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PART A: BASIC SCIENTIFIC CONCEPTS
3. BIOLOGICAL CONCEPTS
3.1 Basic Biology
Biology is the study of living things. The characteristics common to organisms (living things) are
the following:
1.
Movement (changing of position)
2.
Nutrition (Feeding; the ability to use material around for energy and growth)
3.
4.
5.
6.
7.
Sensitivity (Detect changes in environment – stimuli – and react accordingly)
Respiration (The production of energy from food)
Growth (Increase in size and addition of new cells)
Reproduction (The ability to produce its own kind)
Excretion (Removal of waste products)
3.2 The Cell
The cell is the building block of living matter. Cells vary in size – from microscopic (can only be
seen with a microscope) to much larger.
3.2.1 Different types of cells
The body is made up of a lot of cells. All are important but not all of them do the same jobs. This
concept is known as division of labour or distribution of work, which means that different
cells do different jobs.
Therefore, if a cell has to do a special job, it needs a special structure. This concept is called
differentiation of structure for specialisation of function. E.g. red blood cells, nerve cells, zygote,
etc. Cells are 3-dimensional and have different shapes (e.g. spherical, cylindrical). The
differences in shape are not due to whether the cell is a plant or animal cell but according to the
job the cell does.
Despite the difference in structure, each cell contains several basic structures (called
organelles) within.
Cells contain a nucleus. The nucleus contains DNA, which contains information which makes
an organism what it is and act as it does. DNA contains instructions for life. Some cells do not
have a proper nucleus since the DNA is floating in the cytoplasm and is not surrounded by the
nuclear membrane (membrane around the nucleus). These are called prokaryotic cells
(prokaryotes) and have several basic differences from other cells which have a nucleus and
are called eukaryotic cells (eukaryotes).
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Cells also contain cytoplasm. This is an area which contains particles in it (e.g. food reserves.)
Mitochondria are also found in the cytoplasm. They generate energy for the cell’s living
processes (i.e. respiration takes place in the mitochondria). The cell membrane is a layer
outside the cell which stops the contents of the cell from escaping and controls the substances
which are allowed to enter and leave the cell. Several other organelles are present such as
chloroplasts (perform photosynthesis) and others.
3.2.2 Prokaryotic and Eukaryotic Cells
The diagram below shows a typical bacterium.
Prokaryotic and eukaryotic cells have several differences apart from the presence/ absence of
the nucleus. These are listed in the table below.
Prokaryotic cells
Eukaryotic cell
Small
Usually larger
DNA in cytoplasm (no nucleus)
DNA in the nucleus
No mitochondria present
Mitochondria always present
No chloroplasts present
Chloroplasts present in plant cells
Cell wall present
Cell wall present (plants)/ absent (animals)
Flagella or pili present
pili absent, flagella sometimes present
Store glycogen and fats
Stores starch and oils.
Ribosomes small
ribosomes large
3.2.3 Animal and Plant Cells
Animal and plant cells are different from each other. Typical examples of the cells are given in
the diagrams below:
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Animal cell
Plant cell
A summary of differences is given in the table below. All other characteristics are similar or
identical.
Animal cell
Plant cell
Irregular shape
Normally regular shaped (rectangular)
No chloroplasts present
Chloroplasts present therefore carries out
photosynthesis.
Cell wall present
Cell wall absent
Store glycogen and fats
Stores starch and oils.
Therefore plant cells have a type of organelle called the chloroplast. This is a spherical organelle
which gives plants its colours, and is the site where light is harvested for photosynthesis. In fact
photosynthesis occurs in the chloroplast.
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The cell wall is a layer outside of the cell membrane which is very rigid and not easily
destroyed.In fact it is like a wall and thus keep the shape of the cell rigid unlike in animal cells. It
protects the cell, preventing it from bursting if it expands.
