THE OPEN UNIVERSITY OF TANZANIA
FACULTY OF SCIENCE TECHNOLOGY AND ENVIRONMENTAL STUDIES
OEV 107: GENERAL BIOLOGY
EXTENDED COURSE OUTLINE
DR. EMMANUEL S.P. KIGADYE
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THE OPEN UNIVERSITY OF TANZANIA
FACULTY OF SCIENCE TECHNOLOGY AND ENVIRONMENTAL STUDIES
OEV 107: GENERAL BIOLOGY
EXTENDED COURSE OUTLINE
By
Dr. Emmanuel S.P. Kigadye
B.Sc., M.Sc., Ph.D. (Dar)
Senior Lecturer in Biology
Open University of Tanzania
2008
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COURSE RATIONALE
Since students enrolled in science stream of the environmental studies may have different
backgrounds, there is need to provide sufficient knowledge in modern biology to better
understand environmental studies. This course is designed to provide students with basic
background information on major groups of living things including plant kingdom and
animal kingdom. Also the course will provide an introduction to microbiology.
Learning outcomes
On completion of the course students will be able to:
•
Describe the origin and nature of life,
•
Clearly explain the distinctive features between plants and animals,
•
Apply diversity and unity knowledge in living world to study forms and
organization of the different organisms,
•
Identify major groups of microorganisms and their economic importance.
Mode of Assessment
Continuous Assessment
Written assignments
15%
Timed tests
25%
Final examination
60%
Students are encouraged to read reference text books to supplement the material in this
course outline. In addition, since this outline contains no illustrations students are advised
to study illustrations in different text books.
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TABLE OF CONTENT
COURSE RATIONALE................................................................................................... iii
TABLE OF CONTENT.....................................................................................................iv
LECTURE ONE.................................................................................................................1
ORIGIN OF LIFE...............................................................................................................1
1.0 Introduction ..............................................................................................................1
1.1 Ideas about the origin of life..................................................................................2
1.2 Early Earth............................................................................................................3
1.3 Simple Single Celled Animals to Complex Organisms ..........................................4
SUMMARY ...................................................................................................................8
REFERENCES ...............................................................................................................9
LECTURE TWO ..............................................................................................................10
PLANT AND ANIMAL CLASSIFICATION ...............................................................10
1.0 Introduction ............................................................................................................10
2.0 Principles and Methods of classification..................................................................11
2.1 Nomenclature......................................................................................................11
2.2 Binomial nomenclature .......................................................................................12
2.3 Trinomial nomenclature ......................................................................................12
3.0 Theories of Taxonomy ............................................................................................13
3.1 Traditional Evolutionary Taxonomy....................................................................14
3.2. Phyogenetic Systematics/Cladistics ....................................................................14
3.3. Species Concept .................................................................................................15
3.4 The kingdoms of life ...........................................................................................15
SUMMARY .................................................................................................................16
REFERENCES .............................................................................................................17
LECTURE THREE ..........................................................................................................18
THE PLANT KINGOM ...................................................................................................18
1.0 Introduction ............................................................................................................18
2.0 Characteristics of Plants ..........................................................................................19
3.0 Plant Life Cycle ......................................................................................................20
4.0 Plant classification ..................................................................................................20
4.3 The Gymnosperms ..............................................................................................25
4.4 The Angiosperms ................................................................................................27
SUMMARY .................................................................................................................29
REFERENCES .............................................................................................................30
LECTURE FOUR ............................................................................................................31
THE ANIMAL KINGDOM..............................................................................................31
1.0 Introduction ............................................................................................................31
2.0 Body structure.........................................................................................................32
3.0 Animal classification...............................................................................................36
3.1. Phylum Cnidaria ...............................................................................................39
3.3 Phylum Platyhelminthes.....................................................................................43
3.3.3. Economic Importance of Phylum Platyhelminthes...........................................46
3.4. THE PSEUDOCOELOMATES ........................................................................47
SUMMARY .................................................................................................................48
References ....................................................................................................................50
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LECTURE FIVE ..............................................................................................................51
PHYLUM MOLLUSCA...................................................................................................51
1.0. Introduction ...........................................................................................................51
2.0 Characteristics of Phylum Mollusca ........................................................................52
2.0.Classification of Phylum Mollusca..........................................................................53
2.1 Class Caudofoveata.............................................................................................53
2.2 Class Solenogastres.............................................................................................53
2.4 Class Polyplacophora ..........................................................................................54
2.5 Class Scaphopoda (tusk shells)............................................................................54
2.6 Class Gastropoda (Snails and relatives) ...............................................................55
2.7 Class Bivalvia (the bivalves) ...............................................................................55
2.8 Class Cephalopoda (squids and octopuses)..........................................................55
3.0 Economic importance of Phylum Mollusca .............................................................56
PHYLUM ANNELIDA ....................................................................................................57
4.0 Introduction ............................................................................................................57
4.1 Characteristics of Phylum Annelida.....................................................................58
4.2 Classification of Phylum Annelida ......................................................................58
4.3 Economic importance of Phylum Annelida..........................................................60
SUMMARY .................................................................................................................60
REFERENCES .............................................................................................................62
LECTURE SIX.................................................................................................................63
PHYLUM ARTHROPODA..........................................................................................63
1.0 Introduction ............................................................................................................63
2.0.Why Arthropods are the most diverse animals.........................................................64
3.0.Characteristics of Phylum Arthropoda.....................................................................65
4.0.Classification of phylum Arthropoda.......................................................................66
4.1 Subphlum Trilobita (triobite)...............................................................................66
4.2 Subphylum Chelicerata .......................................................................................66
4.3 Subphylum Crustacea..........................................................................................67
4.4 Subphylum Uniramia ..........................................................................................68
5.0 Economic importance of Insects..............................................................................73
SUMMARY .................................................................................................................74
REFERENCES .............................................................................................................75
LECTURE SEVEN ..........................................................................................................76
PHYLUM ECHINORDERMATA AND HEMICHORDATA ..........................................76
1.0 Introduction ............................................................................................................76
2.0 Characteristics of Phylum Echinodermata ...............................................................77
3.0 Classification of Phylum Echinodermata. ................................................................78
3.1 Class Crinoidea ...................................................................................................78
3.2 Class Asteroidea..................................................................................................78
3.3.Class Ophiuroidea ...............................................................................................78
3.4.Class Echinoidea.................................................................................................79
3.5.Class Holothuroidea ............................................................................................79
4.0 PHYLUM HEMICHORDATA ...............................................................................79
4.1 Introduction.........................................................................................................79
4.2 Classification of Phylum Hemichordata...............................................................80
4.4 Economic importance of Echinoderms and Hemichordates .................................80
SUMMARY .................................................................................................................81
REFERENCES .............................................................................................................83
LECTURE EIGHT ...........................................................................................................84
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PHYLUM CHORDATA ..................................................................................................84
1.0 Introduction ............................................................................................................84
2.0 The Non-vertebrate chordates .................................................................................85
2.1 Subphylun Urochordata.......................................................................................85
2.2 Subphylum Cephalochordata...............................................................................86
2.3 Subphylum Vertebrate.........................................................................................86
3.0.Overview of the evolution of vertebrates.................................................................88
4.0.Classification of Phylum Chordata ..........................................................................89
4.1. Group Protochordata (Acrania) ..........................................................................89
4.2 Group Craniata....................................................................................................90
SUMMARY .................................................................................................................91
REFERENCES .............................................................................................................93
LECTURE NINE..............................................................................................................94
CELL BIOLOGY .............................................................................................................94
1.0.Introduction ............................................................................................................94
2.0 Prokaryotic cells .....................................................................................................95
2.1 Eukaryotic cells...................................................................................................96
SUMMARY ............................................................................................................... 101
REFERENCES ........................................................................................................... 102
LECTURE TEN ............................................................................................................. 103
BASIC BIOLOGY OF VIRUSES BACTERIA AND FUNGI ........................................ 103
1.0 Introduction .......................................................................................................... 103
2.0 VIRUSES ............................................................................................................. 104
2.1The structure of viruses ...................................................................................... 105
2.2.Bacteriophages.................................................................................................. 105
2.3.Diseases viruses ................................................................................................ 105
2.4.Viroids .............................................................................................................. 106
3.0.BACTERIA .......................................................................................................... 106
3.1 Prokaryotes versus Eukaryotes .......................................................................... 107
3.2.The structure of bacteria.................................................................................... 108
3.3 Kinds of Bacteria .............................................................................................. 109
3.4 Economic importance of Bacteria...................................................................... 110
4.0 FUNGI.................................................................................................................. 111
4.1 The structure of fungi........................................................................................ 112
4.2 Fungi reproduction............................................................................................ 112
4.3 Nutrition ........................................................................................................... 112
4.4 Fungi classification ........................................................................................... 112
4.5 Economic importance of Fungi.......................................................................... 113
SUMMARY ............................................................................................................... 113
REFERENCES ........................................................................................................... 114
LECTURE ELEVEN...................................................................................................... 115
ELEMENTS OF BIOENERGETICS AND BIOCHEMISTRY....................................... 115
1.0.Introduction .......................................................................................................... 115
2.0.The Flow of Energy in Living Things ................................................................... 116
2.1 Oxidation-Reduction Reactions ......................................................................... 117
3.0 The laws of thermodynamics................................................................................. 117
3.1 The First law of Thermodynamics ..................................................................... 117
3.2 The second law of Thermodynamics ................................................................. 118
4.0..Entropy ................................................................................................................ 118
5.0 ENZYMES ........................................................................................................... 119
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5.1 How enzymes work........................................................................................... 120
5.2 Factors affecting Enzyme activity...................................................................... 120
6.0.Enzyme Cofactors................................................................................................. 121
7.0.What is ATP ......................................................................................................... 122
8.0 Biochemical Pathways .......................................................................................... 123
SUMMARY ............................................................................................................... 124
REFERENCES ........................................................................................................... 125
LECTURE TWELVE ..................................................................................................... 126
RESPIRATION .............................................................................................................. 126
1.0 Introduction .......................................................................................................... 126
2.0.The mechanisms of respiration in animals ............................................................. 127
3.0.Cellular respiration................................................................................................ 128
3.1 Glycolysis ......................................................................................................... 129
3.2 Aerobic Respiration .......................................................................................... 130
3.3 Anaerobic Respiration....................................................................................... 131
SUMMARY ............................................................................................................... 131
REFERENCES ........................................................................................................... 132
LECTURE THIRTEEN .................................................................................................. 133
NUTRITION AND GROWTH....................................................................................... 133
I.0 Introduction ........................................................................................................... 133
2.0 Animal Essential Nutrients.................................................................................... 133
3.0.Plants Essential Elements...................................................................................... 134
SUMMARY ............................................................................................................... 136
REFERENCES ........................................................................................................... 136
LECTURE FOURTEEN................................................................................................. 137
GENETICS .................................................................................................................... 137
1.0 Introduction .......................................................................................................... 137
2.0 Mendel and the Garden Pea................................................................................... 138
2.1 Mendel Experimental Design ............................................................................ 138
2.2 The F1 Generation ............................................................................................. 139
2.3 The F2 Generation ............................................................................................. 139
3.0 Mendel’s Model of Heredity ................................................................................. 140
4.0 Mendel First law of Heredity: Segregation ............................................................ 141
4.1 Mendel’s Second Law of heredity: Independent Assortment ............................. 141
5.0 Epistasis................................................................................................................ 141
6.0 Pleitropy ............................................................................................................... 143
7.0 Environmental Effects........................................................................................... 143
8.0.Mutations.............................................................................................................. 143
SUMMARY ............................................................................................................... 144
REFERENCES ........................................................................................................... 145
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LECTURE ONE
ORIGIN OF LIFE
1.0 Introduction
Before, we can address the origin of life; we must first consider what qualifies something as
“living”. What is life? This is a difficulty question to answer, largely because life itself is
not a simple concept. If you try to write a definition of life you will find that it is not an
easy task, because of the loose manner in which the term is used. However, all living things
on earth are share certain general properties, these properties define what we mean by life.
The following fundamental properties are shared by all organisms on earth.
i.
Cellular organization. All organisms consist of one or more cells: complex,
organized assemblages of molecules enclosed within membranes.
ii.
Sensitivity. All organisms respond to stimuli, though not always to the same
stimuli in the same ways.
iii.
Growth. All living things assimilate energy and use it to grow, a process called
metabolism. Plants, algae, and some bacteria use sunlight to create covalent
carbon-carbon bonds from CO2 and H2O through photosynthesis. This transfer
of the energy in covalent bonds is essential to life on earth.
iv.
Development. Multicellular organisms undergo systematic gene-directed
changes as they grow and mature.
v.
Reproduction. All living things reproduce, passing on traits from one
generation to the next. Although some organisms live for a very long time, no
organism lives forever, as far as we know. Because all organisms die, ongoing
life is impossible without reproduction.
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vi.
Regulation. All organisms have regulatory mechanisms that coordinate internal
processes.
vii.
Homeostasis. All living things maintain relatively constant internal conditions,
different from their environment.
viii.
Heredity. All organisms on earth posses a genetic system that is based on the
replication of a long complex molecule called DNA. This mechanism allows for
adaptation and evolution over time, also distinguishing the characteristics of
living things.
Objectives: At the end of this lecture you should be able to:
1. Describe the origin and nature of life
2. List the eight properties of life
3. Explain how the early earth was like
4. Make link between earth earliest living organisms to modern day organisms.
1.1 Ideas about the origin of life
The question on how life originated on earth is difficulty to answer. However, there are, in
principle at least three possibilities.
1. Special creation: Life-forms may have been put on earth by supernatural or divine
forces. The hypotheses of special creation, that a divine God created life are is at the
core of most major religions.
2. Extraterrestrial origin: Life may not have originated on earth at all; instead, life
may have infected earth from some other planet. The hypotheses of panspermia
proposes that meteors or cosmic dust may have carried life forms to earth, perhaps
as an extraterrestrial infection of spores originating on a planet of a distant stars. For
example recent discovered suggestions of fossils in rocks from Mars, and the
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discovery of liquid water on under the surface of mars and Jupiter’s ice shrouded
moon Europa might lend credence to this idea.
3. Spontaneous Origin: Life may have evolved from inanimate matter, as association
among molecules became more and more complex. Most scientists tentatively
accept the hypotheses of spontaneous origin that life evolved from inanimate matter.
In this view. In this view, the force leading to life was selection. As changes in
molecules increased their stability and caused them to persist longer, these
molecules could initiate more and more complex associations culminating in the
evolution of cells.
Spontaneous origin is the only testable hypotheses of life’s
origin currently available and it provides the only scientific explanation that can be
tested and potentially disproved.
1.2 Early Earth
After the big bang the primitive earth cooled and its rocky crust formed, volcanoes
blasted enormous amounts of material and gases from the earth’s molten core skyward.
These gases formed a cloud around the earth and were held as an atmosphere by the
earth’s gravity. The early atmosphere contained carbon dioxide and hydrogen gas, along
with significant amount of water. It also might have contained compounds in which
hydrogen atoms were bounded other light elements (sulphur, nitrogen and carbon).
These compounds would have been hydrogen sulphide (H2S), ammonia (NH3), and
methane (CH4). In addition the atmosphere was thought to be rich in hydrogen gas (H2).
The availability of hydrogen atoms and their associated electrons made such an
atmosphere to be referred as a reducing atmosphere.
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Earth’s first organisms emerged and lived at very high temperatures. Debris from the
forming solar system slammed into early earth from 4.6 to 3.8 billion years ago,
keeping the surface molten hot. As the bombardment slowed down, temperatures
dropped down. By about 3.8 billion years ago, ocean temperatures are thought to have
dropped to a hot 49-88°C (120-190°F). Between 3.8 and 3.5 billon years ago, life first
appeared on earth. The first step in the origin of life, therefore, probably took place in a
hot reducing atmosphere, devoid of gaseous oxygen, and different from the atmosphere
that exists now. On earth today, we are shielded by from the effects of solar radiation by
the ozone gas (O3) in the upper atmosphere.
ACTIVITY
Molecules that are the building blocks of living organisms form spontaneously under
conditions designed to simulate those of primitive earth. Read on experiments designed
for testing the spontaneous origin hypotheses.
1.3 Simple Single Celled Animals to Complex Organisms
1.3.1 Microfossils
The fossils found in ancient rocks show an obvious progression from simple to complex
organisms, beginning about3.5 billion years ago. The earliest evidence of life appears in
microfossils, fossilized forms of microscopic life about 1-2 micrometers in diameter,
single-celled, lacked external appendages, and having little evidence of internal
structure. The microfossil physically resembles present day bacteria. Organisms with
simple body plan are called prokaryotes, from Greek words meaning “before nucleus”,
because they lack a nucleus a spherical body characteristics of more complex cells of
eukaryotes (see Lecture nine). Judging from fossil records eukaryotes did not appear
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until about 1.5 billion years ago. Therefore, for at least 2 billion years-nearly half the
earth-bacteria were the only organism hat existed.
1.3.2 Ancient Bacteria: Archaebacteria
Most organisms living today are adapted to the relatively mild conditions of present day earth. However if we look in unusual environments; we encounter organisms that
are quite remarkable, differing in form and metabolism from other living things.
Sheltered from evolutionary alteration in unchanging habitats that resemble earth’s
early environment, these living relics are the surviving representatives of the first ages
of life on earth. In places such as the oxygen free depths of the black sea or the boiling
waters of hot water springs and deep sea vents, we can find bacteria living at very high
temperatures without oxygen.
These unusual bacteria are called archaebacteria, from the Greek words “ancient
ones.” Among the first to be studied in details have the methanogens, or methane
producing bacteria, among the most primitive bacteria that exist today.
These
organisms are typically simple in form and are able to grow only in an oxygen-free
environment, in fact oxygen poison them. Therefore, they grow without air
“anaerobically”. The methane producing bacteria convert CO2 and H2 into methane
gas (CH4). They resemble all other bacteria in that they posses hereditary machinery
based on DNA, a cell membrane composed of lipid molecules, an exterior cell wall, and
a metabolism based on an energy-carrying molecule called ATP. However, the
resemblance ends at that point.
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The methane-producing bacteria are survivors from an earlier time when oxygen gas
was absent. Other bacteria that fall in into this category are some of those that live in
very salty environments like the Dead Sea (extreme halophiles) or salt lovers. Those
that live in very hot environments such as hydrothermal volcanic vents under the ocean
are called extreme thermophiles (or heat lovers).
1.3.3 Eubacteria
The second major groups of bacteria, the eubacteria, have very strong cell wall and a
simpler architecture. Most bacteria living today are eubacteria. Included in this group
are bacteria that have evolved the ability to capture the energy of light and transform it
into the energy of chemical bonds within cells. These organisms are photosynthetic, as
are plants and algae.
One type of photosynthetic eubactreria that has been important in the history of life on
earth is the cynobacteria, sometimes called “blue-green algae”. The have the same
kind of chlorophyll pigments that is most abundant in plants and algae, as well as other
pigments that are blue or red. Cynobacteria produce oxygen as a result of their
photosynthetic activities, and when they appeared at least 3 billion years ago, the played
a decisive role in increasing in increasing the concentration of free oxygen in earth’s
atmosphere from below 1% to the current 21%. As the concentration of oxygen
increased, so did the amount of ozone in the upper layers of the atmosphere. The
thickening of ozone layer afforded protection from most of the ultraviolet radiation
from the sun, radiation that is highly destructive to proteins and nucleic acids. Certain
cynobacteria are also responsible for the accumulation of massive limestone deposits.
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1.3.4 Eukaryotic cells
In rocks of about 1.5 billion years old, we begin to see the first microfossils that are
noticeable different in appearance from the earlier simpler forms. These cells are much
larger than bacteria and have internal membranes and thicker walls. Cells more than 10
micrometers in diameter rapidly increased in abundance. Some fossilized cells 1.4
billion years old are as much as 60 micrometers in diameters; others, 1.5 billion years
old, contain what appear to be small membrane-bound structures. Many of these fossils
have elaborate shapes, some exhibit highly branched filaments, tetrahedral
configurations, or spines.
These early fossils mark a major event in the evolution of life: A new kind of organism
had appeared. These new cells are called eukaryotes, from Greek word for “true” and
“nucleus,” because they posses an internal structure called nucleus. All organisms other
than the bacteria are eukaryotes. The early eukaryotes rapidly evolved to produce all of
the diverse organisms that inhabit the earth today, including humans.
1.3.5 Multi-cellular organisms
Some single eukaryotic cells began living in association with others, in colonies.
Eventually individual members of the colony began to assume different duties, and the
colony began to on characteristics of a single individual. Multicellularity has arisen
many times among the eukaryotes. Practically every organism big enough to see with
unaided eye is multicellular, including all animals and plants. The great advantage of
Multicellularity is that it fosters specialization: some cells devote their entire energies to
one task, other cells to another. Few innovations have had a great impact on the history
of life as the specialization made possible by Multicellularity.
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SUMMARY
All living things are characterized by cellular organization, growth, reproduction, and
heredity. Other properties commonly exhibited by living things include movement and
sensitivity to stimuli. Of the three hypotheses of how life might have originated, only
the theory of spontaneous provides a scientifically testable explanation. When life
appeared on earth, the atmosphere was devoid of oxygen gas and very hot. It is believed
that these conditions fuelled chemical reactions that gave rise to life. All bacteria living
today are members of either Archaebacteria or Eubacteria. For at least the first 2 billon
years of life on earth, all organisms were bacteria. About 1.5 billion years ago, the first
eukaryotes appeared. Biologist place living organisms into six general categories called
kingdoms. The two most ancient kingdoms contain bacteria.
Self Assessment questions
1. List eight fundamental properties of living things.
2. Describe how the early earth was like.
3. Discuss on the earliest cells.
4. What was the major event in the evolution of life?
5. Explain how multicellularity promoted the diversity of living things
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REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston.
1284pp.
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LECTURE TWO
PLANT AND ANIMAL CLASSIFICATION
1.0 Introduction
There are about 30 million kinds of organisms known to science and more remain to be
discovered, named and classified. Of the 30 million known kinds only about 2 million have
been identified named and classified. These organisms are not only enormous in number,
but also present diversity of size, structure, mode of reproduction, modes of life, and
ecological and geographical distribution over earth. This great diversity of life on earth
today have attempted biologist to categorize similar organisms in order to better understand
them and indicate their relationship to other living organisms. This endeavour gave rise to
the science of taxonomy.
The early scientist classified things according to size, colour or whether they possessed a
soul. The Greek philosopher and biologist Aristotle was the first to classify organisms
based on their structural similarities. Following the renaissance in Europe, the English
naturalist John Ray (1627-1705) introduced a more complex system of classification and a
new concept of species. Rapid growth of systematic in the eighteenth century culminated in
the work of Carolus Linnaeus (1707-1778), who produced our current scheme of
classification.
