Why do big organisms need exchange surfaces when single cells can rely on diffusion alone?
Surface area to volume ratio and how it changes with size, why large organisms need specialised exchange surfaces and transport systems, and the adaptations of exchange surfaces such as alveoli, villi and root hairs.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on surface area to volume ratio and exchange surfaces, covering how the ratio changes with size, why large organisms need exchange surfaces and transport systems, and the adaptations of alveoli, villi and root hairs.
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What this dot point is asking
OCR wants you to calculate and compare surface area to volume ratios, explain why large organisms need exchange surfaces and transport systems, and describe the adaptations of exchange surfaces such as alveoli, villi and root hairs.
Surface area to volume ratio
Every cell needs to take in oxygen and nutrients and remove waste. These move by diffusion across the surface. Whether the surface is big enough depends on the surface area to volume ratio: the surface area compared with the volume it has to supply.
As an organism gets bigger, the volume increases faster than the surface area. So a large organism has a low surface area to volume ratio, while a small organism has a high one. You can show this with cubes: the bigger the cube, the smaller its ratio.
Why size matters
A single-celled organism is tiny and has a high surface area to volume ratio. Diffusion across its surface is fast enough to supply all of it, because the distances are short.
A large multicellular organism has a low surface area to volume ratio, and many of its cells are deep inside, far from the surface. Diffusion across the outer surface alone would be far too slow to supply them. So large organisms need:
- Specialised exchange surfaces to take in substances (lungs for oxygen, intestines for nutrients).
- Transport systems to carry substances around the body (the blood and circulatory system; xylem and phloem in plants).
Adaptations of exchange surfaces
Efficient exchange surfaces share key features that speed up diffusion. Learn the three core ideas: large surface area, short diffusion distance, and a maintained concentration gradient.
- Alveoli (lungs). A huge total surface area (millions of tiny air sacs); walls one cell thick (short distance); a rich supply of capillaries (maintains the gradient); moist surfaces so gases dissolve. Ventilation keeps fresh air flowing.
- Villi (small intestine). Many finger-like folds give a large surface area; thin walls; a good blood supply to carry absorbed nutrients away.
- Root hairs (plants). Long thin extensions give a large surface area for absorbing water and mineral ions; a thin cell wall and membrane; a large number of them.
The reason these features work comes straight from the factors affecting diffusion. A larger surface area means more particles can cross at once. A thinner surface means a shorter distance for particles to diffuse, which is faster. A good blood supply (or ventilation in the lungs) carries the diffusing substance away on the far side, so the concentration there stays low and the concentration gradient stays steep, which keeps diffusion fast. If the gradient were allowed to fall, diffusion would slow down, so maintaining it is just as important as the surface area and the thinness.
This is why the lungs combine a large surface area of alveoli with constant ventilation (breathing in and out): breathing in brings fresh, oxygen-rich air to the alveoli, and breathing out removes the carbon dioxide, so the gradients for both gases are kept steep. In the same way, the constant flow of blood through the capillaries around the alveoli carries oxygen away and brings carbon dioxide, maintaining the gradients on the blood side too.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksExplain why a single-celled organism can rely on diffusion across its surface to supply its needs, but a large animal cannot. Use the idea of surface area to volume ratio.Show worked answer →
A B2 explanation worth 4 marks.
A single-celled organism is very small, so it has a large surface area compared with its volume (a high surface area to volume ratio). Diffusion across its surface is fast enough to supply all its cells with oxygen and nutrients and to remove waste, because the distances are short.
As an organism gets larger, its volume increases faster than its surface area, so the surface area to volume ratio falls. The surface is now too small, and the distances too great, for diffusion alone to supply every cell quickly enough. So large animals need specialised exchange surfaces (such as lungs) and transport systems (such as blood) to move substances. Markers reward the link from a falling ratio to the need for exchange surfaces and transport systems.
OCR 20214 marksA cube has sides of 2 cm. Calculate its surface area, its volume, and its surface area to volume ratio. Then state two features that make the alveoli in the lungs efficient exchange surfaces.Show worked answer →
A 4-mark calculation plus recall.
Surface area of a cube cm squared. Volume cm cubed. Surface area to volume ratio (or ). Reward all three values with correct working.
Two features of alveoli: a large surface area (many tiny air sacs); thin walls (one cell thick) for a short diffusion distance; a good blood supply (many capillaries) to maintain the concentration gradient; moist surfaces so gases dissolve. Any two are accepted.
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