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3.3 DNA, Genes and Chromosomes
We know so far that most cells contain a nucleus. The purpose of the nucleus is to hold DNA,
which contains information which makes an organism what it is. DNA (which stands for
Deoxyribose Nucleic Acid) contains instructions for life. Our DNA is the cause of many of our
physical characteristics (e.g. eye colour, gender, hair colour, etc.)
DNA has the following structure (it looks like a twisted ladder):
DNA is made up of units called genes. A gene is a section of the DNA which codes for a
particular protein, which in turn codes for a particular characteristic (e.g. blood type). A number
of genes may work together for the production of a particular characteristic (e.g. height, eye
colour).
In order to save space, DNA is coiled up very tightly into loops
which make up units known as chromosomes. Chromosomes
therefore consist of very long stretches of DNA. Different
organisms have a different number of chromosomes –
humans, for example, have 23 pairs of chromosomes.
Genes are inherited from parents – for example, humans get 23 chromosomes from one parent
and a complementary set of another 23 chromosomes from the other parent. This is why people
resemble both their mother and their father in one way or another – because they have inherited
characteristics from both parents.
Some genes may be affected by the environment – for instance, a person may have genes for
tallness, but s/he is not fed well when young. As a result, the person does not grow as tall as
his/her genes would want him/her to.
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The same gene may exist in different forms – for example the gene for blood type may be in 3
forms: A, B or O. Each of the forms is known as an allele. Thus an allele is one of the different
forms that a gene may take.
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3.4 Cell Division
Cells divide in order to produce new cells. There are two ways in which cells divide, as
explained below. One method, called mitosis, occurs when normal body cells produce more of
the same. On the other hand meiosis occurs when the body wants to produce gametes, the sex
cells used to reproduce.
3.4.1 Mitosis
The process known as mitosis occurs as follows:
The first step to occur copying of DNA (DNA replication) so that two copies of each
chromosome are present. Then organelles divide to produce twice the number of organelles.
The nucleus also divides thus giving two nuclei, each with an identical set of the replicated
chromosomes.
One of each copy of the organelles then separates to different sides of the cell and the cell
divides (cell division). Eventually, two identical cells known as daughter cells are produced
from the mother.
3.4.2 Meiosis
The process of meiosis occurs in order to produce gametes (e.g. sperm and eggs in humans).
When two gametes unite, they produce a complete organism. Therefore, each gamete must
have half the usual number of chromosomes. For example, humans usually have 46
chromosomes – however, the gametes of humans have 23 chromosomes each. When two
gametes meet and fuse, the chromosomes add up together (23 + 23 = 46). Therefore, the new
baby will have 46 chromosomes just like every human being. 23 of its chromosomes come from
its mother, while 23 of its chromosomes come from its father.
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Meiosis is very important since it produces variations in the genes, and hence no individual is
the same as the next. Random fertilisation also ensures variation. This is important if a species
is to survive and for evolution.
The process of meiosis occurs as follows (only 1 pair of chromosomes is shown):
In meiosis, first the chromosomes replicate – thus each chromosome is seen to be formed of
two chromatids. (This is the same as DNA replication which occurs in mitosis). The two
chromosomes separate to different sides of the cell and the cell divides (meiotic division 1). The
resulting cell therefore contains only one of each pair of chromosomes – some of these would
be maternal (had been given to the organism by the mother) or paternal (had been given to the
organism by the father.) This first division is essentially the same as that of mitosis.
The next step is for the two chromatids of each chromosome to separate and the cell divides
again. The resulting cell contains just one of each pair of chromosomes – therefore, in the case
of humans, this cell would contain 23 chromosomes (not the usual 46.) These cells, which
contain half the usual number of chromosomes (and are therefore haploid), are called
gametes. When two gametes fuse together (e.g. an egg and a sperm cell) during fertilisation,
the two ‘half-sets’ of chromosomes unite to form one full set, and a new individual (which is
diploid) is formed. The first cell formed after fertilisation is called the zygote.
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When the zygote is formed, it divides by mitosis, and different cells take up specialised
structures according to their function (e.g. bone cells, brain cells, etc.) However, each cell
continues to have the 46 chromosomes it had originally.
