What does Edexcel GCSE Biology Topic 8 cover?

Topic 8 covers the need to transport substances, why large organisms need exchange surfaces and a transport system using surface area to volume ratio, how the alveoli are adapted for gas exchange, Fick's law, the structure and function of the blood, the blood vessels, the heart and circulation, aerobic and anaerobic respiration, and cardiac output.

Why do large organisms need exchange surfaces and a transport system?

Large organisms have a small surface area to volume ratio and long diffusion distances, so diffusion across the outer surface alone is too slow to supply their cells. They need exchange surfaces with a large surface area, such as the alveoli, and a transport system, the blood, to carry substances to and from every cell. Single-celled organisms have a large ratio and rely on diffusion.

How are the alveoli adapted for gas exchange?

The alveoli have a large total surface area (millions of air sacs), thin walls one cell thick for a short diffusion distance, a good blood supply to keep a steep concentration gradient, and a moist surface so gases dissolve. These adaptations make diffusion of oxygen into the blood and carbon dioxide out of it fast and efficient.

What is the difference between aerobic and anaerobic respiration?

Aerobic respiration uses oxygen, fully breaks glucose down to carbon dioxide and water, and releases a large amount of energy. Anaerobic respiration happens without oxygen and releases much less energy: in muscles it produces lactic acid, and in yeast and plants it produces ethanol and carbon dioxide.

How do you calculate cardiac output?

Cardiac output equals stroke volume multiplied by heart rate. Stroke volume is the blood pumped per beat and heart rate is the beats per minute, so cardiac output is the total volume pumped per minute. It rises during exercise as both stroke volume and heart rate increase.

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Edexcel GCSE Biology Topic 8 Exchange and transport in animals: a complete overview of exchange surfaces, the blood, the heart and respiration

A deep-dive Edexcel GCSE Biology guide to Topic 8 Exchange and transport in animals. Covers surface area to volume ratio and exchange surfaces, the alveoli, Fick's law, the blood and blood vessels, the heart and circulation, respiration, and cardiac output, with the core practical and exam patterns Edexcel repeats.

Generated by Claude Opus 4.814 min readTopic 8Updated 2026-06-02

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What Topic 8 actually demands
Exchange surfaces and gas exchange
The blood, vessels and heart
Respiration
How Topic 8 is examined
Check your knowledge

What Topic 8 actually demands

Exchange and transport in animals explains how substances get into, around and out of the body. The examiners test the surface area to volume argument, the adaptations of the alveoli, the structure-to-function of the blood, vessels and heart, the comparison of aerobic and anaerobic respiration, and cardiac-output calculations. This guide ties together the three dot-point pages, each with its own practice questions.

Exchange surfaces and gas exchange

Organisms must exchange oxygen, carbon dioxide, water and dissolved food with their surroundings. Small organisms manage by diffusion because they have a large surface area to volume ratio; large ones have a small ratio and long diffusion distances, so they need exchange surfaces and a transport system. The alveoli are adapted with a large surface area, thin walls, a good blood supply and a moist surface. Fick's law (Biology only) shows that the rate of diffusion rises with surface area and concentration difference and falls with diffusion distance.

The blood, vessels and heart

Blood has four parts: red blood cells (oxygen, via haemoglobin, no nucleus, biconcave), white blood cells (immune defence), plasma (the carrier liquid) and platelets (clotting). The three vessels are arteries (thick walls, high pressure, away from the heart), veins (thinner walls, valves, towards the heart) and capillaries (one cell thick, for exchange). The heart is a double pump: the right side sends blood to the lungs, the left side to the body. Cardiac output $=$ stroke volume $\times$ heart rate.

Respiration

Respiration is the exothermic reaction that releases energy from glucose, continuously, in all cells. Aerobic respiration uses oxygen, gives carbon dioxide and water, and releases a lot of energy. Anaerobic respiration gives less energy: lactic acid in muscles (with an oxygen debt), and ethanol and carbon dioxide in yeast and plants. The rate of respiration is measured with a respirometer in the core practical.

How Topic 8 is examined

Surface area to volume. Explaining why large organisms need exchange surfaces and transport, with simple ratio calculations.
Adaptations. How the alveoli, red blood cells, vessels and heart suit their functions.
Calculations. Cardiac output and surface area to volume ratios, plus Fick's law (Higher).
Comparisons. Aerobic versus anaerobic respiration.

Check your knowledge

A mix of recall and calculation questions covering Topic 8. Attempt them under timed conditions, then check against the solutions.

Explain why a large animal needs a transport system but a single-celled organism does not. (2 marks)
Give two adaptations of the alveoli for gas exchange. (2 marks)
State the function of platelets and of plasma. (2 marks)
Explain how a red blood cell is adapted to carry oxygen. (3 marks)
State the general rule for whether a blood vessel is an artery or a vein. (1 mark)
A heart has a stroke volume of $75\ cm^{3}$ and a heart rate of $72$ beats per minute. Calculate the cardiac output. (2 marks)
Compare the energy released and products of aerobic and anaerobic respiration in muscle. (3 marks)
Give the products of anaerobic respiration in yeast. (1 mark)

Solutions

Q1: A large animal has a small surface area to volume ratio and long diffusion distances, so diffusion alone is too slow; a single cell has a large ratio and short distances, so diffusion is enough.
Q2: Any two of: large surface area, thin (one-cell) walls, good blood supply, moist surface.
Q3: Platelets help the blood clot at a wound; plasma is the liquid that transports cells, dissolved food, carbon dioxide, urea and hormones.
Q4: It is packed with haemoglobin to bind oxygen, has no nucleus to leave more room for haemoglobin, and has a biconcave shape giving a large surface area for fast diffusion. Any three.
Q5: Arteries carry blood away from the heart; veins carry blood towards the heart.
Q6: Cardiac output $= 75 \times 72 = 5400\ cm^{3}$ per minute ( $5.4$ litres per minute).
Q7: Aerobic uses oxygen, gives carbon dioxide and water, and releases a large amount of energy; anaerobic happens without oxygen, gives lactic acid, and releases much less energy.
Q8: Ethanol and carbon dioxide.

Sources & how we know this

Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)