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
4SPNO[WHZZLZ[OYV\NOSLUZ
THNUPMPJH[PVU_
3SPNO[WHZZLZ[OYV\NO[OLZSPKL
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2SPNO[WHZZLZ[OYV\NO[OLMPS[LY
HUKJVUKLUZLY
1SPNO[YLMSLJ[ZVUTPYYVY
HUK[YH]LSZ\W[V[OLL`LWPLJL
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
Z[YPH[PVUZPUJLSSZ
JHUZOVY[LUVYSLUN[OLU
is concerned with contraction of
the muscle, while a nerve cell
is specialised to transmit nerve
[OLZLJLSSZ
PUZ\SH[L[OL
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:
4\ZJSLJLSSZ
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 Examples of tissues, organs and systems in plants and animals.
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.
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 ❯
:\IZ[HUJLZTV]PUNV\[VM
[OLJLSS;OLZL^LYLTHKL
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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
PU[OLISVVK
VULJLSS[OPJR
Figure 8.13 Diffusion of gases between
the lungs and the blood.
ISVVKJHWPSSHY`
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ISVVKJHWPSSHY`^P[OISVVK
YPJOPUV_`NLU
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
PUZPKLJLSS
‹UL[MSV^VM^H[LY
intoJLSS
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flaccid cell
‹V\[ZPKLO`WLY[VUPJ[V
TVYLJVUJLU[YH[LK
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‹UL[MSV^VM^H[LYout
of[OLJLSS
‹JLSSTLTIYHULW\SSZ
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
‹JLSSPUO`WLY[VUPJ
ZVS\[PVU
‹UL[MSV^VM^H[LY
V\[VMJLSS
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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
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Autotrophic nutrition
Heterotrophic nutrition
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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.
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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
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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.
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JOSVYVWO`SSPZWYLZLU[
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1 HPYZ\YYV\UKPUN[OLSLHM
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*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
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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
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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
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[OLHPYZWHJL^OLYLP[Z
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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
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*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[
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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