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3.5 Inherited and Non-Inherited Variation
It is not just our genes which make us who we are. Variation arises also from environmental
factors.
For example, our weight is not just dependent on our genes but also on how much we eat and
how much exercise we do. Our skin colour is partially genetically-determined, but it also
depends on how much we expose ourselves to the sun. Scars, for example, are not the result of
our genes but the result of accidents which have happened during our life. Our hair length or nail
length are also dependent on the environment (in this case on the people who cut them) and not
on our genes. The above are all examples of non-inherited variation.
On the other hand, variation which is only caused by our genes (e.g. eye colour, blood type,
gender) is called inherited variation. Thus, it is both our genes and the environment which
make us who we are and which give rise to variation (differences) among us. Variation
(especially inherited variation), is very important for evolution
3.5.1 Continuous and Discontinuous Variation
Variation, whether inherited or non-inherited, can be of two types: continuous or
discontinuous.
An example of continuous variation is height.
If a graph is plotted as follows, the resulting
line will be a smooth curve with the shape as
follows. This is a characteristic of continuous
variation:
Other examples of continuous variation are:
weight, intelligence and fruit size. These may
all be influenced by the environment.
In discontinuous variation, on the other hand,
the graph that would be plotted would be a histogram (bar chart) as follows:
The graph above shows an example of a discontinuous variation – blood type. Around 46% of
people have a blood type O, 42% have blood type A, 9% have blood type B and 3% have blood
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type AB. The graph which is plotted cannot be a smooth curve (as it is for continuous variation),
because there are no intermediates. In addition, the blood group cannot be changed by the
environment.
3.5.2 Evolution: A Result of Variation
As we have seen, organisms are genetically different thus producing inherited variation.
However, the environment also causes variation. In both cases, these forms of variation may
cause a survival advantage (the particular individual survives better and competes in a more
efficient manner) and ultimately lead to evolution (a change in anatomy, physiology or behaviour
of an organisms over a period of time).
Taking an example, an organism such as a gazelle may have a variation in neck height. Thus
there are individuals with a long neck and others with a short neck. However, it might turn out
that the trees which these gazelles feed on become taller, or else the lower branches are eaten
by smaller animals. Therefore, the gazelles with the longer neck can reach the best food and
survive, while those with shorter necks die. In biological terms, those with short necks are
selected against and the others are selected for. If this process repeats itself over several
thousands of years, the environmental pressures are favoring gazelles with longer necks and
therefore, all individuals with short necks are eliminated. Thus, a new species with long necks
are formed.
Taking another example, chimps are capable of using simple tools, such as branches to gather
insects such as ants. Supposedly a chimp learns how to use other tools such as rocks to break
open nuts, whilst the others don’t know how to use them. The former chimp has obtained
another food source. And therefore has a survival advantage. Continued development of other
tools may eventually lead to a new species.
Therefore, a particular characteristic may favour survival of a particular organism and lead to
evolution.
A particular good example of evolution that occurred in recent times has been shown by the
peppered moth. For years the story of the peppered moth, Biston betularia, has provided one of
the best-known examples of natural selection in action. The light-colored form of the moth,
known as typica, was the predominant form in England prior to the beginning of the industrial
revolution. Shown at left, the typica moth's speckled wings are easy to spot against a dark
background, but would be difficult to pick out against the light-coloured bark of many trees
common in England.
Around the middle of the 19th century, however, a new form of the moth began to appear. The
first report of a dark-colored peppered moth was made in 1848. By 1895, the frequency in
Manchester had reached a reported level of 98% of the moths. This dark-colored form is known
as carbonaria, and (as shown at right), it is easiest to see against a light background. As you
can well imagine, carbonaria would be almost invisible against a dark background, just as typica
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would be difficult to see against a light background. The increase in carbonaria moths was so
dramatic that many naturalists made the immediate suggestion that it had to be the result of the
effects of industrial activity on the local landscape. In fact, industrial activity releases large
amounts of soot into the atmosphere, which settles on trees. In this way the typical form would
be more visible and is selected against, whereas the black form is camouflaged and thus
protected from predators.
