2 Organisation of the organism

Focus
In the previous chapter you recognised the characteristics present in all living organisms and used
a mnemonic to help you remember these. You were introduced to reasons for classifying organisms
into groups and the use of the binomial system of naming species. You had the opportunity to develop
your own dichotomous keys based on identifiable features. Then you learned about some of the main
animal and plant groups. In this chapter you will discover the main differences between animal, plant
and bacterial cells, as well as the functions of their parts. Within an organism there are levels of
organisation. By the end of the chapter you will be able to name these and describe examples from
animals and plants. Why are cells different shapes? What jobs do they do? How can we work out their
magnification when looking at them? By studying the chapter carefully and following the practical
suggestions you should be able to answer these questions.

Cell structure and organisation

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?

Cell structure
If a very thin slice of a plant stem is cut and thin slice taken from the tip of a plant shoot and
studied under a microscope, the stem appears to photographed through a microscope. It is 60 times
consist of thousands of tiny, box-like structures. larger than life, so a cell which appears to be 2 mm
These structures are called cells. Figure 2.1 is a long in the picture is only 0.03 mm long in reality.

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 Cell structure and organisation

You can cut sections through plant structures quite easily

just by using a razor blade. Cutting sections of animal
structures is more difficult because they are mostly soft
and flexible. Pieces of skin, muscle or liver, for example,
first must be soaked in melted wax. When the wax goes
solid it is then possible to cut thin sections. The wax is
dissolved away after making the section.
When sections of animal structures are examined
under the microscope, they too are seen to be
made up of cells, but they 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 times to show the cells clearly.
The sections are often treated with dyes, called
stains, in order to make the structures inside the
cells show up more clearly.

▲ Figure 2.1 Longitudinal section through the tip of a

plant shoot (×60). The slice is only one cell thick, so
light can pass through it and allow the cells to be
seen clearly

Thin slices like this are called sections. If you cut

along the length of the structure, you are taking a
longitudinal section (Figure 2.2(b)). Figure 2.1 shows a
longitudinal section, which passes through two small
developing leaves near the tip of the shoot, and two ▲ Figure 2.3 Transverse section through a kidney tubule (×700).
A section through a tube will look like a ring (see Figure
larger leaves below them. The leaves, buds and stem are
2.7(b)). In this case, each ‘ring’ consists of about 12 cells
all made up of cells. If you cut across the structure, you
make a transverse section (Figure 2.2(a)). Making sections is not the only way to study cells.
Thin strips of plant tissue, only one cell thick, can
be pulled off stems or leaves (experiment 1, pages
30–31). Plant or animal tissue can be squashed or
smeared on a microscope slide (experiment 2,
page 31), or treated with chemicals to separate the
cells before studying them.
There is no such thing as a typical plant or animal
cell because cells vary a lot in their size and shape
depending on their function. However, it is possible
to make a drawing, like that in Figure 2.4, to show
the features that are present in most cells. All cells
(a) transverse section (b) longitudinal section
have a cell membrane, which is a thin boundary
▲ Figure 2.2 Cutting sections of a plant stem enclosing the cytoplasm. Most cells have a nucleus.

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 2 Organisation of the organism

nucleus and type of chemical reactions that take place

cell membrane in the cells. Some enzymes are attached to the
membrane systems of the cell, while others float
freely in the liquid part of the cytoplasm.
cytoplasm
Cell membrane
This is a thin layer of cytoplasm around the outside
of the cell. It stops the cell contents from escaping
and controls which substances can enter and leave
the cell. In general, oxygen, food and water are
allowed to enter; waste products are allowed to
leave; and harmful substances are kept out. In this
way the cell membrane maintains the structure and
mitochondria granules
chemical reactions of the cytoplasm.
▲ Figure 2.4 A group of liver cells. These cells have all the
characteristics of animal cells Nucleus (plural: nuclei)
Most cells contain one nucleus, which is usually seen
Cytoplasm as a rounded structure covered by a membrane and
Under the ordinary microscope (light microscope), fixed in the cytoplasm. In drawings of cells, the
cytoplasm looks like a thick liquid with particles nucleus may be shown darker than the cytoplasm
in it. In plant cells it may be seen to be flowing because, in prepared sections, it takes up certain
about. The particles may be food reserves like oil stains more strongly than the cytoplasm. The
droplets or granules (small particles) of starch. function of the nucleus is to control the type and
Other particles are structures known as organelles, quantity of enzymes produced by the cytoplasm. In
which have special functions in the cytoplasm. In this way it regulates the chemical changes that take
the cytoplasm, large numbers of chemical reactions place in the cell. As a result, the nucleus controls
are taking place, which keep the cell alive by what the cell will be, for example, a blood cell, a
providing energy and making substances that the liver cell, a muscle cell or a nerve cell.
cell needs. When existing cells divide, new cells are
The liquid part of cytoplasm is about 90% water, produced. The nucleus controls cell division, as
with molecules of salts and sugars dissolved in it. shown in Figure 2.5. A cell without a nucleus
Suspended in this solution there are larger molecules cannot reproduce. Inside the nucleus are thread-like
of fats (lipids) and proteins (see Chapter 4). Fats and structures called chromosomes, which can be seen
proteins may be used to build up the cell structures, most easily at the time when the cell is dividing (see
like the membranes. Some of the proteins are Chapter 17 for a fuller account of chromosomes and
enzymes (see Chapter 5). Enzymes control the rate cell division).