Objectives: At the end of this lecture you should be able to:
1. Describe the scientific method of naming organisms
2. Distinguish between taxonomy and phylogeny
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3. Describe the kinds of tools used to establish phylogenetic relationships
4. List the six kingdom of organisms
2.0 Principles and Methods of classification
Organisms are arranged into ascending series of groups of increasing inclusiveness or
hierarchical system of classification. The major categories or taxa (singular-taxon), into
which organisms are grouped are given one of several standard taxonomic ranks to indicate
the general inclusiveness of each group. The hierarchy of taxonomic ranks has been
expanded considerably since Linnaeus’s time. It now includes seven mandatory ranks in
descending series: Kingdom, phylum, class, order, family, genus and species. All organisms
being classified must be placed into at least seven taxa, one at each of the mandatory ranks.
Taxonomist has options of subdividing these seven ranks even further to recognize more
than seven taxa (super class, subclass, infraclass, super order, suborder, and others) for any
particular group of organisms. For very large and complex groups, such as fishes, insects
and some plants, these additional ranks are needed to express different degrees of
evolutionary divergence.
2.1 Nomenclature
Nomenclature is the process of giving a scientific name to an organism once it is identified.
Organisms are given common names in different parts of the world and the same common
name may be used for different kinds of organisms. Even within one country common
names can confuse. The cat flea, sand flea, snow flea, and water flea have little in common.
Despite haring part of their common names, each of these organisms is in a different major
group. A scientific name is universally used for a particular species or a particular group of
11
organisms. The formation and use of scientific names of organisms classified as animals are
governed by the International Code of Zoologist Nomenclature (ICZN); of those classified
as plants (including fungi) by the International Code of Botanical Nomenclature(ICBN);
and of those classified as bacteria (including actinomycetes) by the international code of
Nomenclature of Bacteria (ICNB).
2.2 Binomial nomenclature
The names of species consist of two two terms and are therefore called binomial, binomial
or binary. Each species has a latinezed name composed of two words (hence binomial)
written in italics (underlined if handwritten or typed). The fist word is the name of the
genus, written in capital initial letter. The name of a genus is always a noun, and the
species epithet is usually an adjective that must agree in gender with the genus. For
instance, the scientific name of a common robin is Turdus migratorius (L. turdus, thrush;
migratorius for the migratory habit). A species epithet never stands alone; the complete
binomial must be used to name a species. Names of genera must refer only to single groups
of organisms; a single name cannot be given to two different genera, however, to denote
different and unrelated species.
2.3 Trinomial nomenclature
The system of nomenclature is employed to name the subspecies. The subspecies name is
also a Latin name or Latinized word and follows the name of the species to which it
belongs, for example mountain gorilla is Gorilla beringei. The full scientific name of
subspecies is, therefore, a trinomial name consisting of three names – generic, specific and
sub-specific names.
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3.0 Theories of Taxonomy
There are two current popular theories taxonomy; traditional evolutionary taxonomy and
phylogenetic systematics (cladistics). Both are based on evolutionally principles.
However, they differ on how evolutionally principals are used. These differences have
important implication on how we use taxonomy to study evolutionally processes. The
relationship between a taxonomic group and a phylogenetic tree or cladogram is important
for both theories. This relationship can take one of three forms: monophly, paraphly or
polyphyly. A taxon is monophyletic if it includes the most recent common ancestors of all
members of a group and all descendants of that ancestor. A taxon is paraphyletic if it
includes the most recent common ancestors of all members of a group and some but not all
descendants of that ancestor.
A taxon is polyphyletic if it does not include the most recent common ancestor of all
members of a group; this situation requires the group to have had at least two separate
evolutionary origins, usually requiring independent evolutionally acquisition of a diagnostic
feature. For example, if birds and mammals were grouped in taxa called Homoeothermy,
we would have a polyphyletic taxon because birds and mammals descend from two quite
separate amniotic lineages that have evolved homeothermy independently. The most recent
common ancestor of birds and mammals is not homeothermy and does not occur in the
polyphyletic Homeothermia just described. Both evolutionary and cladistic taxonomy
accept their classification. They differ regarding acceptance of paraphyletic groups.
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3.1 Traditional Evolutionary Taxonomy
Traditional evolutionary taxonomy incorporates two different evolutionary principles for
recognizing taxa: Common descent and amount of adaptive evolutionary change, as
shown on a phylogenetic tree. Evolutionary taxa must have a single evolutionary origin and
must show unique adaptive features. Evolutionary taxa may be either monophyletic or
paraphyletic. Recognition of paraphyletic taxa requires, however, that taxonomies distort
patterns of common descent. An evolutionary taxonomy of anthropoid primates provides a
good example. This taxonomy places humans (genus homo) and their immediate fossil
ancestors in family Hominidae and places chimpanzees (genus Pan), gorillas (genus
Gorilla), and orangutans (genus Pongo) in family Pongidae.
However, pongidae genera Pan and Gorilla share more recent common ancestry with
Hominidae than they do with the remaining Pongid genus, Pongo. Family Pongidae is
therefore paraphyletic because it does not include humans, who also descend from its most
recent common ancestor. Evolutionary taxonomists nonetheless recognize Pongid genera as
single, family level grade of arboreal, herbivorous primates having limited mental capacity;
in other words, they show a family level adaptive zone. Humans are terrestrial, omnivorous
primates who possess greatly expanded mental and cultural attributes, thereby comprising a
distinct adaptive zone at the taxonomic level of a family.
3.2. Phyogenetic Systematics/Cladistics
A second and stronger challenge to evolutionary taxonomy is one known as phylogenetic
systematics or cladistics. As the first name implies, this approach emphasizes the criterion
of common descent and, as the second name implies, it is based on the cladogram of a
group being classified. This approach to taxonomy was first proposed in 1950 by German
14
entomologist Willi Hennig. All taxa recognized by Hennig’s cladistic system must be
monophyletic. We saw previously how evolutionary taxonomist recognition of primate
families Hominidae and Pongidae distorts genealogical relationships to emphasize adaptive
uniqueness of the Hominidae. Because the most recent common ancestor of the
paraphyletic family Pongidae is also an ancestor of family Hominidae, recognition of
Pongidae is incompatible with cladistic taxonomy.
Cladists denote the common descent of different taxa by identifying sister taxa. Sister taxa
share more recent common ancestry with each other than either one does with any other
taxon. The sister group of humans appears to be chimpanzees, with gorillas forming a sister
group to humans and chimpanzees combined. Orangutans are the sister group of the clade
that contains humans, chimpanzees, and gorillas; gibbons form the sister group of the clade
that contains orangutans, chimpanzees, gorillas and humans.
3.3. Species Concept
Species are groups of organisms that remain relatively constant in their characteristics, can
be distinguished from other species, and do not normally interbreed with other species in
nature. Scientist have described and named a total of 1.4 million species, but many more
actually exists.
3.4 The kingdoms of life
The earliest classification system recognized only two kingdoms of living things: animals
and plants. But biologist discovered microorganisms and learned more about other
organisms; they added kingdoms in recognition of fundamental differences discovered
15
among organisms. Moat biologist now uses a six-kingdom system of first proposed by Carl
Woose of the University of Illinois. The six kingdoms are as follows:
i.
Kingdom Archaebacteria: Prokaryotes that lack a peptidoglycan cell wall,
including the methanogens and extreme halophiles and thermophiles.
ii.
Kingdom Eubacteria: Prokaryotic organisms with a peptidoglycan cell wall,
including cyanobacteria, soil bacteria, nitrogen-fixing bacteria, and pathogenic
(disease-causing) bacteria.
iii.
Kingdom Protista: Eukaryotic, primarily unicellular (although algae are
multicellular), photosynthetic or heterotrophic organisms, such as amoebas and
paramecium.
iv.
Kingdom Fungi: Eukaryotic. Mostly multicellular (although yeast are
unicellular), heterotrophic, usually non-motile organisms, with cell walls of
chitin such as mushrooms.
v.
Kingdom Plantae: Eukaryotic, multicellular, non-motile, usually terrestrial,
photosynthetic organisms, such as tree, grasses and mosses.
vi.
Kingdom Animalia: Eukaryotic, multicellular, motile, heterotrophic organisms,
such as sponges, spiders, newts, penguins, and humans.
SUMMARY
For scientific communication, it is important to have one standard set of names. Biologist
use the binomial system devised by Carl Linnaeus nearly 250 years ago. Species are
grouped into genera, genera into families, families into orders, orders into classes, and
classes into phyla. Phyla are the basic units within kingdoms, such a system is hierarchical.
Species are groups of organisms that differ from one another in recognizable ways and
generally do not interbreed with one another in nature. A cladogram presents the
16
evolutionary history of a group of organisms, without making any judgments about the
reative importance of different characteristics. Taxonomists often choose to emphasize
certain characteristics in classifying organisms.
Self Assessment questions
i.
What do you understand by the term nomenclature?
ii.
What is binomial nomenclature?
iii.
What is trinomial nomenclature?
iv.
Define the following terms: polyphyletic and paraphyletic
v.
Distinguish between evolutionary and cladistic taxonomy
vi.
Define the term species
vii.
Discuss on the kingdom of life
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Makonda, F.B.S. (1995) Plant Kingdom. OBT 101. The Open University Of Tanzania.
63pp.
17
Mamiro, D.P. and Mamiro, P.R.S. (2000) Taxonomy. OBL 301. The Open University of
Tanzania. 70pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
LECTURE THREE
THE PLANT KINGOM
1.0 Introduction
Plants dominate all terrestrial communities; they provide most of our food, directly and
indirectly, as well as our shelter, clothing, and medicines. Most of nearly 300,000 species of
existing plants are photosynthetic, capturing energy from the sun and transforming it into
chemical bonds. Photosynthesis also produces the oxygen essential to respiration of every
living organism.
Plants and fungi are the only major groups of organisms that are primarily terrestrial. They
dominate almost every part of the terrestrial landscape, except for extreme deserts polar
regions, and top of the highest mountains. Most plants are protected from desiccation (the
tendency of organism to loose water to the air) by a waxy cuticle secreted onto their
exposed surfaces. The cuticle is relatively impermeable and provides an effective barrier to
water loss.
Special structures in this barrier allow the entry of carbon dioxide, which is necessary for
photosynthesis, and exit oxygen by diffusion. These tiny slit-like eye-shaped pores are
called stomata. At an early stage of their evolution plants also developed a special kind of
18
relationship with fungi that has been a key to their successful occupation of terrestrial
habitats. Mycorrhizae (a symbiotic association of fungi with plants) are found in 80% of
all plants and frequently seen in fossils of the earliest plants. Many other features, such as
efficient water and food conducting systems, developed and contributed to the success of
plants on land. Roots, shoots and leaves expanded areas of photosynthetically active tissue.
Specialization in key reproductive features improved the chances for survival of embryonic
tissues within seeds as they were dispersed. In this chapter, we will discuss characteristics
of the plant phyla, focusing on difference in their life cycles.
Objectives: at the end of this lecture you should be able to:
1. Understand the nature and characteristics common to most plants
2. Recognise the characteristics of the major plant groups.
3. Understand structure of seed plants.
2.0 Characteristics of Plants
All plants have similar characteristics:
•
Multicellular and have cells specialized to form tissue and organs
•
Contain chlorophylls a and b and carotenoids, store reserve food as starch,
and have cellulose cell walls.
•
Have sex organs with an outer layer of non reproductive cells, which prevent
desiccation of developing gametes.
•
Protect the developing embryo from drying out by providing it with water
and nutrition within the female reproductive structure.
•
Have a life cycle that is described as alternation of generation.
19
3.0 Plant Life Cycle
Plants live alternatively between two different forms during their life cycle “alternation of
generation”. One stage of their life cycle is diploid (2n) stage called the sporophyte
because special cells of this stage undergo meiosis and form numerous haploid (n) spores.
The release of e spores allows the plant to be dispersed through the environment and
explore new areas. In plants, spores are reproductive cells that are capable of developing
into a haploid, multi-cellular adult without fusion with another cell. In land plants, a hard
shell covers the spore. When spores land in suitable areas they germinate into their
alternative stages in the life cycle, the gametophyte. Gametophytes are composed of
haploid cells and look very different from the sporophyte plant. They are involved in the
sexual reproduction of the plant since it is the gametophyte that produces male or female
gametes. When the gametes unite at the time of fertilization, the newly formed embryo
undergoes mitosis and grows into the sporophyte form of the next generation.
4.0 Plant classification
Plants can be divided into for four major groups as follows:
Bryophytes
Phylum Hepatophyta: liverworts
Phylum Bryophyata: mosses
Phylum Antocerotophyta: hornworts
Seedless Vascular plants
20
Phylum Psilotophyta: whisk ferns
Phylum Lycopodophyta: club mosses
Phylum Equisetophyta: horsetails
Phylum Pteridophyta: ferns
Gymnosperms
Phylum Pinophyta: conifers
Phylum Cycadophyta: cycads
Phylum Ginkgophyta: maidenhair trees
Phylum Gnetophyta: gnetophytes
Angiosperms
Phylum Magnoliophyta: flowering plants
Class Magnoliopsida: dicots
Class Liliopsida: monocots
The Bryophytes
Plants can be further divided into main group; bryophytes and vascular plants. The
bryophytes consist of three groups: hornworts, liverworts and mosses (see section 4.0).
Bryophytes lacks vascular tissue (specialized means of transporting water and organic
nutrients. Although they often have a “leafy” appearance, these plants do not have true
roots, stems and leaves. Which by definition must contain true vascular tissue. Therefore,
bryophytes are said to have root like, stem like and leaf like structures.
21
The gametophyte is the dominant generation in bryophytes, it is the generation we
recognize as the plant. Flagellated sperm swim to the vicinity of the egg in a continuous
film of water. The sporophyte, when present, is attached to and derives its nourishment
from the photosynthetic gametophyte.
Bryophytes are quite small; the largest is no more than 20 cm tall. This characteristic is
linked to the lack of efficient means to transport water to any height. And because sexual
reproduction involves flagellated sperm, bryophytes are usually found in moist habitats.
Nevertheless, mosses compete well in harsh environment because the gametophyte can
produce asexually; allowing mosses to spread into stressful and even dry habitats. If need
be mosses can dry up and later, when water is available begin to photosynthesize again.
ACTIVITY
Read on the structure and lifecycle of mosses and liverworts
4.1.1 Economic importance of Bryophytes
Some bryophytes such as sphagnum also called bog or peat, has commercial importance.
This has special nonliving cells that can absorb moisture, which is why peat moss is often
used in gardening to improve the water holding capacity of the soil. In some areas where
the ground is wet and acidic, such as bogs, dead mosses, especially sphagnum, accumulate
and do not decay. This accumulated moss, called peat, can be used as fuel.
4.1.2 Adaptation of Bryophytes
Bryophytes are small and simple plants generally found in most habitats for two reasons,
they lack vascular tissue and their sexual reproduction involves flagellated sperm.
22
Bryophytes do not have the complexity of, nor are they found in the variety of habitats
occupied by, the vascular plants. Nevertheless, mosses are better than flowering plants at
living on stone walls and on fences and even in the shady cracks of hot exposed rocks.
They slowly convert rocks to soil that can be used for the growth of other organisms. For
these particular microhabitats, there seems to be a selective advantage to being small and
simple.
Vascular Plants
Vascular plants include ferns and their allies (seedless), gymnosperms and angiosperms.
Vascular tissue in these plants consists of xylem which conducts water and minerals up
from the soil and phloem which transports organic nutrients from one part of a plant to
another. The vascular plants have true roots, stems and leaves. The roots absorb water from
the soil and the stem conducts water to the leaves. Xylem with its strong-walled cells,
supports the body of the plant against the pull of gravity. The leaves are fully covered by a
waxy cuticle except where it is interrupted by stomata’s, little pores whose size can be
regulated to control water loss.
The sporophyte generation is dominant in vascular plants. This is the generation that has
vascular tissue and the other features just discussed. Another advantage of having a
dominant sporophyte relates to its being diploid. If a family gene is present, it can be
masked by a functional gene. The greater the amount of genetic material, the greater the
possibility of mutations, that will lead to increased variety and complexity. Indeed, the
vascular plants are complex, extremely varied and widely distributed.
23
The seedless vascular plants (ferns and their allies) disperse the species by producing
windblown spores. When the spores germinate, a relatively large gametophyte is
independent of the sporophyte for its nutrition. In these plants flagellated sperms are
released by antheridia and swim in a film of external water to the archegonia, where
fertilization occurs. Because the water dependent gametophyte is independent of the
sporophyte, these plants cannot wholly benefit from the adaptations of the sporophyte to a
terrestrial environment.
4.2.1 Ferns
Ferns have about 12,000 species, they are widely distributed and most abundant in warm,
moist tropical regions but they are also found in the northern regions and in dry, rocky
places. They range in size from those that are low growing and resemble mosses to those
that are tall tress. The fronds (leaves) in particular can vary. The royal ferns have fronds
that can stand six feet tall; those of the Genus maidenhair fern are branched with broad
leaflets. And those of the heart tongue fern are strap like and leathery. In nearly all ferns,
the leaves first appear in a curled-up form called fiddlehead, which unrolls as it grows.
4.2.2 Economic importance of Ferns
Ferns are used by florist in decorative bouquets and as ornamental plants in the home and
garden. Wood from tropical the tropical fern tree is often used as a building material
because it resists decay, particularly by termites. Ferns have medicinal values: many
Indians use ferns as an astringent during child birth to stop bleeding, and maidenhair fern is
the source of an expectorant.
24
4.2.3 Adaptation of Ferns
Ferns have true roots, stems, and leaves. The well-developed leaves fan out, capture solar
energy, and photosynthesize. The water dependent gametophyte, which lacks vascular
tissue, is separate from the sporophyte. Flagellated sperm require an outside source of water
in which to swim to the eggs in the archergonia. Once established, some ferns, like the
bracken fern Pteridium aquilinum, can spread into drier areas by means of vegetative
(asexual) reproduction. Ferns also spread by means of the rhizomes growing horizontally in
the soil, producing the fiddleheads that grow up as much as fronds.
4.3 The Gymnosperms
There are four groups of gymnosperms: conifers (division Pinophyta), Cycads (division
Cycadophyta), ginkgo (division Ginkgophyata) and gnetophytes (Division Gnetophyta).
The seeds in gymnosperms are not covered; they are exposed on the surface of sporophylls,
leaves that bear sporangia. (In flowering plants, seeds are enclosed within a fruit).
Reproductive organs are borne in cones on which the sporophylls are spirally arranged.
Other than these features, the four divisions of gymnosperms have little in common.
Seed plants like a few of their predecessors, produce heterospores called microspores and
megaspores. There are also separate micro gametophytes (male) and mega gametophytes
(female). A microspore develops into a pollen grain, which is the immature micro
gametophyte, while retained within a microsporangium. After they are released, pollen
grains develop into mature, sperm bearing micro gametophytes. Pollination is the transfer
of pollen to the vicinity of the mega gametophyte. The sperm is delivered to the egg
through a pollen tube: therefore, no external water is required for fertilization.
25
Independence from external water when reproducing is a major evolutionary trend in the
plant kingdom.
A megaspore develops into an egg-bearing mega gametophyte while still retained within an
ovule. An ovule is the sporophyte structure that holds the mega sporangium and then the
mega gametophyte. After fertilization, the zygote becomes an embryonic plant enclosed
within the ovule, which then becomes the seed.
NOTE: Mega gametophyte and micro gametophyte are dependent to a degree upon the
diploid sporophyte. This means that the sporophyte can evolve into diverse forms without
any corresponding changes in the gametophyes. The seed disperse the sporophyte among
seed plants.
In see plants, seeds disperse the sporophytte, which is the diploid generation. A seed is the
mature ovule; it contains an embryonic sporophyte and stored food enclosed by a protective
seed coat derived from the integument (coverings) of the ovule. Seeds are resistant to
adverse conditions, such as dryness or temperature extremes. They contain an embryo that
is already partially developed and a food reserve that supports the emerging seedlings until
it can exist on its own. The survival value of seeds contributed greatly to the success of
seed plants and their present dominance among plants.
ACTIVITY: Read on the structure and life cycles of conifers (division Pinophyta), cycads
(division Cycadophyta), ginkgo (division Ginkgophyta) and the gnetophytes (divison
Gnetophyta).
4.3.1 Economic importance of Gymnosperms
26
Conifers are widely distributed on the earth’s surface and are economically important. They
supply much of the wood used for building construction and paper production. They also
produce many valuable chemicals, such as those extracted from resin, a viscous liquid
substance that protects the conifers from attack by fungi and insects.
4.3.2 Adaptation of Gymnosperms
Gymnosperms have well developed roots and stems. Many are tall trees that can withstand
heat, dryness, and cold. The reproductive pattern of conifers has several important
innovations not found in plants we have considered so far. There is no need of external
water fertilization to occur. Pollen grains are transferred by wind, and the growth of the
pollen tube delivers sperm to an egg. Enclosure of the dependent megagametophyte in an
ovule protects it during its development and shelters the developing zygote as well. Finally,
the embryo is protected by seed and is provided with a store of nutrients that supports
development for the first period of its growth following germination. All of these factors
increase the chance for reproduction success on land.
4.4 The Angiosperms
Angiosperms (phylum Mangoliophyta) are the flowering plants: their seeds are enclosed by
fruits. With 235, 000 species, they are an exceptionally large and successfully group of
plants. This is six times the number of species of all other plant groups combined.
Angiosperms live ain all sorts of habitats, from fresh water to desert and from the frigid
north to the torrid tropics. They range in size from the tiny, almost microscopic, duckweed
to eucalyptus that are over 100 meters tall. It would be impossible to exaggerate the
27
importance of angiosperms in our every day lives. They provide us with clothing, food,
medicines and many commercially valuable products.
Angiosperms are divided into two groups: dicotyledons (class Magnoliopsida) and
monocotyledons (class Liliopsida). The dicotyledons are either woody or herbaceous and
they have flower parts usually in fours and fives, net-veined leaves, vascular bundles
arranged in a circle within the stem, and two cotyledons or seed leaves. Dicots families
include many familiar plant groups, such as the buttercup, mustard, maple, cactus, pea, and
rose families. The rose family include roses, apples, plums, pears, cherries, peaches,
strawberries, raspberries and a number of other shrubs.
The monocotyledons are almost always herbaceous and have flower in the stem, and are
almost always herbaceous and have flower parts in threes, parallel-veined leaves scattered
vascular bundles in the stem, and one cotyledon or seed leaf. Monocots families include the
lily, palm, orchid, iris, and grass families. The grass family includes wheat, rice corn
(maize) and other agricultural important plants.