Typical form
Black form
3.5.3 Sex Determination in Humans
Sex determination is whether a person will be male or female.
The last pair of chromosomes (the 23rd pair) are known as the ‘sex’ chromosomes because they
determine the gender of the person. Women have the genotype XX which makes them women,
while men have the genotype XY which makes them men. Thus, it is the second allele which
determines whether a person will be a male or a female, since the first X allele is present in
everyone.
Consider therefore a woman and a man – the woman produces gametes of one type only: type
X, while the man produces gametes of two types: type X and type Y. Therefore, the following
can happen upon fertilisation:
X
X
Y
XX
XY
This means that there is a:
- 50% chance that a child will be female (and have the XX genotype); and
- 50% chance that a child will be male (and have the XY genotype.).
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3.6 Classification
All organisms are grouped into groups according to specific characteristics. This is known as
classification or systematics.
When an organism is classified, it is placed into progressively smaller groups (as follows):
In order for biologists worldwide to be able to communicate, a common means of naming
organisms was needed. Thus, the binomial nomenclature was developed. This consists of giving
a Latin name to the organism, which is made up of the genus name followed by the species
name. So, in the case of humans, the scientific name is:
Homo sapiens
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Note the following:
The scientific name is written in italics (if using a computer), or with the words underlined
-
separately (if writing manually), i.e. Homo sapiens;
The first letter of the genus name is capitalised; and
The first letter of the species name is in small letters.
A few examples:
Xiphias gladius: Swordfish (pixxispad)
Octopus vulgaris: Octopus (qarnit)
Musca domestica: House fly (dubbiena)
Pinus halepensis: Aleppo pine
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3.6.1 The 5 Kingdoms
Living things are divided into 5 kingdoms according to their characteristics and degree of
organisation:
MONERA: single cell which is not fully developed (prokaryotic: they don’t even have a
proper nucleus). E.g. bacteria, blue green algae.
PROTISTA: single cell; fully developed. E.g. Protozoa (Amoeba)
FUNGI: Many types of cells. E.g. Moulds, mushrooms (Agaricus campestris), yeast.
PLANTS: Many differentiated (specialised) cells; Produce their own food.
ANIMALS: Many differentiated cells; Feed on ready-made food.
Viruses are considered as being borderline between living and non-living since they don’t have
the required tools to reproduce themselves (and they attack cells and use these cells to
reproduce).
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3.6.2 Kingdom Monera: The Bacteria Kingdom
The Bacteria Kingdom (Monera) consists of bacteria. Bacteria are microscopic single-celled
organisms that are only visible with a light microscope. Bacteria are known as prokaryotes (all
other organisms are called eukaryotes), as they have a very simple structure which is unique
since, unlike other cells:
it has no nucleus; and
it has no mitochondria.
Although bacteria do not have mitochondria, they still carry out respiration and this is done by
specialised parts of the cell membrane (also known as the plasma membrane).
In addition, bacterial cells are different from plant cells since:
they have no cellulose cell wall (the cell wall is made of nitrogen-containing compounds);
and
they have no chloroplasts.
The following is a generalised diagram of a bacterium:
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Some bacteria have flagellae (singular: flagellum), which allow them to move.
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Not all bacteria are harmful (unlike viruses). For example, some bacteria play a role in:
Decomposing dead organisms and sewage;
The circulation of nitrogen;
Industry: to make dairy products (yoghurt, cheese, butter) and vinegar; and
The production of antibiotics (e.g. Streptomyces)
3.6.3 The Protist Kingdom
Protists (also known as protoctists) are single-celled organisms with a fully-developed cell.
Therefore they have a proper nucleus (unlike bacteria) and are considered to be eukaryotes.
Protists are considered to be the ancestors of plants and animals, i.e. some protists are
unicellular ‘primitive forms’ of plants, while other protists are unicellular ‘primitive forms’ of
animals.