(a) Animal cell about to (b) The nucleus divides first. (c) The daughter nuclei separate (d) Two cells are formed – one
divide. and the cytoplasm pinches may keep the ability to
off between the nuclei. divide, and the other may
become a specialised cell.
▲ Figure 2.5 Cell division in an animal cell

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 Cell structure and organisation

Plant cells cell sap, a watery solution of sugars, salts and

A few generalised animal cells are shown in Figure 2.4, sometimes pigments. This large, central vacuole
while Figure 2.6 is a drawing of two palisade cells from pushes the cytoplasm outwards so that it forms just
a plant leaf. (See ‘Leaf structure’ in Chapter 6.) a thin lining inside the cell wall. It is the outward
pressure of the vacuole on the cytoplasm and
cell wall cell wall that makes plant cells and their tissues
firm (see ‘Osmosis’ in Chapter 3). Animal cells may
sometimes have small vacuoles in their cytoplasm,
chloroplast but they are usually produced to do a special job
and are not permanent.
cytoplasm
nuclear Chloroplasts
membrane Chloroplasts are organelles that contain the green
vacuole nucleus substance chlorophyll (see Chapter 6).

chloroplast

cell
membrane
▲ Figure 2.6 Palisade cells from a leaf
vacuole
Plant cells differ from animal cells in several ways
cytoplasm
because they have extra structures: a cell wall,
chloroplasts and sap vacuoles. cell wall

Cell wall
The cell wall, which is outside the membrane,
contains cellulose and other compounds. It is non-
living and allows water and dissolved substances to
pass through it. The cell wall is not selective like
(a) longitudinal section (b) transverse section
the cell membrane. (Note that plant cells do have
a cell membrane, but it is not easy to see or draw ▲ Figure 2.7 Structure of a palisade mesophyll cell. It is
important to remember that, although cells look flat
because it is pressed against the inside of the cell
in sections or in thin strips of tissue, they are three-
wall (see Figure 2.7).) dimensional and may seem to have different shapes
Under the microscope, plant cells are quite depending on the direction in which the section is
distinct and easy to see because of their cell walls. cut. If the cell is cut across it will look like (b); if cut
In Figure 2.1 it is only the cell walls (and in some longitudinally it will look like (a)
cases the nuclei) that can be seen. Each plant cell
has its own cell wall but the boundary between two The shape of a cell when seen in a transverse
cells side by side does not usually show up clearly. section may be quite different from when the
So, cells next to each other appear to be sharing the same cell is seen in a longitudinal section, and
same cell wall. Figure 2.7 shows why this is so. Figures 8.4(b)
and 8.4(c) on page 132 show the appearance of
Vacuole cells in a stem vein as seen in transverse and
Most mature plant cells have a large, fluid-filled longitudinal sections.
space called a vacuole. The vacuole contains

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 2 Organisation of the organism

