Biology and Health Sciences

Key unit
competence
To be able to explain specialisation of cells, and the link between levels of organisation
in multicellular organisms

Cross-cutting issue

Comprehensive sexuality education: Understanding what healthy cells are and how they function in
the body, enables us to live a happy, healthy life.

At the end of this unit, you should be able to:
• Recall that a cell is a basic structure of an organism
• Describe the diff erent types of cells in this unit and state their functions
• Describe the diff erent cell structures found in animals and plants and how
they relate to their functions
• Identify diff erent levels of organisation in multicellular organisms
• Observe and illustrate diff erent types of cells and tissues under light
microscope or micrographs
• Categorize plant and animal tissues using observation of micrographs or
slides
• Illustrate well labelled structures of xylem and phloem tissue from slides or
micrographs of sections of vascular plants
• Sequence the levels of organization of multicellular organisms from the
simplest to the most complex
• Diff erentiate the relationship between the structure and function of
specialised cells
• Appreciate the complexity of life from the tiny cell through to the tissue,
organ, system and organism levels of organisation
• Be aware that an organism is a complex organisation of cells, an important
unit of life.

Oral activity

Figure 6.1 Red blood cells are part of blood and are specialised
to transport oxygen.

In pairs, think back to what you learnt in the last two units. Th en, brainstorm the answers
to these questions.
1. Why is the cell referred to as the ‘basic
unit of life’?
2. Are all cells the same? If not, in what
ways are they diff erent?
3. What structures do plant cells have to
help them carry out photosynthesis?
4. Are all organisms made up of many
cells? What do we call single-celled
organisms? What do we call organisms
that consist of many cells? Give an
example of an organism that has many
cells in its body.

Topic 2: Organisation and maintenance of life

Cell specialisation
In the previous unit, you learnt about plant and animal cells. Not all cells
look the same or work in the same way. Th ere are many different types
of cells and each type can carry out different functions. For example, in
animals, cells in muscles are specialised to bring about movement. These
cells contain lots of mitochondria so that they can make energy for the
muscles to work.
Blood cells in animals are another specialised type of cell. They
transport oxygen. A plant’s leaf cells are specialised to perform
photosynthesis, whereas its root cells are adapted to take in water and
nutrients from the soil. Th e cells are able to do these different functions
because they are slightly different in structure from one another.
In this unit you will find out more about some specialised cells in
animals and in plants. You will also learn how their structure is linked to
their functions.

Hint
Understanding how our bodies are made up can be useful in health related careers
in Rwanda.

Examples of some specialised animal cells
In this section, we look at examples of different types of animal cells
whose structures have been adapted to enable them to perform different
functions.

Ciliated cells
Ciliated cells have tiny hair-like structures, called cilia, on their surfaces.
Th ere are also other specialised cells, called goblet cells, amongst the
ciliated cells. Goblet cells make a substance called mucus.
Ciliated cells line the surfaces of some parts of the body, such as the air
passages. Air passages take air to and from the lungs. Th e cilia trap dust
particles and stop them from going into the lungs. Mucus from the goblet
cells also traps dust.
Ciliated cells are also found in the female reproductive organs. You will
learn about their function in this part of the body later on.

Nerve cells
Nerve cells, or neurons, are specialised to conduct nerve impulses in the
nervous system. They carry information from one part of the body to
another part.
Nerve cells are long and thin, and are grouped together in bundles
to form nerves. Their shape makes them suitable for conducting nerve
impulses over long distances.

Unit 6: Levels of organisation in multicellular organisms

Red blood cells
Red blood cells are specialised cells that are found in the blood. They
transport oxygen around the body.
Red blood cells are small and have an unusual shape. They are referred to
as biconcave discs. This shape gives them a large surface area on which oxygen
molecules can bond. Red blood cells do not have a nucleus. They are packed
full of a special substance called haemoglobin. Haemoglobin attracts oxygen
molecules, which makes red blood cells well suited for carrying oxygen around
they body. The cells are elastic, so they can easily squeeze through narrow
tubes in the blood system.

Exercise 6.11. Give the meaning of each of these words.
a) specialised c) neurons
b) cilia d) haemoglobin
2. a) What is the function of ciliated cells?
b) List two ways in which ciliated cells are adapted to
their function.
3. a) What is the function of red blood cells?
b) List two ways in which red blood cells are suited to
their function.
c) Make a neat, labelled drawing of some red blood cells.