-
The Kingdom Protista is divided into two main groups:
The Protozoa;
The Protophyta.
Protozoans (-zoa means related to animals) are animal-like protists. They feed by taking in and
digesting food, just like animals do. E.g. Amoeba, Paramecium.
Protophytes (-phyta means related to plants) are plant-like protists. They possess chloroplasts
and make their food by photosynthesis. E.g. Euglena, Chlamydomonas.
Common examples of protists include some types of plankton, which are very small organisms
eaten by many sea creatures (e.g. baleen whales).
Amoeba
Amoeba is a protozoan (an animal-like protist which feeds on ready-made food). The Amoeba
genus consists of many different species, which are all slightly different. The largest species are
only just visible to the naked eye (around 1 mm in diameter.)
A common example
is Amoeba proteus. It
is found on the
surface
mud
of
ponds.
This
organism
is
irregularly-shaped
and does not have a
fixed shape (i.e. the
shape can change
from
moment
to
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moment). Amoeba has a number of pseudopodia (‘false feet’), which it enlarges in order to crawl
forward.
Pseudopodia are also used to eat prey with (this is known as ingestion). Amoeba realises that
there is food by means of vibrations carried out by its prey. Digestion occurs in the food
vacuoles. The removal of waste products is known as egestion and this process is the reverse
of ingestion.
The following diagrams show Amoeba feeding (ingestion and egestion):
The contractile vacuole is used to remove extra water and any waste products dissolved in it.
Reproduction occurs by simple binary fission (division in two).
Respiration is aerobic (using Oxygen), and Oxygen and carbon dioxide diffuse through the
surface.
Euglena
Euglena is a protophyte (a plant-like protist which makes its own food by photosynthesis).
Euglena viridis is found in polluted freshwater. Where there are many Euglenas present, the
water becomes green.
Euglena has a flagellum which it uses for swimming.
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The eyespot (also known as the stigma) is a light-sensitive spot, which is used by the Euglena to
detect light. Euglena then swims towards light in order to carry out photosynthesis.
3.6.4 The Fungus Kingdom
We have so far said that Fungi are multicellular organisms with non-differentiated cells.
However, there are some organisms that are unicellular but have fungal characteristics and are
therefore placed in this kingdom (e.g. yeast).
Examples of Fungi include:
Mushrooms
Toadstools
Moulds
Yeasts
(N.B. The difference between mushrooms and toadstools is that mushrooms are edible while
toadstools are not.)
Fungi are heterotrophic, since they feed on ready-made food. They may either feed on dead
organic matter (and be saprotrophic) or else on living organisms, causing them harm but not
death (and be parasitic).
Saprotrophic fungi, together with bacteria, are the decomposers in most food webs and help to
recycle essential nutrients in ecosystems. They can decompose, for example, apples and bread.
Examples of parasitic fungi are those that cause mildew (a disease of crops), athlete’s foot (a
disease of the skin of the human foot), and trush (another human disease).
Fungi are made of microscopic threads called hyphae (singular: hypha), which
form a network called a mycelium. In most fungi, the hyphae do not have
dividing walls (septa) within them, therefore they are called aseptate or nonseptate. As a result, each hypha has a large number of nuclei within it.
Fungal hyphae grow horizontally within the material in which the fungus is growing (like the
roots of plants), however, unlike plants, the material in which they grow (the substratum) need
not be soil. They can grow on dead wood, dung, skin, wet bread, etc., and therefore take
nutrients from the substratum they are growing on.
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Feeding occurs when the fungus releases enzymes into the substratum and digests it (external
digestion). Then, the products of digestion are absorbed by the hyphae.
In order to reproduce, fungi form a special vertical hypha called a sporangiophore (or fruiting
body) and spores are formed asexually from this. A number of spores are enclosed together in a
sporangium, which is a sac at the tip of the sporangiophore. The spores are then released and
spread by air or other methods. When they land on a suitable substratum, they germinate to
produce a mycelium.