▼ 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 and nucleus
site of chemical reactions
cell membrane a partially permeable layer that around the cytoplasm prevents cell contents from
Animal and plant cells

forms a boundary around the escaping

cytoplasm
controls what substances enter
and leave the cell
nucleus a circular or oval structure inside the cytoplasm controls cell division
containing DNA in the form of
controls cell development
chromosomes
controls cell activities
mitochondria circular, oval or slipper-shaped inside the cytoplasm responsible for aerobic respiration
organelles
ribosomes small, circular structures attached inside the cytoplasm protein synthesis
to membranes or lying free
cell wall a tough, non-living layer made around the outside of prevents plant cells from bursting
of cellulose surrounding the cell plant cells
Plant cells only

allows water and salts to pass

membrane
through (freely permeable)
vacuole a fluid-filled space surrounded by a inside the cytoplasm of contains salts and sugars
membrane plant cells
helps to keep plant cells firm
chloroplast an organelle containing chlorophyll inside the cytoplasm of traps light energy for
some plant cells photosynthesis

When studied at much higher magnifications with in the cytoplasm can be seen clearly. They have
the electron microscope, the cytoplasm of animal and recognisable shapes and features.
plant cells no longer looks like a structureless jelly. It Figure 2.8(c) is an electron micrograph of a plant
appears to be organised into a complicated system of cell. As well as the organelles already named and
membranes and vacuoles. Ribosomes are one of the described, other organelles are also present, like
organelles present. They may be held on a membrane chloroplasts and a cell wall.
but can also be found free in the cytoplasm. They
build up the cell’s proteins (see Chapter 4).
Mitochondria are tiny organelles, which may mitochondrion
cell
appear slipper-shaped, circular or oval when nuclear pore membrane
viewed in section. In three dimensions, they may nucleus
be spherical, rod-like or extended. They have an
outer membrane and an inner membrane with
many inward-pointing folds. Mitochondria are most
frequent in regions of rapid chemical activity. cytoplasm
They are responsible for releasing energy from
food substances through the process of aerobic
respiration (see Chapter 12). ribosomes on
Note that prokaryotes do not possess membranes
mitochondria in their cytoplasm. (a) diagram of a liver cell (×10 000)
Figure 2.8(a) is a diagram of an animal cell ▲ Figure 2.8 Cells at high magnification
magnified 10 000 times. Figure 2.8(b) is an
electron micrograph of a liver cell. Organelles

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 Cell structure and organisation

cell found in plant and animal cells but have the same
membrane cytoplasm
function of protein synthesis.

ribosomes chromosome
on membranes cell wall (single DNA
strand coiled up)

nucleus
nuclear pore
ribosome glycogen
mitochondrion granule

plasmid

(b) electron micrograph of two liver cells (×10 000)

cytoplasm
flagellum in
some bacteria 0.001mm

nucleus ▲ Figure 2.9 Generalised diagram of a bacterium

cell wall
ribosomes
cell membrane
cytoplasm
mitochondrion

chloroplast
(c) electron micrograph of a plant cell (×6 000)
▲ Figure 2.8 Cells at high magnification (continued)

Test yourself ▲ Figure 2.10 Longitudinal section through a bacterium

1 a What structures are usually present in both (×27 000). The light areas are coiled DNA strands. There
animal and plant cells? are three of them because the bacterium is about to
b What structures are present in plant cells but divide twice (see Figure 2.11)
not in animal cells?
2 What cell structure is mainly responsible for
controlling the entry and exit of substances into or
out of the cell? (a) bacterial cell (b) chromosome replicates
3 How does a cell membrane differ from a cell wall?

Bacterial cell structure (c) cell divides (d) each cell divides again
Bacteria (singular: bacterium) are very small
▲ Figure 2.11 Bacterium reproducing. This is asexual
organisms that are single cells not often more than
reproduction by cell division (see ‘Asexual reproduction’
0.01 mm in length. They can be seen only at high in Chapter 16 and ‘Mitosis’ in Chapter 17)
magnification under a microscope.
They have a cell wall made of a complicated Each bacterial cell contains a single chromosome
mixture of proteins, sugars and fats. (You will made of a circular strand of DNA (see Chapter 4 and
remember that plant cell walls are made of ‘Chromosomes, genes and proteins’ in Chapter 17). The
cellulose.) Inside the cell wall is the cytoplasm, chromosome is not surrounded by a nuclear membrane
which may contain granules (small particles) of but is coiled up to fill a small part of the cell, as
glycogen, fat and other food reserves (see Figure shown in Figure 2.10. There are also smaller circular
2.9). Large numbers of ribosomes float freely in the structures called plasmids, which are also made of
cytoplasm. They are smaller than the ribosomes DNA. Plasmids are used by scientists in the process of
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 2 Organisation of the organism

genetic modification because it is relatively easy to called flagella, projecting from them. The flagella can
insert genetic material into them (see Chapter 21). flick and so move the bacterial cell about.
Bacteria can be different shapes: they may be The functions of the structures in a bacterium are
spherical, rod-shaped or spiral. Some have filaments, shown in Table 2.2.
▼ Table 2.2 Summary: the parts of a bacterial cell