Sperm cells
Sperm cells are male sex cells. They are made inside the male reproductive
system in structures called the testes.
Sperm cells are specialised for joining, or fusing, with
the female sex cell, the egg. Sperm cells have a tail, called a
flagellum, which they use to swim. Because sperm cells can
move, we say that they are motile. There are large numbers
of mitochondria in the middle region of a sperm cell. These
mitochondria make energy to help the flagellum to move.
The flagellum enables the sperm cell to swim towards the
egg when it is inside the female reproductive system.
In the head of the sperm, there are special enzymes
inside the acrosome. These enzymes break down the
membrane around the female egg cell. The sperm nucleus
contains genetic material. The nucleus enters the egg
cell and fuses with the nucleus of the female egg during
fertilisation.

Topic 2: Organisation and maintenance of life

Egg cells
Egg cells are female sex cells. They are also called ova (singular = ovum).
They are produced in the ovaries of the female reproductive system.
Egg cells are large cells that have a large nucleus, and they can be seen
with the naked eye. The cytoplasm contains many nutrients. Each egg cell
has a special cell membrane that allows only one male sperm cell to pass
through it. There is genetic material in the egg’s nucleus. The egg and sperm
nuclei fuse to form a single cell,
called a zygote.

Exercise 6.2

1. Give the meaning of each of these words.
a) flagellum
b) motile
c) acrosome
d) zygote
2. a) What is the function of sperm cells?
b) List three ways in which sperm cells are suited to
their function.
c) Make a neat, labelled drawing of a sperm cell.
3. a) What is the function of egg cells?
b) What happens when a sperm cell meets an egg cell?
c) How is the egg cell suited to its function?

Unit 6: Levels of organisation in multicellular organisms

Unit 6: Levels of organisation in multicellular organisms

Xylem cells
Xylem cells are sometimes called xylem vessels. They transport water
up from the roots to the other parts of the plant.
Xylem cells are non-living cells. The end of each cell has holes in
its wall and the cells are arranged one on top of the other, to form a
long narrow tube. These tubes, rather like long straws, are ideal for
transporting water.
Phloem cells
Phloem is a living tissue. It transports food made by the leaves during
photosynthesis to all parts of the plant. Figure 6.8 shows the difference
between the xylem and phloem. In the xylem, water flows in one
direction only from the roots to the leaves. In the phloem, the food
flows in both directions.
Mesophyll cells
Most cells inside a leaf are found between the upper and lower
surfaces. (see Figure 6.9). This region of the leaf is called the mesophyll.
Mesophyll cells are specialised for photosynthesis, which is their main
function. Look at the diagram below, which shows the inside of a leaf.
There are two types of mesophyll cells and they are found in
different places in the mesophyll. These cells are called palisade
mesophyll cells and spongy mesophyll cells. Palisade mesophyll cells
are long, thin cells which are full of chloroplasts. They are found near
the upper surface of the leaf. It is easy for them to trap lots of light in
this position. Spongy mesophyll cells also have lots of chloroplasts and
they are loosely packed in the leaf. This makes it easy for gases, such as
carbon dioxide, which is needed for photosynthesis, to move into and
out of these cells.

Topic 2: Organisation and maintenance of life

Unit 6: Levels of organisation in multicellular organisms 63

Exercise 6.3
1. Draw a mind map to show what you have learnt about these animal
cells: ciliated cells, red blood cells, nerve cells, sperm cells, egg cells.
Refer to pages 59 to 61 for help.
2. Draw a mind map to show what you have learnt about these plant
cells: root hair cells, xylem cells, mesophyll cells. Refer to pages 61
and 62 for help.

Experiment 6.1
Work in groups.
You will need: a microscope; prepared slides or micrographs
Procedure
1. Use a microscope to look at the prepared slides of different tissues
that your teacher will give you. Remember the rules for using a
microscope. If your school does not have slides, use the micrographs
that your teacher gives you.
2. Identify the tissues shown in the slides/micrographs. Decide which
slides/micrographs show plant tissue and which show animal tissue.
Discuss the reasons for your choice in your group.
3. Can you identify specialised cells in the tissue specimen?
4. Draw labelled biological diagrams of each tissue specimen in the
slides/micrographs.

