2.2.1 Surface Area: Volume Ratio and Transport · 2. Scaling Up | Ocr Gateway GCSE Combined Science A J250: Biology

Section 1 of 9Exchange and size

Single-celled organism: gas exchange by diffusion (Amoeba)

Every living cell must exchange substances with its surroundings: oxygen and nutrients in, carbon dioxide and wastes out.

Exchange and size

For a single-celled organism such as an amoeba this is easy,

because substances can diffuse across the cell surface membrane to reach all parts of the cell quickly.

In a large multicellular organism most cells are far from the outside, so diffusion alone is far too slow to supply the cells in the middle.

The key idea is the surface area to volume ratio (SA:V). As an organism gets bigger its volume increases faster than its surface area, so the ratio gets smaller.

Section 2 of 9Calculating SA:V

For a cube of side length L: surface area = 6 x L x L and volume = L x L x L. Worked comparison:

Side length	Surface area	Volume	SA:V ratio
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1
3 cm	54 cm²	27 cm³	2 : 1

The cube that is three times longer has a ratio only a third as large. To simplify a ratio divide both numbers by the volume, giving a figure such as 6 : 1. A larger ratio means a greater area of surface for each unit of volume that needs supplying.

Section 3 of 9Why large organisms need special systems

Examples of Diffusion in Living Organisms

A multicellular organism has a small SA:V, a long diffusion distance and a high demand for oxygen and food because many cells are respiring. Diffusion across the outer surface cannot meet this demand. Large organisms therefore have two features:

Why large organisms need special systems

Specialised exchange surfaces

Specialised exchange surfaces with a large surface area,

such as alveoli in the lungs, villi in the small intestine, gills in fish and root hairs in plants.

Transport systems

Transport systems that carry substances between the exchange surfaces and the cells,

such as the circulatory system in animals and xylem and phloem in plants.

Flatworms and other flat organisms can manage without these because their thin, flat shape keeps the SA:V high and the diffusion distance short.

Section 4 of 9Adaptations of exchange surfaces

Surface Area : Volume — 3D Step-by-Step Simulation

An efficient exchange surface has a large surface area (folding or many small units), a thin membrane (often one cell thick) to make the diffusion distance short, a good blood supply to carry substances away and keep the concentration gradient steep, and sometimes ventilation to bring fresh air or water to the surface.

Adaptations of exchange surfaces

In the lungs, alveoli give a huge area, have walls one cell thick, are surrounded by capillaries, and are ventilated by breathing.

The small intestine is long and has villi with microvilli, each villus containing blood capillaries and a lacteal.

Section 5 of 9Applying SA:V to animals and plants

Transport systems in a flowering plant and the human body

SA:V helps to explain body shape.

1

Small animals such as mice lose heat quickly because of a large SA:V, so they eat a lot and have thick fur.

2

Large animals such as elephants have a small ratio, retain heat well, and use large ears to lose it.

3

Cold-climate animals tend to be compact with short limbs and ears.

4

In plants, flat leaves give a large area to absorb light and carbon dioxide, and the thin leaf gives a short diffusion path.

This topic links directly to gas exchange, digestion, and the circulatory system, which together keep cells supplied.

Section 6 of 9Worked reasoning and further examples

Consider two different blocks of agar jelly, one with side 1 cm and one with side 3 cm, placed in dilute acid.

1

The small block has a SA:V of 6:1 and the acid reaches its centre almost at once, so it changes colour right through very quickly.

2

The large block has a ratio of 2:1 and takes much longer.

3

This model shows why a cell that grows too large would starve in the middle: the surface cannot supply enough for the larger volume.

4

It is one reason why cells divide rather than grow indefinitely,

and why large organisms are built from many small cells rather than being one giant cell.

Shape also matters, not only size.

1

A long thin or flattened shape, like a leaf or a flatworm, gives a much higher ratio than a sphere of the same volume.

2

Organisms of the same size can therefore differ in how well they exchange materials.

3

Cells such as root hair cells and cells lining the gut change their shape by folding or elongating,

increasing the surface area without increasing the volume much.

4

When asked to calculate, always show the formulae, substitute the values, and give the ratio in its simplest form.

Section 7 of 9Exam-style questions

Small Intestine, Villus & Blood Capillary

Blood vessels linking heart, lungs and organs

Capillary bed and capillary wall one cell thick

Single-celled protist: gas exchange by diffusion

Calculate the SA:V ratio of a cube with sides of 4 cm. [3]3 marks
Explain why a large animal needs a circulatory system but a single-celled organism does not. [3]3 marks
Give two features of the alveoli that make gas exchange efficient. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Surface area to volume ratio	The surface area of an object divided by its volume; falls as size increases.
Diffusion distance	How far particles have to travel to cross an exchange surface or reach a cell.
Exchange surface	A specialised area where substances move between an organism and its surroundings.
Alveolus	A tiny air sac in the lung where gas exchange happens.
Villus	A finger-like projection in the small intestine that increases the area for absorption.
Multicellular	Made of many cells.
Concentration gradient	The difference in concentration across an exchange surface.

Section 9 of 9Exam technique

1

Saying that bigger organisms have a bigger SA:V; the ratio actually gets smaller as size increases.

2

Quoting only surface area, not the ratio, when comparing organisms of different size.

3

Writing the units of SA:V incorrectly or forgetting that area is cm² and volume cm³.

4

Describing diffusion distance as "length" of the organism rather than the distance between the exchange surface and the cell.

5

Saying that exchange surfaces are thin "to let more through" without linking to diffusion distance.