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Biology

D G Mackean
Dave Hayward
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Cambridge O Level

Biology

D G Mackean
Dave Hayward
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Contents
How to use this book
Scientific Enquiry

1 Characteristics and classification of living organisms

2 Cells
3 Movement in and out of cells
4 Biological molecules
5 Enzymes
6 Plant nutrition
7 Human nutrition
8 Transport in plants
9 Transport in animals
10 Diseases and immunity
11 Gas exchange in humans
12 Respiration
13 Excretion in humans
14 Coordination and response
15 Drugs
16 Reproduction
17 Inheritance
18 Variation and selection
19 Organisms and their environment
20 Human influences on ecosystems
21 Biotechnology and genetic modification
Theory exam-style questions
Practical exam-style questions
Alternative to practical exam-style questions
Glossary
Index
 2 Cells
In the previous chapter you recognised the bacterial cells, as well as the functions of their
characteristics present in all living organisms parts. Within an organism there are levels of
and used a mnemonic to help you remember organisation. By the end of the chapter you will
them. You were introduced to reasons for be able to name these levels of organisation and
classifying organisms into groups and the use of describe examples from animals and plants. Why
the binomial system of naming species. You had are cells different shapes? What jobs do they
the opportunity to develop your own dichotomous do? How can we work out their magnification
keys based on identifiable features. Then you when looking at them? By studying the chapter
learned about some of the main animal and carefully and following the practical suggestions
plant groups. In this chapter you will discover you should be able to answer these questions.
the main differences between animal, plant and

Cell structure and function

Cell structure
FOCUS POINTS
★ What are the structures and functions of plant, animal and bacterial cells?
★ How do you identify cell structures in diagrams and images of animal, plant and bacterial cells?
★ What are the differences between a plant and an animal cell?
★ How are new cells produced?
★ What are the specific functions of these specialised cells?
– ciliated cells
– root hair cells
– palisade mesophyll cells
– neurones
– red blood cells
– sperm and egg cells (gametes)
★ What are the meanings of the terms cell, tissue, organ, organ system and organism?

If a very thin slice of a plant stem is cut and a transverse section (Figure 2.2(a)). If you cut
studied under a microscope, the stem appears to along the length of the structure, you are taking
consist of thousands of tiny, box-like structures. a longitudinal section (Figure 2.2(b)). Figure 2.1
These structures are called cells. Figure 2.1 is a shows a longitudinal section, which passes through
thin slice taken from the tip of a plant shoot and two small developing leaves near the tip of the
photographed through a microscope. It is 60 times shoot, and two larger leaves below them. The
larger than life, so a cell which appears to be 2 mm leaves, buds and stem are all made up of cells. If
long in the picture is only 0.03 mm long in life. you cut across the structure, you make a transverse
Thin slices like this are called sections. If section (Figure 2.2(a)).
you cut across the structure, you are making

4
 Cell structure and function

▲ Figure 2.3 Transverse section through a kidney tubule

(×700). A section through a tube will look like a ring (see
Figure 2.17(b)). In this case, each ‘ring’ consists of about
12 cells
▲ Figure 2.1 Longitudinal section through the tip of a plant
shoot (×60). The slice is only one cell thick, so light can Making sections is not the only way to study cells.
pass through it and the cells can be seen clearly Thin strips of plant tissue, only one cell thick,
can be pulled off stems or leaves (Experiment 1,
page 12). Plant or animal tissue can be squashed
or smeared on a microscope slide (Experiment 2,
page 13) or treated with chemicals to separate the
cells before they are studied.
There is no such thing as a typical plant or
animal cell because cells vary a lot in size and shape
depending on their function. However, it is possible
to make a drawing like Figure 2.4 to show features
which are present in most cells. All cells have a cell
membrane, which is a thin boundary enclosing the
(a) transverse section (b) longitudinal section
cytoplasm. Most cells have a nucleus.
▲ Figure 2.2 Cutting sections of a plant stem nucleus
cell membrane
You can cut sections through plant structures quite
easily using a razor blade. Cutting sections of animal
structures is more difficult because they are mostly cytoplasm
soft and flexible. Pieces of skin, muscle or liver, for
example, must first be soaked in melted wax. When the
wax goes solid it is possible to cut thin sections. The
wax is dissolved away after the section has been cut.
When sections of animal structures are examined
under the microscope, they, too, are seen to be made up
of cells but these cells are much smaller than plant cells
and need to be magnified more. The photomicrograph
of kidney tissue in Figure 2.3 has been magnified 700 mitochondria granules

times to show the cells clearly. The sections are often ▲ Figure 2.4 A group of liver cells. These cells have all the
treated with dyes, called stains, to make the structures characteristics of animal cells
inside the cells show up more clearly.