Name of part Description Where found Function

cytoplasm jelly-like, contains particles and surrounded by the contains cell structures, e.g. ribosomes, circular
organelles cell membrane DNA, plasmids
cell a partially permeable layer that around the prevents cell contents from escaping
membrane 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 around the outside prevents the cell from bursting, allows water and
(not made of cellulose) that of the bacterial cell salts to pass through (freely permeable)
surrounds the cell membrane

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
For safe experiments/demonstrations which the incurve of each leaf there is an epidermal
are related to this chapter, please refer to the layer which can be peeled off (Figure 2.12(a)).
Biology Practical Skills Workbook that is also
l Using forceps, peel a piece of epidermal tissue
part of this series.
from the incurve of an onion bulb leaf.
Safety l Place the epidermal tissue on a glass
l Eye protection must be worn. microscope slide.
l Take care when using a scalpel, follow your l Using a scalpel, cut out a 1 cm square of tissue
teacher’s guidance. (throw away the rest) and arrange it in the
l Take care using the iodine and methylene centre of the slide.
blue stains – they will stain skin and clothing. l Add two to three drops of iodine solution.
(This stains any starch in the cells and makes
Looking at cells different parts of the cells distinct.)
1 Plant cells – preparing a slide of onion l Using forceps, a mounted needle or a wooden
epidermis cells splint, support a cover-slip with one edge
The onion contains a very useful source of resting near to the onion tissue, at an angle of
epidermal plant tissue which is one cell thick. about 45° (Figure 2.12(b)).
This makes it quite easy to set up as a temporary l Gently lower the cover-slip over the onion
slide. The onion is made up of fleshy leaves. On tissue. Try to avoid trapping any air bubbles.

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 Cell structure and organisation

(Air bubbles reflect light when viewing under

the light microscope, hiding the features you
are trying to see.)
l Leave the slide for about 5 minutes. This
allows the iodine stain to react with the
specimen. The iodine stains the cell nuclei
pale yellow and the starch grains blue.
l Place the slide on to the microscope stage,
choose the lowest power objective lens
and focus on the specimen. Increase the
magnification using the other objective lenses.
Under high power, the cells should look like
those shown in Figure 2.13.
▲ Figure 2.13 Onion epidermis cells
An alternative tissue is rhubarb epidermis (Figure 2 Plant cells – preparing cells with
2.12(c)). You can strip this off from the surface of chloroplasts
a stalk and treat it in the same way as the onion
tissue. If you use red epidermis from rhubarb l Using forceps, remove a leaf from a
stalk, you will see the red cell sap in the vacuoles. moss plant.
l Place the leaf in the centre of a microscope
slide and add one or two drops of water.
l Place a cover-slip over the leaf.
l Examine the leaf cells with the high power
objective of a microscope. The cells should
look like those shown in Figure 2.14.

(a) peel the epidermis from the inside of an onion bulb leaf

(b) place the epidermis on to the slide, adding 2–3 drops of

iodine solution and carefully lower a coverslip on to it ▲ Figure 2.14 Cells in a moss leaf (×500). The vacuole
occupies most of the space in each cell. The
chloroplasts are limited to the layer of cytoplasm lining
the cell wall

3 Animal cells – preparing human

cheek cells
Human cheek cells are constantly being wiped
off the inside of the mouth when the tongue and
food rub against them, so they can be collected
(c) peel a strip of red epidermis from a piece of rhubarb skin
easily for use in a temporary slide.
▲ Figure 2.12 Looking at plant cells

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 2 Organisation of the organism

