SA:V ratio at Higher Level: The same statement as SL. At HL the ratio also explains heat loss in whole animals, and leads on to the ways cells raise their SA:V on the next page.
Practise this as you read
- Calculate SA:V for cube models.
- Explain and suggest effects of size on exchange and heat loss.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Start Studying Free Full access to Aimnova Pro · cancel anytime
The big idea: As a cell gets bigger its volume grows faster than its surface area, so the SA:V ratio falls and exchange cannot keep up.
The points to remember
- Materials enter and leave across the surface, so the rate of exchange depends on the surface area.
- The need for exchange (oxygen used, waste made) depends on the volume of the cell.
- When a cell doubles its length, its area is multiplied by 4 but its volume by 8, so SA:V halves.
- A large cell would not exchange materials fast enough, so cells stay small or divide.
- A cube is a simplified model of a cell: real cells are not cubes, but area and volume scale the same way.
Remember it as: Bigger cell, smaller ratio, slower exchange per unit of cell.
| For a cube of side L | Formula |
|---|---|
| Surface area | 6 × L × L |
| Volume | L × L × L |
| SA:V ratio | surface area ÷ volume |
Real example: a mouse and an elephant: A mouse has a much larger SA:V ratio than an elephant, so it loses heat across its skin far faster for its size. To stay warm, each gram of mouse uses energy much faster than each gram of elephant.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
Try Smart Study FreeYour first topic is free to keep • No credit card required
How it is asked: Expect a Paper 1A item on how SA:V and the rate of exchange change as a cell grows, a cube calculation, or a Paper 2 Explain or Suggest about size, heat or exchange.
IB-style questionExplain[3 marks]
An amoeba divides when it reaches about 0.5 mm across. Explain why cells divide when they reach a certain size.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.