Biology · Lesson 2

The cell: life's building block

JAMBWAECNECO

What the syllabus asks

JAMB: Living organisms: the cell, its structure and functions, cell as a living unit

WAEC: Classification and organisation of life; the cell: cell structure and functions of organelles, plant vs animal cells, cell theory, levels of organisation, cell properties (diffusion, osmosis, active transport), cell division (mitosis and meiosis)

NECO: Classification and organisation of life; the cell: cell structure and functions of organelles, plant vs animal cells, cell theory, levels of organisation, cell properties (diffusion, osmosis, active transport), cell division (mitosis and meiosis)

What you will learn

State the cell theory and describe levels of organisation.
Identify organelles and state their functions.
Compare plant and animal cells.
Explain diffusion, osmosis and active transport with examples.
Describe turgidity, plasmolysis and haemolysis.
Outline mitosis and meiosis and their significance.

Key points

Cell theory: all organisms are made of cells; the cell is the unit of life; cells come from cells.
Organisation: cell → tissue → organ → system → organism.
Key organelles: nucleus (control), mitochondria (respiration), ribosomes (proteins), chloroplasts (photosynthesis), membrane (selective entry).
Plant cells: cellulose wall, chloroplasts, large vacuole; animal cells: centrioles, glycogen.
Diffusion: high → low, passive; osmosis: water through a selectively permeable membrane towards the concentrated side.
Hypotonic: turgor (plants) or haemolysis (animals); hypertonic: plasmolysis or crenation.
Active transport uses ATP against a gradient; mitosis gives 2 identical diploid cells, meiosis 4 different haploid gametes.
Unicellular organisms (Amoeba, Euglena, Paramecium) show that one cell can perform all life processes; the contractile vacuole removes excess water (osmoregulation).
Surface area to volume ratio falls as size increases, which is why cells stay small and large organisms need exchange surfaces.

Start here: Discovering the cell

In 1665, the English scientist Robert Hooke looked at a thin slice of cork through his home-made microscope. He saw tiny box-like compartments and called them "cells", after the small rooms in a monastery. What he saw were actually the empty walls of dead plant cells.

Over the next two centuries, better microscopes showed that every plant and animal is made of cells. By 1839, Schleiden (studying plants) and Schwann (studying animals) proposed the cell theory, and Virchow later added that new cells only come from existing cells.

Cell theory: (1) All living things are made of one or more cells. (2) The cell is the basic unit of structure and function in living things. (3) All cells come from pre-existing cells.

Think of a house in your area. The cell is like a single block. One block can't make a house, but every wall, room and storey is built from blocks. Similarly, every tissue, organ and organism is built from cells. A bacterium is one cell; your body has about 37 trillion.

Levels of organisation

Cells → tissues (groups of similar cells doing one job, such as muscle tissue) → organs (different tissues working together, such as the heart or a leaf) → systems (organs working together, such as the digestive system) → organism.

Worked example 2.1

Arrange in order of increasing complexity: kidney, nephron cell, excretory system, kidney tissue, human.

Cell: nephron cell. Tissue: kidney tissue. Organ: kidney. System: excretory system. Organism: human.

nephron cell → kidney tissue → kidney → excretory system → human

Magnification

Total magnification = eyepiece magnification × objective magnification. With a ×10 eyepiece and a ×40 objective, the image is magnified ×400. Also: actual size = image size ÷ magnification.

Worked example 2.2

A cell appears 12 mm long in a drawing magnified ×400. Find its actual length in micrometres (1 mm = 1000 µm).

12 ÷ 400 = 0.03 mm.
0.03 × 1000 = 30 µm.

30 µm

⚠️ Common mistake: adding the eyepiece and objective magnifications (10 + 40 = 50). They multiply.

Try it yourself. Which organism is a single cell: Amoeba, a mango tree, or a grasshopper?

Show solution

Amoeba (unicellular).

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Practice questions

1. Which organelle is the site of aerobic respiration?

Mitochondrion
Ribosome
Golgi body
Nucleolus

Show answer

A: Mitochondrion
Mitochondria are the "powerhouse" where ATP is made.

2. Which structure is found in plant cells but not animal cells?

Cell membrane
Mitochondrion
Cytoplasm
Cellulose cell wall

Show answer

D: Cellulose cell wall
Plant cells have a cellulose wall, chloroplasts and a large vacuole.

3. Protein synthesis takes place in the

vacuoles
centrioles
ribosomes
lysosomes

Show answer

C: ribosomes
Ribosomes assemble amino acids into proteins.

4. A red blood cell placed in distilled water will

become plasmolysed
swell and burst
shrink
remain the same

Show answer

B: swell and burst
Water enters by osmosis (higher water potential outside); animal cells have no wall, so they burst (haemolysis).

5. The movement of molecules from a region of lower to higher concentration using energy is

active transport
diffusion
osmosis
plasmolysis

Show answer

A: active transport
Moving against the gradient needs energy (ATP): active transport.

Key words

Cell: The smallest unit of life that can carry out all life processes.
Organelle: A specialised structure inside a cell, such as the nucleus or a mitochondrion.
Cytoplasm: The jelly-like material inside the cell membrane where many reactions occur.
Cell membrane: A thin, selectively permeable boundary controlling what enters and leaves the cell.
Cell wall: A rigid outer layer of cellulose in plant cells.
Nucleus: The organelle containing chromosomes (DNA) that controls cell activities.
Mitochondrion: The site of aerobic respiration; releases energy as ATP.
Chloroplast: The organelle containing chlorophyll; the site of photosynthesis.
Ribosome: A tiny organelle where proteins are made.
Vacuole: A fluid-filled sac; large and permanent in plant cells.
Diffusion: The net movement of particles from a region of higher to lower concentration.
Osmosis: The diffusion of water through a selectively permeable membrane from a dilute to a concentrated solution.

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