ACTIVITY: Read on the structure of the flower and function of the flower parts
4.4.1 Economic importance of Angiosperms
Angiosperms provide the food that sustains most animals on land, including humans.
Humans also use plant fibres to produce cloth, to provide firewood, and to supply
construction materials. Plants, oils, spices and drugs are derived from different parts of
various angiosperm plants.
28
4.4.2 Adaptation of Angiosperms
Angiosperms have true roots, stems, and leaves. The vascular tissue is well developed and
leaves are generally broad. Angiosperms are found in all sorts of habitats; some have even
returned to the water. The reproductive organs are in the flowers, which often attract
animals pollinators; their ovules are located in ovaries that develop into fruits. Therefore,
angiosperms produce covered seeds. Fruits often help with dispersal of seeds.
SUMMARY
Plants are adapted to living on land. In general they tend to have features that allow them to
live and reproduce on land. Plants unlike green algae protect the embryo from drying pot.
During an alternation of generations life cycle, one generation is dominant over the other.
In bryophytes the dominant gametophyte is water dependent because it lacks vascular tissue
and produces flagellated sperm. Bryophytes (liverworts and mosses) lack specialized
vascular tissue and have a dominant gametophyte. Fertilization requires an outside source
of moisture, and they are often found in moist locations.
The seedless vascular plants are dispersed by windblown spores; those that produce seeds
are dispersed by seeds. The seedless vascular plants include ferns and their allies. The
sporophyte is dominant and the species is dispersed by spores. The independent
nonvascular gametophyte produces flagellated sperm. Seed plants produce micro- and
megagametophytes. This has led to two evolutionary events: replacement of flagellated
sperm by pollen grains, and the production of seeds for dispersal of the sporophytes and the
species. Gymnosperms share common characteristics: the seeds are naked (not covered).
Conifers, with their ability to live under extreme conditions, are still a very prevalent
29
and successful group. Conifers are the most prevalent of the gymnosperms. In conifers
life cycle, windblown pollen grains replace flagellated sperm. Following fertilization, the
seed develops from the ovule, a structure that is on a cone. The seeds are uncovered and
dispersed by the wind. Angiosperms are the flowering plants. The flower both attracts
animals (e.g. insects) that aid in pollination and produce seeds enclosed by fruits, which aid
dispersal.
Self Assessment questions
1. List the characteristics of plants
2. What are general characteristics of bryophytes?
3. What are general characteristics of vascular plants?
4. Differentiate the life cycle of a seedless plant and a seed plants
5. List and describe the four phylum’s of gymnosperms
6. What is the economic importance of gymnosperms?
7. Explain why flowering plants are the dominant plants today.
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
30
Makonda, F.B.S. (1995) Plant Kingdom. OBT 101. The Open University Of Tanzania.
63pp.
Mamiro, D.P. and Mamiro, P.R.S. (2000) Taxonomy. OBL 301. The Open University of
Tanzania. 70pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
LECTURE FOUR
THE ANIMAL KINGDOM
1.0 Introduction
Animals are multicellular eukaryotes that are heterotrophic by ingesting food. Animals
produce heterogametes (eggs and sperm) and follow the diplontic life cycle in which the
adult is always diploid. In this cycle meiosis produce haploid gametes, which join to form a
zygote that develop into an adult. Today there are four million known species of animals,
ranging in size from microscopic rotifers, 40 micrometers in length, to giant blue whales,
30 metres long. Animals not only vary in size, but also inhabit widely diverse habitats, from
the frigid Arctic to the scorching desert, from the dry African savanna to the South America
tropical rainforest, and from violet tidal zones to the ocean depths. The following are
characteristics common to all animals:
i.
Animals are multicellular organisms
31
ii.
The bodies of animals are usually composed of groups of cells organized into
tissues, organs and organ systems.
iii.
All animals cells have nucleus but lack cell walls
iv.
Animals are heterotrophic. They do not carry on photosynthesis
v.
During all parts of their life’s animals are able to move from place to place or
move one part of their body with respect to other parts.
vi.
Animals respond quickly and appropriately to changes in their environment.
vii.
Sexual reproduction is a characteristic of animals, although many animals
reproduce asexually as well as sexually.
Objectives: The end this lecture you should be able to:
i.
List the characteristics of Animals
ii.
Draw a phylogenetic tree that shows the nine major animal phyla.
iii.
Describe the way of life and the anatomical features of different classes of
animals
iv.
Describe the life cycles of several invertebrates of economic importance to man
v.
Compare the phyla in terms of body plan, type of symmetry. Number of tissue
layers, level of organization, and presence of coelom.
2.0 Body structure
One of the criteria used to classify animals is type of symmetry. Asymmetry means that the
animal has no particular symmetry. Radial symmetry means that the animal is organized
circularly and, just as a wheel, two identical halves are obtained no matter how the animal
32
is sliced longitudinally. They have a body design in which the parts of the body are
arranged around a central axis in such a way that any plane passing through the central axis
divides the organism into halves that are approximate mirror images e.g. phyla Cnidaria
(jelly fish, sea anemone and corals) and Ctenophora (comb jellies). Radially symmetrical
animals tend to be attached to a substrate; that is, they are sessile. This type of symmetry is
useful to these animals since it allows them to reach out in all direction from one centre.
Bilaterally symmetry means the animal has definite right and left halves that are mirror
image of each other: only one longitudinal cut down the centre of the animal will produce
two equal halves. A bilaterally symmetry body plan has a top and a bottom, better known
respectively as the dorsal and ventral portions of the body. It also have a front and, or
anterior end, and a back, or a posterior end. Bilaterally symmetry constitutes a major
evolutionary advance. This unique form of organization allows parts of the body to evolve
in different ways, permitting different organs to be located in different parts of the body.
Bilaterally symmetry animals tend to be active and to move forward at an anterior end.
They are therefore efficient in seeking food and mates and avoiding predators.
One of the main events during the development of animals is the establishment of the germ
layers from which all other structures are derived. Although a total of three layers are seen
in most animal embryos, some animals only have two germ layers: ectoderm and
endoderm. Such animals have the tissue level of organization. Animals with three germ
layers – ectoderm, mesoderm and endoderm – have an organ level of organization.
The second key transition in the evolution of the animal body plan was the evolution of the
body cavity. The evolution of efficient organ systems within the animal body depended
33
critically upon a body cavity for supporting organs, distributing materials and fostering
complex developmental interactions. The presence of a body cavity allows the expansion
and sometimes lengthening of portions of the digestive tract. This longer passage allows for
storage of undigested food, longer exposure to enzymes for more complete digestion, and
even storage and the final of processing of food remnants. Such an arrangement allows
animals to eat a great deal when it is safe to do so and then to hide during the digestive
process, thus limiting the animals exposure to predators.
An internal body cavity also provides space within which the gonads (ovaries and testes)
can expand, allowing the accumulation of large numbers of eggs and sperm. Such
accumulation helps to make possible all of the diverse modification of breeding of breeding
strategy that characterizes the more advanced phyla of animals. Furthermore, large numbers
of gametes can be released when the conditions are as favourable as possible for the
survival of the young animals.
Three kinds of body cavities are found in bilaterally symmetrical animals. Acoelomates
have no body cavity. Pseudocoelomates have a body cavity called pseudocoel located
between the mesoderm and endoderm. Coelomates in which the fluid filled body cavity
develop within the mesoderm. In coelomate animals the gut is suspended, along with other
organs systems of animals, within the coelome. The coelome is surrounded by a layer of
epitheal cells entirely derived from the mesoderm. The portion of the epithelium that lines
the outer wall of the coelome is called the parietal peritoneum, and the portion that covers
the internal organs suspended within the cavity is called the visceral peritoneum.
34
The third and key transition in animal body plan is involved the subdivision of the body
plan into segments. During the animal early development, these segments becomes most
obvious in the mesoderm but later are reflected in the ectoderm and endoderm as well. Two
advantages results from early embryonic segmentation:
i.
In annelids and other highly segmented animals, each segment may go on to
develop a more or less complete set of adult organ systems.
ii.
Locomotion is far more effective when individual segments can move
independently because the animal as a whole has more flexibility of movement.
Segmentation underlies the organization of all advanced animals. In some adult arthropods
the segments are fused, but segmentation is usually apparent in their embryological
development. In vertebrates, the backbone and muscular areas are segmented. In addition
segmentation leads to specialization of parts because the various segments can become
differentiated for specific purposes.
The fourth key feature in animal body plan is seen in two outwardly dissimilar large phyla,
Echinodermata (star fish) and Chordata (vertebrates). They are members of a group called
the deuterostomes. Members of other coelomic animal’s phyla are called protostomes.
Deuterostomes are a group of coelomate animals in which the second embryonic opening
is associated with the mouth and the first embryonic opening, the blastopore, is associated
with the anus. Protostomes are a group of coelomate animals in which the first embryonic
opening, the blastopore, is associated with the mouth and the anus develop at the other end.
Deuterostomes differ in many aspects of embryo growth, including the plane in which the
cells divide. Perhaps most importantly, the cells that make up an embryonic protostome
each contain a different portion of the regulatory signals resent in the egg, so no one cell of
35
the embryo (or adult) can develop into complete organism. In marked contrast, any of the
cells of a deuterostomes can develop into a complete organism. All animals are believed to
be descendants from protoctists; however, the sponges may have evolved separately from
the rest of the animals.
3.0 Animal classification
The animal kingdom is traditionally divided into two subkingdoms. In the simpler
subkingdom, Parazoa, the animals lack symmetry and possess neither tissues nor orgsns.
In the more advanced subkingdoms, Eumetazoa, the animals are symmetrical and possess
tissues.
The phylum is the largest category of the animal kinddom. Animal phyla are often grouped
together to produce additional, informal taxa intermediate between phylum and kingdom.
These taxa are based on embryological and anatomical characters that reveal phylogenetic
affinities of different animal phyla. The traditional higher-level grouping of true animals
phyla is as follows:
Branch A (Mezoa)
Phylum Mesozoa, the mezoa
Branch B (Parazoa)
Phylum Porifera, the Sponges
Phlum Placozoa
Branch C (Eumetazoa) divided into two grades: Grade I, (Radiata)
Phylum Cnidaria
36
Phylum Ctenophora
Grade II (Bilateria) all other Phyla categorised into two divisions:
Division A (Protostomia),
Acoloemates:
Phylum Platyhelminthes
Phylum Gnathostomulida
Phylum Nermertea
Pseudocoelomates
Phylum Rotifera
Phylum Gastrotricha
Phylum Kinorhyncha
Phylum Nematoda
Phylum Nematomorpha
Phylum Acanthocephala
Phylum Entoprocta
Phylum Priapula
Phylum Loricifera
Eucoelomates
Phylum Mollusca
PhylumAnnelida
Phylum Arthropoda
Phylum Echiurida
Phylum Sipunculida
Phylum Tardigrada
Phylum Pentastomida
37
Phylum Onychophora
Phylum Pogonophora
Division B (Deuterostomia)
Phylum Phoronida
Phylum Ectoprocta
Phylum Chaetognatha
Phylum Brachiopoda
Phylum Echinodermata
Phylum Hemichodarta
Phylum Chordata
3.1 The Sponges
There are perhaps 5000 species of marine sponges, phylum Porifera and about 150 species
that life in fresh water. Sponges probably represent the most primitive animals, possessing
neither tissue-level development nor body symmetry. The cellular organization hints at the
evolutionary ties between the unicellular protistis and the multicellular animals. Sponges
are unique in the animal kingdom in possessing choanocytes, special flagellated cells whose
beating drives water through the body cavity.
3.1.1 Characteristics of Phylum Porifera
i) Multicellular; body a loose aggregation of cells of mesenchymal origin
ii) Body with pores (ostria), canals, and chambers that serve for passage of water
iii) All aquatic, mostly marine
iv) Symmetry radial or none
38
v) Epidermis of flat pinacocytes; interior surface lined with flagellated collar cells
(Choanocytes) that create water currents
vi) Gelatinous protein matrix called mesohyl (mesoglea) contains amebocytes,
collencytes, and skeletal elements
vii)
Skeletal structure of fibrillar collagen ( aprotein) and calcareous or siliceous
crystalline spicules (sponging) fibrils
viii)
No organs or true tissues; digestion intracellular, excretion and respiration by
diffusion
ix)
Nervous system absent
x)
Asexual reproduction by buds or gemmules and sexual reproduction by eggs and
sperms; free swimming ciliated larvae
3.1. Phylum Cnidaria
Although members of phylum Cnidaria are more highly organized than sponges, they are
still relatively simple animals. Most are sessile; those that are unattached, such as jelly fish
can swim only feebly. Cnidaria are radiate animals, which are characterized by primary
radial symmetry, which is ancestral to eumetazoan. They have two well defined germ
layers, ectoderm and endoderm. The body plan is sac like, and body walls composed of two
distinct layers, epidermis and gastrodermis, derived from ectoderm and endoderm
respectively. Polymorphism; polyp (sessile and attached) stage medusa (free swimming)
stage has widened their ecological possibilities of occupying a benthic (bottom) and pelagic
(open water) habitat by the same species. Polymorphism also widens the possibility of
structural complexity.
3.1.1. Characteristics of the Phylum Cnidaria
39
i)
Entirely aquatic, some fresh water but mostly marine
ii)
Radial symmetry or bilateral symmetry around a longitudinal axis with oral and
aboral ends; no definite head
iii)
Two basic individuals: polyp and medusa
iv)
Exoskeleton or endoskeleton of chitinous, calcareous or protein component in some
v)
Body with two layers epidermis and gastrodermis with mosoglea (diploblastic);
mesoglea with cells and connective tissue (ectomesoderm) in some
vi)
Gastrovascular cavity(often branched or divided with septa) with single opening that
serve as both mouth and anus; extensible tentacles usually encircling the mouth or
oral cavity
vii)
Special stinging cells organelles called nematocysts in either epidermis or
gastrodermis or in both.
viii)
Nerve net with symmetrical and asymmetrical synapses; with some sensory organs,
diffuse conduction.
ix)
Muscular system (epitheliomuscular type) of an outer layer of longitudinal fibers at
base of epidermis and an inner one of circular fibers at base of gastrodermis
x)
Asexual reproduction by budding (in polyp) or sexual reproduvction by gametes (in
all medusa and some polyp); sexual forms monoecious or dioecious; planula larva,
holoblastic indeterminate cleavage
xi)
No excretory or respiratory system
xii)
No coelomic cavity
3.1.2. Classification of the Phlum Cnidaria
Class Hydrozoa
Examples: Hydra , Obelia. Physalia, Tubularia
40
Characteristics
i) Solitary or colonial
ii) Asexual polyp and sexual medusa, one of the type may be suppressed.
iii) Hydranth with no mesenteries
iv) Medusa when present with a velum
v) Both fresh water and marine
ACTIVITY: Read on the life cycle of Obelia showing alternation of polyp (asexual) and
medusa (sexual) stages.
Class Scyphozoa
Examples: Aurelia, Cassioopeia, Rhizostome
Characteristics
i) Solitary
ii) Polyp stage reduced or absent,
iii) Bell shaped medusa without velum
iv) Gelatinous mesoglea and much enlarged
v) Margin of bell umbrella typical with eight notches that are provided with sebnse
organs
vi) All marine
Class Cubozoa
Examples: Tripedalia, Carybdea, Chironex, chiropsalmus
Characteristics
41
i) Solitary
ii) Polyp stage reduced
iii) Bell shaped medusa square in cross section
iv) Tentacles or group of tentacles hanging from a blade like pedalium at each corner of
the umbrella
v) All marine
Class Anthozoa: three subclasses;
Subclass Zoantharia (sea anemones): Examples: Metridium, anthopleura, Tealia,
Astrangia, Acropora
Subclass Ceriantipatharia: Examples: Cerianthus, Antipathes, Stichopathes
Subclass Octocorallia: Examples: Tubipora, Alcyonium, Gorgonia, Plexaura, Renilla
Characteristics
i) All polyp no medusa
ii) Solitary or colonial
iii) Enteron subdivided by mesenteries or septa bearing nematocysts
iv) Gonads endodermal
v) All marine
Economic importance
i) Some species of the cnidarians are involved in coral reef formation. Coral reefs are
the most productive of all ecosystems and their diversity of life forms is rivalled
only by tropical rain forests
ii) Part of ecosystem food web
iii) Have environmental importance
42
3.3 Phylum Platyhelminthes
Platyhelminthes were derived from an ancestor that probably had many cnidarians-like
characteristics, including a gelatinous mesoglea. Nonetheless, replacement of gelatinous
mesoglea with cellular, mesodedermal parenchyma laid the basis for a more complex
organization. Parenchyma is a form of tissue containing more cells and fibres than the
mesoglea of cnidarians.
The acoelomates, represented by the flatworms, are the most primitive bilaterally
symmetrical animals and the simplest animals in which organs occur. All of the higher
animals are fundamentally bilaterally symmetrical. Among bilaterally symmetrical animals,
different parts of the body are specialized in relation to different functions. Among
bilaterally symmetrical animals, those with the simplest body plan are the acoelomates:
they lack any internal cavity other than the digestive tract. In this lecture we will focus our
discussion of the acoelomates on the largest phylum of the group, the flat worms.
3.3.1 Characteristics of the phylum Platyhelminthes
i. Three germ layers (triploblastic)
ii. Bilateral symmetry: definite polarity of anterior and posterior end
iii. Body flattened dorsoventrally, in most; oral and genital apertures mostly on ventral
surface
iv. Epidermis may be cellular or syncytial (ciliated in some); rhabdites in epidermis of
most Turbellaria, epidermis a syncytial tegument in monogenia, Treamatoda,
Cestoda, and some Turbellaria.
43
v. Muscular system of mesodermal origin, in form of a sheath of circular, longitudinal,
and oblique layers beneath the epidermis or tegument
vi. No internal body space (acoelomate) other than digestive system incomplete
(gastrovascular type); absent in some
vii. Digestive system incomplete (gastrovascular type), absent in some
viii.
Nervous system consisting of a pair of anterior ganglia with longitudinal
nerve cords
ix. Simple sense organs, eye spots in some
x. Excretory system of two lateral canals with branches bearing flame cells
(protonephridia), lacking in some forms
xi. Respiratory, circulatory and skeletal systems lacking, lymph channels with free cells
in some trematodes
xii. Most forms monoecious; reproductive system complex, usually with well developed
gonads, ducts, and accessory organs; internal fertilization; lifecycle simple in free
swimming forms and those with single hosts; complicated life cycle often involving
several hosts in many internal parasites
xiii.
Class Turbellaria mostly free living; classes Monogenia, Trematoda and
Cestoda entirely parasitic
3.3.2 Classification of Phylum Platyhelminthes
Class Tuberllaria
Examples: Dugesia (planaria), Microstomum, Planocera
Characteristics
44
i. Usually free living with soft flattened bodies, covered ciliated epidermis containing
secreting cells and rod like bodies (rhabdites)
ii. Mouth usually on ventral surface sometimes near centre of body
iii. No body cavity except intracellular spaces in parenchyma
iv. Mostly hermaphrodite, some have asexual fission
v. A paraphyletic taxon
Class Trematoda
Examples: Fasciola, Clonorchis, Schistosoma
Characteristics
i) Body covered with syncytisl tegument without cilia
ii) Body life like or cylindrical in shape
iii) Usually with oral and ventral suckers
iv) No hooks
v) Alimentary canal usually with two main branches
vi) Mostly monoecious
vii) Life cycle complex, with first host a mollusc final host usually a vertebrate, parasitic
in all Classes of vertebrate
Class Monogenea
Examples: Dactylogyrus, Polystomia, Gyrodactylus
Characteristics
i) Body covered with syncytisl tegument without cilia
45
ii) Body usually leaf like or cylindrical in shape
iii) Posterior attachment organ with hooks, suckers, or clamps, usually in combination
iv) Monoecious
v) Simple lifecycle. With single host usually with free swimming, ciliated larva
vi) All parasitic, mostly on skin or gills of fish
Class Cestoda
Examples: Diphyllobothrium, Hymenolepsis, Taenia
Characteristics
i) Body covered with conciliated, syncytisl tegument
ii) Tape like body shape
iii) Scolex with suckers or hooks, sometimes both for attachment
iv) Body usually divided into series of proglottids; no digestive tract
v) Usually monoecious
vi) Parasitic in all classes of vertebrates
vii) Lifecycle complex with two hosts or more hosts, fist host may be vertebrate or
invertebrate
3.3.3. Economic Importance of Phylum Platyhelminthes
Some flat worms are very important pathogens of humans and domestic animals. Examples
of flukes infecting humans and domestic animals are: The blood flukes (Schistosoma
species), Chinese liver fluke (Clonorchis sinensis), Lung flukes (Paragonimus species),
Intestinal flukes (Fasciola species) and sheep liver fluke (Fasciola hepatica).
46
Common Cestodes of human are: beef Tape worm (Taenia saginata), pork Tape worm
(Taenia solium), fish Tape worm (Diphyllobothrium latum), dog Tape worm (Dipylidium
caninum), dwarf Tape worm (Hymenolepsis nana), unicolar hydatid (Echinococcus
granulosus), and multilocular hydatid (Echinococus multilocularis).
3.4. THE PSEUDOCOELOMATES
All bilaterally symmetrical animals except solid worms posses an internal body cavity.
Among them, seven Phyla are characterized by their possession of a pseodocoel. Only one
of the seven, phylum Nematoda, includes a large number of species. In all pseudo
coelomates, the pseudocoel serves as a hydrostatic skeleton: one that gains its rigidity from
being filled with fluid under pressure. The animal muscle can work against this skeleton
thus making the movement of pseudocoelomtes far more efficient than those of the
acoelomates.
3.4.1.Phylum Nematoda
Nematodes are present in nearly every conceivable kind of ecological niche. Approximately
12,000 species have been named. They live in the sea, in fresh water, and in soil, from
Polar Regions to the tropics, and from mountaintops to the depth of the seas. Nematodes
also are of parasites virtually every type of animal and many plants. The effect of nematode
infestation on crops, domestic animals and humans make this phylum one of the most
important of all animal groups.
