Gas Exchange — A-Level Biology Revision

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What is Gas Exchange?

Gas exchange is the physical process by which gases move passively by diffusion across a surface. In A-Level Biology, this focuses on the exchange of oxygen and carbon dioxide between an organism and its environment. Efficient gas exchange surfaces, such as the alveoli in the lungs or the gills in fish, share common features: a large surface area, a thin diffusion pathway, and a steep concentration gradient maintained by a good blood supply and ventilation.

Board notes: Gas exchange in humans, fish, and insects is a core topic for all A-Level Biology boards (AQA, Edexcel, OCR). The level of detail on lung volumes and the control of breathing may vary. AQA and OCR place a strong emphasis on the interpretation of spirometry data.

Worked example

Common mistakes

• 1Confusing ventilation with gas exchange. Ventilation is the movement of the environmental medium (air or water) over the respiratory surface (e.g., breathing), which maintains the concentration gradient for diffusion, but it is not the diffusion process itself.
• 2Forgetting the role of surfactant in the alveoli. Surfactant is a substance that reduces the surface tension of the fluid lining the alveoli, preventing them from collapsing during exhalation.
• 3Not being able to explain the countercurrent flow mechanism in fish gills. Water flows over the gill lamellae in the opposite direction to the blood flow in the capillaries, which maintains a steep concentration gradient for oxygen diffusion along the entire length of the lamella.

Gas Exchange exam questions

Exam-style questions for Gas Exchange with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• How is the structure of the alveoli adapted for gas exchange?

  Alveoli have a very large total surface area, a wall that is only one cell thick, and a rich network of capillaries, all of which maximise the rate of diffusion of oxygen into the blood and carbon dioxide out of the blood.

• What is the Bohr effect?

  The Bohr effect is the phenomenon where an increase in carbon dioxide concentration in the blood leads to a decrease in the affinity of haemoglobin for oxygen. This results in more oxygen being released to respiring tissues where CO2 levels are high.