Transport and exchange surfaces — Edexcel GCSE Biology
Why organisms must move substances in and out, why large animals need exchange surfaces and a transport system, and how alveoli work.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Substances must be transported into and out of an organism and its cells. Oxygen, dissolved food molecules, water and mineral ions are needed by cells, while carbon dioxide from respiration and urea are waste products that must be removed. In a large animal many cells are far from the outside, so blood transports these substances.
The surface area : volume ratio compares how much surface an organism has with its size, and it gets smaller as organisms get bigger. A single-celled organism has a large ratio, so diffusion across its surface is fast enough. Multicellular organisms have a small ratio, so they need exchange surfaces and a transport system.
Alveoli are tiny air sacs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out. Millions of alveoli give a very large surface area, and walls one cell thick give a short diffusion distance. A rich capillary network and ventilation keep a steep concentration gradient, and the moist lining helps gases dissolve.
Exam questions
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Quick check questions
Why must urea be transported out of the body's cells?
Show answer
It is a waste product that is carried in the blood to the kidneys and removed in urine.
Does a large animal have a larger or smaller surface area : volume ratio than a single cell?
Show answer
Smaller.
Why do multicellular organisms need a transport system?
Show answer
Many cells are far from the surface, so diffusion alone is too slow to supply them.
State two ways alveoli are adapted for gas exchange.
Show answer
Any two of: large surface area, walls one cell thick, moist lining, good blood supply, ventilation.
Frequently asked questions
How are alveoli adapted for gas exchange?
Alveoli are adapted for gas exchange by having a very large surface area, from millions of alveoli, and walls only one cell thick for a short diffusion distance. A rich network of capillaries and ventilation keep a steep concentration gradient, and a moist lining lets gases dissolve, which helps them to diffuse.
Why is the surface area to volume ratio important for cells?
The surface area to volume ratio matters because substances enter and leave across the surface. A single-celled organism has a large ratio and a short diffusion distance, so diffusion is fast enough. A large multicellular organism has a small ratio and many cells deep inside the body, so diffusion across its body surface alone would be far too slow.
How do you calculate surface area to volume ratio?
To calculate the surface area to volume ratio, divide the surface area by the volume and write the answer in the form x : 1. For a cube with 3 cm sides, the surface area is 6 × 9 = 54 cm² and the volume is 27 cm³, so the ratio is 54 ÷ 27 = 2 : 1.
What substances need to be transported in the body?
Cells need oxygen for aerobic respiration, dissolved food molecules such as glucose and amino acids for energy, growth and repair, and water and mineral ions. Waste products must be removed: carbon dioxide from respiration, and urea, which is carried in the blood to the kidneys. In humans, blood is the transport medium.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026