3.4.2 Characteristics of Phylum Nematoda
i. Body bilaterally symmetrical, cylindrical in shape
ii. Body covered with a secreted., flexible, non living cuticle
47
iii. Motile cilia and flagella completely lacking; some sensory endings derived
from cilia present
iv. Muscles in body wall running in longitudinal directions only
v. Excretory system of either one or more gland cells opening by an excretory
pore, a canal system without gland cells, or both gland cells and canals
together; flame cell protonepridia lacking
vi. Pharynx usually muscular and triradiate in cross section
vii. Male reproductive tract opening into rectum to form a cloaca; female
reproductive tract opening a separate gonopore
viii.Fluid in pseudocoel enclosed by cuticle forming a hydrostatic skeleton
ACTIVITY: Read on the life cycle of Ascaris lumbricoides and Necator americanus
4.4.3.Economic importance of the Nematodes
Nearly all vertebrates and many invertebrates are parasitized by the nematodes. A number
of theses are very important pathogens of humans and domestic animals. Many nematodes
are very prevalent in Tropical Africa. The common parasitic nematodes includes: Ascaris
lumbricoides
(roundworms),
Necator
americanus
and
Ancyclostoma
duodenale
(hookworms), Entoroboius vermicularis (Pin worm), Trichinella spiralis (trichina worms)
and Trichuris trichura (Whip worms)
SUMMARY
48
Animals are hterotrophic, multicellular, and usually have the ability to move. Almost all
animals reproduce sexually. The Kingdom Animalia is divided into two subkingdoms:
Parazoa, which includes only the asymmetrical phylum Porifera, and Eumatazoa. Most
animals are characterized by a symmetrical appearance. In few instances, the symmetry is
radial: a mid body axis can be drawn in any two-dimensional plane. Most animals exhibit
bilateral symmetry: the body can be divided in only one plane. Bilaterally symmetrical
animals include three major evolutionary lines: acoelomates; which lack a body cavity,
pseudocoelomates; which develop a body cavity (psuedocoel) between the mesoderm and
endoderm; and coelomates, which develop body cavity (coelom) within the mesoderm.
The sponges are characterized by specialized, flagellated cells called choanocytes. Theu do
not possess tissue or organs, and most species lack symmetry in their body organization.
Animals with radial symmetry body plan and distinct tissues include the phyla Cnidaria and
Ctenophora. Acoelomates are the most primitive bilaterally symmetrical animals. They lack
an internal cavity, except for digestive system, and are the simplest animals that have
organs-like structure made up of two or more tissues. The most prominent phylum of the
acoelomates, Platyhelminthes, includes the free-living flatworms and the parasites flukes
and tapeworms. Pseuodocoelmates, represented by the nematodes (phylum Nematoda),
have a body cavity that develops between the mesoderm and the endoderm. This
pseudocoel gives these worms enhance powers of movement.
Self Assessment questions
1. What are the characteristics that distinguish animals from other living organisms?
2. How do the two subkingdoms of animals differ in terms of symmetry and body
organization?
49
3. Why sponges are considered as one of the most primitive groups of animals?
4. Describe the two body forms present in phylum Cnidaria.
5. How are tapeworms different from flukes?
6. Why are the nematodes structurally unique in the animal world?
References
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
50
LECTURE FIVE
PHYLUM MOLLUSCA
1.0. Introduction
Molluscs are a major group of the true coelomate animals. They belong to the protosome
branch, or schizocoelous coelomates, and have cleavage and mosaic development.
Mollusca is one of the largest animal phyla after Arthropoda. There are nearly 50,000 living
species and some 35,000 fossil species. The name mollusc indicates one of their distinctive
characteristics, a soft body. This diverse group include organisms as different as chitons,
clams and octopuses. The group range from fairly simple organisms to some most complex
invertebrates, and in size from almost microscopic to giant squids Architeuthis harveyi.
Molluscs are found in a great range of habitats, from tropics to polar seas, at altitudes
exceeding 7000 m, in ponds, lakes and streams, on mudflats, in pounding surf, and in open
oceans from the surface to abyssal depths. Most live in the sea, and they represent a variety
of lifestyles, including bottom feeders, burrowers, borers and pelagic forms. The phylum
includes some of the sluggish and some of swiftest and most active invertebrates. It
includes herbivorous grazers, predaceous carnivores, and ciliary filter feeders. The
enormous variety, great beauty and availability of the shells of molluscs have made shells
collecting a popular hobby.
Objectives: The end this lecture you should be able to:
i.
List the characteristics of Phylum Mollusca
51
ii.
Describe the way of life and the anatomical features of different classes of the
phylum Mollusca
2.0 Characteristics of Phylum Mollusca
i. All organ system are present and well developed
ii. Body bilaterally symmetrical (bilateral asymmetry in some), unsegmented,
usually with definite head
iii. Ventral body wall specialized as muscular foot, variously modified but used
chiefly for locomotion
iv. Dorsal body wall forms the mantle, which encloses the mantle cavity, which is
modified into gills or lung, and secretes the shell (shell absent in some)
v. Surface epithelium usually ciliated and bearing mucous glands and sensory
nerve endings
vi. Coelom mainly limited to the area around the heart
vii. Complex digestive system; rasping organ (radula) usually present, anus usually
emptying into mantle cavity
viii.
Open circulatory system (mostly closed in cephalopods), heart usually three
chambered, two in most gastropods. Posses Blood vessels, and sinuses,
respiratory pigments in blood
ix. Gaseous exchange by gills, lung, mantle, or body surface
x. Usually one or two kidneys (metanephridia) opening into the mantle cavity
xi. Nervous system of paired cerebral, pleural, pedal and visceral ganglia, with
nerve cords and sub-epidermal plexus; ganglia centralized in nerve ring in
polyplacophorans, gastropods, and cephalopods
52
xii. Sensory organs of touch smell, taste, equilibrium and vision (in some), eyes
highly developed in cephalopods
xiii.
Most dioecious, some hermaphrodite. Many aquatic and marine molluscs
pass through free swimming Trochophore and veliger larvae
2.0.Classification of Phylum Mollusca
2.1 Class Caudofoveata
Examples: Chaetoderma and Limifossor
Characteristics:
i. Wormlike, shell head and excretory organs absent
ii. Radula usually present
iii. Mantle covered with chitinous cuticle and calcareous scales, oral pedal shield
near anterior mouth, mantle is cavity posterior end with pair of gills
iv. Sexes separate
2.2 Class Solenogastres
Examples: Neomenia
Characteristics:
i. Wormlike, shell head and excretory organs absent
ii. Radula usually present
iii. Mantle usually covered with scales or spicules, mantle cavity posterior
iv. Without true gills, but sometimes with secondary respiratory structures, foot
represented by long narrow ventral pedal groove
v. Hermaphroditic
2.3 Class Monoplacophora
53
Example: Neopilina
Characteristics
i. Body bilaterally symmetrical with a broad flat foot
ii. A single limpet like shell
iii. Mantle cavity with five or six pairs of gills
iv. Large coelomic cavity
v. Radula present
vi. Six pairs of nephridia, two which are gonoducts
vii. Separate sexes
2.4 Class Polyplacophora
Examples: Chaetopleura (Chitons)
Characteristics
i. Elongated, flattened body with reduced head
ii. Bilaterally symmetrical, radula present, shell of eight dorsal plates
iii. Foot broad and flat
iv. Gills multiple, along side of body between foot and mantle
v. Sexes usually separate, with trochophore but no veliger larvae
2.5 Class Scaphopoda (tusk shells)
Example: Dentalium
i. Body enclosed in a one piece tubular shell open at both ends
ii. Conical foot
iii. Mouth with radula and tentacles
54
iv. Head absent
v. Mantle for respiration
vi. Sexes separate, trochophore larvae
2.6 Class Gastropoda (Snails and relatives)
Examples: Busycon, Polinices, Physa, Helix, Aplysia
Characteristics
i. Body asymmetrical, usually in a coiled shell (shell uncoiled or absent is some)
ii. Head well developed with radula
iii. Foot large and flat
iv. Dioecious or monecious, some with trochophore, but most with veliger, some
without larva
2.7 Class Bivalvia (the bivalves)
Examples: Mytilus, Venus, Bankia
Characteristics
i. Body enclosed in two-lobed mental, shell of two lateral valves of variable size and
form, with dorsal hinge
ii. Head greatly reduced but mouth with labial palps, no radula, no cephalic eyes
iii. Gills plate like
iv. Foot usually wedge shaped, sexes usually separate, typically with trochophore larva
and veliger larvae
2.8 Class Cephalopoda (squids and octopuses)
Examples: Loligo, Octopus, Sepia Nautilus
55
Characteristics
i. Shell often reduced or absent
ii. Head well developed with eyes and a radula
iii. Head with arms or tentacles
iv. Foot modified into a funnel
v. Nervous system of well developed ganglia, centralized to form a brain
vi. Sexes separate, with direct development (no larvae)
3.0 Economic importance of Phylum Mollusca
A group as large as molluscs would naturally affect humans in some way. A wide variety of
Molluscs are used as food. Pearls, both natural and cultured, are produced in the shells of
clams and oysters, most of them in marine oyster, found around eastern Asia. Some
molluscs are destructive. Burrowing shipworms which are bivalves of several species, do
great damage to wooden ships and wharves. To prevent the ravages of shipworms, wharves
must be either creosoted or built of concrete. Snails and slugs often damage garden and
other vegetation. In addition, many snails serve as intermediate hosts for serious parasites. A
certain boring snail, the oyster drill, rivals sea stars in destroying oysters.
56
PHYLUM ANNELIDA
4.0 Introduction
Annelida consists of the segmented worms. It is large phylum numbering approximately
15,000 species, he most familiar of which are earthworms and freshwater worms (class
Oligochaeta) and leeches (class Hirudinea). However, approximate two-third of the phylum
is composed of marine worms (class Polychaeta), which are less familiar to most people.
Among the latter are many curious members: some are strange, even ugly, whereas others
are graceful and beautiful. They include the clam worms, plumed worms, parchment
worms, scale worms and many others. Annelids are true coelomates and belong to the
protosome branch, with spiral mosaic cleavage. They are a highly developed group in
which the nervous system is more centralized and the circulatory system more complex
than those of phyla we have studied so far.
Annelids are worldwide in distribution, occurring in the sea, freshwater, and terrestrial soil.
Some marine annelids live quietly in tubes or burrow into bottom mud or sand. Some of
these feed on organic matter in the mud through which they burrow; others feed on
suspended particles with elaborate ciliary or mucous devices for trapping food. Many are
predators either pelagic or hiding in crevices of coral or rock except, when hunting. Fresh
water annelids burrow in mud or sand, live among vegetation, or swim about freely. The
most familiar annelids are terrestrial earthworms, which move about through soil. Some
leeches are bloodsuckers, and others are carnivores, most of them live in fresh water.
57
4.1 Characteristics of Phylum Annelida
i.
Body metamerically segmented; symmetry bilateral
ii.
Body wall with outer circular and inner longitudinal muscle layers, outer transparent
moist cuticle secreted by epithelium
iii.
Chitinous setae, often present on fleshy appendages called parapodia, setae absent in
leeches
iv.
Coelom (schizocoel) well developed and divided by septa, except in leeches,
coelomic fluid supplies turgidity and functions as a hydrostatic skeleton
v.
Blood system closed and segmentally arranged, respiratory pigments (haemoglobin,
hemerythrin or chlorocruorin) often present, amebocytes in blood plasma
vi.
Digestive system complete and not metamerically arranged
vii.
Respiratory gas exchange through skin, gills, of parapodia
viii.
Excretory system typically a pair of nephridia for each metamere
ix.
Nervous system with a double ventral nerve cord and pair of ganglia with lateral
nerves in each metamere: brain, a pair of dorsal cerebral ganglia with connectives to
cord
x.
Sensory system of tactile organs, taste buds statocysts (in some), photoreceptor
cells, and eyes with lenses (in some)
xi.
Hermaphroditic or separate sexes, larvae, if present are trochophore type, asexual
reproduction by budding in some, spiral and mosaic cleavage.
4.2 Classification of Phylum Annelida
Class Polychaeta
Example: Nereis, Aphrodita, Glycera, Arenicola, Chaetoptersus and Amphitrite
i.
Mostly marine
58
ii.
Head distinct and bearing eyes and tentacles
iii.
Most segments with parapodia (lateral appendages) bearing tufts of many setae
iv.
Clitellum absent
v.
Sexes separate, gonads transitory, asexual budding in some
vi.
Trochophore larva
Class Oligochaeta
Examples: Lumbricus, Stylaria, Aelosima, Tubfex
Characteristics
i.
Body with conspicuous segmentation, number of segments variable
ii.
Setae; few per metamere, no parapodia
iii.
Head absent
iv.
Coelom spacious and usually divided by intersegmental septa
v.
Hermaphroditic development, no larva
vi.
Chiefly terrestrial and fresh water
Class Hirudinea
Examples: Hirudo, Placobdella and Macrobdella
Characteristics
i.
Body with fixed numbers of segment, usually 34 with many annuli
ii.
Body usually with anterior and posterior suckers
iii.
Clitellum present
iv.
No parapodia, setae absent (except Acanthobdella)
v.
Coelom closely packed with connective tissue and muscle
vi.
Developent direct, hermaphrodite
59
vii.
Terrestrial, fresh water and marine
4.3 Economic importance of Phylum Annelida
Much of the economic importance of annelids is inderict, deriving from their ecological
roles. Many are members of grazing food cahains or detritus food cahains, serving as prey
for other organisms of more direct interest to humans, such as fishes. Cnsequenly a lifely
market in some polychaeta and oligochaetes as fish bait thrives.
Burrow of earth worms increase drainage and aeration of soils, and migration of worms
help mix soil and distribute organic matter to deeper layers. Some marine annelids that
burrow, serve an analogous role in the sea, lugworms (Arenicola) are sometimes called
“earthworms of the sea”. New medical use of leeches have revived the market in bloodsucking leeches and established “leech farms” where these organisms are raised in
captivity.
SUMMARY
Mollusca is one the largest and most diverse of all phyla, its members ranging in size from
very small organisms to largest of invertebrates. Their basic body division are the head,
foot and the visceral mass, usually covered by a shell. The majority are marine but some are
freshwater, and a few are terrestrial. They occupy a variety of niches, a number are
economically important, and a few are medically important as host of parasites. Molluscs
are coelomates, although theircoelom is limited in the area around the heart. The
evolutionary development of a coelom was important because it enabled better organization
of visceral organs and, I many of the animals that have it, an efficient hydrostatic skeleton.
The mantle and mental cavity are important characteristics of molluscs. The mantle secretes
60
the shell and overlies a pat of the visceral mass to form a cavity housing gills. The mantle
cavity has been modified as a lung in some molluscs. The radula, a protrusible, tongues like
organ with teeth is used for feeding except in bivalve and some solenogasters. The
primitive larva of molluscs is the trochophore, and most marine molluscs have a more
derived larva, the veliger.
Phylum Annelida is a large, cosmopolitan group containing polychaetes, earthworms and
freshwater oligochaetes, and leeches. Certainly the most important structure innovation
underlying diversification of this group is metamerism, a division of the body into a series
of similar segments, each of which contain a repeated arrangement of many organs and
systems. The coelom is also highly developed in annelids, and this, together with the septal
arrangement of fluid filled compartments and well-developed body-wall musculature, is a
an effective hydrostatic skeleton for precise burrowing and swimming movements.
Self Assessment Questions
1. What is the basic body plan of a mollusc?
2. What is radula? How is it used in different types of Molluscs
3. What part of the mollusc is represented by the coelom
4. What is a trochophore? What is a veliger?
5. What evolutionary advantages does segmentation confer upon an organism
6. What are annelid setae? What function do they serve?
7. What are parapodia? What class of annelid possess them? What is their function?
8. How are annelids developmentally similar to molluscs? What is the likely
evolutionary connection between these phyla?
61
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
62
LECTURE SIX
PHYLUM ARTHROPODA
1.0 Introduction
Phylum Arthropoda embraces the largest assemblage of living animals on earth. It includes
spiders, scorpions, ticks, mites, crustaceans, millipedes, centipedes, insects, and some
smaller groups. In addition there is a rich fossil record extending back to the mid-Cambrian
period.
Arthropods are eucoelomate protosomes with well developed organ system, and their
cuticular exoskeleton containing chitin is a prominent characteristic. Like annelids, they
are conspicuously segmented; their primitive body pattern is a linear series of similar
somites, each with a pair of jointed appendages. However, unlike annelids, arthropods
have exaggerated their segmentation theme: variations occur in the pattern of somites and
appendages in the phylum. Often somites are combined or fused into functional groups,
called tagmata, for specialized purposes. Appendages, too, are frequently differentiated
and specialized for walking, swimming flying, or eating.
Arthropods are found in all types of environment from low ocean depths to very high
altitudes and from the tropics far into both north and south Polar Regions. Some species are
adapted for life on land or in fresh, brackish, and marine waters. Others live in or on plants
and other animals. Most species use flight to varying degrees to move among their
favoured habitats. Some live in places where no other animals could survive. Although all
feeding types (carnivores, omnivorous and herbivorous) occur in this vast group, the
63
majority are herbivore. Most aquatic arthropods depend on algae for their nourishment, and
most land forms live chiefly on plants. There are many parasites. In diversity of ecological
distribution arthropods are the most diverse animals in world.
Objectives: At the end this lecture you should be able to:
i.
Explain why arthropods are the most diverse animals
ii.
Describe the way of life and the anatomical features of different classes of the
phylum Arthropoda
iii.
Describe the characteristics of the major insect orders
iv.
Describe the economic importance of insects
2.0.Why Arthropods are the most diverse animals
Arthropods have achieved great diversity, number of species, wide distribution, variety of
habitats and feeding habits, and power of adaptation to changing conditions. The following
are some of the structural and physiological patterns that have been helpful to them:
• A versatile exoskeleton. The skeleton is the cuticle, an outer covering secreted by
the epidermis.
• Segmentation and appendages for more efficient locomotion
• Air piped directly to cells. Most arthropods have a highly efficient tracheal system
of air tubes. Aquatic arthropods breathe mainly by some form of gills.
• Highly developed sensory organs
• Complex behaviour patterns
• Reduced competition through metamorphosis
64
3.0.Characteristics of Phylum Arthropoda
i.
Bilateral symmetry: metameric body, tagmata of head and trunk; head,
thorax and trunk; thorax, and abdomen; or cephalothorax and abdomen
ii.
Appendages jointed; primitively, one pair to each somite (metamere), but
number often reduced: appendages often modified for specialized function
iii.
Exoskeleton of cuticle containing protein, lipid, chitin, and often calcium
carbonate secreted by underlying epidermis and shed (molted) at intervals
iv.
Muscular system complex, with exoskeleton for attachment, striated muscles
for rapid action, smooth muscles for visceral organs, no cilia
v.
Coelom reduced, most of body cavity consisting of hemocoel (sinuses, or
spaces, in the tissues) filled with blood
vi.
Complete digestive system, mouth parts modified from appendages and
adapted for different methods of feeding
vii.
Circulatory system open, with dorsal contractile heart, arteries, and
hemocoel
viii.
ix.
Respiration by body surface, gills, trachea (air tubes), or book lungs
Paired excretory glands called coxal, antennae, or maxillary glands present
in some, some with other excretory organs, called malpighian tubules
x.
Nervous system similar to annelid plan, with dorsal brain connected by a
ring around the gullet to a double nerve chain of ventral ganglia, fusion of
ganglia in some species, well developed sensory organs
xi.
Sexes usually separate, with paired reproductive organs and ducts, usually
internal fertilization, oviparous or ovoviviparous, often with metamorphosis,
pathogenesis in few forms. Growth with ecdysis.
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4.0.Classification of phylum Arthropoda
4.1 Subphlum Trilobita (triobite)
All extinct forms, body divided by two longitudinal furrows into three lobes, distinct head,
thorax, and abdomen, biramous.
4.2 Subphylum Chelicerata
First pairs of appendages chelicerates, have pairs of pedipalps and for pairs of legs, no
antennae, no mandibles, cephalothorax and abdomen often with segments fused.
Class Merostomata
Aquatic chelicerates, Have cepahalothorax and abdomen only and compound lateral eyes.
Appendages with gills, sharp telson. Subclasses: Eurypterida (all extinct) and Xiphosurida
the horse shoe crab.
Class Pycnogonida (sea spiders)
Small (3 to 4cm), but some reach 500mm, body chiefly cephalothorax, tiny abdomen,
usually four pairs of long walking legs (some with five or six pairs). One pair of subsidiary
legs (ovigers) for egg bearing. Mouth on long proboscis. Four simple eyes. No respiratory
of excretory system. Example: Pycnogonium.
Class Arachnida
Scorpions, spiders, mites, ticks and harvestmen. Have four pairs of legs. Segmented or
unsegmented abdomen with or without appendages and generally distinct from
cephalothorax. Respiration by gills, trachea or book lungs. Excretion by malphigian tubules
or coxal glands. Have a dorsal bi-lobed brain connected to ventral gangilionic mass with
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nerves and simple eyes. Sexes are separate and chiefly oviparous. No true metamorphosis.
Examples: Argiope, and Centruroides.
4.3 Subphylum Crustacea
Crustaceans, they are mostly aquatic, with gills. Cephalothorax, usually with dorsal
carapace. Have biramous appendages modified for various functions. Head appendages
consisting of two pairs of maxillae, sexes usually separate, development primitively with
nuplius stage.
Class Brachiopoda
Brachiopods: Have flattened, leaf like swimming appendages (phyllopodia) with
respiratory function. Examples: Triops, lynceus and Daphnia.
Class Maxillopoda
Ostracods and copepod, branchiurans and barnacles: Five cephalic, six thoracic and usually
four abdominal somites no typical appenegaes on abdomen. Unique maxillopodian eye.
Examples: Cypris, Cyclops, Ergasilus, Argulus and Balanus.
Class Malacostraca
Shrimps, Cray fishes, Lobsters and Crabs: usually with eight thoracic and six abdominal
somites, each with a pair of appendages. Examples; Armadillidium, Gammarus,
Megacytiphanes, Grapsus, Homarus and Panulirus.
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4.4 Subphylum Uniramia
Insects and Myriapods. All appendages uniramous. Head appendages consisting of one pair
of antennae, one pair of mandibles, and one or two pairs of maxillae.
Class Insecta
Insects: Body with distinct head, thorax and abdomen. Have a pair of antennae. Mouth
parts modified for different food habits head of six fused somites, thorax of three somites.
Abdomen with variable number of wings (sometimes with one pair or none) and three pairs
of jointed legs. Separate sexes, usually oviparous. Gradual or abrupt metamorphosis.
Insects are dived into orders chiefly on the basis of morphology and developmental
features. Entomologists do not agree on names of orders or on the limits of each order.
Some tend to combine and others to divide groups. However, the following synopsis of
major orders is one that is rather widely accepted.
Order Protura
Very small (1 to 5 mm), have no eyes or antennae. Appendages on abdomen as well as
thorax. Live in soil and dark, humid places. Develop through gradual metamorphosis.
Order Diplura
Usually they are less than 10 mm in length. Pale and have no eye. Have a pair of long
terminal filaments or pairs of caudal forceps. Live in damp humus or rotting logs. Have a
direct development.
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Order Collembolla
Spring tails and snow fleas. Small (5 mm or less). No eyes. Respiration by trachea or body
surface. A spring organ folded under abdomen for leaping. Abundant I soil. Sometimes
swarm on pond surface film or on snow banks in spring. Direct development.