5
2 Cells

Cytoplasm the cell is dividing (see Chapter 17 for a fuller account

Under an ordinary microscope (light microscope), of chromosomes and cell division).
cytoplasm looks like a thick liquid with particles
in it. In plant cells it may be seen to flow about.
The particles may be food reserves such as oil
droplets or granules (small particles) of starch. Other
particles are structures known as organelles, which
have special functions in the cytoplasm. In the
cytoplasm, large numbers of chemical reactions are (a) Animal cell about to (b) The nucleus divides first.
divide.
taking place which keep the cell alive by providing
energy and making substances that the cell needs.
The liquid part of cytoplasm is about 90%
water with molecules of salts and sugars dissolved
in it. Suspended in this solution there are larger
molecules of fats (lipids) and proteins (see
Chapter 4). Fats and proteins may be used to build (c) The daughter nuclei separate (d) Two cells are formed – one
up the cell structures, such as the membranes. and the cytoplasm pinches may keep the ability to
off between the nuclei. divide, and the other may
Some of the proteins are enzymes (see Chapter become specialised.
5). Enzymes control the rate and type of chemical
▲ Figure 2.5 Cell division in an animal cell
reactions which take place in the cells. Some
enzymes are attached to the membrane systems of Plant cells
the cell, while others float freely in the liquid part
of the cytoplasm. A few generalised animal cells are shown in Figure 2.4,
while Figure 2.6 is a drawing of two palisade cells
Cell membrane from a plant leaf. (See ‘Leaf structure’ in Chapter 6.)
This is a thin layer of cytoplasm around the outside cell wall
of the cell. It stops the cell contents from escaping
and controls the substances which can enter and
leave the cell. In general, oxygen, food and water chloroplast
are allowed to enter; waste products are allowed to
leave and harmful substances are kept out. In this cytoplasm
nuclear
way the cell membrane maintains the structure and membrane
chemical reactions of the cytoplasm. vacuole nucleus

Nucleus (plural: nuclei)

Most cells contain one nucleus, which is usually seen
as a rounded structure covered by a membrane and
fixed in the cytoplasm. In drawings of cells, the
nucleus may be shown darker than the cytoplasm ▲ Figure 2.6 Palisade cells from a leaf
because, in prepared sections, it takes up certain Plant cells differ from animal cells in several ways
stains more strongly than the cytoplasm. The because they can have extra structures: cell wall,
function of the nucleus is to control the type and chloroplasts and sap vacuoles.
quantity of enzymes produced by the cytoplasm. In
this way it regulates the chemical changes which Cell wall
take place in the cell. As a result, the nucleus The cell wall, which is outside the membrane, contains
controls what the cell will be, for example, a blood cellulose and other compounds. It is non-living and
cell, a liver cell, a muscle cell or a nerve cell. allows water and dissolved substances to pass through.
The nucleus controls cell division, as shown in The cell wall is not selective like the cell membrane.
Figure 2.5. A cell without a nucleus cannot reproduce. (Note that plant cells do have a cell membrane, but it
Inside the nucleus are thread-like structures called is not easy to see or draw because it is pressed against
chromosomes, which can be seen most easily when the inside of the cell wall (see Figure 2.7).)
6
 Cell structure and function

Under the microscope, plant cells are quite

distinct and easy to see because of their cell walls. chloroplast
In Figure 2.1 it is only the cell walls (and in some
cases the nuclei) which can be seen. Each plant cell
cell has its own cell wall but the boundary between membrane

two cells side by side does not usually show up

clearly. So, cells next to each other appear to share vacuole
the same cell wall. cytoplasm
Vacuole cell wall

