Biology and Health Sciences

Key unit                                                                                  

To be able to differentiate between
animal and plant cells using a light microscope.

Cross-cutting issue
Standardisation
culture: When chemical products are purchased, they should be of a high standard or quality. It’s important that they will be disposed of or recycled to avoid harming our environment and human life in general.

At the end of this unit, you should be able to:
• Explain the role of a cell in a living organism
• Describe the structure of a plant and animal cells
• Identify the diff erent parts of the cell
• Explain the uses of the various structures seen under the light microscope in
the plant and animal cell
• Explain that cells with high rates of metabolism contain large numbers of
mitochondria for suffi cient energy
• Organize a science practical setup according to given instructions
• Prepare slides of human cheek cells and epidermal cells of an onion
• Explain the diff erences in the structure of plant and animal cells seen under a
light microscope
• Demonstrate that plant and animal cells diff er in shape
• Appreciate the importance of cells in organisms
• Show perseverance when observing slides of plant and animal cells
• Pay attention while handling delicate slides and sharp instruments to avoid
injury.

Oral activity:

In your groups, think about what you learnt in the previous unit and earlier
grades. Th en, brainstorm the answers to these questions.
1. Arrange these structures in the correct order, starting with the smallest
structure: tissue,  organ, cell,organism.
2. What different types of microscopes are available for biologists to use
to view cells?
3. How would a biologist choose the type of microscope he or she should use
to view a cell?

Figure 5.1 A plant cell, as seen under a high-power
                 microscope. 

Topic 2: Organisation and maintenance of life

The cell
Cells are the basic units of all living organisms. Cells are made of molecules
such water, proteins, carbohydrates and fats. Th ese molecules are made
up of atoms such as carbon, hydrogen and oxygen. Cells are the building
blocks of living organisms.
Some organisms, such as bacteria, are made up of only one cell. Th ese
are called unicellular organisms (‘uni’ means ‘one’). Other organisms,
such as humans and trees, are made up of many cells. Th ey are called
multicellular organisms (‘multi’ means ‘many’).
Because cells are too small to be seen with the naked eye, scientists
use microscopes to see the structure of cells. Sometimes they use powerful
microscopes called electron microscopes to view structures that are too
small to be seen with a light microscope.

Hint
Microscopes are useful for identifying organisms that cause
diseases in Rwanda

Structures found in plant and animal cells
All cells have the same basic structure. Cells contain cytoplasm and
organelles. Th e cytoplasm is a living, jelly-like substance in which the
organelles are found. Many chemical reactions take place inside the
cytoplasm. It is made mainly from water, in which substances such as
sugars and gases are dissolved. Th e organelles in the cytoplasm carry
out special functions inside the cell. Examples of organelles include the
nucleus, chloroplasts and mitochondria. A cell membrane surrounds the
cytoplasm.

Figure 5.2 shows typical plant and animal cells and their organelles.

Experiment 5.1
Work in groups.
You will need: a microscope; prepared slides of onion epidermis and
human cheek cells; drawing paper
Procedure
1. Look at Activity 4.1, on page 43, to remind yourself how to use a
microscope to view specimens.
2. View the prepared slides that your teacher will give you.
3. Make careful drawings of the two different types of cells using
Figure 5.3, below, to help you. Only draw the structures that you
can see on the slide.

Questions
1. Mention two things that you did to ensure that you avoided any
injuries whilst doing this activity.
2. How do the two types of cells differ in shape?
3. Which structures are present in the onion cells but absent in the
cheek cells?
4. What was the total magnification that you used to view
these specimens?

Figure 5.3 A micrograph of an onion epidermis (A) and human cheek cells
(B), as seen under a light microscope

Topic 2: Organisation and maintenance of life

Unit 5: Plant and animal cells 53

There are many different types of plant and animal cells. Although they all
have slightly different structures (depending on their functions), they all
have these organelles:
• cell membrane
• nucleus
• mitochondria
• vacuole
• cytoplasm.

Cell membrane
The cell membrane surrounds the cytoplasm and keeps the cell contents
in place. It is selectively permeable, which means that it can control the
movement of substances into and out of the cell.
Nucleus
The nucleus is the control centre of the cell. It is easily seen inside the cell.
The nucleus is round and found near the centre of an animal cell and on the
side of a plant cell.