Order Thysanura
The Silverfish and bristletails. Small to medium size. Have large eyes and a long antennae.
Three long terminal cerci. Live under stones and leaves and around human habitations.
Direct development.
Order Ephemeroptera
Mayflies. Have membranous wings, fore wings larger than hind wings. Adult mouth parts
vestigial. Nymph aquatic, with lateral tracheal gills. Hemimetabolous development.
Order Odonata
Dragon flies. Among the most primitive of insects orders. Have two pairs of transparent
wings. Have a large, long and slender body. Have chewing mouth parts. Hemimetablous
development.
Order Orthoptera
Grasshoppers, locusts, crickets cockroaches, walking sticks and playing mantis. Wings
when present, with fore wings thickened and hind wings folded like a fan under forewings.
Chewing mouth parts. Hemimetablous development.
69
Order Isoptera
Termites. Small, membranous, narrow wings similar in size with few veins. Wings shed at
maturity, erroneously called ‘white ants’. Distinguishable from true ants by broad union of
thorax and abdomen. Complex social organization. Hemimetabolous development.
Order Mallophaga
Biting lice. As large as 6 mm. They are wingless and have chewing mouth parts. Legs
adapted for clinging to host. Live on birds and mammals. Hemimetabolous development.
Order Anoplura
Sucking lice. Depressed body as large as 6 mm. Wingless, mouth parts for piercing and
sucking. Adapted for clinging to warm blooded host. Include the head louse, body louse,
cab louse and others. Hemimetabolous development.
Order Hemiptera
Heteroptera: the true bugs. Size 2 to 100 mm. Wings present or absent. Fore wings with
basal portion leathery, apical portion membranous, hind wings membranous. At rest wings
are held over the abdomen. Have piercing and sucking mouth parts. Many with odorous
scent glands. They include the water scorpions, water striders, bed bugs, squash bugs
assassin bugs, chinch bugs, stink bugs, plant bugs, lace bugs and others. Hemitabolous
development.
Order Homoptera
70
Cicadas, aphids, scale insects and mealy bugs, leafy hoppers and tree hoppers. Wings
present or absent, if winged, either membranous or thickened fore wings and membranous
hind wings. Wings held roof like over body. Have piercing and sucking mouth parts. All
are plant eaters some very destructive. Few serve as source of shellac, dyes etc. Some with
complex life histories. Hemimetabolous development.
Order Neuroptera
Dobson flies, lion flies and lacewings. Medium to large size, similar membranous wings
with many cross veins. Chewing moth parts for biting and chewing. Include ground
beetles, stag beetles, dung beetles, diving beetles, boll weevils, fire flies and lady bird
beetles. Hemimetabolous development.
Order Coleoptera
Beetles, fire flies and weevils. The largest order of animals. Fore wings (elytra) thick,
hard, opaque, membranous hind wings folded under forewings at rest. Mouth parts for
biting and chewing. Include ground beetles, carrion beetles whirling beetles, darkling
beetles, stag beetles, dung beetles, diving beetles, boll weevils, fire flies, lady bird beetles
and others. Holometabolous development.
Order Lepidoptera
Butterflies and months. Membranous wings covered with overlapping scales, wings
coupled at base, mouth arts a suckling tube, coiled when not in use. Larva (caterpillar) with
chewing mandibles for plant eating stubby pro-legs on abdomen and silk glands foe
spinning cocoons. Antennae knobbed in butterflies and usually plumed in moths.
Holometabolus development.
71
Order Diptera
True flies: Single pair of wings, membranous and narrow, hind wings reduced to
inconspicuous balancers (halters). Sucking mouth parts or adapted for sponging or lapping
or piercing. Legless larva called maggots or when, aquatic, wigglers. Include crane flies,
mosquitoes, month flies, midges, fruit flies, fresh flies. House flies gnats and many others.
Order Trichoptera
Caddis flies: Small, soft bodied, wings well veined and hairy, folded roof like over hairy
body. Chewing mouth parts, aquatic larvae construct case of leaves, sand, gravel, bits of
shell, o plant matter, bound together with secreted silk or cement. Some make silk feeding
nets attached to rocks in streams. Holometabolous development.
Order Siphonaptera
Fleas: Small, wingless, bodies laterally compressed. Legs adapted for leaping. Withpout
eyes. Ectoparisitic on birds and mammals. Larvae legless and scavengers. Holometabolous
development.
Order Hymenoptera
Ants, bees and wasps: Very small to large insects. Membranous, narrow wings coupled
distally. Subordinate hind wings. Mouth parts for biting and lapping up liquid. Ovipositor
somestomes modified into stinger, piercer, or saw. Both social and solitary species occur.
Most larvae legless, blind and maggot like. Holometabolous development.
72
ACTIVITY:
Distinguish
Between
Hemimetabolous
and
Holometabolous
insect
development.
5.0 Economic importance of Insects
Insects are necessary for cross-pollination of many crops. Bees pollinate many food crops
per year in the world. In addition, some insects produce useful materials: honey and bee
wax from bees, silk from silkworms, and shellac from wax secreted by lac insects. Many
predacious insects, such as tiger beetles, aphid lions, ant lions, playing mantis and lady bird
beetles, destroy harmful insects. Some insects control harmful ones by parasitizing them or
laying egg where their young, when hatched may consume the host. Dead animals are
quickly consumed by maggots hatched from eggs laid on carcasses.
Harmful insects include those which eat and destroy plants and fruits, such as
grasshoppers, chinch bugs, corn borers, boll weevils, grain weevils and scores of others.
Practically every cultivated crop has several insect pests. Humans expend enormous
resources in all agricultural activities, in forestry and in the food industry to counter insects
and the damage they engender.
Lice, blood-sucking flies, warble flies, bot flies, and many other attack humans or domestic
animals or both. Malaria, carried by the Anopheles mosquito is still one of the world’s
major diseases. Mosquitoes also transmit yellow fever and Lymphatic filariasis. Fleas carry
plague. House flies are vectors of typhoid and cholera. Tsetse flies carry African sleeping
sickness and certain blood sucking bugs, Rhodnius and related genera transmit Chagas’
disease. There is tremendous destruction of food, clothing and property by weevils,
73
cockroaches, ants, clothes months, termites, and carpet beetles. Not the least of insects
pests are bed bugs etc.
The beneficial roles of insects in our environment is often overlooked. In recent years
methods of control other than chemical insecticides have been under intense investigations,
experimentation and development. Several types of biological control using insects have
been developed. Introduction of natural predators or parasites of insect pests has had some
success. Another approach to biological control is to interfere with reproduction or
behaviour of insect pests.
SUMMARY
Arthropoda is the largest, most abundant and diverse phylum in the world. Arthropods are
metameric, coelomate protostome with well developed organ systems. Most show marked
tagmatization. They are extremely diverse and occur in all habitats capable of supporting
life. Perhaps more than any other single factor, success of arthropods is explained by
adaptation made possible by their cuticular exoskeleton. Other important elements in their
success are jointed appendages, tracheal respiration efficient sensory organs, complex
behaviour, metamorphosis and ability to fly. Insects are important to human’s welfare,
particularly because they pollinate food crops, control population of other harmful insets
by predation and parasitism. Insects also serve as food to other animals. Many insects are
harmful to human interests because they fed on crops and many are carriers of important
diseases affecting humans and domestic animals.
Self Assessment Questions
74
1. List some characteristics of Arthropods that clearly distinguish them from Annelids.
2. Name the subphyla of Arthropods and give a few example of each.
3. Differentiate the following from each other: Diplopoda, Chilopoda and insecta.
4. What different modes of feeding are found in insects, and how are these reflected in
their mouth parts?
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
75
LECTURE SEVEN
PHYLUM ECHINORDERMATA AND HEMICHORDATA
1.0 Introduction
Echinodermata, along with Chordata and Hemichordata (acorn worms and ptrebranchs) are
deuterostomes. Typical deuterstomes embryogenesis is shown only in chordates such as
amphioxus (next lecture), but these shared characters and molecular evidence support
monophyl of Deuterostomia.
Echinoderms are marine forms and include classes Asteroidea (sea stars or star fishes),
Ophiuroidea (bristle stars, Echinoidea (sea urchins), Holothuroidea (sea cucumbers) and
Crinoidea (sea lilies). Echinoderms have a combination of characteristics found in no other
phylum.
i.
An endoskeleton of plates or ossicles
ii.
Water vascular system
iii.
Pedicillariae
iv.
Dermal branchiae and
v.
Secondary radial or bi-radial symmetry
The water vascular system and dermal ossicles have been particularly important in
determining evolutionary potential and limitations of this phylum. Their larvae are bilateral
and undergo a metamorphosis to a radial adult.
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Phylum Ehinodernmata belongs to the Deusterostomia branch of the animal kingdom, the
members of which are enterocoelous coelomates. The other phyla are Hemichordata and
chordate (see next lectures). Deutersostomes have the following features in commaon: anus
development from or nar the blastopore, and mouth developing elsewhere,. Coelom budded
off from the archentoron (entercoel). Radial and indeterminate cleavage and mesoderm
derived from or with endoderm from entorocoelic pouches.
Objectives: At the end this lecture you should be able to:
i.
List the characteristics of the echinoderms
ii.
Depict the position of Echinoderms in the animal kingdom
iii.
Describe the biological contribution of Echinoderms
iv.
Name and characterize the classes of phylum echinodermata
2.0 Characteristics of Phylum Echinodermata
i.
Body not metameric, adult with radial, pentamerous symmetry characterized with
five or more radiating areas.
ii.
No head or brain, few special sensory organs.
iii.
Nervous system with circumoral ring and radial nerves.
iv.
Endoskeleton of dermal calcareous ossicles with stereom structure, covered by an
epidermis (ciliated in most). Pedicellariae (in some).
v.
A water vascular system of coelomic origin that extends from the body surface as
a series of tentacles like projections (podia or tube feet).
vi.
Locomotion by tube feet, which project from ambulacral areas, or by movement of
spines or by movement of arms, which project from central disc of the body.
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vii.
Digestive system usually complete, axial or coiled. Anus absent in ophiuroids.
viii.
Coeleom extensive, forming the perivisceral cavity and the cavity of the watervascular system. Coelom of enterocoelous type.
ix.
Respiration by dermal brancjhiae, tube feet, respiratory tree (holothuroids, and
bursae (ophiuroids).
x.
Excretory organs absent
xi.
Sexes separate
xii.
Development through free swimming, bilateral, larval stages (some with direct
development). Metamorphosis to radial adult or sub-adult form.
3.0 Classification of Phylum Echinodermata.
3.1 Class Crinoidea
Sea lilies and feather stars: Five arms branching at base and bearing pinnules. Ciliated
ambulacral grooves on oral surface with tentacles-like tube feet for food gathering, spines,
madreporite and pedicelllariae absent. Examples: Antedon., Nemaster, Comantheria.
3.2 Class Asteroidea
Sea stars: Star shaped, with arms not sharply demarcated from the central disc. Ambulacral
grooves open, with tube feet on oral side. Tube feet often with suckers. Anus and
madreporite aboral. Pecillariae present. Examples: Asterias and Pisaster.
3.3.Class Ophiuroidea
Brittle stars and basket stars: Star shaped, with arms sharply demarcated from central disc.
Ambulacral groove closed, covered by ossicles. Tube feet without suckers and not used for
locomotion. Pedicillariae absent. Examples: Ophiura and Astrophyton.
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3.4.Class Echinoidea
Sea urchins, sea biscuits and sand dollars. More or less globular or disc shaped, with no
arms, compact skeleton or test with closely fitting plates. Movable spines. Ambulacral
grooves closed. Tube feet often with suckers. Pedicellariae present. Examples: Arbacia,
Strongylocetrotus, lytechinus and Meoma.
3.5.Class Holothuroidea
Sea Cucumbers: Cucumber shaped with no arms, spine absent. Microscopic ossicles
embedded in muscular body wall. Anus present, ambulacral grooves closed. Tube feet with
suckers. Circumoral tentacles (modified tube feet). Pedicillariae absent. Medreporite plate
internal. Examples: Sclerodactyla, Parastichopus and Cucumaria.
4.0 PHYLUM HEMICHORDATA
4.1 Introduction
Hemichordates are marine animals formerly considered a subphylum of chordates, based on
possession of gills slits a rudimentary notochord and a dorsal nerve cord. However
zoologist now agrees that the so called hemichordates ‘notochord’ are really an evigination
of their mouth cavity and not homologous with the chordate notochord, so hemichordates
are considered a separate phylum. Hemichodates belong to deuterostome branch of the
animal kingdom and are enterocoelous coelomates with radial cleavage. Hemichordates
show both echinoderm and chordate characteristics.
A chordate plan of structure is suggested by gill slits and a restricted dorsal tubular nerve
cord. Similarity to Echinoderms is seen in larval characteristics. A tubular nerve cord in the
79
collar zone may represent an early stage of the condition in chordates: a diffuse net of nerve
cells is similar to the un-centralized, sub-epithelial plexus of echinoderms. Gill slits in the
pharynx, also characteristic of chordate, serve primarily for filter feeding and only
secondarily for breathing and are thus comparable to gill slits in proto-chordates.
4.2 Classification of Phylum Hemichordata
Class Enteropneusta
Acorn worms: Sluggish worm like animals that live in burrows under stones, usually in
mud or sand flats of intertidal zones. Has a mucus cover body divided into tonguelike
proboscis, a short collar, and a long trunk (protosome, mesosome and metasome).
Class Pterobranchia
The basic body plan of Pterobranchia is similar to that of Enteropneusta, but certain
differences are correlated with the sedentary mode of life of pterobranches. Only two
genera are known in any detail. In both genera arms with tentacles contain an extension of
coelomic compartments of the mesosome, as in a lophophore. One genus has a single pair
of gill slits, and the other has none. Both live in tubes from which they project their
proboscis and tentacles to feed by mucociliarly mechanism. Some species are dioecious
others monoecious, and sexual reproduction occurs by budding.
4.4 Economic importance of Echinoderms and Hemichordates
The Echinoderms are important both biologically and geologically: biologically because
few other groupings are as abundant in the biotic desert of the deep sea, as well as the
shallower oceans, and geologically as their ossified skeletons are major contributors to
many limestone formations, and can provide valuable clues as to the geological
80
environment. Further, it is held by some that the radiation of echinoderms was responsible
for the Mesozoic revolution of marine life. Echinoderms are regarded as a delicacy; and for
children sea-urchin skeletons are as popular a collecting object as brightly coloured starfish
are fascinating.
Sea cucumbers are also considered a delicacy in some countries of south east Asia;
particularly popular are the pineapple roller Thelenota ananas (susuhan) and the red
Halodeima edulis. They are well known as bêche de mer or Trepang in China and
Indonesia. The sea cucumbers are dried, and the potentially poisonous entrails removed.
The strong poisons of the sea cucumbers are often psychoactive, but their effects are not
well studied. It does appear that some sea cucumber toxins restrain the growth rate of
tumour cells, which has sparked interest from cancer researchers. The calcareous tests of
echinoderms are used as a source of lime by farmers in areas where limestone is
unavailable; indeed 4,000 tons of the animals are used annually for this purpose. This trade
is often carried out in conjunction with shellfish farmers, for whom the starfish pose a
major irritation by eating their stocks.
SUMMARY
Phyla Echinoderamata Chordata, and Hemichordata show characteristics of Deuterostomia.
Echinoderms are important marine group sharply distinguished from other animals. They
have penta-radial symmetry but were derived fro bilateral ancestors. Echinoderms are
unique, exclusively marine group of organisms in which deuterostomia development and an
endoskeleton are seen for the fist time. Echinoderms are characterized by a secondary radial
symmetry and a five part body plan. They have characteristic calcium-rich plates called
81
ossicles and unique water vascular system that includes hollow tube feet. Sea stars also
called starfish are five armed, mobile predators. Brittle stars are dwellers of the deep ocean
floor and shallow waters, where they pull themselves along two arms at a time, like oars
rowing a boat. Sand dollars and sea urchin lack arms but have five part symmetry. Sea
cucumbers are soft-bodied slug-like animals without arms. Arrow worms are among the
most abundant primary carnivores in marine food web. Hemichordates were formally
considered chordates because their buccal diverticulum was considered a notochord. In
common with chordates some of them do have gill slits and a hollow nerve cord.
Hemichordates show affinities with chordates, echinoderms and they are the likely sister
group of chordates.
82
Self Assessment Questions
1. What constellation of characteristics of echinoderms is found in no other phylum?
2. Define the following: Pedicellariae and madreporite.
3. Give three examples of how echinoderms are important to humans
4. Hemichordates were once considered chordates. Why?
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
http://en.wikipedia.org/wiki/echinoderm. 24th August 2008.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
83
LECTURE EIGHT
PHYLUM CHORDATA
1.0 Introduction
The chordates are deuterostomes coelomates whose nearest relations in the animal kingdom
are the echinoderms, also deuterostomes. The chordates are the best known and most
familiar of all animal groups. There are 43,000 species of chordates, a phylum that includes
birds, reptiles, amphibians, fishes and mammals. Four principle features characterize the
chordates and have played an important role in the evolution of the phylum.
i.
A single hollow nerve cord runs just beneath the dorsal surface of the animal. In
vertebrates, the dorsal nerve cord differentiates into the brain and spinal cord.
ii.
A flexible rod, the notochord, forms on the dorsal side of the primitive gut in the
early embryo and is present at some stage in the life cycle in all chordates. The
notochord is located just below the nerve chord. The notochord, which persists
throughout the life of some invertebrates chordates, becomes surrounded and then
and then replaced during embryological development in most vertebrates by the
vertebrae column that form around the nerve chord.
iii.
Pharyngeal slits connect the pharynx, a muscular tube that links the mouth cavity
and oesophagus, with the outside. In most vertebrates, the slits do not actually
connect to the outside and are better termed as pharyngeal pouches. Pharyngeal
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pouches are present in the embryos of all vertebrates but re lost later in the
development of terrestrial of terrestrial vertebrates. The presence of these structures
in all vertebrate embryos provides a clue to the aquatic ancestry of the group.
iv.
Chordates have a postnatal tail that extends beyond the anus, at least during their
embryonic development. Nearly all other animals have a terminal anus.
All vertebrates have all four of these characteristics at some time in their lives. For example
humans have pharyngeal slits, a dorsal nerve cord, and a notochord as embryo. As adults
humans retain only the nerve cord and one pair of pharyngeal slits, which are the
Eustachian tubes that connects the throat to the middle ear.
Objectives: At the end this lecture you should be able to:
i.
Distinguish chordates from invertebrates
ii.
Explain why vertebrates are more successful than invertebrates
iii.
Describe the characteristics the major of chordates
iv.
Characterize the classes of phylum chordate
2.0 The Non-vertebrate chordates
2.1 Subphylun Urochordata
The urochordates, commonly called tunicates, number some 3000 species. They are found
in all seas from shoreline to great depths. Most are sessile as adults, although some are free
living. The name tunicate is suggested by the usually tough, non living tunic that surrounds
the animal. As adults tunicates are highly specialized chordates, for in most species only
larvae forms, which resemble a microscope tadpole, bears the entire chordate hallmark.
During adult metamorphosis, the notochord (which in larvae is restricted to the tail, hence
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the group name urochordata). And tail disappears, while dorsal nerve cord becomes
reduced to a single ganglion. Urochordate are traditionally divided into three classes:
Ascidiacea, larvacea and Thaliacea.
ACTIVITY: Read on the characteristics of the three classes of the urochordates.
2.2 Subphylum Cephalochordata
Cephalochordates are marine lancelets: slender, laterally compressed, translucent animals
about 5 to 7 cm in length that inhabit sandy bottoms of coastal waters around the world.
Lancelets originally bore the generic name Amphioxus later surrendered by priority to
Branchiostoma. Amphioxus is especially interesting because it has the four distinctive
characteristics of chordates in simple form. No other chordate shows the basic diagnostic
characteristic of chordates so well. In addition, to the four chordate anatomical hallmarks,
Amphioxus posses several structural features that foretell the vertebrate plan.
2.3 Subphylum Vertebrate
The third subphylum of chordates is the largest and diverse Vertebrata. This monophyletic
group shares the basic chordate plan characteristics with the other two subphyla, but in
addition it reveal novel homologies that other do not share. Vertebrates are chordates with
a spinal column. The name vertebrate comes from the individual bony segments that make
up the spine, which are called vertebrae. Vertebrates differ from the tunicates and lancelets
in two important respects.
i.
Vertebral column. In vertebrates the notochord becomes surrounded, and then
replaced during the course of embryological development by bony vertebral
column.
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ii.
Head. In all vertebrates except the earliest fishes, there is a distinct and well
differentiated head, with a skull and brain. For this reason, the vertebrate are
sometimes called the craniates chordates.
In addition to these two key characteristics, vertebrates differ from other chordates in other
important respects.
i.
Neural crest. A unique group of embryonic cells called neural crest contributes to
the development of many vertebrate structures. These cells develop on the crest of
the neural tube as it forms by invagination and pinching together of the neural plate.
Neural crest cells then migrate to various locations in the developing embryo, where
they participate in the development of a variety of structures.
ii.
Internal organs. Among the internal organs of vertebrates, livers, kidneys, and
endocrine organs are characteristic of the group. All vertebrates have a heart and a
closed circulatory system.
iii.
Endoskeleton. The endoskeleton of most vertebrate is made up of bone (in a few,
like sharks, there is flexible cartilage instead). The vertebrate endoskeleton makes
possible the great size and extraordinary powers f movement that characterize this
group.
iv.
Integument. Basically of two divisions, an outer epidermis of stratified epithelium
from ectoderm and an inner dermis of connective tissue derived from mesoderm.
v.
Many muscles. Attached to the skeleton to provide for movement.
vi.
Complete digestive system. Ventral to the spinal column and provided with a large
digestive glands, liver and pancreas.
vii.
Well developed coelom. Largely filled with visceral systems.
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viii.
Excretory system: consisting of paired kidney (mesonephric type in adults),
provided with duct to drain waste to the cloaca or anal region.
ix.
Highly differentiated brain: 10 or 12 pairs of cranial nerves usually with both
motor and sensory functions: a pair of spinal nerves for each primitive myotome; an
autonomic nervous system in control of involuntary function of internal organs.
Paired special sense organs.
x.
Endocrine system of ductless glands secreted through the body.
xi.
Nearly always have separate sexes, each sex containing paired gonads with ducts
that discharge their products either into the cloaca or into special openings near the
anus.
3.0.Overview of the evolution of vertebrates
The first vertebrates evolved about 470 million years ago in the oceans as fishes without
jaws or paired fins. Many of them looked like flat hot dog; with a hole in one end and a fin
at the other. Jawed fishes became the dominant creatures in the sea, some of them growing
larger than cars. Their descendants, the amphibians, invaded the land. Salamander like
amphibians and other, much larger now extinct amphibians were the first vertebrate to live
successful on land.
