Once a cell commits to a job, everything about it can change to suit that job — its shape, which organelles it keeps, even its size. But size has a hard limit, and that limit is the reason you are made of trillions of cells rather than one enormous one.
📚 What you need to know
In multicellular organisms, cells become specialised for specific functions. This is the division of labour.
Specialisation happens after fertilisation, by differentiation, so that different tissues can develop in the embryo.
Structural adaptations include the shape of the cell and the organelles it contains (or does not contain).
Cell size is itself an adaptation, and specialised cells vary enormously — from about 8 µm to 300 mm.
Metabolic requirements depend on volume; the rate of exchange depends on surface area.
As a cell gets bigger its surface area to volume ratio (SA:V) decreases, so exchange cannot keep up with demand.
This constrains cell size: growth must stop and the cell must divide, which gives rise to multicellular organisms.
NOS: models are simplified versions of complex systems — agar cubes model the effect of SA:V on diffusion.
Differentiation and the division of labour
A eukaryotic cell in a complex organism does not try to do everything. It becomes specialised for one job, and other cells specialise for theirs. Biologists call this the division of labour, borrowing the phrase from economics, and it works for the same reason: doing one thing well beats doing everything adequately.
The process starts after fertilisation, as the embryo develops different tissues. The development of these distinct specialised cells is called differentiation, and it works by switching different genes on and off. Every cell keeps the same DNA; what changes is which parts of it are used.
Two kinds of structural adaptation matter most:
The shape of the cell. A nerve cell is drawn out into a metre-long axon; a red blood cell is squashed into a disc.
The organelles it contains, or does not contain. A cell that makes large amounts of protein will have many ribosomes and extensive rough ER. A red blood cell throws away its nucleus entirely to make room for haemoglobin.
Whenever you are asked how a cell is adapted, run through the same three questions: what shape is it and why, which organelles are unusually abundant or missing and why, and what size is it and why. Nearly every mark scheme in this topic is built from those three.
Cells come in wildly different sizes
Each size is explained by a job. Small to squeeze through capillaries, long to carry a signal or exert a force, large to store food for an embryo.
Cell
Size feature
Why that helps
Red blood cell
Very small
Can move through narrow capillaries
Active white blood cell
Larger than an inactive one
Makes room for rough ER and Golgi apparatus to synthesise antibodies
Sperm cell
Long, with a narrow streamlined head
Movement towards the egg, with reduced resistance
Egg cell
The largest volume of any human cell
Stores food reserves for the early embryo
Nerve cell
Large cell body and a very long axon
The cell body supports protein synthesis; the axon carries impulses over distance
Muscle cell
Long and wide
Length and diameter allow it to exert force during contraction
Why cells cannot simply grow bigger
Here is the argument in one line: a cell’s demands depend on its volume, but its ability to meet them depends on its surface area. Those two do not grow at the same rate.
Metabolic reactions happen in the cytoplasm, so a cell with a larger volume has higher metabolic requirements.
All exchange with the outside — nutrients in, waste out — happens across the plasma membrane, so the rate of exchange depends on surface area.
Double the length of a cube and the surface area goes up four times, but the volume goes up eight times. The SA:V ratio falls.
Read the bottom row from left to right: 6, then 3, then 2. The ratio falls even though the surface area is growing.
What this means for real cells
Feature
A small cell (high SA:V)
A large cell (low SA:V)
Metabolic requirements
Relatively low
Higher, because there is more cytoplasm
Surface area available
Large enough for enough nutrients and gases in, and waste out
Not large enough for a sufficiently high rate of exchange
Diffusion distance
Short, so substances reach every organelle quickly
Long, so substances cannot reach the centre fast enough
Outcome
Can survive by simple diffusion at the cell surface
Growth must stop and the cell must divide
Single-celled organisms manage perfectly well because their SA:V ratio is high. Once cells cannot grow any further, the only route to a bigger organism is to have more cells — which is where multicellular organisms come from.
But being multicellular creates the same problem at a larger scale: the organism as a whole now has a poor SA:V ratio. Multicellular organisms have therefore evolved adaptations to get round it:
Gas exchange systems and digestive systems, which provide enormous internal surfaces for exchange with the environment
Circulatory systems, which transport substances efficiently within the body rather than relying on diffusion
The one-line version. The rate of metabolism depends on the mass or volume of the cell; the rate of exchange depends on the surface area. Write that sentence into any SA:V answer and the rest follows.
Modelling SA:V with agar cubes
This is the nature of science point for this sub-topic: models are simplified versions of complex systems. Scientists use models to represent things that cannot easily be investigated directly, and can then experiment on the model to test predictions.