Note: l Place the slide on to the microscope stage,

choose the lowest power objective lens
Check local guidance to whether observing
and focus on the specimen. Increase the
cheek cells is permitted. Use appropriate
magnification using the other objective
precautions to treat contaminated items with
lenses. Under high power, the cells should
disinfectant or by autoclaving.
look like those shown in Figure 2.15, but less
l Rinse your mouth with water. This will remove magnified.
any fragments of food. l When you have completed the ‘Test yourself’
l Take a cotton bud from a freshly opened pack. section, place your used slide in laboratory
Rub the cotton bud lightly on the inside of your disinfectant before washing.
cheek and gums to collect some cheek cells
in saliva. 4 Animal cell – preparing human skin cells
l Rub the cotton bud on to the centre of a clean You can try another method of obtaining cells if
microscope slide, leaving a sample of saliva. the previous method is not suitable.
Repeat if the sample is too small. Then drop
l Wash your wrist well, then press some
the cotton bud into a container of absolute
transparent sticky tape on to the cleaned area
alcohol or disinfectant.
of skin.
l Add two to three drops of methylene blue dye.
l Remove the tape and stick it to a
(This stains parts of the cheek cells to make
microscope slide.
nuclei more visible.)
l Place the slide on the microscope stage.
l Using forceps, a mounted needle or wooden
l Look for cells. You should be able to see nuclei
splint, support a cover-slip with one edge
in them.
resting near to the cheek cell sample, at an
l If you add a few drops of methylene blue
angle of about 45°. Gently lower the cover-slip
solution before putting the tape on the slide,
over the tissue. Try to avoid trapping any air
the cells take up the stain and it makes the
bubbles. (Air bubbles reflect light when viewing
nuclei more distinct.
under the light microscope, hiding the features
you are trying to see.) Practical work questions
l Leave the slide for a few minutes. This allows
the methylene blue stain to react with 1 In experiment 1, what cell structures could
the specimen. you identify in the onion cells you observed?
2 a For experiment 2, explain why the
chloroplasts appear to be pressed against
the cell wall of the cell.
b Why are the chloroplasts green?
3 For experiment 2, explain why it is necessary
to use a stain when preparing specimens
of cells.
4 In experiments 3 and 4, the skin cells are
animal epidermal cells. Plants also have
epidermal cells. Compare a human skin
epidermal cell with an upper epidermal cell of
a leaf (see Figure 6.26 on page 104).
What cell structures do leaf epidermal
cells have which are not present in human
▲ Figure 2.15 Cells from the lining epithelium of the epidermal cells?
cheek (×1 500)

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 Cell structure and organisation

cilia
Test yourself
6 Make a large drawing of one cell and label
the following parts: cell wall, cell membrane,
cytoplasm, nucleus.
nucleus
7 Make a note of the magnification of the eyepiece
and objective lenses of your microscope.
8 Copy and complete the table by
a writing the magnification of the eyepiece lens
b writing the magnification of the objective lens
of your microscope which you used to make
(a) ciliated cells
your drawing These cells form the lining of the nose and windpipe, and the tiny
c calculating total magnification provided by the cytoplasmic ‘hairs’, called cilia, are in a continual flicking movement,
microscope. which creates a stream of fluid (mucus) that carries dust and bacteria
through the bronchi and trachea, away from the lungs.

magnification of the eyepiece lens

root hair
magnification of the objective lens (very thin cell wall)
total magnification provided by the
microscope
9 Estimate how much bigger your drawing is than
the image you see through the microscope. Use
these figures to calculate the total magnification of
your drawing.

Specialisation of cells
(b) root hair cell
When cells have finished dividing and growing, most These cells absorb water and mineral salts from the soil. The hair-like
become specialised and have specific functions. projection on each cell penetrates between the soil particles and offers
When cells are specialised: a large absorbing surface. The cell membrane is able to control which
dissolved substances enter the cell.
» they do one special job cell wall
» they develop a distinct shape
» special kinds of chemical changes take place in
their cytoplasm.
chloroplast
The changes in shape and the chemical reactions
enable the cell to carry out its special function. Red
cytoplasm
blood cells and root hair cells are just two examples nuclear
membrane
of specialised cells. Figure 2.16 shows a variety of
specialised cells. vacuole nucleus
The specialisation of cells to carry out special
functions in an organism is sometimes called
‘division of labour’ within the organism. Similarly,
the special functions of mitochondria, ribosomes
and other cell organelles may be called division of
labour within the cell. (c) palisade mesophyll cells
These are found underneath the upper epidermis of plant leaves.
They are columnar (quite long) and packed with chloroplasts to trap
light energy. Their function is to make food for the plant by
photosynthesis using carbon dioxide, water and light energy.
▲ Figure 2.16 Specialised cells (not to scale)

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 2 Organisation of the organism

acrosome
mid-piece
nucleus

tail

nerve fibre

(f) sperm cell

Sperm cells are male sex cells. The front of the cell is oval shaped
and contains a nucleus which carries genetic information. There is a
tip, called an acrosome, which secretes enzymes to digest the cells
around an egg and the egg membrane. Behind this is a mid-piece
which is packed with mitochondria to provide energy for movement.
The tail moves with a whip-like action, enabling the sperm to swim.
Their function is reproduction, achieved by fertilising an egg cell.

