3.3.1 Surface area to volume ratio

Surface area to volume ratio is the geometric constraint that decides how an organism's body size shapes the way it exchanges substances with its environment, and the way it loses heat. The ratio falls as size grows; biology has to work around it.

Surface area grows with the square of size; volume grows with the cube.

The ratio is geometric, not biological. Any three-dimensional object's volume increases with the cube of its linear dimension, while its surface area increases only with the square. The consequence is direct: surface area to volume ratio always falls as size increases. Biology does not get to negotiate this; it has to work around it.

Write larger SA:V or smaller SA:V with the comparison spelled out. Large SA:V alone misses the comparative mark when two organisms are named in the question.

Single-celled organisms exchange substances directly across the body surface.

A single-celled organism has a high surface area to volume ratio, and the plasma membrane functions as the exchange surface. Oxygen and respiratory substrates enter by diffusion across the membrane; carbon dioxide and waste leave by the same route. No specialised exchange structures are required.

Larger organisms cannot supply every cell by surface diffusion alone.

Two changes converge as body size grows. The ratio falls, so the outer surface shrinks relative to the interior volume it contains. And the diffusion pathway from the environment to internal cells lengthens — distances that diffusion alone cannot bridge fast enough to sustain respiration. Even a fully permeable outer surface would not be enough.

For aquatic or thin-bodied organisms that survive without a transport system, the credited answer is short diffusion pathway to all cells. Pores and stomata score zero.

Smaller bodies lose heat faster, so they respire at a higher rate to stay warm.

The heat-loss chain is the topic's most heavily credited sequence. A smaller organism has a larger surface area to volume ratio; heat is lost faster through that proportionally large surface; respiration must run faster to replace the heat and keep body temperature constant.

1. The smaller organism has a larger surface area to volume ratio than the larger organism. State the comparison; do not state the ratio in isolation.
2. More heat is lost per gram of body mass, so the rate of heat loss is faster across the proportionally larger surface.
3. A higher rate of respiration releases heat to replace what is lost, so the smaller organism maintains a constant body temperature.

Write more heat lost per gram or faster rate of heat loss. Heat is lost more easily, without a rate or quantity dimension, is ignored.

Write releases heat or releases energy. Don't write produces heat or produces energy — both are rejected wherever they appear.

Stopping after the heat-loss step is the most consistent mark drop in this topic. The chain is three steps; the third — respiration releasing heat to maintain body temperature — is what AQA credits as the metabolic-rate consequence.

Pitfall — The two chains share the opening step only

The two chains share the opening step. They do not share the rest.

The metabolic-rate chain runs through heat loss and finishes on respiration releasing heat to maintain body temperature. The specialised-exchange chain runs through diffusion pathway and finishes on the need for an internal exchange surface. Writing diffusion pathway into a metabolic-rate answer, or heat loss into a specialised-system answer, scores zero on the affected mark.

Read the question's framing first. Body temperature, metabolic rate, oxygen consumption, two warm-blooded organisms compared — heat-loss chain. Advantage of a specialised system, thin-bodied organism, why a flat-bodied animal survives without lungs — diffusion-pathway chain.

An efficient exchange surface has three features.

Fick's principle states that the rate of diffusion rises with surface area and concentration gradient, and falls with diffusion pathway length. An efficient exchange surface follows directly from those three terms: large area, short pathway, and a gradient kept steep.

Diffusion pathway is the credited phrase for the distance substances must cross. The mark-scheme entry counts it as a discrete term.

Key terms

• surface area to volume ratio
• ratio
• surface area
• volume
• metabolic rate
• heat loss
• releases heat
• diffusion pathway
• respiration
• larger SA:V