CSEC® Examinations
Biology for CSEC® Examinations is part of a well-established series
of books aimed at students preparing for their CSEC Science studies.
Rejuvenated in a third edition, Biology for CSEC® Examinations features
comprehensive, systematic coverage of the latest CSEC syllabus (2013).
Written by an expert team of science educators, this revised edition
benefits from a new, clear and accessible design and the most up
to date scientific information.
Also available in the CSEC Science series:
Key features of the CSEC Science series:
Linda Atwaroo-Ali is Head of Science at St Joseph’s Convent
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3rd Edition
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‘O’ and ‘A’ levels all over the world, and has taught chemistry at
School and University levels for over forty years.
Linda Atwaroo-Ali
• Intuitive and easy-to-follow format makes it simple to study a whole
topic, or to find answers to specific problems
• Regular consolidation (in-text questions and exam preparation) checks
understanding and reinforces learning
• New group-work feature tests students’ investigative and problemsolving skills and demonstrates real-world applications of key
syllabus points
• Practical activities and experiments throughout the text encourage
hands-on learning
• Dedicated School-Based Assessment section gives step-by-step tips
to maximise success in the CSEC coursework.
Biology for CSEC® Examinations
3rd Edition for the 2013 syllabus
Biology
for CSEC® Examinations
Linda Atwaroo-Ali