(d) nerve cells

These cells are specialised for conducting
electrical impulses along the fibre, to and
from the brain and spinal cord. The fibres
are often very long and connect distant jelly coat
parts of the body to the CNS, e.g. the foot nucleus
cell membrane
and the spinal column. Chemical reactions
cause the impulses to travel along the fibre.
cytoplasm
containing yolk
cell body follicle cells
droplets
nucleus
(g) egg cell
Egg cells (ova, singular: ovum) are larger than sperm cells and
are spherical. They have a large amount of cytoplasm, containing
yolk droplets made up of protein and fat. The nucleus carries
genetic information. The function of the egg cell is reproduction.

cytoplasm containing haemoglobin ▲ Figure 2.16 Specialised cells (not to scale) (continued)

Test yourself
10 In what way does the red blood cell shown in
(e) red blood cells Figure 2.16(e) differ from most other animal cells?
These cells are distinctive because they have no nucleus
when mature. They are tiny disc-like cells that contain
11 Why does the cell shown in Figure 2.7(b) appear to
a red pigment called haemoglobin. This readily combines have no nucleus?
with oxygen and their function is the transportation of
oxygen around the body.

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 Cell structure and organisation

Tissues, organs, organ systems

and the organism

FOCUS POINTS
★ Definitions of tissues, organs, organ systems
and organism

Some microscopic organisms are made of one cell

only (see ‘Features of organisms’ in Chapter 1). These
(b) cells forming a small tube
can carry out all the processes needed to keep them E.g. a kidney tubule (see page 211). Tubules such as this carry
alive. The cells of the larger plants and animals liquids from one part of an organ to another.
cannot survive on their own. A muscle cell could not
obtain its own food and oxygen. Other specialised
cells provide the food and oxygen needed for the
muscle cell to live. Unless these cells are grouped
together in large numbers and made to work
together, they cannot stay alive.
Tissues
A tissue, like bone, nerve or muscle in animals, and
epidermis, xylem or pith in plants, is made up of (c) one kind of muscle cell
large numbers of cells. These are often just a single Forms a sheet of muscle tissue. Blood vessels, nerve fibres and
connective tissues will also be present. Contractions of this kind of
type. The cells of each type have a similar structure muscle help to move food along the food canal or close down
and function so that the tissue itself has a special small blood vessels.
function. For example, muscles contract to cause
movement, xylem carries water in plants. Figure 2.17
shows how some cells are arranged to form simple
tissues. Some forms of tissues are epithelium,
tubes, sheets and glands.
Key definitions
A tissue is a group of cells with similar structures
working together to perform a shared function.

(d) cells forming part of a gland

The cells make chemicals, which are released into the central space
and carried away by a tubule such as that shown in (b). Hundreds
of cell groups like this would form a gland like the salivary gland.
▲ Figure 2.17 How cells form tissues

(a) cells forming an epithelium

A thin layer of tissue, e.g. the lining of the mouth cavity. Different
types of epithelium form the internal lining of the windpipe, air
passages, food canal, etc., and protect these organs from physical
or chemical damage.

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 2 Organisation of the organism

Organs brain
Organs are made of several tissues grouped together
to make a structure with a special job. For example,
the stomach is an organ that contains tissues made
spinal
from epithelial cells, gland cells and muscle cells. cord nerve
These cells are supplied with food and oxygen
brought by blood vessels. The stomach also has a
nerve supply. The heart, lungs, intestines, brain and
eyes are further examples of organs in animals. In
flowering plants, the root, stem and leaves are the
organs. Some of the tissues of the leaf are epidermis,
palisade tissue, spongy tissue, xylem and phloem
(see Chapter 8).

Key definitions
An organ is a structure made up of a group of tissues
working together to perform a specific function.