Organisation in multicellular organisms
There are four levels of organisation in multicellular organisms such as
plants and animals: cells, tissues, organs and organ systems. An organism
is made up of many organ systems which enable it to function for life.
Cells
These are the most basic units of life. There are many different types
of cells in a multicellular organism. They are specialised to do a
certain function.

Unit 6: Levels of organisation in multicellular organisms

Tissues
In multicellular organisms, similar cells are grouped together to form
tissues. A tissue is a group of specialised cells that have a similar structure
and function. For example, muscle tissue is composed of muscle cells,
which function to help the animal move, by contracting and relaxing.
Examples of tissues in plant leaves are xylem, phloem and mesophyll.
As you have learnt, xylem is made up of xylem cells, which are able to
transport water. Phloem transports food in the plant. Xylem and phloem
are found in leaf veins. Mesophyll tissue is made up of mesophyll cells and
makes food.
Organs
Organs are structures that are made up of tissues. For example, your
stomach is an organ for digesting your food. A particular organ may
contain several different tissues. For example, your stomach contains
muscle tissue (for mixing up the food) as well as glandular tissue (which
produces digestive juices). Animal organs include the skin, heart, liver,
brain, lungs and kidneys. Roots, stems, flowers, leaves and fruits are plant
organs.
Organ systems
Organ systems are the highest level of organisation. A system consists
of several organs working together to perform a function of life. For
example, your digestive system consists of your stomach and your intestines
(amongst other organs), which function to digest your food. In plants, the
leaves, stems and roots work together to form a transport system.

  Topic 2: Organisation and maintenance of life

Figure 6.11 Levels of organisation of a multicellular plant organism: cell, tissue,
organ, organ system

Homework
1. Define each of these words.
a) cell
b) tissue
c) organ
d) organ system
2. Give an example for each of the words in question 1.
3. Make a simple drawing showing organisation in multicellular
organisms. Use Figure 6.10, on page 64, and Figure 6.11, above,
to help you.
4. Name two animal tissues and two plant tissues.

Advantages of specialisation of cells
Specialisation of cells occurs only in multicellular organisms and not in
unicellular organisms.
Cell specialisation gives these advantages to multicellular organisms.
• It enables them to grow bigger.
• It enables them to carry out complex processes. Different cells carry out
different functions.
• Specialised cells can work together to form tissues, organs and organ
systems.

Exercise 6.4
1. Arrange these structures in the correct order, starting with the
smallest.

organ	tissue	cell	organ	system organ

2. Give definitions for each of these words.
a) specialisation
b) permeable
c) zygote
d) motile

Topic 2: Organisation and maintenance of life

Checklist of learning
In this unit, I have learned that:
In multicellular organisms, cells are specialised to perform certain functions.
Examples of specialised cells in animals include ciliated cells, nerve cells, red blood cells, sperm cells
and egg cells; examples in plants include root hair cells, xylem cells and mesophyll cells.
A group of similar cells is called a tissue.
Plant tissues, such as mesophyll tissue and xylem and phloem tissue, consist of similar cells that are
structurally adapted to their functions.
Mesophyll tissue in the leaves contains cells with lots of chloroplasts, where the reactions of
photosynthesis take place; xylem tissue transports water around the plant.
Animal tissues, such as muscle tissue, consist of similar cells that work together to perform a
function.
Organs consist of many tissues that work together to do a certain function.
Animal organs include the skin, heart, liver, brain, lungs and kidneys; plant organs include roots,
stems, flowers, leaves and fruits.
An organ system consists of many organs working together, for example, the digestive system in
animals and the transport system in plants.
The specialisation of cells in multicellular organisms gives them advantages over unicellular
organisms.

Peer assessment

Complete the questions, and then swap your answers with a friend for marking.
1. What is another name for nerve cells?
2. Which organelles are found in high numbers in muscle cells? Why is this so?
3. Explain the functions of each of these cells: a) red blood cells b) xylem cells.
4. Look at the micrographs, and then answer these questions.
a) Identify each type of cell
shown in pictures A to D.
b) Identify each type of cell
as either an animal cell or
a plant cell.
c) Give one function for each
type of cell.
d) Explain how cell D is
suited to its function

A B

CD