Lesson Summary

This lesson introduces the structure of plant and animal cells and explains the functions of their major organelles. Learners will identify key cell components such as the cell membrane, nucleus, cytoplasm, mitochondria and ribosomes. The lesson also develops practical laboratory skills by guiding students to observe plant and animal cells under a microscope using appropriate staining techniques.

Understanding cell structure is fundamental in Biology because cells are the basic units of life and perform essential processes required for the survival of organisms.

Key Concepts

Cell

Cell organelle

Cell membrane

Cell wall

Cytoplasm

Nucleus

Mitochondria

Ribosomes

Chloroplast

Sap vacuole

Microscopy

Staining techniques

Notes

1. The Cell as the Basic Unit of Life

A cell is the smallest unit of life capable of carrying out all essential biological functions.

All living organisms are made up of cells.

Cells perform activities such as:

• respiration

• growth

• reproduction

• transport of substances

• response to environmental stimuli

Cells arise from pre-existing cells through cell division.

Cells may exist as:

• unicellular organisms (e.g. protozoa)

• multicellular organisms (e.g. plants and animals)

2. Structure of an Animal Cell

Animal cells consist of several structures known as organelles, each performing a specific function.

Cell Surface Membrane

The cell surface membrane surrounds the cell.

Functions:

• controls movement of substances into and out of the cell

• maintains the internal environment of the cell

• protects the cell from external damage

The membrane is selectively permeable, meaning it allows some substances to pass while restricting others.

Cytoplasm

The cytoplasm is a jelly-like substance that fills the cell.

Functions:

• contains enzymes that control metabolic reactions

• supports organelles

• allows transport of materials within the cell

Most chemical reactions in the cell occur in the cytoplasm.

Nucleus

The nucleus is usually the largest organelle in the cell.

Functions:

• controls the activities of the cell

• contains genetic material (DNA)

• regulates cell division

• directs protein synthesis

Because of this role, the nucleus is often described as the control centre of the cell.

Mitochondria

Mitochondria are rod-shaped organelles responsible for aerobic respiration.

Functions:

• release energy from glucose

• supply energy needed for cell activities such as movement, growth and active transport

Cells that require more energy contain large numbers of mitochondria.

Ribosomes

Ribosomes are small particles found in the cytoplasm.

Functions:

• site of protein synthesis

• produce enzymes and structural proteins required by the cell

Although small, ribosomes are essential for cell survival.

3. Structure of a Plant Cell

Plant cells contain most organelles found in animal cells but also possess additional structures.

Cell Wall

The cell wall is a rigid outer layer surrounding the cell membrane.

Functions:

• provides structural support

• maintains the shape of the cell

• prevents the cell from bursting when water enters

The cell wall is mainly composed of cellulose.

Chloroplasts

Chloroplasts are green organelles found in plant cells.

Functions:

• contain chlorophyll

• carry out photosynthesis

Photosynthesis allows plants to produce glucose using:

• carbon dioxide

• water

• sunlight

Sap Vacuole

The sap vacuole is a large fluid-filled sac.

Functions:

• stores water, nutrients and waste materials

• maintains turgor pressure which helps keep the plant upright

4. Comparison Between Plant and Animal Cells

These differences reflect the different functions of plants and animals.

5. Microscopic Examination of Cells

Cells are too small to be seen with the naked eye. They are studied using microscopes.

Students may examine:

• plant cells such as onion epidermis

• animal cells or protozoa

Staining Techniques

Stains are used to make cell structures easier to see.

Examples:

Staining improves visibility of structures such as the nucleus and cytoplasm.

Practical Observation of Cells

Steps when observing cells under a microscope:

1. Prepare a thin specimen (e.g. onion epidermis).

2. Place the specimen on a microscope slide.

3. Add a drop of stain.

4. Cover with a cover slip.

5. Observe under low magnification.

6. Adjust focus to view cell structures clearly.

Learners should be able to identify structures such as:

• cell wall

• cell membrane

• nucleus

• cytoplasm

• chloroplasts

Exam Technique

If asked to:

Identify cell structures

Label organelles clearly and accurately on diagrams or photomicrographs.

Describe functions

State the organelle and link it directly to its biological function.

Example:

“Ribosomes are the site of protein synthesis.”

Describe practical procedures

Mention:

• preparation of slide

• staining technique

• use of microscope

Avoid vague statements such as “look at the cells”.

Common Mistakes

Confusing the cell membrane with the cell wall

Saying mitochondria “produce energy” instead of releasing energy through respiration

Forgetting that ribosomes synthesize proteins

Failing to describe the role of staining in microscopy

Listing cell structures without explaining their functions

EXAM PRACTICE

Section A – Core Understanding

1. Identify two structures found in both plant and animal cells.

2. State the function of the cell surface membrane.

Section B – Application & Explanation

3. Explain the function of mitochondria in a cell.

4. Explain two differences between plant and animal cells.

Section C – Evaluation

5. “The nucleus is the most important organelle in a cell.”

Discuss this statement.

Examiner Commentary & Answer Guidance

Question 1

Strong answers should correctly identify structures such as:

• nucleus

• cytoplasm

• cell membrane

• mitochondria

Incorrect answers often include plant-only structures such as chloroplasts.

Question 3

High-quality answers should:

• identify mitochondria as the site of aerobic respiration

• explain that energy is released from glucose

• link energy to cell activities

The command word Explain requires linking structure to function.

Question 5

High-level responses should:

• recognise the nucleus controls cell activities

• explain that other organelles such as mitochondria and ribosomes are also essential

A balanced judgement must be provided.