Amphibians in turn, gave rise to the first reptiles about 300 million years ago. Within 50
million years, reptiles better suited than amphibians to living out of water, replaced them as
the dominant land animal on earth. With the success of reptiles, vertebrates truly came to
dominate the surface of the earth. Many kids of reptiles evolved, ranging in size from
smaller than a chicken to bigger than a truck. Some flew, and others swam. Among them
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evolved reptiles that gave rise to the two great lines of terrestrial] vertebrates; dinosaurs
(and their bird descendants) and mammals. For over 150 million years, dinosaurs
dominated the face of the earth. Over all these centuries (over a million centuries) the
largest mammals was no bigger than a cat. Then about 65 million years ago, the dinosaurs
abruptly disappeared, for reasons that are still hotly debated. Mammals quickly took their
place becoming in turn abundant and diverse.
Vertebrates are a diverse group, containing members adapted to life in aquatic habitats, on
land and in the air.
4.0. Classification of Phylum Chordata
There are nine principal classes of living vertebrates. Five of the classes are fishes that live
in water, and four are land dwelling tetrapods, animals with four limbs.
4.1. Group Protochordata (Acrania)
Subphylum Urochordata (Tunicata).
Tunicates: Notochord and nerve cord in free swimming larva only. Ascidian, adults sessile.
Encased in tunic.
Subphylum Cephalochordata (Amphioxus)
Lancelets; Nerve cord and notochord found along entire length of body and persist
throughout life. Fish like in form.
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4.2 Group Craniata
4.2.1 Subphylum Vertebrata
Bony or cartilaginous vertebrae surrounding spinal cord (vertebrae absent in agnathans).
Notochord only in embryonic stages, persist in some fishes, also may be divided into two
groups (super classes) according to presence of jaws.
Super class Agnatha (without jaws). Cyclostomata: the hag fishes and lampreys. Without
true jaw or paired appendages. Probably a paraphyletic group.
Class Myxini. Hag fishes: Terminal mouth with four pairs of tentacles, buccal
funnel absent, nasal sac with duct to pharynx, five to 15 pairs of gill pouches,
partially hermaphroditic.
Class Cephalaspidomorphi (Petromyzones). Lampreys: Suctorial mouth with
honry teeth, nasal sac not connected to mouth, seven pairs of gill pouches.
Super class Gnathostomata. Jawed fishes: all tetrapods. Have jaws and usually have
paired appendages.
Class Chondrichthytes. Cartilaginous fishes (sharks, skates, rays and chimaeras).
Cartilaginous skeleton, teeth not fused to jaws and usually replaced. Five to seven
gills with separate openings, no operculum, no swim bladder.
Class Actinopterygii. Ray finned bony fishes. Ossified skeleton, single gill opening
covered by operculum. Paired fins supported primarily by dermal rays. Limb
musculature within body. Swim bladder mainly a hydrostatic organ, if present.
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Class Sarcopterygii. Lobe finned bony fishes. Ossified skeleton, single opening
covered by operculum, paired fins with sturdy internal skeleton and musculature
within the limb. Diphercercal tail. usually with lung like swim bladder.
Class Amphibia. Amphibians. Ectothermic tetrapods, respiration by lungs, gills,
or skin. Development through larval stages, skin moist containing mucous glands
and lacking scales.
Class Reptilia. Reptiles: Ectothermic tetrapods possessing lungs embryo develops
within shelled egg. No larval stages. Skin dry, lacking mucous glands, and covered
by epidermal scales. A paraphyletic group.
Class Aves. Birds: Endothermic vertebrates with front limbs modified for flight,
body covered with feathers, scales on feet.
Class Mammalia. Mammals: Endothermic vertebrates, possessing mammary
glands, body more or less covered with hair. Well developed brain.
ACTIVITY: Read on the ecology and the economic importance of the various classes in
the phylum Chordata.
SUMMARY
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Phylum Chordata is named for the rod like notochord that forms a stiffening body axis at
some stage in the life cycle of every chordate. All chordates share four distinctive hallmarks
that set them apart from all other phyla: notochord, dorsal tubular nerve cord, pharyngeal
pouches, and postnatal tail. Two of the three chordate subphyla are invertebrates and lack a
well developed head. They are Urochordata (tunicates) and Cephalochordata (lancelets).
Chordates have evolutionary affinities to echinoderms. Chordates have a greater
fundamental unity of organs systems and body plan than many invertebrate phyla.
Subphylum vertebrate includes the backboned members of the animal kingdom. The living
jawless vertebrates: hagfishes and lampreys, actually lack vertebrae but are included with
the vertebrates because they share numerous homologies. As a group vertebrate are
characterized by having a well developed head and by their comparatively large size, high
degree of motility and distinctive body plan, which embodies several distinguishing
features that permitted their exceptional adaptive radiation. Most important of these are
living endoskeleton, which allow continuous growth and provides a study frame work for
efficient muscle attachment and action. A pharynx perforated with slits with vastly
increased respiratory efficiency. A complex nervous system with clear separation of brain
and spinal cord and possession paired limbs for locomotion efficiency.
Self Assessment Questions
1. What are the four primary characteristics of the chordates?
2. What re the three subphyla of chordates? Give example of ach.
3. List the classes of vertebrates and give examples of each.
4. Distinguish mammals from other classes.
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REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
93
LECTURE NINE
CELL BIOLOGY
1.0.Introduction
In lecture 1.3 we were introduced to ancient cells, prokaryotic and eukaryotic cells. In the
present we are going to learn on some basic principles of cell biology. All organisms are
composed of cells. Some organisms are made up of a single cell too small to be seen with
unaided eye. While others are, like human beings are composed of many cells. Cells are the
smallest living things, the basic unit of organization of all organisms. In the present lecture
we will take a close look at the structure of cells, how they communicate with the
environment, grow and reproduce. The general plan of cellular organization varies in the
cells of different organisms, but despite these modifications, all cells resemble each other in
certain fundamental ways. In terms of structural organization cells can be divided into two
groups; prokaryotic and eukaryotic cells. Prokaryote cells include bacteria, blue green
algae, and mycoplasma.
Eukaryotic type of cells is found in organisms such as fungi,
plants and animals.
OBJECTIVES: At the end of the present chapter you should be able to
i.
Describe the structure of the cell
ii.
Explain how cell communicate with the environment
iii.
Explain how cells grow
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iv.
Describe cells reproduce
2.0 Prokaryotic cells
Prokaryotic cells are simple cells, their organization is fundamentally similar. They are
small cells surrounded by a membrane and enclosed within a rigid cell wall, with no
distinct interior compartments. Sometimes bacterial cells adhere in chains or masses, but
the individual cells function independently of another. Most prokaryote are enclosed by a
strong cell wall composed of peptidoglycan. No type of eukartotic cells possesses cell walls
with this type of chemical composition.
Bacteria may be classified into two type’s base on differences in their cell wall detected by
the Gram staining procedure. Gram positive bacteria have a thick, single layered cell wall
that retains the Gram stain within the cell, causing the stained cell to appear purple under a
microscope. Gram negative bacteria have a multi layered wall which does not retain the
Gram stain. The susceptibility of bacteria to antibiotics often depends on the structure of
their cell walls.
Prokaryotes have simple internal organization, there are few if any internal compartments
such as simple organelles like ribosomes. Most have no membrane bounded organelles,
they do have true nucleus. The entire cytoplasm of the prokaryotic cell is one unit with no
internal support structures. The strength of the cell comes from the rigid cell wall. Have one
chromosome, a simple circle of DNA. Some prokaryotes have a flagellum (plural flagella)
95
which is used for movement. They may be one or more per cell or none, depending on the
species. Only few eukaryotic cells have flagella.
2.1 Eukaryotic cells
Eukaryotic cells are far more complex than prokaryotic cells. The have internal
compartmentalization. The interior of eukaryotic cells contain numerous organelles,
membranes bounded structures that close off compartments within which multiple
biochemical processes can proceed simultaneously and independently. Plant cells often
have a large membrane bounded sac called a central vacuole, which stores proteins,
pigments and waste materials.
Both plant and animal cells contain vesicles, smaller sacs that store and transport a variety
of materials. Inside the nucleus, the DNA is wound tightly around protein packaged units
called chromosomes. All eukaryotic cells are supported by an internal protein scaffold, the
cytoskeleton. While the cells of animals and some protistis lack cell walls, the cells of
fungi, plants and many protistis have strong cell walls composed of cellulose or chitin
fibres embedded in a matrix of other polysaccharide and proteins. This composition is very
different from the peptidoglycan that makes up the bacterial cell walls. Let as know
examine the structure and function of the internal components of eukaryotic cells in more
details.
The Nucleus
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The largest and easily seen organelle, the nucleus is the storehouse of the genetic
information that directs all of the activities of a living eukaryotic cell. Most eukaryotes
posses a single nucleus although, the cells of fungi and some other groups, may have
several to many nuclei. Mammalian erythrocytes (red blood cells) lose their nuclei when
they mature. Many nuclei exhibits a dark stain zone called the nucleolus, which is the
region where intensive synthesis of ribosomal RNA is taking place.
The Nuclear Envelope
The surface of the nucleus is bounded by two phospholipid bi-layer membranes, which
together make yup the nuclear envelope. The outer membrane of the nuclear envelope is
continuous with the cytoplasm’s interior membrane system, called the endoplasmic
reticulum. Scattered over the surface of the nuclear envelope like craters on the moon, are
shallow depressions called nuclear pores. Nuclear pores are filled with proteins that act as
molecular channels, permitting certain molecules to pass into and out of the nucleus.
The chromosomes: Packing DNA
In both prokaryote and eukaryotes, DNA contains the hereditary specifying cell structures
and function. However, unlike the DNA of prokaryotes, The DNA of eukaryotes is divided
into several linear chromosomes. The chromosomes are associated with packaging
proteins called histones.
The Endoplasmic Reticulum
The largest of internal membranes is called the endoplasmic reticulum (ER). ER is
composed of a lipid bi-layer embedded with proteins. It weaves in sheets of through the
interior of the cell, creating a series of channels and interconnections between its folds. The
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ribosome rich regions of the ER appear rough, like the surface of sandpaper and they are
therefore called the rough ER. The ER region is devoted to protein synthesis. The regions
of the ER with relatively few bound ribosomes are referred to as smooth ER. The
membranes of the smooth ER contain many embedded enzymes that catalyze the synthesis
of a variety of carbohydrates and lipids.
The Golgi apparatus: Delivery system of the cell
Occurs at various locations within the endomembrane system, they are flattened stacks of
membranes called Golgi bodies, which are found often interconnected with each other.
The number of Golgi bodies a cell contains ranges from one or few in protists, to 20 or
more in animal cells and several hundreds in plant cells. They are especially abundant in
glandular cells, which manufacture and secrete substances. Collectively the Golgi bodies
are referred to as the Golgi apparatus. The Golgi apparatus is the delivery system of the
cell. It collects, packages, modifies, and distributes molecules that are synthesised at one
location within the cell and used at another. Proteins and lipids manufactured on the rough
and smooth ER membranes are transported and modified through the Golgi apparatus.
Vesicles: Enzyme store houses.
Lysosomes, membrane bounded digestive vesicles, are also components of the
endomembrane system and seem to arise from the Golgi apparatus. They contain in a
concentrated mix the digestive enzymes of the cell, which catalyze the rapid breakdown
of proteins, nucleic acids, lipids and carbohydrates. In addition to breaking down organelles
and other structures within cells, lysosomes also eliminate particles (including other cells)
that cells have engulfed in a process called phagocytosis.
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Peroxisomes (Detoxifiers of Hydrogen peroxide), contain enzymes that catalyzes the
removal of electrons and associated hydrogen atoms. Hydrogen Peroxide is dangerous to
cells because of its violent chemical reactivity. Peroxomes contain the enzyme catalase,
which breaks down hydrogen Peroxide into harmless water and oxygen.
Ribosomes: Sites of Protein Synthesis
Ribosomes provide a frame work for protein synthesis in the cytoplasm. During protein
synthesis, ribosomes attaches to the messenger RNA (mRNA) transcribed from a gene and
use the information to direct the synthesis of a protein. Ribosomes are made up of several
molecules of a special form of RNA called ribosomal RNA or rRNA, bound within a
complex of several dozens different proteins. Ribosomes are among the most complex
molecular assemblies found in cells. Bacterial ribosomes are smaller than eukaryotic
ribosomes.
Organelles that contain DNA
Among the most interesting cell organelles are those in addition to the nucleus that contain
DNA: Mitochondria, Chloroplasts and Centrioles. Mitochondria are typically tubular or
sausage shaped organelles about the size of bacteria and found in all types of eukaryotic
cells. Mitochondria re bounded by two membranes: a smooth outer membrane and an inner
one folded into numerous contiguous layers called cristae. The cristae partition the
mitochondrion into two compartments: a matrix, lying inside the inner membranes and an
outer compartment, or intermediate space lying between the two mitochondrial membranes.
On the surface of the inner membranes, and also submerged within it, are proteins that carry
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out oxidative metabolism, the oxygen requiring process by which energy in
macromolecules is stored in ATP.
Chloroplast: where photosynthesis takes. The photosynthetic cells of plants and other
organisms that carry out photosynthesis (using light energy to manufacture organic
molecules) typically contain from one to several hundred chloroplasts. The Chloroplast
contains the pigment chlorophyll that gives most plants their green colour. The chloroplast
body is enclosed, like the mitochondrion, within two membranes that resemble that of
mitochondria.
However, chloroplast is larger and more complex than mitochondria. In addition to the
outer and inner membranes, which lie in close association with each other, chloroplast have
a closed compartment of stacked membranes called grana (singular granum), which lie
internal to the inner membrane. A chloroplast may contain a hundred or more grana, and
each granum may contain from few to several dozens disk shaped structures called
thylakoids. On the surface of the thylakoids are the light-capturing photosynthetic
pigments. Surrounding the thylakoids is a fluid matrix called stroma. Like mitochondria,
chloroplast contain DNA, but many of the genes that specify chloroplast components are
also located in the nucleus.
Centrioles: Microtubules Assembly Centres, are barel shaped organelles found in cells of
animals and most protists, they occur in pairs. The pair is refereed to as centrosome. At
least one centriole contains DNA, which apparently is evolved in producing their structural
proteins. Centrioles help to assemble microtubules. Microtubules influence cell shape,
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moves the chromosomes in cell division and provide the functional internal structure of
flagella and cilia.
The Cytoskeleton: Frame work of the Cell
The cytoplasm of all cells is crisscrossed by a network of proteins fibres that support the
shape of the cells and anchors organelles to fixed locations. This network is called the
cytoskeleton. Individual fibres form by polymerization as identical protein subunits attract
one another chemically and spontaneously assemble into long chains. Fibres disassembles
in the same way as subunit after another breaks away from one end of the chain. Cells from
plants and animals contain three types of cytoskeleton fibres, each formed from a different
kind of subunit. Actin filament, Microtubules and Intermediate filaments.
SUMMARY
The cell is the smallest unit of life. All living things are made up of cells. The cell sib
composed of a nuclear region, which holds the hereditary apparatus, enclosed within the
cytoplasm. In all cells cytoplasm is bounded by a membrane of phospholipid and protein.
Eukaryotic cells are far more complex than prokaryotic cells. A eukaryotic cell is organized
into three principal zones the nucleus, the cytoplasm and the plasma membrane. Located in
the cytoplasm are numerous organelles that which perform specific functions for the cell.
Mitochondria and chloroplasts are part of the energy processing systems.
Self Assessment Questions
1. How are prokaryotes different from prokaryotes?
2. What is endoplasmic reticulum?
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3. Describe the basic structure of mitochondria?
4. What of eukaryotic cells contain mitochondria?
5. What type of eukaryotic cells contains chloroplast?
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
102
LECTURE TEN
BASIC BIOLOGY OF VIRUSES BACTERIA AND FUNGI
1.0 Introduction
The present lecture offer students an introduction to the vast knowledge of viruses, bacteria
and fungi. Viruses which are not cells have been of inestimable importance in the
development of biochemistry and molecular biology. These relatively simple systems have
helped biologist to understand the working of more complex organisms and have also
provided the biological tools to engineer cellular activities for human purpose. Many
people think of Bacteria as the germs that cause disease. However, most bacteria do not
cause disease. Bactria help to recycle vital chemicals in the ecosystem, therefore, they are
essential for the well being of l[all organisms. Fungi were once classified as plants as
plants. However, close examination shows that fungi and plants do not share enough
similarities to be classified in the same kingdom. Characteristics of that distinguishes from
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plants include cell walls composed of chitin, multinucleated cells, lack of chlorophyll and
differences in spore structure.
OBJECTIVES: At the end of this lecture you should be able to:
i.
Describe the basic biology of viruses, bacteria and fungi
ii.
Distinguish prokaryotic and eukaryotic cells
iii.
Discuss the importance of bacteria and viruses to health and biotechnology
iv.
Discuss the importance of fungi to animal and plant health.
2.0 VIRUSES
Viruses are not considered to be living organisms, as they cannot reproduce independently
and can only replicate themselves by gaining entry into a cell and using that cell machinery.
A virus is more accurately considered a detached fragment of a genome. Because of their
disease producing potential, viruses are important biological entities. Viruses cause AIDS,
polio, flu and many other important human and plant diseases. Viruses are made of RNA or
DNA surrounded by a protein coat. Many animal viruses form an envelope around the
capsid rich in protein, lipids and glycoprotein molecules. Individual kind of viruses
contains only a single type of nucleic acid, ether DNA or RNA. They are able to reproduce
because they carry genes that are translated into proteins by the cells genetic machinery,
proteins that lead to the production of more viruses. Viruses also lack ribosomes and all of
the enzymes necessary for protein synthesis and energy metabolism.
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The simple structure of viruses, the large numbers that are produced in an infection of a
cell, and the fact that their genes are related to those of their hosts have led many scientists
to study viruses in attempts to unravel the nature of genes and how they work. In future it is
expected that viruses will be one of the principal tools used to experimentally carry genes
from one organism to another.
2.1The structure of viruses
The simplest viruses consist of a single molecule of nucleic acid surrounded by a capsid,
which is made up of one to a few different protein molecules repeated many times. In more
complex viruses, there may be several different kinds of molecules of either DNA or RNA
in each virus particles and many different kinds of proteins. Most viruses have rod like or
threads like appearance. Isometric viruses have a roughly spherical shape. Most viruses are
icosahedral in basic structure. It is the most efficient symmetrical arrangement that linear
subunits can take to form a shell with maximum internal capacity.
2.2.Bacteriophages
Bacteriophages are viruses that infect bacteria. They are diverse both in structure and
function and can be found only in bacterial hosts. Many of these bacteriophages are large
and complex, with relatively large amount of of DNA and proteins. Some of them have
been named as members of the “T” series. The T-series bacteriophages are all virulent
viruses, multiplying within infected cells and eventually lysing (rupturing) them.
2.3.Diseases viruses
Among the diseases that viruses cause are influenza, small pox, infectious hepatitis, yellow
fever, polio rabies, and AIDs, as well as many other diseases not well known. Viruses not
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only cause human diseases but also cause major losses in agriculture, forestry and in the
productivity of natural ecosystems.
2.4.Viroids
Viroids are tiny naked molecules of RNA, only a few hundreds nucleotides long, that are
important infectious disease agents in plants. A recent viroid outbreak killed over ten
million coconut palms in the Philippines. It is not clear how viroids cause disease.
However, one important evidence is that viroid nucleotides sequence resembles the
sequence of introns within ribosomal RNA genes. These sequences are capable of
catalysing excision from DNA.
3.0.BACTERIA
The simplest organisms living on earth today are bacteria, and biologists think they closely
resemble the first living organisms to evolve on earth. Too small to see with unaided eye,
bacteria are most abundant of all organisms are the only one characterized with prokaryotic
cellular organizations. Life on earth could not exist without bacteria. Bacteria make
possible many of essential functions of the ecosystems, including the capture of nitrogen
from the atmosphere, and in many aquatic communities, photosynthesis. Indeed bacteria
photosynthesis is thought to have been responsible for generating much the oxygen in the
earth’s atmosphere, Bacteria are also responsible for many serious human diseases. An
understanding of bacteria is thus essential for both scientific and practical reasons.
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3.1 Prokaryotes versus Eukaryotes
Prokaryotes or bacteria differ from eukaryotes in numerous important points. These
differences represent some of the most fundamental distinctions that separate any group of
organisms.
i.
Multicellularity: All bacteria are fundamentally single celled. Eukaryotes are
multicellular.
ii.
Cell size: Most bacterial cells are one micrometer or less in diameter. Most
Eukaryotic cells are well over 10 times that size.
iii.
Chromosome: Eukaryotic cells have a membrane bound nucleus containing
chromosome made up of nucleic acids and protein. Bacteria do not have membrane
bound nucleus. Have naked circular DNA localized in a zone of the cytoplasm
called the nucleoid.
iv.
Cell division and genetic recombination: Cell division in eukaryotes takes place
by mitosis and involves spindles made up of microtubules. Cell division in bacterial
takes place mainly by binary fission. True sexual reproduction occurs only in
eukaryotic cells and involves syngamy and meiosis, with alternation of diploid and
haploid forms. Despite lack of sexual reproduction bacteria do have mechanisms
that lead to the transfer of genetic material.
v.
Internal compartmentalization: The cytoplasm of bacteria unlike that of
eukaryotes contains no internal compartments or cytoskeleton and no organelles
except ribosomes.
vi.
Flagella: Bacterial flagella are simple, composed of single fibre of the protein
flagellin. Eukaryotic flagella and cilia are complex and have 9 + 2 structure of
microtubules.
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vii.
Metabolic diversity: In photosynthetic eukaryotes, the enzymes for photosynthesis
are packed in membrane bound organelles. Only one kind of photosynthesis occurs
in eukaryotes, and involves the release of oxygen. In photosynthetic bacteria, the
enzymes for photosynthesis are bound to cell the membrane. These bacterial have
several different patterns of anaerobic and anaerobic photosynthesis, involving the
formation of end products such as sulphur, sulphate and oxygen.
3.2.The structure of bacteria
Bacterial cell walls are usually made up of peptidoglycan. Bacteria are mostly simple in
form, varying mainly from straight and rod-shaped (bacilli) or spherical (cocci) to long and
spirally coiled (spirilla). Some bacteria form branched filaments or form erect structures
that release spores. Some rod shaped and spherical bacteria, adhere end to end after they
have divided, forming chains.
Bacteria have slender, rigid, helical flagella. Flagella may be distributed all over the surface
of the bacterium in which they occur, or may be confined to one or both ends of the cell.