Agar cubes are not shaped like cells and do not have membranes. But the scale factors behave in exactly the same way, so the effect on diffusion is real and measurable.
🧩 The agar cube method
Make up coloured agar containing Universal Indicator, or very dilute sodium hydroxide solution plus Universal Indicator, and let it set.
Cut it into cubes of the required dimensions, for example 0.5 cm, 1 cm and 2 cm along each side.
Calculate the surface area, volume and SA:V ratio of each cube and record them before you start.
Place each cube in a boiling tube containing the same measured volume of dilute hydrochloric acid. The acid must be at a higher molarity than the alkali so the colour change is visible.
Time how long each cube takes to change colour all the way through — or measure how far the colour change travels into the block in a fixed time, such as five minutes.
Convert the times to rates using rate = 1 ÷ time, then plot rate of diffusion against SA:V ratio.
Cube
Surface area / square cm
Volume / cubic cm
SA:V
Time / s
Rate / per s
A (0.5 cm)
1.5
0.125
12 : 1
176
0.0057
B (1 cm)
6
1
6 : 1
259
0.0039
C (2 cm)
24
8
3 : 1
384
0.0026
The conclusion writes itself: as the SA:V ratio of the cube increases, the rate of diffusion through it also increases. Cube C had the smallest ratio and the slowest rate; cube A had the largest ratio and the fastest rate.
Watch the units. Rate here is 1 ÷ time, so it is measured per second. A common slip is to plot the raw times instead of the rates — which gives a graph sloping the opposite way and a conclusion that says the reverse of what you mean.
Worked examples
WE 1
Calculating SA:V
A cube-shaped model cell has sides of 4 cm. Calculate its surface area to volume ratio. (2 marks)
Step 1: surface area
A cube has 6 faces, each 4 × 4 = 16, so the surface area is 6 × 16 = 96 square cm.
Step 2: volume
4 × 4 × 4 = 64 cubic cm.
Step 3: the ratio
96 ÷ 64 = 1.5, so the ratio is 1.5 : 1.
1.5 : 1always show both figures before dividing — a wrong answer with correct working still scores
WE 2
Explaining the limit on cell size
Explain why cells cannot continue to grow indefinitely. (4 marks)
Point 1: what demand depends on
Metabolic reactions happen in the cytoplasm, so metabolic requirements depend on the cell’s volume.
Point 2: what supply depends on
Exchange of nutrients, gases and waste happens across the plasma membrane, so it depends on surface area.
Point 3: the mismatch
As a cell grows, volume increases faster than surface area, so the SA:V ratio falls and exchange cannot keep pace with demand.
Point 4: the extra problem
The diffusion distance to the centre also becomes too long. Growth must therefore stop and the cell must divide.
demand grows with volume, supply grows with surface area, and volume wins4 marks means four separate points — do not merge steps 1 and 2 into one sentence
WE 3
Evaluating the model
Agar cubes are used to model the effect of SA:V ratio on diffusion into cells. Suggest one strength and one limitation of this model. (2 marks)
Strength
The scale factors are the same as in real cells, so the relationship between SA:V and rate of diffusion can be measured and tested directly.
Limitation
Agar cubes are not shaped like cells and have no plasma membrane, so they cannot model selective permeability or any active transport.
the geometry is realistic; the biology is not“suggest a limitation” wants a biological difference, not “the cubes might be cut unevenly”
💡 Exam tips
Say metabolism depends on volume, exchange depends on surface area. It is the sentence the mark scheme is built around.
Give two consequences of a low SA:V: not enough exchange, and too long a diffusion distance.
For SA:V calculations, show the surface area and the volume separately before dividing.
Remember that differentiation switches genes on and off; the DNA itself does not change.
Have named cell-size examples ready: the red blood cell is small to fit capillaries, the egg cell is large to store food.
In the agar practical, rate = 1 ÷ time. Plot the rate, not the time.
⚠ Common mistakes
Saying big cells have a small surface area. They have a large surface area — just not large enough relative to their volume.
Writing the ratio the wrong way round. It is surface area to volume, so it gets smaller as the cell grows.
Claiming differentiated cells lose genes. They keep the full genome; only expression differs.
Forgetting the diffusion distance. It is a separate problem from the ratio itself.
Saying specialisation happens before fertilisation. It happens after, as the embryo develops.
Treating a model as if it were the real thing. Agar cubes model geometry, not cell biology.
Up next: Specialised Cells — if a low SA:V ratio is the problem, some cells have evolved remarkable ways to push their own ratio back up.
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