Figure 5.4 The nucleus, as seen using an electron microscope,
is the control centre of the cell.

The nucleus plays an important role when the cell divides to make new
cells. It contains the hereditary, or genetic, information that is passed from
parents to their offspring during reproduction.

Unit 5: Plant and animal cells

Mitochondria
Mitochondria (singular = mitochondrion) are the organelles inside plant
and animal cells where the reactions of cellular respiration take place.
Energy that the cell can use is made in the mitochondria. Some cells,
depending on their function, have more mitochondria than others. For
example, muscle cells (which need a lot of energy to work) have several
thousand mitochondria.
Vacuoles
Vacuoles are organelles that usually contain fluid. They are surrounded by
a membrane. Animal cells have small vacuoles or no vacuoles. The fluid
in animal vacuoles often contains substances such as enzymes, or stored
food or waste products. Plant cells have large vacuoles that take up a lot of
space inside the cell. Plant vacuoles usually contain water and dissolved
substances, such as mineral salts and food molecules. Plant vacuoles are
important for keeping the cell rigid.
Activity 5.1
Work in pairs.
Use the library or the Internet to find out about the structure and the
functions of each of the following cell organelles: nucleus, mitochondrion,
ribosome, Golgi body, endoplasmic reticulum, lysosome and chloroplast.

Exercise 5.1
1. Define each of these terms:
a) unicellular
b) multicellular
c) organelle
2. Give one example each of a unicellular and a multicellular organism.
3. What is the function of each of the following?
a) the cell membrane
b) the nucleus
4. Which organelle in the cell makes energy?
5. a) What is sometimes stored inside vacuoles?
b) What is the difference between the vacuoles found in animal
cells and those found in plant cells?

Topic 2: Organisation and maintenance of life

Structures that are found only in plant cells
Two important structures are found in plant cells but not in animal cells:
cell walls and chloroplasts.
Cell wall
The cell wall is a rigid structure found around the outside of plant cells. It is
not living, and it is made up of a substance called cellulose.
The cell wall gives the plant cell its shape. Because the cell wall is rigid, it
protects the contents of the cell. It is fully permeable, which means that
substances can move freely into and out of the cell through the cell wall.
Chloroplasts
Chloroplasts are the organelles found in plant cells that carry out the
process of photosynthesis. They contain a green pigment called chlorophyll.
Chloroplasts are also found in the cells of algae and
some bacteria.

Exercise 5.2
1. a) What is the function of the cell wall in plants?
b) What substance is the cell wall made of?
2. a) What is the function of chloroplasts?
b) What pigment is found inside chloroplasts?

Differences in the structure of plant and animal cells
Although plant and animal cells have many features in common, there are
some obvious differences. Table 5.1 lists the main differences between plant
and animal cells.

Table 5.1 The main differences between plant and animal cells

Unit 5: Plant and animal cells

Homework

Copy this table, and then complete it.

Cells are the basic units of life; they are the building blocks of living organisms.
Some organisms consist of only one cell (unicellular), whilst other are made up of many cells
(multicellular).
We can view cells using either a light microscope or an electron microscope.
All cells contain cytoplasm and organelles such as a nucleus, mitochondria, a cell membrane and
vacuoles.
The cytoplasm is a jelly-like substance in which the organelles are found inside the cell.
The cell membrane surrounds the cytoplasm and controls what goes in and out of the cell.
The nucleus is the control centre of the cell and contains hereditary material.
Mitochondria are the organelles in the cell where energy is made; cells that need more energy, such
as those found in muscles, have more mitochondria.
Vacuoles are membrane-bound organelles that contain fluid; plants have large vacuoles and animals
have no vacuoles or small vacuoles.
Only plant cells have a cell wall, which gives the cell a rigid shape and allows substances to pass in
and out of the cell.
Chloroplasts are organelles in plant cells where the reactions of photosynthesis take place.

Self-assessment

1. Match the organelles in Column A with the correct function in Column B.

2. Give the meaning of each of these terms.
a) permeable   c) enzymes
b) hereditary   d) photosynthesis

3. Draw a table to show the differences between a plant and an animal cell. Include these headings: Shape, Outer covering, Organelles, Vacuoles.