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Module 3: Exchange and transport
Quick questions on Exchange surfaces and gas exchange: surface-area-to-volume ratio, alveoli, gills and tracheae - OCR A-Level Biology A
6short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is features of an efficient exchange surface?Show answer
These features all increase the rate of diffusion, in line with Fick's law: rate is proportional to (surface area times concentration difference) divided by diffusion distance.
What is gas exchange in bony fish?Show answer
Fish use gills: stacks of gill filaments, each bearing many thin lamellae with a rich blood supply. The crucial adaptation is the counter-current system: water flows over the lamellae in the opposite direction to blood flow within them. Because water is always more oxygenated than the blood it meets, a diffusion gradient is maintained along the whole length of the lamella, so oxygen diffuses into the blood across the entire surface. In a parallel system the two would reach equilibrium partway and exchange would stop.
What are gas exchange in insects?Show answer
Insects have a tracheal system: air enters through spiracles (valved pores) into a network of tracheae, which branch into fine tracheoles that deliver oxygen directly to respiring tissues. Gas exchange is mainly by diffusion down the tracheoles; some larger or active insects also ventilate by abdominal pumping movements, and at high activity fluid is withdrawn from the tracheole ends to bring air closer to the cells. Insects do not use their blood (haemolymph) to transport oxygen.
What is q1?Show answer
State three features of an efficient gas-exchange surface. [3 marks]
What is q2?Show answer
Explain why ventilation helps maintain a high rate of gas exchange in the alveoli. [2 marks]
What is q3?Show answer
Name the valved pores through which air enters an insect's tracheal system. [1 mark]
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