How are gas exchange surfaces adapted in mammals, fish, insects and plants?
Adaptations for gas exchange: the features of an efficient exchange surface; surface-area-to-volume ratio; gas exchange in mammals, fish (counter-current flow), insects and plants; and ventilation.
A focused answer to the Eduqas Component 3 statement on gas exchange. Covers the features of an efficient exchange surface, surface-area-to-volume ratio, gas exchange in mammals, fish (counter-current flow), insects and plants, and ventilation.
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What this dot point is asking
Eduqas wants you to state the features of an efficient exchange surface, explain surface-area-to-volume ratio, and describe gas exchange in mammals, fish, insects and plants, including ventilation. This opens Component 3's core content.
Features of an efficient exchange surface
Surface-area-to-volume ratio
As an organism gets bigger, its volume rises faster than its surface area, so its surface-area-to-volume (SA:V) ratio falls. A small organism (or single cell) has a high SA:V, so its surface is large enough and diffusion distances short enough to supply all cells directly. A large organism has a low SA:V and long diffusion distances, so it needs specialised exchange surfaces and a transport system.
Gas exchange in mammals, fish, insects and plants
- Mammals: air is ventilated into the lungs and reaches the alveoli. Alveoli give a very large surface area, walls one cell thick (with thin capillary walls alongside) for a short diffusion distance, a moist lining, and a rich blood supply that maintains the gradient.
- Fish: water passes over the gills, where each filament carries lamellae with a large surface area. Counter-current flow (water and blood flowing in opposite directions) keeps the water beside the blood always richer in oxygen, so the gradient is maintained along the whole lamella.
- Insects: a tracheal system of tubes (tracheae and tracheoles) carries air directly to the tissues; spiracles open to the outside, and movements ventilate the system.
- Plants: gases diffuse through stomata (opened and closed by guard cells) into the air spaces of the spongy mesophyll, where the large internal surface area allows exchange with photosynthesising and respiring cells.
Ventilation
Ventilation maintains the concentration gradient by moving the external medium. In mammals, the diaphragm and intercostal muscles change the thorax volume and pressure to draw air in (inspiration) and push it out (expiration). In fish, the mouth and operculum work together to pass a continuous one-way flow of water over the gills.
Examples in context
Example 1. Why insects have a tracheal system, not lungs. Their small size and the tracheal tubes deliver oxygen straight to respiring cells without needing blood to carry it, which is one reason insect size is limited by how far air can diffuse down the tracheoles.
Example 2. Emphysema and surface area. In emphysema, alveolar walls break down, merging alveoli and reducing the surface area for gas exchange, so less oxygen diffuses into the blood, directly linking the alveolar adaptation to a disease.
Try this
Q1. State three features of an efficient gas exchange surface. [3 marks]
- Cue. Large surface area; thin (short diffusion distance); moist; a maintained concentration gradient (any three).
Q2. Explain why a large organism needs a specialised gas exchange surface. [2 marks]
- Cue. Its surface-area-to-volume ratio is small and diffusion distances long, so the body surface cannot supply all cells by diffusion alone.
Q3. State what is meant by counter-current flow in a fish gill. [1 mark]
- Cue. Water and blood flow in opposite directions across the lamella.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksExplain how the counter-current flow of water and blood in a fish gill makes gas exchange efficient.Show worked answer →
In a gill, water flows over the lamellae in the opposite direction to the blood flowing through them (counter-current flow).
This means that, along the whole length of the lamella, the water always has a higher oxygen concentration than the blood next to it.
So a diffusion gradient for oxygen is maintained along the entire length of the gill, not just at one end.
As a result, oxygen diffuses into the blood along the whole lamella, and the blood can become almost as saturated as the incoming water (much more than with parallel flow).
Markers reward water and blood flowing in opposite directions, the oxygen gradient being maintained along the whole length, and more oxygen diffusing into the blood than with parallel (concurrent) flow.
Eduqas 20214 marksExplain why a single-celled organism can rely on diffusion across its surface for gas exchange, but a large mammal needs specialised exchange surfaces and a transport system.Show worked answer →
A single-celled organism has a large surface-area-to-volume ratio, so its surface area is large enough relative to its volume for diffusion to supply all its cells.
Diffusion distances are also very short, so oxygen reaches all parts of the cell quickly.
A large mammal has a small surface-area-to-volume ratio, so its body surface is too small relative to its volume to supply all cells by diffusion alone.
Diffusion distances are also too long, so it needs specialised exchange surfaces (large area, thin) such as alveoli, plus a transport system to carry gases to and from the cells.
Markers reward the large SA:V ratio and short diffusion distance for the small organism, and the small SA:V ratio and long diffusion distance requiring exchange surfaces and a transport system for the mammal.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)