AQA A-Level Biology: Exchange and Transport in Organisms

Gas-exchange surfaces in animals share a common set of structural features that maximise the rate of diffusion.

Choose one of the following gas-exchange surfaces:

• the mammalian alveolus
• the fish gill (with counter-current flow)
• the insect tracheal system

Describe and explain how your chosen surface is adapted for efficient gas exchange. In your answer, link each adaptation to the factors in Fick's law:

$\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$rate of diffusion∝diffusion distancesurface area×concentration difference​

A student investigated how the percentage saturation of haemoglobin with oxygen varies with the partial pressure of oxygen (pO₂). Measurements were taken on two blood samples from the same mammal: one at a normal carbon dioxide concentration, and one at a higher carbon dioxide concentration.

The results are shown below.

pO₂ / kPa	Saturation, normal CO₂ / %	Saturation, higher CO₂ / %
1	8	4
2	26	14
3	57	35
4	80	60
6	92	82
9	98	96

(a) Describe how the results for the higher CO₂ concentration differ from those at the normal CO₂ concentration. (2 marks)

(b) Explain what causes this difference (the Bohr shift) and why it is an advantage at respiring tissues. (3 marks)

(c) At a respiring tissue the pO₂ is 3 kPa. Use the data to calculate the extra percentage of oxygen that is unloaded from the haemoglobin because of the higher CO₂ concentration. (1 mark)

Two cube-shaped "model organisms", A and B, are used to investigate the relationship between body size and surface area.

• Model A has sides of length 1 cm.
• Model B has sides of length 4 cm.

(a) For each model, calculate the surface area, the volume and the surface-area-to-volume (SA:V) ratio. Show your working and give appropriate units. (3 marks)

(b) Using your results, explain why large, multicellular organisms need specialised exchange surfaces and transport systems, whereas a single-celled organism does not. (2 marks)

The mountain vole is a small mammal that lives high in the mountains, where the partial pressure of oxygen (pO₂) in the air, and therefore in the lungs, is low. It is closely related to a species of lowland vole that lives at sea level.

Biologists found that the haemoglobin of the mountain vole has a higher affinity for oxygen than the haemoglobin of the lowland vole.

Suggest and explain how the mountain vole's high-affinity haemoglobin is an advantage in its habitat. In your answer refer to the loading of oxygen at the gas-exchange surface and to the position of its oxygen dissociation curve compared with the lowland vole. (5 marks)

Tissue fluid is formed when fluid is forced out of the blood plasma at the arteriole end of a capillary bed and is mostly reabsorbed at the venule end. Plasma proteins are normally too large to leave the capillary.

A patient has a reduced concentration of plasma proteins in their blood. Over time, fluid accumulates in their tissues, causing swelling (oedema).

Explain how a reduced concentration of plasma proteins affects the formation and reabsorption of tissue fluid, and why this leads to a build-up of tissue fluid. In your answer refer to hydrostatic pressure and to the effect of plasma proteins on water potential. (4 marks)

An alveolus in the lung is in close contact with a blood capillary.

Describe how an oxygen molecule in the air inside the alveolus reaches and combines with the haemoglobin in a red blood cell in the adjacent capillary. (3 marks)