1.1.1Structure and Functions in Living Organisms: Part 2

The Need for a Transport System — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

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📚 Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Hub / Topical Revision

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Part 1 of 7Exchange in single-celled and multicellular organisms

All living cells need a supply of oxygen and nutrients such as glucose, and must remove waste products such as carbon dioxide and urea.

Diffusion

In a single-celled organism such as an amoeba, these substances move directly across the cell surface by diffusion, the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

This is enough because the cell is tiny: the distance to the centre is very short and the surface area is large compared with the volume.

Part 2 of 7Surface area to volume ratio

As an organism gets bigger, its volume increases much faster than its surface area, so the surface area to volume ratio decreases. Consider cubes: a cube with 1 cm sides has surface area 6 cm² and volume 1 cm³, a ratio of 6:1. A cube with 2 cm sides has surface area 24 cm² and volume 8 cm³, a ratio of 3:1. A cube with 3 cm sides has surface area 54 cm², volume 27 cm³, a ratio of 2:1.

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

A large organism has a surface that is too small compared with its volume to supply all the cells, and the cells deep inside are too far from the surface for diffusion alone to be fast enough.

Part 3 of 7Why large animals need transport systems

Multicellular animals therefore need two things:

Why large animals need transport systems

Specialised exchange surfaces

Specialised exchange surfaces with a large surface area,

such as the alveoli in the lungs and the villi in the small intestine,
where substances enter and leave the body.

Transport system

A transport system to carry substances between the exchange surfaces and all the cells of the body,

such as the circulatory system with blood as the transport medium.

Diffusion is a slow process over long distances: the time taken increases rapidly as the distance increases, so diffusion cannot supply the cells of a human body, which are centimetres or more from the surface.

Why large animals need transport systems

Blood carries oxygen from the lungs and glucose from the intestine to the cells, and carries carbon dioxide and urea away.

Active animals with a high metabolic rate need a particularly efficient system,

which is why mammals have a double circulation with a four-chambered heart.

Part 4 of 7Substances transported and examples across the animal kingdom

Substance	From	To
Oxygen	Lungs	All respiring cells
Carbon dioxide	Respiring cells	Lungs
Glucose, amino acids	Small intestine	All cells, liver
Urea	Liver	Kidneys
Hormones	Endocrine glands	Target organs
Heat	Active organs	Skin, around the body

The need for transport depends on size and activity.

A tiny flatworm has a flattened body so that no cell is far from the surface, and diffusion is enough.

An earthworm is longer but has a thin, moist skin and a simple blood system.

Insects use a network of tubes that carry air to the cells.

Fish have gills and a single circulation,

while mammals and birds, with high metabolic rates and a need to maintain body temperature, have a double circulation.

In each case the pattern is the same: the greater the demand and the lower the surface area to volume ratio, the more elaborate the transport system.

Part 5 of 7How transport meets the needs of cells

The blood must supply oxygen and glucose at a rate that matches the rate of respiration in tissues.

During exercise the muscles need much more, so the heart rate and breathing rate rise and blood is diverted to them.

Waste carbon dioxide must be removed because it dissolves to form an acid that lowers the pH of blood and interferes with enzymes.

Transport also distributes hormones from glands to target organs and carries heat from the organs that generate it,

which helps with temperature control.

These links show why the circulatory system is central to many topics in this unit.

Part 6 of 7Calculations and adaptations

Questions may ask for surface area to volume ratio of a shape, or compare animals.

Calculations and adaptations

A small mammal such as a mouse has a large surface area compared with volume,

so it loses heat quickly and must eat a lot,
whereas an elephant has a small ratio and keeps warm more easily,
which is why elephants have large ears to lose heat.

Flatworms have a flat shape that keeps diffusion distances short, so they do not need a circulatory system.

Simple organisms rely on diffusion; complex ones modify their shape internally, with folding and branching, to increase the surface available for exchange.

Plants have comparable issues and use xylem and phloem for transport.

Part 7 of 7Exam-style questions

Calculate the surface area to volume ratio of a cube with sides of 4 cm. Explain what happens to the ratio as organisms get larger. [3]3 marks
Explain why an earthworm can rely on diffusion for gas exchange but a human cannot. [3]3 marks
Name two substances carried in the blood from the small intestine and state where they are used. [2]2 marks

Key terms

Term	Meaning
Diffusion	Net movement of particles from higher to lower concentration.
Surface area to volume ratio	Surface area of an object divided by its volume.
Concentration gradient	Difference in concentration between two regions.
Exchange surface	Area specialised for exchanging substances with a large surface area and thin walls.
Circulatory system	System of vessels and heart that transports substances around the body.
Metabolic rate	Rate at which the body carries out energy-releasing reactions.

Avoid these mistakes

Saying "large animals have a smaller surface area". They have a smaller surface area to volume ratio, but a bigger total surface area.

Thinking diffusion needs energy. It is passive.

Confusing diffusion with osmosis; osmosis is the diffusion of water across a partially permeable membrane.

Stating that cells inside a large animal never get oxygen by diffusion; they do, but only across a short distance from capillaries.

Forgetting to say that transport systems also remove waste.

Last updated: 20 October 2026