Therefore, the fungal mycelium is mostly hidden from human view and remains undetected until
the fruiting bodies are formed. An example of a fruiting body is the edible mushroom Agaricus
campestris.
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Common uses of fungi include:
As food (e.g. mushrooms);
Production of alcohol (yeasts);
To make dough rise (yeasts); and
Production of antibiotics (e.g. Penicillium produces penicillin).
Yeasts
Yeasts are unicellular fungi. Only a few species can form true hyphae – most consist of separate
spherical cells, as follows:
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They respire anaerobically (without oxygen) in a process called fermentation. Fermentation is
important for the production of alcohol. The reaction occurring is the following:
C6H12O6  2CO2 + 2C2H5OH + 118 kJ
sugar
carbon
dioxide
alcohol
energy
Yeast (‘hmira’) is also used to make bread rise.
Reproduction occurs by budding.
Bread mould
When bread is left to become stale, it goes mouldy. This is due to the growth of a filamentous
fungus, Rhizopus or Mucor, whose structure is similar to that of a typical fungus (as outlined
above).
3.6.5 The Plant Kingdom
Plants are multicellular organisms. They have flowers, leaves, stems and roots. They carry out
photosynthesis to produce energy from sunlight.
The following are the 4 main plant phyla:
1. Bryophyta
The Bryophytes have a very simple structure (known as a thallus) having no proper roots,
leaves or stems. On the lower surface, there are hair-like structures (called rhizoids) which are
used to anchor them to the ground and to absorb moisture, and no roots. They do not have
vascular tissue specialised for the flow of water and food, since they are only one or a few cells
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thick, therefore do not need vascular tissue. However, they are usually small in size due to the
absence of vascular tissue.
They grow in moist places and in order to reproduce, they need a watery medium (the male
gametes have flagella and swim in water to reach the female gamete). Therefore, their spread is
limited due to this dependency.
e.g. Liverworts
&
mosses.
2. Ferns
Have roots stems and leaves, similar to those of flowering plants. Since they have vascular
tissue, they can become quite large (in fact they are usually much larger than mosses).
On their surface, they have a waxy layer which allows them to live in drier areas, however,
reproduction still requires a damp environment.
3. Conifers
The conifers (or gymnosperms) are able to conserve water. They reproduce by seeds that are
formed in cones and not by means of spores (as in bryophytes and ferns). Seeds are larger than
spores and have a protective seed coat around them. In the case of conifers, the seeds are
formed in cones (not in flowers) and are not enclosed in an ovary.
Male and female cones are found on the same plant. The male cones produce pollen which falls
or is blown onto the female cone, which is fertilised and produces seeds.
e.g. pine, cypress.
4. Flowering plants
The flowering plants are also known as the angiosperms. Their seeds are formed in flowers,
and enclosed in an ovary. There are two main groups in this phylum: The monocots
(monocotyledons) and the dicots (dicotyledons). (N.B. A cotyledon is a modified leaf in a seed,
which plays a part in supplying food in the growing plant embryo.)
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Left: A dicotyledonous leaf
Right: A monocotyledonous leaf
3.6.6 The Animal Kingdom
The animal kingdom is divided into a lot of phyla. The following are the most important 7:
1. Coelenterata
The Coelenterates (or Cnidarians) have a sac-like body with tentacles and stinging cells. They
live in a watery (aquatic) environment.
e.g.
Sea anemones
Jellyfish
Corals are skeletons of coelenterates.
2. Platyhelminthes
Flatworms. These are flat and have a very thin body to facilitate the diffusion of oxygen. They do
not have a circulatory system. Many are animal parasites.
e.g.
Taenia (Tapeworm)
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3. Nematoda
The Nematodes (roundworms) have a long thread-like body which is round in cross-section.
Some live in soil, but many are plant or animal parasites.e.g. Ascaris
4. Annelida
The Annelids are the segmented worms (true worms). They have a long segmented body and a
digestive tract with a mouth and anus.
e.g. Earthworm
The leech (Hirudo medicinalis) was used in the past for medicinal purposes to combat
headaches. It is a blood sucker.
