Explain why a large surface area to volume ratio makes it more difficult for an organism to maintain a stable internal environment.

OCR A-Level Biology A (H420) — 5.1 Communication and homeostasis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Multicellular organisms have evolved complex communication and homeostatic systems. The need for such systems is linked, in part, to the relationship between an organism's size and its surface area relative to its volume.

Model answer (4 marks)

A large surface area to volume ratio means a greater relative surface area across which heat or substances can be lost or gained by diffusion or conduction.

Consequently internal conditions such as temperature, water potential and ion concentration change more rapidly or fluctuate more.

Therefore corrective homeostatic responses must be triggered more frequently or with greater magnitude to counteract deviations from the set point.

This requires a greater expenditure of energy or metabolic resources to maintain homeostasis, making it more difficult to sustain a stable internal environment.

Examiner tips

Use the exact phrase "surface area to volume ratio" and link it to diffusion/conduction. Show the chain: larger SA:V → faster loss/gain → rapid change in internal conditions → more frequent/stronger homeostatic responses → higher energy cost. Keep each point brief and directly tied to the marks.
Use bullet‑style or numbered points to match the four marks, ensuring each sentence covers one of the scheme points.

Common mistakes

Confusing a large SA:V ratio with a small one; students often state the opposite. Failing to mention the energy/metabolic cost of maintaining homeostasis. Using vague terms like "more changes" without linking to diffusion or conduction.

Mark scheme (4 marks)

A large SA:V ratio means a greater relative surface area across which heat or substances can be lost or gained by diffusion/conduction
Internal conditions (e.g. temperature, water potential, ion concentration) therefore change more rapidly / fluctuate more
This means corrective homeostatic responses must be triggered more frequently or with greater magnitude to counteract deviations from the set point
This requires a greater expenditure of energy / metabolic resources to maintain homeostasis, making it more difficult to sustain a stable internal environment

Key terms in this question

surface area to volume ratio · internal environment