Most mature plant cells have a large, fluid-filled

space called a vacuole. The vacuole contains
cell sap, a watery solution of sugars, salts and
sometimes pigments. This large, central vacuole
pushes the cytoplasm outwards so it forms just a
thin lining inside the cell wall. It is the outward (a) longitudinal section (b) transverse section
pressure of the vacuole on the cytoplasm and cell
▲ Figure 2.7 Structure of a palisade mesophyll cell. It is
wall which makes plant cells and their tissues firm important to remember that, although cells look flat
(see ‘Osmosis’ in Chapter 3). Some animal cells may in sections or in thin strips of tissue, they are three-
sometimes have small vacuoles in their cytoplasm, dimensional and may seem to have different shapes
but they are usually produced to do a special job depending on the direction in which the section is
and are not permanent. cut. If the cell is cut across it will look like (b); if cut
longitudinally it will look like (a)
Chloroplasts
The shape of a cell when seen in a transverse
There are organelles that contain the green substance section may be quite different from when the same
chlorophyll, called chloroplasts (see Chapter 6). cell is seen in a longitudinal section and Figure 2.7
▼ Table 2.1 Summary: the parts of a cell

Name of part Description Where found Function

cytoplasm jelly-like, with particles and enclosed by the cell contains the cell organelles, e.g.
organelles in membrane mitochondria, nucleus
site of chemical reactions
cell membrane a partially permeable layer around the prevents cell contents from escaping
that forms a boundary around cytoplasm
controls what substances enter and
the cytoplasm
leave the cell
Animal and nucleus a circular or oval structure inside the cytoplasm controls cell division
plant cells containing deoxyribonucleic
controls cell development
acid (DNA) in the form of
chromosomes controls cell activities
mitochondria circular, oval or slipper- inside the cytoplasm responsible for aerobic respiration
shaped organelles
ribosomes small, circular structures, inside the cytoplasm protein synthesis
attached to membranes or
lying free
cell wall a tough, non-living layer made around the outside prevents plant cells from bursting
of cellulose surrounding the of plant cells
allows water and salts to pass
cell membrane
through (freely permeable)
Plant cells
vacuole a fluid-filled space inside the cytoplasm contains salts and sugars
only
surrounded by a membrane of plant cells
helps to keep plant cells firm
chloroplast an organelle containing inside the cytoplasm traps light energy for photosynthesis
chlorophyll of some plant cells

7
2 Cells

shows why this is so. Figures 8.4(b) and 8.4(c) on cell

membrane cytoplasm
page 123 show the appearance of cells in a stem
vein as seen in transverse and longitudinal section.
When studied at much higher magnifications with ribosomes on
an electron microscope, the cytoplasm of animal and membrane
plant cells no longer looks like a structureless jelly. It
appears to be organised into a complicated system of
membranes and vacuoles. Ribosomes are some of the nucleus
organelles present. They may be held on a membrane nuclear pore
but can also be found free in the cytoplasm. They
build up the cell’s proteins (see Chapter 4). mitochondrion
Mitochondria are tiny organelles, which may
appear slipper-shaped, circular or oval when viewed in
section. In three dimensions, they may be spherical,
(a)
(a) electron
electron micrograph oftwo
twoliver
livercells
cells (×10 000)
rod-like or extended. They have an outer membrane micrograph of (×10 000)

and an inner membrane with many inward-pointing

folds. Mitochondria are most common in regions
of rapid chemical activity. They are responsible for
nucleus
producing energy from food substances through the cell wall
process of aerobic respiration (see Chapter 12). ribosomes
Note that prokaryotes do not have mitochondria cell membrane
in their cytoplasm. cytoplasm
mitochondrion
Figure 2.8 is a diagram of an animal cell
magnified 10 000 times. Figure 2.9(a) is an
electron micrograph of two liver cells. Organelles
in the cytoplasm can be seen clearly. They have chloroplast
recognisable shapes and features. (b) electron micrograph of a plant cell (×6 000)
Figure 2.9(b) is an electron micrograph of a plant ▲ Figure 2.9 Cells at high magnification
cell. As well as the organelles already named and
described, other organelles are also present such as
chloroplasts and a cell wall. Test yourself
1 a What structures are usually present in both
mitochondrion animal and plant cells?
cell b What structures are present in plant cells but
nuclear pore membrane
not in animal cells?
nucleus 2 What cell structure is mainly responsible for
controlling the entry and exit of substances into or
out of the cell?
3 How does a cell membrane differ from a cell wall?

cytoplasm

ribosomes on membrane

▲ Figure 2.8 Diagram of a liver cell (×10 000)

