SA:V Ratio, Body Size and Metabolic Rate
How the surface area to volume ratio explains heat loss, body shape and metabolic rate in animals, and why large animals need exchange systems. Includes the 4-mark exam question, a worked model answer and the examiner trap on small animals having a lower metabolic rate.
Chapters (8)
Exam question [4 marks]
A shrew is a very small mammal. Per gram of body mass, its metabolic rate is much higher than that of an elephant. Explain this difference, and explain why an elephant needs specialised exchange and transport systems.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | The shrew has a large surface area to volume ratio, so it loses heat quickly (to its surroundings) | "big SA:V", "large surface area compared with volume", "loses heat fast" | "shrew has a large surface area" with no reference to volume; "shrew is cold" |
| 2 | So it needs a high metabolic rate / respires faster to release heat and maintain body temperature | "more respiration to generate heat", "uses energy faster to stay warm" | "shrews are more active"; "shrew has a high metabolic rate" with no link to heat loss |
| 3 | The elephant has a small surface area to volume ratio, so diffusion alone is too slow / the diffusion distance is too long to supply all its cells | "long diffusion distance", "surface too small for its volume", "cannot meet needs by diffusion across the body surface" | "elephant has a small surface area" with no reference to volume; "elephant is too big to diffuse" with no reasoning |
| 4 | So it needs a specialised exchange organ (e.g. lungs) with a large surface area AND a mass transport system (e.g. circulatory system); both are needed for the mark, though an answer saying a blood system carries oxygen from an exchange surface to cells earns it | "lungs / gills", "blood system carries oxygen to cells" | "elephant has organs"; "heart" or "lungs" named with no function |
Marks 1 and 2 are a chain (ratio, then the response to heat loss); mark 2 needs the link to heat. Marks 3 and 4 are a chain too (ratio, then the adaptation). Do not credit "small animals are more active" or any answer saying small animals have a lower metabolic rate.
Model answer (4/4)
The shrew has a large surface area to volume ratio, so it loses heat to its surroundings quickly. It therefore needs a high metabolic rate, respiring faster to release heat and keep its body temperature constant. The elephant has a small surface area to volume ratio, so diffusion across its body surface is too slow to supply all its cells with oxygen. It therefore needs a specialised exchange organ such as lungs, with a large surface area, and a mass transport system such as the circulatory system to carry oxygen to its cells.
Examiner trap
- Saying small animals have a lower metabolic rate
- Not linking body shape to SA:V
Key terms
- metabolic rate
- body shape
- heat loss
- exchange system
- mass transport
- surface area to volume ratio
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:27Mouse and elephant compared
Quick reminder. A small body has a large surface area compared with its volume. A big body has a small surface area compared with its volume. As an animal gets bigger, the ratio falls. Keep that in your head, because everything today follows from it.
0:45Heat loss in small animals
First, metabolic rate. That is the rate at which an organism uses energy, and we can measure it by its rate of oxygen uptake or carbon dioxide produced. A small mammal has a large surface area for its volume. So it loses heat very quickly to its surroundings. To stay warm, it has to make heat just as fast, and it does that by respiring quickly.
So, per gram, a small mammal has a high metabolic rate, and that means it needs a lot of oxygen and a lot of food. It is not that small animals are busier. It is that they leak heat.
1:24Adaptations to body shape
Body shape matters too. Animals in cold places tend to be compact, with small ears and short limbs. That keeps the surface area to volume ratio low, so they lose less heat. Animals in hot places often have large ears and long, thin limbs. That raises the surface area to volume ratio, so they lose heat more easily.
1:48Why large bodies need systems
Now the second half. A large organism has a small surface area for its volume. Cells deep inside are a long way from the outside. Diffusion alone is far too slow over that distance, and the body surface is too small to take in enough oxygen for all that volume. So large animals cannot rely on the body surface. They need adaptations.
2:12Exchange organs in large animals
The first adaptation is a specialised exchange organ, like the lungs. Folded and branched, it gives a very large surface area, so enough oxygen can diffuse in. The second is a mass transport system, like the circulatory system. It carries oxygen from the lungs to every cell, and carries carbon dioxide back, much faster than diffusion could.
3:10Examiner trap and recap
The trap. Never say small animals have a lower metabolic rate. Per gram, theirs is higher. Recap. Bigger body, lower ratio, so special exchange and transport systems. Smaller body, higher ratio, so faster heat loss and a higher metabolic rate.
Next time, we see how two very different organisms exchange gases: a single cell, and an insect with its tracheal system.
Watch first
This lesson builds on:
- Surface Area to Volume Ratio Explained · AQA 7402 · 3.3.1
Keep going
More from 3.3 Exchange with the environment
- Surface Area to Volume Ratio Explained · AQA 7402 · 3.3.1
- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
- Fish Gills and the Counter-current Principle · AQA 7402 · 3.3.2
- Gas Exchange in Leaves: Mesophyll and Stomata · AQA 7402 · 3.3.2
All 10 3.3 Exchange with the environment lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-052.