The Cell — Science Sage

Cell Theory

The cell theory is one of the fundamental principles of biology. It states three things:

All living organisms are made of one or more cells.
The cell is the basic unit of life — the smallest unit that can carry out all life processes.
All cells come from pre-existing cells (cells only arise from other cells).

Unicellular organisms (like amoeba and bacteria) consist of a single cell. Multicellular organisms (like humans and trees) are made of trillions of specialised cells working together.

Parts found in ALL cells

Cell membrane — a thin, flexible layer that surrounds every cell. It controls what enters and exits the cell.
Cytoplasm — a jelly-like fluid that fills the cell and holds the organelles in place.
Nucleus — the control centre of the cell. Contains DNA, which carries the genetic instructions for the cell.
Mitochondria — the "powerhouse" of the cell. Releases energy from food through cellular respiration.
Ribosomes — tiny structures where proteins are made.

Plant cells vs Animal cells

Plant and animal cells share many features, but plant cells have three additional structures:

Structure	Plant cell	Animal cell
Cell membrane	✓	✓
Cytoplasm	✓	✓
Nucleus	✓	✓
Mitochondria	✓	✓
Ribosomes	✓	✓
Cell wall	✓ (rigid, made of cellulose)	✗
Chloroplasts	✓ (contain chlorophyll for photosynthesis)	✗
Large vacuole	✓ (stores water, keeps cell firm)	✗ (small or absent)

Memory trick: Plant cells have a Wall, Chloroplasts, and a big Vacuole (WCV)

Why are cells so small?

Cells are microscopic because they need a large surface area to volume ratio. A smaller cell can exchange materials (oxygen, nutrients, waste) across its membrane more efficiently relative to its size. If cells were too large, their centres would not receive enough oxygen.

Specialised cells have specific shapes that suit their function. Red blood cells are disc-shaped to carry oxygen. Nerve cells are long and thin to transmit electrical signals. Root hair cells have a large surface area to absorb water.

Answer in your exercise book. Draw and label diagrams where required.

Question 1 · (3 marks)

State the three statements that make up the Cell Theory.

Question 2 · (5 marks)

Draw and label a plant cell showing at least 5 structures. Name the function of each structure you label.

Question 3 · (3 marks)

List THREE structures found in a plant cell but NOT in an animal cell. Explain the function of each.

Question 4 · (2 marks)

Which organelle is called the "powerhouse" of the cell? Describe what it does.

Question 5 · (2 marks)

Explain why cells are microscopic in size. Use the term "surface area to volume ratio" in your answer.

Question 6 · (6 marks)

Four cube-shaped "model cells" have different side lengths. For a cube, surface area SA = 6 × side², and volume V = side³.

Side length	1 mm	2 mm	3 mm	4 mm
Surface area (mm²)	6	24	54	96
Volume (mm³)	1	8	27	64
SA : V ratio	6	3	2	1.5

(a) Describe the pattern in the SA:V ratio as the cube's side length increases. (2 marks)
(b) A real cell needs to absorb oxygen and nutrients through its surface (membrane) fast enough to supply its entire volume of cytoplasm. Using the table, explain why a cell with a side length of 4 mm would struggle to survive, while a cell with a side length of 1 mm would not. (2 marks)
(c) Explain how this pattern supports the idea that cells must stay microscopic in size. (2 marks)

The Cell — Unit of Life

Cell Theory

Parts found in ALL cells

Plant cells vs Animal cells

Why are cells so small?

Cell Explorer

Build-a-Cell