Pili are other kinds of hair-like outgrowth that occur on the cells of some bacteria. They are
shorter than flagella, pili help the bacterial cell to attach to appropriate substrate and
exchange genetic information. The number and arrangement of flagella and pili are useful
aids in bacterial identification. Some bacteria form thick walled endospores around their
chromosomes and a small portion of the surrounding cytoplasm when they are exposed to
nutrient-poor condition. These endospores are highly resistant to environmental stress,
especially heat, and can germinate to form new individual after decades or even centuries.
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The internal structure of the bacteria cell is basically the prokaryotic cell organization.
Bacterial cells lack nuclei and do not carry out mitosis, instead dividing by transverse
binary fusion. Bacteria cells lack the extensive functional compartmentalization seen within
eukaryotic cells.
3.3 Kinds of Bacteria
As we learned in lecture 1.3.2 bacteria is split into two lines early in the history of life, so
different in structure and metabolism that they are as different from each other as either is
from eukaryotes. The differences are so fundamental that biologist assign the two groups of
bacteria to separate domains. One domain consists of the archaebacteria (ancient bacteria)
The only survivors are confined to extreme environments that may resemble habitats on the
early earth. The other more ancient domain consists of the eubacteria (true bacteria). It
includes nearly all of the named species of bacteria. Archaebacteria and eubacteria differ in
four ways.
i.
Cell wall: the cell wall of eubacteria is constructed of carbohydrates-protein
complexes called peptidoglycan. The cell wall of archaebacteria lack peptidoglycan.
ii.
Plasma membrane: the plasma membranes of eubacteria and archaebacteria are
made up of very different kinds of lipids.
iii.
Gene translation machinery: Eubacteria possess ribosomal proteins and an RNA
polymerase that are distinctly different from eukaryotes. However, the ribosomal
proteins and RNA of archaebacteria are very similar to those of eukaryotes.
iv.
Gene architecture: The genes of eubacteria are not interrupted by introns, while at
least some of the genes of archaebacteria do possess introns.
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ACTIVITY: Read on the major groups of bacteria, giving typical examples and key
characteristics.
3.4 Economic importance of Bacteria
Bacteria are responsible for creating the properties of the atmosphere and the soil over
billion
of
years.
Many
bacteria
are
autotrophic,
either
photosynthetic
or
chemoautotrophic and make major contribution to the world carbon balance in terrestrial,
freshwater and marine habitats. Others are heterotrophic and play a key role in world
ecology by breaking down organic compounds. Some of these heterotrophic bacteria cause
major diseases of plants and animals, including humans. One of the most important roles of
bacteria in the world ecosystem relates to the fact that only a few genera of bacteria and no
other organisms have the ability to fix atmospheric nitrogen and thus makes it available
for use by other organisms.
Bacteria are very important in many industrial processes; bacteria are now used in
production of acetic acid and vinegar, various amino acids and enzymes, and especially in
the fermentation of lactose into lactic acid, which coagulates milk protein and is used in the
production of almost all cheese, yogurt, and similar products. Many of the mostly widely
used antibiotics, including streptomycin, aureomycin, and chloromycetin are derived from
bacteria.
Genetic engineering methods are being applied to produce improve strains of bacteria for
commercial use. Bacillus thuringiensis attacks insects in nature, and improved, highly
specific strains of B. thuringiensis have greatly increased its usefulness as a biological
control agent.
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Many costly diseases of plants are associated with particular heterotrophic bacteria.
Almost every kind of plant is susceptible to one or more kinds of bacterial disease. The
symptoms of these diseases vary, but they are commonly manifested as spots of various
sizes on the stems, leaves, flowers or fruits. Fire blight, which destroys pears, apple trees,
and related plants, is a well known example of bacterial disease.
Bacteria causes many diseases in humans, including cholera, leprosy, tetanus, bacterial
pneumonia, whooping cough, and diphtheria. Members of the genus Streptococcus are
associated with scarlet fever, rheumatic fever, pneumonia and other infections.
Tuberculosis another bacterial disease is still a leading cause of death in humans. Some
sexually transmitted disease are also caused by bacteria, these include gonorrhoea, syphilis,
and Chlamydia.
4.0 FUNGI
Of all the bewildering variety of organisms that live on earth, perhaps the most, unusual,
the most peculiarity different from ourselves, are the fungi. The Fungi is a distinct kingdom
of organisms, comprising about 77,000 named species. Although fungi have traditionally
been included in the plant kingdom, they lack chlorophyll and resemble plants only in their
general appearance and lack of mobility. Significant difference between fungi and plants
include the following: Fungi are heterotrophy no fungi carry out photosynthesis, fungi have
filamentous bodies, fungi have non motile sperm some plants have motile sperm with
flagella, fungi have cell walls made up of chitin and fungi have nuclear mitosis the nuclear
envelop does not break down and reform, instead mitosis takes place within the nucleus.
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4.1 The structure of fungi
Fungi exist in form of filamentous, barely visible to the naked eye, which are called hyphae
(singular hypha). These hyphae may be divided into cells by cross walls called septa
(singular septum).A mass of hyphae is called a mycelium (r-plural mycelium). This
mycelium grows through and penetrates its substrate resulting in unique relationship
between the fungus and its environment.
4.2 Fungi reproduction
Fungi reproduce sexually after two hyphae of opposite mating type fuse. Asexual
reproduction by spores is a second common means of reproduction.
4.3 Nutrition
Fungi obtain their food by secreting digestive enzymes into their surroundings and then
absorbing back into the fungus the organic molecules produced by this external digestion.
Many fungi are able to break between glucose subunits and then absorbing the molecules as
food. Other fungi are predators.
4.4 Fungi classification
There are four groups of fungi: Phylum Zygomycota, the zygomycetes. Phlum Ascomycta,
the ascomycetes and the basidiomycetes. The groups of fungi are distinguished primarily by
their sexual reproduction structures. In the zygomcetes the fusion of hyphae leads directly
to the formation of a zygote, which divides by meiosis when it germinates. In the other two
phyla, an extensive growth of dikaryotic hyphae within which are formed the distinctive
king of reproductive cells characteristic of a particular group.
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4.5 Economic importance of Fungi
Some species of Penicilium are sources of the well known antibiotic penicillin, and other
species of the genus give the characteristic flavours and aromas to cheeses. Citric acid is
produced commercially with members of this genus under highly acidic conditions. Fungi
cause skin diseases in humans, including athlete foot and ringworms.
SUMMARY
Viruses are not living organisms, they cannot reproduce outside of living cells, since they
lack the machinery to do so. Viruses detached fragments of bacterial or eukaryotes
genomes that are able to replicate within cells by using the genetic machinery of those cells.
Viruses are basically either helical or isometric. Most isometric viruses are icosahedral.
Viruses are responsible for many human and plant diseases. Bacteria are oldest and
simplest organisms, but they are metabolically much more diverse than all other living
organisms. Bacteria [lay vital roles in cycling nutrients within ecosystems. Certain bacteria
are the only organisms able to fix atmospheric nitrogen into organic molecules, a process
on which all life’s depends. Bacteria differ from eukaryotes in many ways, the most
important of which concern the degree of internal organization within the cells. Most
bacteria have cell walls that consist of network of a network of polysaccharides molecules
connected by polypeptide cross-links. In gram negative bactreria, an outer membrane
containing lies over a thin peptidoglycan layer. Gram positive bacteria lack this membrane
but have a thicker peptodoglycan. Bacteria a rod shaped (bacilli) or spiral (spirilla) in form.
Bacilli or cocci may adhere in small groups or chains. A bacteria cell does not posses a
membrane bounded nucleus but it may exhibit a nucleod region where the bacterial DNA is
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located. The two kingdoms Archaebteria and eubacteria, are made up of the prokaryotes, or
bacteria. Some heterotrophy bacteria cause major diseases. The fungi are distinct kingdom
of eukaryotic organisms characterized by a filamentous growth form, lack of chlorophyll
and motile cells, chitin-rich cell walls and external digestion of food by the secretion of
enzymes. Together with bacteria are the decomposers of the biosphere. Fungal filaments
called hyphae collectively make up the fungus body which is called the mycelium. There
are three types of fungi: Zygomycota, Ascomycota and Basidiomycota. The yeast are a
group of unicellular, mostly ascomycetes. They are commercially important of their roles in
baking and fermentation.
Self Assessment Questions
1. Why are viruses not considerdd as living organisms
2. What is a bacteriophage?
3. What type of microscope is generally required to visualize a virus particle?
4. How are different species of bacteria recognized?
5. How do Arhaebacteria differ from Eubacteria?
6. What is the composition of fungal cell wall?
7. Fungi are nomotile how are they dispersed to new areas?
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
114
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
LECTURE ELEVEN
ELEMENTS OF BIOENERGETICS AND BIOCHEMISTRY
1.0. Introduction
The aim of this is lecture to provide an elementary account of biochemistry. This lecture is
made up selected topics of some importance in biochemistry. Life can be viewed as a
constant flow of energy, channelled by organisms to do the work of living. Each of the
significant properties by which we define life: order, growth, reproduction, responsiveness,
and internal regulation require a constant supply of energy. Life stops if deprived of a
source of energy. Therefore a comprehensive study of life would impossible without
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discussing bioenergetics, the analysis of energy power activities of living systems. In this
lecture we will focus on what it is and how organisms capture, store, and use it.
OBJECTIVES: At the end of this lecture you should be able to
i.
State the laws of thermodynamics
ii.
Define free energy
iii.
Describe how enzyme works
iv.
Explain how ATP stores and power energy requiring reactions
v.
Discuss the organizational units of metabolism.
2.0. The Flow of Energy in Living Things
Energy is defined as the capacity to work. It exists in two states. Kinetic energy (the
energy of motion) and Potential energy (stored energy). There are many forms of energy:
mechanical energy, heat, sound electric current, light or radioactive radiation. There are
many ways to measure energy. The most convenient is in terms of heat because all other
forms of energy can be converted into heat. The study of energy is called
thermodynamics, meaning heat change. The unit of heat most commonly used is the
kilocalorie (kcal). One kilocalorie is equal 1000 calories (cal), and one calorie is the heat
required to raise the temperature of one gram by of water one degree Celsius (ºC).
Energy flow into the biological world from the sun, it is estimated that the sun provide the
earth with more than 13 x 1023
calories
per year, or 40 billion calories per second. Plants,
algae and some certain bacteria capture a fraction of this energy through photosynthesis. In
photosynthesis, energy garnered from sunlight is used to combine small molecules (water
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carbon dioxide) into more complex molecules (sugars). The energy is stored as covalent
bonds between atoms in the sugar molecules.Breaking such a bond require energy to pull
nuclei apart. It takes 98.8 kcal to break one mole (6.023 x 1023) of carbon-hydrogen bond.
2.1 Oxidation-Reduction Reactions
During a chemical reaction, the energy stored in chemical bonds may transfer to new
bonds. In some of these reactions, electrons actually pass from one atom or molecule to
another. When an atom or molecules loses an electron, it is said to be oxidized, and the
process which occurs is called oxidation. On the other hand, when an atom or molecules
gains an electron, it is said to be reduced, and the process is called reduction. Therefore,
chemical reactions of this sort are called oxidation-reduction (redox) reactions. Energy is
transferred from one molecule to the other via redox reaction.
3.0 The laws of thermodynamics
Activities such as running, thinking, singing, reading all involve energy changes. A set of
universal law we call the laws of thermodynamics govern all energy changes in the
universe, from nuclear reaction to the buzzing of a bee.
3.1 The First law of Thermodynamics
The first law of thermodynamics states that “energy cannot be created or destroyed, it only
undergo conversion change from one form to another”. This means that the total amount of
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energy in the universe remains constant, it can change from potential energy to kinetic
energy but it cannot be destroyed.
3.2 The second law of Thermodynamics
The second law states that “disorder (entropy) in the universe is increasing”. For example,
it is much more likely that a column of bricks will tumble over than that a pile of bricks will
arrange themselves spontaneously to form a column. In general, energy transformation
proceed spontaneously to convert matter from a more ordered less stable form, to a less
ordered more stable form.
4.0..Entropy
Entropy is a measure of disorder of a system, so the second law of thermodynamics can
stated as simply as “entropy increases.” When the universe formed 10 t0 20 billion years a
go, it held all the potential energy it will ever have. It has become progressively more
disordered ever since, with energy exchange increasing the amount of entropy in the
universe.
Free energy
Free energy is the energy available to do work. In a molecule within a cell, where energy
is donated by the symbol G (for “Gibb’s free energy”, which limits the system being
considered to the cell). G is equal to the energy contained in a molecule’s chemical bond
(called enthalpy and designated H) minus the energy available because of disorder (called
entropy and given the symbol S) times the absolute temperature T in degrees Kelvin (K =
ºC + 273):
G = H – TS
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When chemical reaction occurs under condition of constant temperature, pressure and
volume as do most biological reactions the change in free energy (∆G) is simply:
∆G = ∆H - T∆S
The change in free energy is a fundamental property of chemical reactions. In some
reactions the ∆G is positive. This means that the products of the reaction contain more free
energy than the reactants; the bond energy (H) is higher or the disorder (S) in the system is
lower. Such reactions do not proceed spontaneously. They require an input of energy and
thus endergonic (inward energy). In other reactions, the ∆G is negative. The product of the
reaction contains less free energy than the reactants, either. Either the reactions tend to
proceed spontaneously. Any chemical reaction will tend to proceed spontaneously if the
difference in disorder (∆S) is greater than the difference in bond energies between reactants
and products (∆H). These reactions release excess free energy as heat and are thus
exergonic (outward energy).
The rate of a reaction depends on the activation energy necessary to initiate it. Catalysts
reduce the activation energy and so increase the rates of reaction. Although they do not
change the final proportions of reactants.
5.0 ENZYMES
Enzymes are biological catalysts. The chemical reactions within living organisms are
regulated by controlling the points at which catalysis takes place. Life itself is, therefore
controlled by catalysts. The agents that carry out most of the catalysts in living organisms
are proteins called enzymes. Thousands of different kinds of enzymes are known each
catalysing one or a few specific chemical reaction. Different types of cells contain different
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enzymes of enzymes, and this difference contributes to structural and functional variation
among cell types.
5.1 How enzymes work
Most proteins are globular with one or more pockets or clefts on surface called active sites.
Substrate binds to the enzyme at these active sites forming an enzyme-substrate complex.
For catalysis to occur within the complex, a substrate molecule must fit precisely into a n
active site. When that happens, amino acid of enzymes ends up in close proximity to certain
to certain bonds of the substrate. This side group interacts chemically with the substrate,
usually stressing or distorting a particular bond and consequently lowering the activation
energy needed to break the bond. The substrate now a product dissociate from the bond.
5.2 Factors affecting Enzyme activity
Any chemical or physical factor that alters an enzyme’s three-dimensional shape, such as
temperature, ph, salt concentration, and the binding of specific regulatory molecules can
affect the enzyme ability to catalyze a reaction.
Temperature
Increases in temperature increase the rate of a reaction. The rate of enzyme-catalyzed
reactions also increases with temperature, but only up to a point known as temperature
optimum. Above the optimum temperature enzymes are denatured.
pH
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Ionic interactions between oppositely charged amino acids residues such as glutamic acid
(negative) and lysine (positive) also hold enzyme together. These enzymes are sensitive to
the hydrogen ion concentration of the fluid the enzyme is dissolved in, because changing
that concentration shifts the balance between positively and negatively charged amino acid
residues. For this reason, most enzymes have a pH optimum that usually ranges from pH 6
to 8.
Inhibitors and activators
Enzyme activity is sensitive to the presence of specific substances that are bind to the
enzyme and cause changes in its shape. Through these substances, a cell is able to regulate
which enzymes are active and which are inactive at a particular time. A substance that
binds to an enzyme and decreases its activity is called an inhibitor. Enzyme inhibition in
two ways: competitive inhibitor competes with the substrate for the same binding site,
displacing a percentage of substrate molecules from the enzyme. Non-competitive
inhibitors bind to the enzyme in location other than the active sites, changing the shape of
the enzyme and making it unable to bind the substrate. Most non-competitive inhibitors
bind to a specific portion of the enzyme called an allosteric site. A substance that binds to
an allosteric site and reduces enzyme activity is called an allosteric inhibitor. Alternatively,
activities bind to allosteric sites and keep the enzyme in their active configuration, thereby
increasing enzyme activity.
6.0.Enzyme Cofactors
Enzyme faction is always assisted by additional chemical components known as cofactors.
When the cofactor is a non-protein organic molecule, it is called a coenzyme. Many
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vitamins are parts of coenzymes. In numerous oxidation-reduction reactions that are
catalyzed by enzymes, the electrons pass in pairs from the active site of the enzyme to a
coenzyme that serves as the electron acceptor. The coenzyme then transfers the electron to
a different enzyme, which release them (and the energy they carry) to the substrate in
another reaction. Often the electron pair with proton (H+) as hydrogen atoms. In this way,
coenzymes shuttle energy in the form of hydrogen from one enzyme to another in a cell.
One of the most important coenzyme is hydrogen acceptor nicotinamide adenine
dinucleotide (NAD+). When NAD+ acquires an electron and a hydrogen atom from the
active site of an enzyme, it is reduced to NADH. The NADH molecule now carries two
energetic electrons and the proton. The oxidation of energy containing molecules, which
provides energy to cells, involves stripping electrons from those molecules donating them
to NAD+.
NOTE: Not all biological catalysts are proteins. The ability of RNA, an informational
molecule to act as a catalyst has stirred great excitement among biologist. It seems RNA
evolved first and catalyzed the formation of the first proteins.
7.0.What is ATP
The chief energy source of all cells is a molecule called Adenosine Triphosphate (ATP).
Most of the energy that plant harvest during photosynthesis is channelled into ATP
production. Likewise, most of the energy stored in fat and starch is used for ATP
production. Cells use the their supply of ATP to power almost every energy requiring
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process they carry out, from supplying activation energy for chemical reactions and actively
transporting substances across membranes to moving through their environment.
ACTIVITY: With aid of a diagram describe the structure of the ATP molecule
How ATP store energy
The key to how ATP stores energy lies in its triphosphate group. Phosphate groups are
highly negatively charged, so they repel one another strongly, therefore unstable. The
unstable bonds holding the phosphate together in the ATP molecules have low activation
energy and are easily broken. When they break they can transfer a considerable amount of
energy. The instability of its phosphate bonds makes ATP an excellent energy donor.
8.0 Biochemical Pathways
Biochemical pathways can be referred to as the organizational units of metabolism. The
total of all chemical reactions carried out by an organism, is called metabolism. Those
reactions expend energy to make or transform chemical bonds are called anabolic
reactions, or anabolism. Reactions that harvest energy when chemical bonds are broken
are called catabolic reactions, or catabolism.
Organisms contain thousands of different kinds of enzymes that catalyze a bewildering
variety of reactions. Many of these reactions in a cell occur in sequences called
biochemical pathways. In such pathways the product of one reaction becomes the
substrate for the next. Biochemical pathways are the organizational units of metabolism, the
elements an organism controls to achieve coherent metabolic activity.. Most sequential
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enzyme steps in biochemical pathways takes place in specific compartments of the cell. The
steps of citric acid cycle for example, occur inside the mitochondria.
SUMMARY
Energy is the capacity to bring about change, to provide motion against a force, or to do
work. Kinetic energy is actively engaged in doing work, while potential energy has the
capacity to do so. Many energy transformations of living things involve transformation of
potential energy into kinetic energy. An oxidation-reduction (redox) reaction is one in
which an electron is taken from one atom or molecules (oxidation) and donated to another
(reduction). The first law of thermodynamics states that amount energy in the universe is
constant; energy is neither lost nor created. The second law of thermodynamics states that
disorder in the universe (entropy) tends to increase. As a result energy spontaneously
converts to less –ordered forms. The rate of a reaction depends on the amount of activation
energy required to break existing bonds. Catalysis is the process of lowering activation
energy by stressing chemical bonds. Enzymes are biological catalysts. ATP is the energy
currency of life. Metabolism is the chemical life of a cell.
Self Assessment Questions
1. What is the difference between anabolism and catabolism?
2. Define oxidation and reduction reactions.
3. State the first and second law of thermodynamics.
4. What is free energy?
5. Distinguish between exergonic and endergonic reactions.
6. Define activation energy
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7. How does the rates of enzyme catalysed reactions affected by temperature?
8. What is ATP?
9. What is biochemical Pathway?
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
125
LECTURE TWELVE
RESPIRATION
1.0 Introduction
Animals get energy out of food molecules using biochemical process called cellular
respiration. While the term cellular respiration pertains to the use of oxygen and production
of carbon dioxide at the cellular level the general term respiration describes the up take of
oxygen from the environment and the disposal of carbon dioxide at the body system level.
Respiration at the body system level involves a host of processes not found at the cellular
level, like the mechanics of breathing and exchange of oxygen and carbon dioxide in the
arteries and veins. Respiration involves the diffusion of gases across plasma membranes.
In plants all active cells respire continuously, often absorbing O2 and releasing CO2 in equal
volumes. However, respiration is much more than a simple exchange of gases. The overall
process is an oxidation reduction-reduction in which compounds are oxidized to CO2 and
the O2 absorbed is reduced to form H2O, starch, fructans, sucrose or other sugars, fats,
organic acids and under some conditions, even proteins can serve as respiratory substrates.
The common respiration of glucose, for example can be written as follows:
C6H12O6 + 6O2 → 6CO2 + 6 H2O + Energy.
Much of the energy released during respiration approximately 686 Kcal per mole of glucose
is heat. When temperature are low this heat can stimulate metabolism and benefit certain
species, but usually it is instead transferred to the atmosphere or soil, with little
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consequence to the plant. Far important than heat is the energy trapped in ATP because this
compound is used for many essential processes of plant life, such as growth and ion
accumulation.
OBJECTIVES: at the end of this lecture you should be able to
i. Explain the mechanism of gas exchange in different kind of animals
ii. Discuss the mechanics of breathing in animals
iii. Describe cellular respiration
iv. Define anaerobic and aerobic respiration
2.0.The mechanisms of respiration in animals
In bony fishes water is forced past gills by the pumping action of buccal and opercular
cavities, or by active swimming in ram ventilation. In the gills, blood flows in opposite
direction to the flow of water. This counter-current flow maximizes gas exchange, making
the fish’s gill an efficient respiratory organ.
Air is piped directly to the body cells of insects, but the cells of terrestrial vertebrates obtain
oxygen by diffusion across the wet membranes of the lungs, which are filled with air in the
process of ventilation. Amphibians force air into lungs by positive pressure breathing,
whereas reptiles and all other terrestrial vertebrates take air into their lungs by expanding
their lungs when they increase rib cage volume through muscular contractions. This creates
sub-atmospheric pressure in the lungs.
