The features of efficient gas exchange surfaces, and gas exchange in humans, fish, insects and plants.

What this dot point is asking

WJEC wants you to state the features that make a gas exchange surface efficient and explain how these features appear in the gas exchange systems of humans, fish, insects and plants, and to use surface area to volume ratios.

Features of an efficient surface

As an organism gets larger, its volume (and so its oxygen demand) rises faster than its surface area, so the surface area to volume ratio falls. A single-celled organism can rely on diffusion across its membrane, but a mammal cannot, which is why large animals evolved dedicated gas exchange organs linked to a transport system.

Humans and fish

In humans, air passes through the trachea, bronchi and bronchioles to the alveoli. The hundreds of millions of alveoli give a surface area of around $70 \text{ m}^2$, each lined by a thin squamous epithelium one cell thick and wrapped in capillaries, with breathing and blood flow keeping the gradient steep.

If the flow were parallel (same direction), blood and water would reach the same oxygen concentration partway along and diffusion would stop, so counter-current flow is much more efficient.

Insects and plants

Insects have a system of air-filled tracheae that branch into fine tracheoles carrying oxygen directly to respiring tissues, bypassing the blood. Spiracles open and close to control water loss, and abdominal pumping movements ventilate the larger tubes during activity.

Plants exchange gases through stomata on the lower leaf surface, opened and closed by guard cells. By day, carbon dioxide diffuses in for photosynthesis and oxygen diffuses out; the leaf's spongy mesophyll air spaces and large internal surface area aid diffusion. Closing the stomata reduces water loss but also limits gas exchange and photosynthesis, a constant trade-off.

Examples in context

Example 1. Emphysema and lost surface area. In emphysema the alveolar walls break down, merging many small alveoli into fewer large ones. This sharply reduces total surface area for diffusion, so the patient becomes breathless even at rest, a real clinical illustration of why large surface area matters for gas exchange.

Example 2. Insect spiracles and water conservation. Desert insects keep their spiracles closed for long periods and open them only briefly in bursts (discontinuous gas exchange). This minimises water loss from the tracheal system while still allowing enough oxygen in, showing the trade-off between gas exchange and water balance that also appears in plant stomata.

Try this

Q1. State three features of an efficient gas exchange surface. [1 mark]

• Cue. Large surface area, short diffusion distance (thin), and a maintained steep concentration gradient.

Q2. Explain why counter-current flow is more efficient than parallel flow in fish gills. [2 marks]

• Cue. Counter-current keeps a diffusion gradient along the whole lamella; parallel flow reaches equilibrium so diffusion stops partway across.

Q3. Explain why a single-celled organism does not need a specialised gas exchange surface. [2 marks]

• Cue. It has a large surface area to volume ratio and a short diffusion distance, so diffusion across its membrane supplies enough oxygen for its small volume.