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54
6×3².
O Level · English Medium · Cambridge · Biology
Nutrition, transport and respiration
Use surface area, volume and diffusion distance to explain exchange.
Original Kidemy further-topic investigation. This optional lesson is shared by related upper pathways; select suitable depth with a teacher. It does not certify complete grade or board coverage.
Learn the idea
For similarly shaped objects, surface area grows with length squared and volume with length cubed. The surface-area-to-volume ratio therefore falls as size increases. Exchange also depends on the gradient, permeability and diffusion distance, not surface area alone.
Worked example
A cube of side 1 has surface6 and volume 1, ratio 6:1. At side 2, surface24 and volume 8, ratio 3:1. The bigger cube has more total surface but less surface per unit volume.
- Calculate ratios consistently.
- Connect structure to exchange.
- State a limitation of the cube model.
Your written practice
Show your working on paper and enter your answer here. If it does not match, compare the model and try again.
Show model answer and explanation
27
3³.
Show model answer and explanation
2
54/27.
Write in your own words. A guardian or teacher should review this response.
Show model answer and explanation
Its volume demand may increase faster than available outer exchange surface, and internal distances may be long.
Link geometry with demand and distance.
Write in your own words. A guardian or teacher should review this response.
Show model answer and explanation
Gradient, distance and permeability can differ, so surface alone does not determine the rate.
Name relevant conditions.
Apply the idea
Compare two sizes of a model organism and explain how folding could increase exchange surface.
Review with a guardian or teacher
- Use the stated evidence or givens.
- Show reasoning rather than only a result.
- Review and explain one correction.
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