Test yourself (a) nervous system

12 a Study Figure 8.7 on page 133 and identify
examples of tissues and an organ.
b Study Figure 7.14 on page 119 and identify
examples of tissues and an organ.

artery
Organ systems
An organ system usually describes a group heart

of organs with closely related functions. For

example, the heart and blood vessels make up vein
the circulatory system; the brain, spinal cord
and nerves make up the nervous system (Figure
2.18). In a flowering plant, the stem, leaves and
buds make up a system called the shoot (Figure
8.1 on page 130).

Key definitions
An organ system is a group of organs with related
functions working together to perform a body function.

(b) circulatory system

▲ Figure 2.18 Two examples of systems in the human body

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 Size of specimens

Organisms Key definitions

An organism is formed by the organs and systems An organism is a living thing that has an organised
working together to make an independent plant or structure, can react to stimuli, reproduce, grow, adapt,
animal (one that can survive by itself). and maintain homeostasis.
An example in the human body of how cells,
tissues and organs are related is shown in
Figure 2.19.

oesophagus
stomach lining

muscle layer
stomach
(b) an organ – the stomach,
from the digestive system
(cut open to show the
small large lining and the muscle layer) gland
intestine intestine
circular
muscle

longitudinal
muscle

(c) tissue – a small piece

of stomach wall with
(a) a system – the digestive system (d) cells – some muscle cells muscle tissue and
of the human organism from the muscle tissue gland tissue
▲ Figure 2.19 An example of how cells, tissue and organs are related

Size of specimens placed on a microscope slide. The light passes through

the objective and eyepieces lenses, magnifying the
image so you can see detail of the specimen. You can
FOCUS POINTS use coarse and fine focus knobs to make the image
★ How do you calculate the magnification and size of clearer. You need to place specimens on microscope
biological specimens using millimetres as units? slides, which may be temporary or permanent
preparations. You can prepare temporary slides quickly,
★ How do you convert measurements between
but the specimens dry out quite rapidly, so they do not
millimetres and micrometres?
store well. You carefully lay a cover-slip (a thin piece of
glass) over the specimen. This helps to keep it in place,
slows down dehydration and protects the objective
The light microscope lens from moisture or stains. To make a permanent
You cannot see most cells with the naked eye. A preparation you usually need to dehydrate the specimen
hand lens has a magnification of up to ×20, but this and fix it in a special resin, for example, Canada Balsam.
is not enough to see the detail in cells. The light These types of slides store well for a long time.
microscope (Figure 2.20) has two convex lenses, with
magnifications of up to ×1 500, although most found in Calculating magnification
school laboratories only magnify to ×400. The eyepiece A lens is usually marked with its magnifying power. This
lens is usually ×10 and there is a choice of objective tells you how much larger the image will be compared to
lenses (usually ×4, ×10 and ×40), set in a nosepiece the specimen’s actual size. So, if the lens is marked ×10,
which can be rotated (turned round). Light, from a you know that the image will be ten times greater than
mirror or a bulb, is projected through the specimen the specimen’s real size. Since a light microscope has

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 2 Organisation of the organism

two lenses, you need to know the magnification of both When doing this type of calculation, you need
lenses. For example, if the specimen is viewed using to make sure that the units of both sizes are the
a ×10 eyepiece lens and ×40 objective lens, the total same. If they are different, convert one to make
magnification will be 10 × 40 = 400. them the same. For example, if the actual size is in
millimetres and the image size is in centimetres,
Key definitions convert the centimetres to millimetres. (There are
Magnification is the observed size of an image divided by 10 millimetres in a centimetre.)
the actual size of the image. In questions you may be asked to calculate
the actual size of a specimen, given a drawing or
eye piece lens photomicrograph and a magnification.
barrel image size
Actual size of the specimen =
magnification

When you give the answer, make sure you quote

the units (which will be the same as those used for
body objective lens
measuring the observed size).
clip
stage
focusing dial Test yourself
13 a In order to see cells clearly in a section of plant
light source tissue, which magnification would you have
to use?
stand A ×5
B ×10
C ×100
D ×1 000
▲ Figure 2.20 A light microscope b What is the approximate width (in millimetres)
When you draw the image, your drawing is of one of the largest cells in Figure 2.3?
14 In Figure 2.3, the cell membranes are not always
usually much larger than the image, so the total clear. Why is it still possible to decide roughly how
magnification of the specimen is even bigger. many cells there are in each tubule section?
image size
Magnification =
actual size of the specimen