8
 Cell structure and function

Bacterial cell structure

Bacteria (singular: bacterium) are very small
organisms which are single cells not often more than
0.01 mm in length. They can be seen only with the
higher powers of the microscope.
They have a cell wall made of a complicated
mixture of proteins, sugars and fats. (You will
remember that plant cell walls are made of cellulose).
Inside the cell wall is the cytoplasm, which may
contain granules (small particles) of glycogen, fat
and other food reserves (see Figure 2.10). Large
numbers of ribosomes float freely in the cytoplasm.
They are smaller than the ribosomes found in plant
▲ Figure 2.11 Longitudinal section through a bacterium
and animal cells but have the same function of
(×27 000). The light areas are coiled DNA strands. There
protein synthesis. are three of them because the bacterium is about to
divide twice (see Figure 2.12)

flagellum Bacteria can be different shapes: they may be

(in some spherical, rod-shaped or spiral. Some have filaments,
bacteria)
called flagella, projecting from them. The flagella
chromosome can flick and so move the bacterial cell about.
(single DNA slime capsule The functions of the structures in a bacterium are
strand
coiled up)
(in some) shown in Table 2.2.

cytoplasm
ribosome

cell wall glycogen granule (a) bacterial cell

plasmid

0.001 mm (b) chromosome replicates

▲ Figure 2.10 Generalised diagram of a bacterium

Each bacterial cell contains a single chromosome,

made of a circular strand of deoxyribonucleic acid (c) cell divides
(DNA) (see Chapter 4 and ‘Chromosomes, genes and
proteins’ in Chapter 17). The chromosome is not
surrounded by a nuclear membrane but coiled up
to fill a small part of the cell, as shown in Figure
2.11. There are also smaller circular structures called (d) each cell divides again
plasmids, that are also made of DNA. Plasmids
▲ Figure 2.12 Bacterium reproducing. This is asexual
are used by scientists in the process of genetic
reproduction by cell division (see ‘Asexual reproduction’
modification because it is relatively easy to insert in Chapter 16 and ‘Mitosis’ in Chapter 17)
genetic material into them (see Chapter 20).

9
2 Cells

▼ Table 2.2 Summary: the parts of a bacterial cell

Name of part Description Where found Function

cytoplasm jelly-like, with particles and surrounded by the contains cell structures, e.g. ribosomes, circular
organelles in cell membrane DNA, plasmids
cell membrane a partially permeable layer that around the prevents cell contents from escaping
surrounds the cytoplasm cytoplasm
controls what substances enter and leave the cell
circular DNA a single circular chromosome inside the cytoplasm controls cell division
controls cell development
controls cell activities
plasmids small, circular pieces of DNA inside the cytoplasm contain genes that carry genetic information to
help the process of the survival and reproduction
of the bacterium
ribosomes small, circular structures inside the cytoplasm protein synthesis
cell wall a tough, non-living layer not around the outside prevents the cell from bursting
made of cellulose surrounding of the bacterial cell
allows water and salts to pass through (freely
the cell membrane
permeable)

Test yourself
4 How is a bacterial cell different from a plant cell?
5 Bacteria and plant cells both have a cell wall. In what way are the cell walls different?

Practical work

Looking at cells l Using forceps, a mounted needle or a wooden

splint, support a coverslip with one edge
1 Plant cells – preparing a slide of onion resting near to the onion tissue, at an angle of
epidermis cells about 45° (Figure 2.13(b)).
The onion contains a very useful source of l Gently lower the coverslip over the onion
epidermal plant tissue which is one cell thick. tissue. Try to avoid trapping any air bubbles.
This makes it quite easy to set up as a temporary (Air bubbles reflect light when viewing under
slide. The onion is made up of fleshy leaves. On the light microscope, hiding the features you
the incurve of each leaf there is an epidermal are trying to see.)
layer which can be peeled off (Figure 2.13(a)). l Leave the slide for about 5 minutes. This

l Using forceps, peel a piece of epidermal tissue allows the iodine stain to react with the
from the incurve of an onion bulb leaf. specimen. The iodine stains the cell nuclei
l Place the epidermal tissue on a glass
pale yellow and the starch grains blue.
l Place the slide on the microscope stage,
microscope slide.
l Using a scalpel, cut out a 1 cm square of tissue
choose the lowest power objective lens
(throw away the rest) and arrange it in the and focus on the specimen. Increase the
centre of the slide. magnification using the other objective lenses.
l Add two to three drops of iodine solution.
Under high power, the cells should look like
(This stains any starch in the cells and makes those shown in Figure 2.14.
different parts of the cells distinct.)

