CHAPTER TWO; CELLS

Activity 9.2: Observing animal cells

Materials: Wooden spatula, clean stick or scalpel, mounted needle, microscope slide and a cover slip, microscope, and a Bunsen burner

Procedure

1. Rinse your mouth with clean water several times in order to remove any food particles.

2. Sterilise the scalpel handle by passing it through a non-luminous flame several times.

3. Let it cool.

4. Gently scrap inside your cheek with the sterilised scalpel handle. A clean wooden spatula may be used instead of a scalpel handle.

5. Place the material you have obtained on a microscope slide.

6. Add a drop of water to the material.

7. Using forceps, a mounted needle, or sharp pencil point, carefully lower a cover slip over the material on the slide.

8. Use a low-power objective lens to observe the specimen under a microscope. 

9. Draw what you see. Compare the diagram below

Functions of the parts of an animal cell

Cell membrane

The cell membrane is a thin layer that encloses the whole cell. The cell membrane is made up of two layers of lipids and protein molecules. This membrane is flexible and semi- permeable. Semi-permeable means that it allows certain substances to pass in or out of the cell. Flexibility of animal cell membrane and the lack of cell wall gives it an irregular shape.

Cytoplasm

This is a jelly-like substance made up of water and dissolved chemical substances. The cytoplasm is the site for many chemical reactions in the cell. Cell organelles such as the vacuoles, nucleus, and mitochondria are suspended in the cytoplasm.

Nucleus

This is a spherical organelle suspended in the cytoplasm. The nucleus consist of nucleolus and a fluid called nucleoplasm. It is surrounded by a membrane called the nuclear membrane.

The functions of the nucleus are to:

(a) determine the chemical processes that take place in the cell;

(b) control the functions of all parts of the cell;

(c) determine the cell’s size, shape, and

functions; and

(d) determine the hereditary characteristics of a cell.

Cell vacuoles

Cell vacuoles are fluid-filled spaces bound by a membrane. Animal cells

have small and temporary vacuoles mainly used to secrete and excrete waste from the cell.

Mitochondria

Mitochondria (singular: mitochondrion) are oval-shaped organelles that have two membranes . The outer membrane is smooth. The inner membrane has folds called cristae. The role of mitochondria is to produce energy for the cell. That is why they are sometimes referred to as ‘power houses of cells’.

The plant cell

Similar to the animal cell, the plant cell has a cell membrane, cytoplasm, nucleus, and mitochondria. These organelles function in the same way as those of an animal cell. However, plant cells have additional structures which serve specific roles in plants.

 These structures are:

The cell wall

This is a strong covering made of cellulose surrounding the cell membrane. The cell wall is fully permeable. It allows the passage of water and minerals. The cell wall protect and support the cell. The cell wall gives the plant cell a definite shape.

Chloroplasts

Chloroplasts are oval organelles that contain green pigments in plants. This pigment is called chlorophyll. It is important in photosynthesis, the process by which green plants make their own food. Chlorophyll absorbs light energy needed for photosynthesis.

Cell vacuole

Plant cells have a large and permanent vacuole that usually occupies the central part of the cell. This vacuole contains sap and is surrounded by a membrane called tonoplast. The cell vacuole is filled with water, causing the cytoplasm to be pushed outwards against the cell wall. This makes the cell firm, hence helps plant cell to maintain their shape.

Activity 9.3: Observing the plant cell

Materials: Onion, knife, forceps, mounted needle, microscope, microscope slide, and cover slip

Procedure

1. Separate a fleshy leaf from an onion bulb.

2. Cut a small square of the leaf.

3. Using forceps, peel off the epidermis from the inner surface of the square.

4. Put it on a microscope slide, and add a drop of water.

5. Using forceps, a mounted needle, or sharp pencil point, carefully lower a cover slip over the epidermis specimen.

6. Use a low-power objective lens to examine the specimen under a microscope.

7. Draw what you see under the microscope

Similarities

Plant and animal cells are similar in that they both have a cell membrane, cytoplasm, a nucleus, cell vacuoles, and mitochondria

Differences

The differences between plant and animal cells are shown

Groups of Cells (Levels of Organisation) 

People in a group can perform more complex tasks than one person alone. Consider what happens in a school system. Just like a body, a school carries out different activities. In order for a school to function properly, there have to be lots of different types of people performing different functions 

Like people, similar cells in our bodies are organised into groups to make them work more effectively.

 - A group of similar cells performing a particular function is a tissue e.g. muscle tissue 

- A group of different tissues form an organ to perform a particular function e.g. heart - A group of different organs form an organ System to perform a particular function e.g. circulatory system - A group of different organ systems form an organism e.g. a human

Cell organisation at various levels carries out specific functions and key life processes in the body. This ensures efficient functioning of the body for the survival of the organism. For example, the reproductive cells (sperm and egg) fuse to develop into an organism that has tissues, organs and systems. 

The diagram below shows the different levels of organisation  in an organism 

Specialized cells

The following are examples of specialised animals cells:

White blood cells

White blood cells can change their shapes so as to engulf and destroy harmful microorganisms. Some contain digestive enzymes which destroy the microorganisms.

Red blood cells

Red blood cells lack nuclei and are bi-concave in shape. This provides a large surface area for transporting oxygen from the lungs to various parts of the body. They also contain haemoglobin which carries oxygen to different parts of the body.

The sperm cells

The sperm cell  fertilize the female egg in order for reproduction to take place. The sperm cell has a tail which enables it to swim to the egg and a pointed head which enables it to penetrate the egg.

Specialized plant cells

The following are examples of specialised plant cells:

Root hair cells

These cells absorb water and mineral salts from the soil. The root hair helps to increase the surface area for absorption.

Guard cell

Guard cells surround the stomata. The inner walls of guard cells are thicker than the outer walls. This makes them expand irregularly. The guard cells control the opening and closing of stomata (singular: stoma) on the leaf. When the guard cells expand, the stoma opens. When they contract, the stoma closes. Stomata are tiny pores used for gaseous exchange and loss of excess water.

Xylem vessels

Xylem vessels are made up of hollow dead cells with walls made of lignin. Lignin is a tough rigid material that makes up the wall of xylem vessels. The cells are connected to form xylem vessels. These vessels transport water and minerals from the root to the leaves and also support the plant in the form of wood. 

Palisade cells

Palisade cells found in plant leaves contain large amounts of chloroplasts. Chloroplasts are the sites for photosynthesis. They contain chlorophyll which traps sunlight energy during photosynthesis

Activity 2.2: Relating the structure of specialised cells to their function 

As earlier mentioned, some cells have special structures/features that enable them to carry out particular functions. In your body, a number of life processes take place, e.g. nutrition, reproduction and respiration. For some of the life processes to be performed, specialised cells are required.

Key questions 

1. What are the specialised cells in your body, and in a plant? 

2. How are they adapted to their functions? 

What you need 

i) Notebook 

ii) Pencil 

Four sets of cards: 

a. The first set is of pictures of five types of specialised cells 

b. The second set has the special features of each specialised cell 

c. The third set gives the functions of each specialised cell 

d. The fourth set has the names of each of the specialised cells 

1. Observe each specialized cell, match it with the correct statement on the card of the specialised feature and the card with the particular function. 

2. Select from the list of names, the one you think matches a particular specialised cell.

3. Record your work in the table below. Present your work to the rest of the class