Gas exchange in single-celled organisms, insects and fishAQA A-Level Biology: Subtopic test

1. 1
   Amoeba is a single-celled organism that lives in freshwater ponds. It takes in oxygen and loses carbon dioxide across its cell surface membrane and has no gas exchange organs.

   (a)

   By which process does oxygen enter Amoeba?

   [1 mark]

   • AActive transport across the cell surface membrane
   • BDiffusion across the cell surface membrane
   • COsmosis across the cell surface membrane
   • DVentilation of the cell by water currents

   (b)

   Why does Amoeba not need a specialised gas exchange system?

   [1 mark]

   • AIt has a large surface area to volume ratio and a short diffusion distance
   • BIt has a low surface area to volume ratio, so it needs little oxygen
   • CIt has a waterproof surface so it loses no water
   • DIt respires anaerobically so needs no oxygen

   (c)

   Explain how a concentration gradient for oxygen is maintained across the cell surface membrane of Amoeba.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A locust is a terrestrial insect. Air enters through openings along the surface of its thorax and abdomen and passes into branching tubes that end next to the muscle cells. The outer surface of the locust is covered by a waterproof layer.

   (a)

   At which structure does oxygen leave the gas exchange system of the locust and enter the respiring cells?

   [1 mark]

   • ASpiracles
   • BTracheae
   • CTracheoles
   • DHaemolymph

   (b)

   What is the main function of tiny hairs around the spiracles of many insects?

   [1 mark]

   • ATo filter out carbon dioxide
   • BTo pump air into the tracheae
   • CTo increase the surface area for gas exchange
   • DTo trap moist air and reduce water loss

   (c)

   Explain how the locust's gas exchange system is a compromise between gas exchange and limiting water loss.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a bony fish, water passes over the gills and flows across the gill lamellae in the opposite direction to the flow of blood in the capillaries inside each lamella. Water entering the gills is 90% saturated with oxygen and leaves 20% saturated. Blood entering the lamellae is 5% saturated with oxygen and leaves 85% saturated.

   (a)

   Explain how counter-current flow allows the fish to absorb a high percentage of the oxygen from the water.

   [3 marks]

   (b)

   Calculate the percentage of the oxygen in the water entering the gills that is absorbed by the fish. Explain why the data show that the flow of blood and water is counter-current, not parallel.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class compares gas exchange in a locust, a terrestrial insect, and a rainbow trout, a fish. Both animals are active and have a high demand for oxygen.

   (a)

   Explain how the gas exchange system of the locust is adapted for efficient gas exchange while limiting water loss.

   [6 marks]

   (b)

   Water contains much less oxygen than air. Explain how the gills of the trout are adapted to obtain enough oxygen from water.

   [6 marks]

   Total for question 4: 12 marks

End of questions