10
 Cell structure and function

Calculating magnification image size

A lens is usually marked with its magnifying power. actual size of the specimen =
magnification
This tells you how much larger the image will be,
compared to the specimen’s actual size. So, if the When you give your answer, make sure you quote
lens is marked ×10, you know the image will be ten the units (which will be the same as those used to
times greater than the specimen’s real size. Since a measure the observed size).
light microscope has two lenses, you need to know
the magnification of both lenses. For example, if the
specimen is viewed using a ×10 eyepiece lens and Test yourself
a ×40 objective lens, the total magnification will be 13 a In order to see cells clearly in a section of plant
10 a × 40 = 400. tissue, which magnification would you use?
A ×5
eyepiece lens B ×10
barrel C ×100
D ×1000
b What is the approximate width (in millimetres)
of one of the largest cells in Figure 2.3?
14 In Figure 2.3, the cell membranes are not always
clear. Why is it still possible to decide roughly how
objective lens many cells there are in each tubule section?
body
clip

stage
Worked example
focusing knob
If you are asked to calculate the magnification of a drawing,
light source e.g. of a cell, you will be told the actual size of the cell and the
diameter of the cell in the drawing.
stand Start by making sure both figures (the observed size and actual
size) use the same units. For example, if the drawing of a cell is
6 cm wide (the observed size) and its actual size is 0.1 mm, you
need to change the cm to mm.
▲ Figure 2.21 A light microscope There are 10 mm in 1 cm, so 6 × 10 = 60 mm.
When you draw the image, your drawing is likely Now use these figures in the equation:
to be much larger than the image, so the total
magnification of the specimen is even bigger. image size
magnification =
actual size of the specimen
image size 60
magnification = =
actual size of the specimen = ×600
0.1
When doing this type of calculation, you need to Now put this into practice
make sure the units of both sizes are the same.
1 The image of a root hair cell is 5.0 cm long. Its actual size
If they are different, convert one to make them is 1.5 mm. Calculate the magnification of the image.
the same. For example, if the actual size is in 2 One of the moss leaf cells in the photomicrograph in
millimetres and the image size is in centimetres, Figure 2.15 is 2.5 cm wide.
convert the centimetres to millimetres. (There are The magnification of the image is ×500. Calculate the
10 millimetres in a centimetre.) actual size of the cell.
In the examination you may be asked to calculate
the actual size of a specimen, given a drawing or
photomicrograph and a magnification.

11
2 Cells

Revision checklist
After studying Chapter 2 you should know and ✔ Cells are often specialised in their shape and
understand the following: activity to carry out special jobs.
✔ Nearly all plants and animals are made up of ✔ The meaning of tissue, organ, organ system and
many microscopic cells. organism.
✔ The structures of plant and animal cells. ✔ How to calculate the magnification and size of a
✔ The functions of the structures in cells. specimen.

12
 Exam-style questions

Exam-style questions
1 The terms tissue, organ and organ system are 5 The diagram shows a human sperm cell.
used when describing the organisation inside an A
organism.
B
Complete the table by:
a defining each term [3] C
b giving one example in a plant and one
mid-piece
example in an animal. [6]
term definition example example in
in a plant an animal
tissue
organ a State the names of parts A, B and C. [3]
b The mid-piece of the sperm cell provides
organ system
energy for the cell. Suggest what type of
2 a Complete the table to compare the parts organelle it contains. [1]
present in a liver cell with those in a c State the function of the sperm cell. [1]
palisade cell. One component has been 6 The diagram shows four specialised cells.
done for you. [5]
part of cell present in present in
palisade cell liver cell
nucleus ✔ ✔ B

b Choose three of the parts and state their

functions.[3] A

3 The diagram shows a drawing of a bacterium.

C D

a Complete the table, using the letters of the

cells to identify them as plant or animal cells. [1]
plant animal
letters

b State two features found in all plant cells

but not in animal cells. [2]
c State one function each of cells A, B, C
0.001 mm
and D. [4]

Label four parts of the cell. [4]

Calculate the magnification of the drawing [2]
4 a Draw a labelled diagram of a named
specialised plant cell. [5]
b Describe the function of the cell. [1]

13
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