Worked example
If you are asked to calculate the magnification of a drawing, 60
e.g. of a cell, you will be told the actual size of the cell and the Magnification = = ×600
0.1
diameter of the cell in the drawing.
Start by making sure that both figures (the observed size and Tasks
actual size) are the same units. For example, if the drawing 1 The image of a root hair cell is 4.5 cm long. Its actual size
of a cell is 6 cm wide (the observed size) and its actual size is is 1.5 mm. Calculate the magnification of the image.
0.1 mm you need to change the cm to mm. 2 One of the moss leaf cells in the photomicrograph in
Figure 2.14 is 2.5 cm wide.
There are 10 mm in 1 cm, so 6 × 10 = 60 mm. The magnification of the image is ×500. Calculate the
Now use these figures in the equation: actual size of the cell.

imagesize 3 The diameter of a drawing of a red blood cell is

Magnification =
actual size of the specimen 14 mm. The actual size of the cell is 7 µm. Calculate
the magnification of the image.

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 Size of specimens

Converting measurements (micron or µm), is used. Figure 2.21 shows a

comparison of the sizes of a range of objects.
Organelles in cells are too small to be measured in
millimetres. A smaller unit, called the micrometre
electron microscope optical microscope unaided eye

water sugar antibody virus bacterium Amoeba a full stop an orange

molecule molecule

10 × 10−4 10 × 10−3 10 × 10−2 10 × 10−1 1.0 10 × 101 10 × 102 10 × 103 10 × 104 10 × 105

size (micrometres)
▲ Figure 2.21 Comparing the sizes of a range of objects

There are: and the observed size is in millimetres, convert the

» 1 000 000 micrometres in a metre millimetres to micrometres.
» 10 000 micrometres in a centimetre
» 1 000 micrometres in a millimetre. Test yourself
15 The tail of a sperm cell is 0.05 mm long. What is
Remember to make sure that the units of both sizes its length in micrometres?
used in a calculation involving magnification are 16 An amoeba is 750 µm long. Calculate its length
the same. So, if the actual size is in micrometres in centimetres.

Revision checklist
After studying Chapter 2 you should know and ✔ Plant cells have a cellulose cell wall and a large
understand the following: central vacuole.
✔ Nearly all plants and animals are made up of ✔ Cells are often specialised in their shape and
microscopic cells. activity to carry out special jobs.
✔ All cells contain cytoplasm surrounded by a ✔ Large numbers of similar cells packed together
cell membrane. form a tissue.
✔ Most cells have a nucleus. ✔ Different tissues arranged together form organs.
✔ Many chemical reactions take place in the ✔ A group of related organs makes up a system.
cytoplasm to keep the cell alive. ✔ The magnification of a specimen can be calculated
✔ Cytoplasm contains organelles, which if the actual size and the size of the image
include mitochondria (respiration), are known.
chloroplasts (photosynthesis) and ribosomes
image size
(protein synthesis). Magnification =
✔ The nucleus directs the chemical reactions in the actual size of the specimen
cell and also controls cell division.

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 2 Organisation of the organism

Exam-style questions
1 The terms tissue, organ and organ systems are 5 The diagram shows a human sperm cell.
used when describing the organisation inside A
an organism. B
Complete the table by C
mid-piece
a defining what each term means [3]
b giving one example in a plant and one in
an animal for each structure. [6]
name of definition example example in
structure in a plant an animal
tissue
a State the names of parts A, B and C. [3]
organ
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 palisade c State the function of the sperm cell. [1]
cell. One component has been done for you. [5] 6 The diagram shows four specialised cells.

part of cell present in palisade cell present in liver

cell
nucleus   B

cell wall
chloroplast A
cytoplasm
membrane
(sap)
vacuole C D

b Choose three of the parts and state a Complete the table, using the letters of the
their functions.[3] cells to identify them as plant or animal cells.[1]
3 The diagram shows a drawing of a bacterium.
plant animal
0.001 mm
letters

b State two features found in all plant cells

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

7 A student used a microscope to study a human cheek

cell. She drew the cell. The drawing was 30 mm
wide. The actual diameter of the cell was 60 µm.
a Label four parts of the cell. [4]
a Calculate the magnification of the
b Calculate the magnification
drawing.[3]
of the drawing. [2]
b The eyepiece of the microscope was ×10
4 a Draw a labelled diagram of a named
and its objective lens was ×40. Calculate
specialised plant cell. [5]
the total magnification of the
b Describe the function of the cell. [1]
microscope.[1]

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