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The avian respiration system is the most efficient among terrestrial vertebrates because it
has unidirectional air flow and cross current blood flow through the lungs. Mammal’s lungs
are composed of alveoli that provide a huge surface area for gas exchange. Air enters and
leaves these alveoli through the same system of airways. Humans inspire by contracting
muscles that insert on the rib cage and by contacting the diaphragm. Expiration is produced
primarily by muscle relaxation and release elastic recoil. As a result, the blood oxygen and
carbon dioxide levels are maintained in a normal range through adjustment in the depth and
rate of breathing.
Breathing serves to keep the blood gases and pH in the normal range and is under the reflex
control of peripheral and central chemoreceptors. These chemoreceptor’s sense pH of the
blood and cerebrospinal fluid, and they regulate the respiratory control centre in the
medulla oblongata of the brain.
Deoxyhaemoglobin combines with oxygen in the lungs to form oxyheamoglobin, which
dissociates in the tissue capillaries to release its oxygen. The degree to which the loading
reaction occurs depends on ventilation. The degree of unloading is influenced by such
factors as temperature and pH. Carbon dioxide combines with water in the tissue
capillaries to form carbonic acid, which is primarily formed within the red blood cells as a
result of an enzymatic reaction. The reverse reaction occurs in the lungs so that CO2 gas can
be exhaled.
3.0.Cellular respiration
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Cells are able to make ATP from the catabolism of organic molecules in two different
ways. In the first called substrate-level phosphorylation, ATP is formed by transferring a
phosphate group directly to ADP from phosphate bearing intermediates. During glycolysis
discussed below, the chemical bonds of glucose are shifted around in reactions that provide
the energy required to form ATP. In the second called aerobic respiration, ATP forms as
electrons are harvested, transferred along the electron transport chain, and eventually
donated to oxygen gas. Eukaryotes produce the majority of their ATP from glucose in this
way.
In most organisms, these two processes are combined. To harvest energy to make ATP
from sugar glucose molecules in the presence of oxygen, the cells carries out a complex
series of enzyme-catalyzed reactions that occur in four stages: the first stage captures
energy by substrate-level phosphorylation through glycolysis, the following three stages
carry out aerobic respiration by oxidizing the end products of oxidation.
3.1 Glycolysis
Stage 1: Glycolysis is the first stage of extracting energy from glucose. It is a ten-reaction
biochemical pathway that produces ATP by substrate-level phosphorylation. The enzyme
that catalyse the glycolysis reactions are in the cytoplasm of the cell, not bound to any
membrane or organelle. Two ATP molecules are used up early in the pathway, and four
ATP molecules are formed by substrate Glycolysis. This yields a net two ATP molecules
for each molecule of glucose catabolised. In addition four electrons are harvested as NADH
that can be used to form ATP by aerobic respiration. Still, the total yield of ATP is small.
When the glycolysis is completed, the two molecules of private that are formed still contain
most of the energy the original glucose molecule held.
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Glycolysis occurs in all organisms and the Glycolytic reaction that form ATP by substratelevel phosphorylation can occur with or without oxygen. In animals, however, the
harvesting of energetic electrons by glycolysis cannot take place indefinitely in the absence
of oxygen. As a result animal cells without oxygen soon die.
3.2 Aerobic Respiration
Stage 2: Pyruvate oxidation. In the second stage, pyruvate, the end product from
glycolysis, is converted into carbon-dioxide and a two-carbon molecule called acetyl CoA.
For each molecule of private converted, one molecule NADH is synthesized.
Stage 3: The Kerbs Cycle. The third stage introduces this acetyl-CoA into a cycle of nine
reactions called the Krebs cycle, named after the British biochemist, Sir Hans Krebs, who
discovered it. In the Krebs cycle, two more ATP molecules are extracted by substrate-level
phosphorylation, and a large number of electrons are removed by NADH.
Stage 4: Electron Transport Chain. In the fourth stage, the energetic electrons carried by
NADH are employed to drive the synthesis of a large amount of ATP by the electrons
transport chain. Pyruvate oxidation, the reactions of the Krebs cycle and ATP production by
electrons transport chains occur inside the mitochondria of all eukaryotes, as well as within
many forms of bacteria. Although plants and Algae can produce ATP by photosynthesis,
they can also produce ATP by aerobic respiration, just as animals and other nonphotosynthetic eukaryotes do.
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3.3 Anaerobic Respiration
In the absence of oxygen, some organisms respire anaerobically, using different inorganic
electrons acceptors than oxygen. For example many bacteria use sulfur, nitrate, or other
inorganic compounds as electron acceptor in place of oxygen. Therefore when oxygen is limiting
NADH and pyruvate begin to accumulate. Under this condition plants carry out fermentation
(anaerobic respiration) forming either ethanol or lactic acid. Alcoholic fermentation is called so
because one of its products is ethanol an alcohol:
C6H12O6 → 2C2H5 OH + 2CO2 + Energy
Yeast, a unicellular fungus, uses aerobic respiration when oxygen is present but switches to
anaerobic respiration if oxygen is absent. Alcoholic fermentation has an economic importance.
Bakers use alcoholic fermentation of yeast to make bread. Alcoholic fermentation is also used to
make wine and beer.
Lactic acid fermentation: Certain animal cells including human muscle cells, convert pyruvate
to lactic acid instead of alcohol. During strenuous exercise, breathing cannot provide the body
with all the oxygen it needs for aerobic respiration. When muscles run out of oxygen, cells
switch from aerobic respiration to lactic acid fermentation.
SUMMARY
Respiration involves the diffusion of gases. Gills are used for respiration by aquatic vertebrates.
Lungs are used for respiration by terrestrial vertebrates. Mammalian breathing is dynamic
process. Blood transport oxygen and carbon dioxide. Cells harvest energy in chemical bonds.
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Cellular respiration oxidizes food molecules. Catabolism of proteins and d fats can yield
considerable energy. Cells can metabolize without oxygen.
Self Assessment Questions
1. What to features of the respiratory system in birds make it the most efficient of all
terrestrial respiratory systems?
2. In what way is the amphibian respiratory system inefficient?
3. Where in a eukaryotic cell does glycolysis occur?
4. With examples, describe anaerobic respiration.
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
132
Salisbury, F.B. and Ross, C.W. (1992) Plant Physiology. Fourth Edition. Wadsworth
Publishing Company. California. 682pp.
LECTURE THIRTEEN
NUTRITION AND GROWTH
I.0 Introduction
The ingestion of food serves two primary functions: it provides a source of energy, and it
provides raw materials the animal is unable to for itself. In plants needs mineral nutrition in order
to live and grow. The present lecture highlights nutrients essential for both animal and plants
livelihood and growth.
OBJECTIVES: at the end of this lecture you should be able to
i.
List essential nutrients necessary for animals livelihood and growth
ii.
List essential nutrients necessary for plants livelihood and growth
iii.
Discuss about growth hormones in both animal and plants
2.0 Animal Essential Nutrients
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Substances that an animal cannot manufacture for itself but which are necessary for its health
must be obtained in the diet and are referred to as essential nutrients. Included among the
essential nutrients are vitamins, certain organic substances required in trace amounts. Some
essential nutrients are required in more than trace amounts. Many vertebrates, for example, are
unable to synthesize one or more of the 20 amino acids used in making proteins. These essential
amino acids must be obtained from proteins in the food they eat. Food also supplies essential
minerals such as calcium, phosphorous and other inorganic substances, including a wide variety
of trace elements, which are required in very small amounts. Among the trace elements are
iodine (a component of thyroid hormone), cobalt (a component of Vitamin B12), zinc and
molybdenum (components of enzymes), manganese, and selenium. Animals obtain trace
elements either directly from plants or from animals that have eaten plants.
3.0.Plants Essential Elements
There are two principal criteria by which an element can be judged essential or nonessential to
any plant. First: an element is essential if the plant cannot complete its life cycle (that is, form
viable seeds) in the absence of the element. Second, an element is essential if it forms part of any
molecule or constituent of the plant that is itself essential in the plant. There are 14 elements
believed to be essential for all angiosperms and gymnosperms, although in fact the nutrient
requirements of only 100 or so (mostly cultivated) species have been investigated.
Addition of oxygen, hydrogen and carbon (from O2 and H2O and CO2) brings the total to 17
elements. With these 17 elements and sunlight, most plants can synthesize all the compounds
they require. But plants also require some organic molecules such as vitamins synthesized by
microorganisms that normally grow on roots, stems or leaves. Plants really are autotrophic and
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make all the organic molecules they need, even though some associated microbes are beneficial
(for example, in mycorrhizae and root nodules).
1.0 Growth Hormones
Animal growth hormones
Growth hormone is a protein hormone of about 190 amino acids that is synthesized and secreted
by cells called somatotrophs in the anterior pituitary. It is a major participant in control of
several complex physiologic processes, including growth and metabolism. Growth is a very
complex process, and requires the coordinated action of several hormones. The major role of
growth hormone in stimulating body growth is to stimulate the liver and other tissues to secrete
Insulin like Growth factors I (IGF-I). IGF-I stimulates proliferation of chondrocytes (cartilage
cells), resulting in bone growth. Growth hormone does seem to have a direct effect on bone
growth in stimulating differentiation of chondrocytes. IGF-I also appears to be the key player in
muscle growth. It stimulates both the differentiation and proliferation of myoblasts. It also
stimulates amino acid uptake and protein synthesis in muscle and other tissues.
Plants growth hormones
Plant hormones (also known as phytohormones) are chemicals that regulate plant growth.
Plant hormones are signal molecules produced within the plant, and occur in extremely low
concentrations. Hormones regulate cellular processes in targeted cells locally and when moved to
other locations, in other locations of the plant. Plants, unlike animals, lack glands that produce
and secrete hormones. Plant hormones shape the plant, affecting seed growth, time of flowering,
the sex of flowers, senescence of leaves and fruits. They affect which tissues grow upward and
which grow downward, leaf formation and stem growth, fruit development and ripening, plant
longevity and even plant death. Hormones are vital to plant growth and lacking them, plants
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would be mostly a mass of undifferentiated cells. Examples of plants hormones include: Abscisic
acid, Auxins, Cytokinins, Ethylene and Gibberellins.
SUMMARY
Essential nutrients are substances that an animal cannot manufacture for itself but which are
necessary for its health and must be obtained from the diet. An element is essential to plant life if
the plant cannot complete its life cycle (that is, form viable seeds) in the absence of the element.
Second, an element is essential if it forms part of any molecule or constituent of the plant that is
itself essential in the plant. Growth hormone is a protein hormone of about 190 amino acids that
is synthesized and secreted by cells called somatotrophs in the anterior pituitary. It is a major
participant in control of several complex physiologic processes, including growth and
metabolism. Plant hormones (also known as phytohormones) are chemicals that regulate plant
growth. Plant hormones are signal molecules produced within the plant, and occur in extremely
low concentrations.
Self Assessment Questions
1. Define the term nutrition
2. List all elements which are essential to an animal life.
3. List all elements which are essential to a plant life
4. Discuss growth hormones in relation to animal life
5. Discuss growth hormones in relation to plant life
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
136
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
Salisbury, F.B. and Ross, C.W. (1992) Plant Physiology. Fourth Edition. Wadsworth
Publishing Company. California. 682pp.
LECTURE FOURTEEN
GENETICS
1.0 Introduction
Every living creature is a product of the long evolutionary history of life on earth. While
organisms share this history, only human beings wonder about the process that led to their
origins. We are still far from understanding everything about our origins, but we have learned a
great deal. Like a partially completed jigsaw puzzle, the boundaries have fallen into place, and
much of the internal structure is becoming apparent. In his lecture we will discuss on piece of the
puzzle, the mystery of heredity. For instance, why do group of people from different parts of the
world often differ in appearance? Why do members of a family tend to resemble one another?
OBJECTIVES: At the end of this lecture should be able to:
i. Define the concept of genetics
ii. Explain Mendelian principle of heredity
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iii. Differentiate between monohybrid and dihybrid
iv. Discuss few example of gene interactions
2.0 Mendel and the Garden Pea
The first quantitative studies of inheritance were carried out by Gregory Mendel, an Austrian
monk. In the garden of a monastery, Mendel initiated a series of experiments on plant
hybridization. The result of these experiments had a permanent change on how human being
viewed heredity.
2.1 Mendel Experimental Design
Mendel was careful to focus only on a few specific differences between the plants he was using
and to ignore the countless other differences he must have seen. For example, he appreciated that
trying to study the inheritance of round seeds versus tall height would be useless. The traits like
apples and oranges are not compatible. Mendel usually conducted his experiments in three
stages:
i. First, he allowed pear plants of given variety to produce progeny by self fertilization for
several generations. Pea plants with white flowers, for example when crossed with each
other, produced only offspring with white lowers, regardless of the number of
generations.
ii. Mendel then performed crosses between varieties exhibiting alternative forms of traits.
For example, he removed the male parts from the flower of a plant that produced white
flowers and fertilized it with pollen from a purple lowered plant. He also carried out
reciprocal cross using pollen from a white-flowered individual to fertilize a flower on pea
plant that produced purple flowers.
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iii. Finally, Mendel permitted the hybrid offspring produced by these crosses to self pollinate
for several generations. By doing so he allowed the alternative forms of a trait to
segregate among the progeny. Then he counted the number of offspring of each type in
each succeeding generation. No other scientist had ever done that before. The
quantitative results Mendel obtained proved to be of supreme importance in revealing the
process of heredity.
Mendel studied seven traits in his experiments: flower colour, seed colour, seed shape, pod
colour, pod shape, flower position and plant height. The seven traits possessed several variants
that differed from one another in ways that were easy to recognize and score. In this lecture we
will examine in detail Mendel’s crosses with flower colour, although other traits were similar,
and produced similar results.
2.2 The F1 Generation
The first filial generation (F1 Generation): In every case the flower colour of the offspring
resembled one of their parents. Thus, in a cross of white-flowered with purple-flowered plants,
the F1 offspring all had purple flowers. Mendel referred to the trait in the F1 plants as dominant
and to the alternative form that was not expressed in the F1 plants as recessive. For each of the
seven pairs of contrasting forms of traits that Mendel examined, one of the pair proved to be
dominant and the other recessive.
2.3 The F2 Generation
After allowing individuals F1 plants to mature and self pollinate, Mendel collected and planted
the seeds from each plant to see what the offspring in the second filial, or F2, generation would
look like. He found that some F2 plants exhibited white flowers, the recessive form of the trait.
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Latent in F1 generation, the recessive form reappeared among some F2 individuals. Believing the
proportion of the F2 types would provide some clue about the mechanism of heredity, Mendel
counted the numbers of each type among the F2 progeny, ¾ of the F2 individuals exhibited the
dominant form of the trait, and ¼ displayed the recessive form. In other words, the dominant:
recessive ratio among the F2 plants was always close to 3:1. Mendel carried out similar
experiments with other traits, such as wrinkled versus round seeds and obtained similar results.
3.0 Mendel’s Model of Heredity
To explain the results, Mendel proposed a simple model. It has become one of the most famous
models in the history of science, containing simple assumptions and making clear predictions.
The model has five elements:
i. Parents do not transmit physiological traits directly to their offspring. Rather they
transmit separate information about the traits, what Mendel called factors. These factors
later act in the offspring to produce the trait.
ii. Each individual receives two factors that may code for the same form or for two
alternative forms of the trait. Each adult individual is diploid, when the individual form
gametes (egg or sperm), they contain only one of each kind of chromosome, and the
gametes are haploid. Therefore only factor for each trait of the adult organism is
contained in the gamete.
iii. Not all copies of a factor are identical. In modern terms, the alternative forms of a factor,
leading to alternative forms of a trait are called alleles. When two haploid gametes
containing exactly the same allele of a factor fuse during fertilization to form a zygote,
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the offspring that develops from that zygote is said to be homozygous: when two haploid
gametes contain different alleles, the individual offspring is heterozygous. In modern
terminology, Mendel factors are called genes.
iv. The two alleles, one contributed by the male gamete and one by the female, do not
influence each other in way, they remain separate. Thus, when an individual matures and
produces its own gametes, the allele’s foe each gene segregate randomly into these genes
gametes as described in point ii.
v. The genes that an individual has are referred to as its genotype; the outward appearance
of the individual is referred to as its phenotype.
4.0 Mendel First law of Heredity: Segregation
Mendel’s models thus account in a net and satisfying way for the segregation ratios he observed.
Its central assumption that alternative alleles of a trait segregate from each other in heterozygous
individuals and remain distinct has since been verified in many other organisms. It is commonly
referred to as Mendel’s First Law of Heredity, or the low of segregation.
4.1 Mendel’s Second Law of heredity: Independent Assortment
When two genes are located on different chromosomes the alleles included in an individual
gametes are distributed at random. The allele for one gene included in the gamete has no
influence on which allele of the other gene is included in the gamete. Such genes are said to
assort independently.
5.0 Epistasis
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Epistasis is the interaction between non-allellic genes (i.e. between loci) on trait. Two or more
genes influence a trait. The production of a phenotype is often controlled by more than one gene.
Such a situation is seen in Zea mays a particular variety of corn. Some commercial variety
exhibit a purple pigment called anthrocyanin in their seed coats, while others do not. In 1918
geneticist R.A. Emerson crossed two pure-breeding corn varieties, nether exhibiting
anthrocyanin pigment. Surprisingly, all of the F1 plants produced purple seeds.
When two of these pigment-producing F1 generations were crossed to produce an F2 generation,
56% were pigment producers and 44% were not. What was happening? Emerson correctly
deduced that two genes were involved in producing pigment, and the second cross had thus been
a dihybrid cross like those performed by Mendel. Mendel has predicted 16 equally possible ways
gametes could combine with each other resulting in genotypes with a phenotypic ration of
9:3:3:1 (9+3+3+1=16). How many of these were in each of the two types Emerson obtained? He
multiplied the fractions that were pigment producer (0.56) by 16 to obtain 7. Thus, Emerson had
a modified ratio of 9:7 instead of the usual 9:3:3:1.
Why was Emerson’s ratio modified? When genes act sequentially, as in a biochemical pathway,
an allele expressed as defective early in the pathway blocks the flow of material through the rest
of the pathway this makes it impossible to judge whether the later steps of the pathway are
functioning properly. Such gene interaction, where one gene can interfere with the expression of
another gene, is the basis of the phenomenon called epistasis. Epistasis affect grain colour
because the purple pigment found in some varieties of corn is the product of a two step
biochemical pathway. Unless both enzymes are active (the plant has a dominant allele for each
of the two genes, A and B) no pigment is produced.
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6.0 Pleitropy
Sometimes, an individual gene allele will have more than one effect on the phenotype. Such an
allele is said to be pleiotropic. Pleiotrophic effects are difficulty to predict; because the genes
that affect a trait often perform other functions we may know nothing about. Pleiotrophic effects
are characteristic of many inherited disorders, such as sickle cell anaemia. In sickle cell anaemia,
a defect in the oxygen-carrying haemoglobin molecules causes anaemia, heart failure, increased
susceptibility to pneumonia, kidney failure, enlargement of the spleen, and may symptoms.
7.0 Environmental Effects
The degree to which an allele is expressed may depend on the environment. Some allele are heat
sensitive, for example. Traits influenced by such alleles are more sensitive to temperature or
light than are the products of other alleles. The arctic foxes, for example, make fur pigment only
when the weather is warm. Similarly, the ch allele in Himalayan rabbits and Siamese cats
encodes a heat sensitive version of tryosinase, one of the enzymes mediating the production of
melanin, a dark pigment. The ch version of the enzyme is inactivated at temperatures above
33ºC. At the surface of the main body and head, the temperature is above 33ºC and the tyrosinase
enzyme is inactive, while it is more active at body extremities such as the tips of the ears and tail,
where temperature is below 33ºC. The dark melanin pigment this enzyme produces causes the
ears, snout, feet, and tail of Himalayan rabbits and Siamese cats to be black.
8.0.Mutations
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Mutations are changes in the hereditary message. Mutations that changes one or a few
nucleotides are called point mutations. They may arise as a result of damage from ionizing or
ultraviolet radiation, chemical mutagens, or errors in pairing during DNA replication. Slipped
mis-pairing, in which non-homologous sequences on homologous chromosomes pair up during
meiosis can lead to deletions. When a deletion interrupts a codon, the result is a frame-shift
mutation, in which the downstream portion of the gene is transcribed out of register. Cancer is a
disease in which the regulatory control that normally restrains cell division is disrupted. A
variety of environmental factors, including ionizing radiation, chemical mutagens, and viruses
have been implicated in causing cancer.
SUMMARY
Mendel solved the mystery of heredity. Mendel quantified his data by counting the numbers of
each alternative type among the progeny of crosses. By counting progeny types, Mendel learned
that the alternatives that were masked in hybrid (the F1generation) appeared only 25% of the
time in the F2 generation. This finding, which led directly to Mendel’s model of heredity, is
usually referred to as the Mendelian ratio of 3:1 dominant to recessive traits. Mendel deduced
from 3:1 ratio that are traits are specified by discrete factors that do not blend. He deduced that
pea plants contain two factors for each trait that he studied (we now know this is because the
plants are diploid). When a plant is heterozygous for a trait, the two factors for that trait are not
the same, and one factor which Mendel described as dominant, determines the appearance, or
phenotype, of the individual. We now refer to Mendel factors as genes and to alternative form of
genes as alleles. When two genes are located on different chromosome, the alleles included in
individual gametes are distributed at random. The allele for gene included in the gamete. Such
genes are said to assort independently. Because phenotypes are often influenced by more than
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one gene, the ratios of alternative phenotypes observed in crosses sometimes deviate from the
simple ratios predicted by Mendel. This is particularly true in epistatic situations, where the
product of one gene masks another. Mutations are changes in the genetic message. Cancer results
from mutations of growth regulating genes.
Self Assessment Questions
1. Describe the three stages in which Mendel conducted his experiments.
2. Explain the Mendel’s model of heredity
3. State the first and second laws of heredity
4. Using specific examples, discuss on modified Mendelian ratios
5. Describe the effect of the environment on heredity
REFERENCES
Brooker, R. (2005) Student Guide to Biology. Mc-Hill, Boston.
Enger, E.D., Ross, F.C. and Bailey, D.B. (2005) Concepts in Biology. 11th Edition.
McGraw-Hill. Boston. 570pp.
Hickman, C.P., Roberts, L.S. and Larson, A. (2003) Animal Diversity. 3rd Edition. Mc-Hill,
Boston. 447pp.
Mader, S.S. (1996) Biology. 5Th Edition. Wm. Brown Publishers. Boston. 908pp.
Raven, H. and Johnson, G.B. (1999) Biology. Fifth Edition. McGraw-Hill, Boston. 1284pp.
145
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