5. Mollusca
The molluscs have a soft unsegmented body. Most have an external or internal shell. Live in
aquatic or moist environments.
-
Three main classes:
Class Cephalopoda: Octopus, squids;
Class Gastropoda: Slugs, snails; and
Class Bivalvia: Mussels, clams
6. Arthropoda
The word ‘Arthropoda’ means ‘jointed limbs’. These are segmented animals, which are
segmented into 3 parts – the head, thorax and abdomen. They have a hard cuticle or
exoskeleton (and no interior skeleton).
Growth occurs as follows:
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The phylum Arthropoda is divided into a number of classes, which include the following:
- Class Insecta: These have 3 pairs of legs, antennae, 2 pairs of wings, and a separate head,
thorax and abdomen. Their waterproof exoskeleton made them very successful in terrestrial
environments. Development involves a complete or an incomplete metamorphosis. e.g. butterfly,
ant, fruit fly
- Class Arachnida: 8 legs. e.g. Spider;
- Class Crustacea: Quite a lot of legs, 2 pairs of antennae. e.g. shrimp, woodlouse, crab; and
- Class Myriapoda: Centipedes and millipedes
7. Vertebrata
The vertebrates have a vertebral column extending to form a tail. They have an internal skeleton
usually made up of bone. They are considered to be the most developed form of life.
The phylum Vertebrata is divided into 5 classes: Fish, Amphibians, Reptiles, Birds & Mammals.
Fish, Amphibians and reptiles are ectothermic (their body temperature rises and falls with that
of the environment), while birds and mammals are endothermic (their bodies produce heat,
therefore their internal body temperature remains constant).
- Fish: Vertebrates adapted for an aquatic environment. Have fins, gills and scales covering the
body. Ectothermic. e.g. Dorado (lampuka), Swordfish (pixxispad), Blue fin tuna (tonna).
- Amphibians: Have thin moist skin without scales. They live on land but lay eggs in water.
Ectothermic.
e.g. Frog, Toad, Newt.
- Reptiles: Very successful terrestrial vertebrates with dry scaly skins. They lay eggs on land in
leathery shells. Ectothermic. e.g. Snakes, lizards.
- Birds: Have a body covered with feathers. The forelimbs are modified into wings. They have
toothless beaks and lay eggs in hard protective shells. Endothermic.
e.g. Swallow, sparrow, robin, mallard.
- Mammals: Have a body covered with hair. Have mammary glands that produce milk to feed
young with. Have external ears. Endothermic.
e.g.
human, dog, cat, rabbit, deer, dolphin
platypus (egg-laying)
kangaroo, koala (marsupials)
3.6.7 Viruses
Viruses are considered to be borderline between living and non-living. They are extremely small,
between 30 and 300 nm (1/100 the size of a bacterium), and can be seen only with an electron
microscope (magnified by 30,000).
J. Henwood
Basic Scientific Concepts- Biology 1
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Viruses have a very simple structure: They have a central core of RNA or DNA (the genetic
material) surrounded by a protein coat. They do not have the usual cellular structure of
organisms. They have no metabolism of their own.
All are parasites (pathogens), and they kill the host cells as they reproduce within them, using
the cell’s energy and materials. This causes disease, e.g. rabies.
Viruses attack specific organisms. Transmission may occur as follows:
- By contaminated water or milk (e.g. polio);
- By droplet transmission (e.g. sneezing);
- By vector (carrier of disease, e.g. mosquito transmits yellow fever); and
- Body fluids (e.g. HIV).
Since viruses are non-living, they cannot be killed – therefore there is no cure for viral infections
or diseases. However, the body may be immunised against viral attack by preparing it for the
attack – this can be done by administering vaccines, e.g. vaccines for polio, rubella, tetanus.
J. Henwood
Basic Scientific Concepts- Biology 1