Gas exchange surfaces and the lungEdexcel International A Level Biology: Subtopic test

1. 1
   Flatworms are flat, leaf-shaped animals less than 1 mm thick. They have no lungs or gills and take in oxygen directly across their body surface. A much larger animal, such as a rabbit, is made of the same types of cell but cannot obtain enough oxygen in this way.

   (a)

   Which feature of the flatworm's body allows it to obtain enough oxygen by diffusion alone?

   [1 mark]

   • AA small surface area to volume ratio
   • BA large surface area to volume ratio
   • CA thick body wall that is impermeable to gases
   • DA long diffusion pathway between the surface and the cells

   (b)

   Which statement best explains why the rabbit needs a specialised gas exchange surface?

   [1 mark]

   • AIts body surface is too thin for diffusion to occur
   • BIts cells use less oxygen than those of the flatworm
   • CThe distance from its body surface to most of its cells is very short
   • DIts surface area is too small relative to its volume for diffusion across the body surface to supply oxygen fast enough

   (c)

   Explain why the flatworm does not need a specialised gas exchange system.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient has a lung disease in which the walls of the alveoli have become thickened by scar tissue. Doctors measure the rate at which oxygen diffuses from the alveoli into the blood and find that it is lower than normal.

   (a)

   According to Fick's law, which change is the most direct cause of the lower rate of oxygen diffusion in this patient?

   [1 mark]

   • AAn increase in the thickness of the exchange surface
   • BAn increase in the surface area of the alveoli
   • CAn increase in the concentration difference across the exchange surface
   • DA decrease in the number of red blood cells

   (b)

   A second patient has emphysema, which halves the total surface area of the alveoli. All other factors in Fick's law are unchanged. What happens to the rate of diffusion?

   [1 mark]

   • AIt doubles
   • BIt is unchanged
   • CIt halves
   • DIt falls to one quarter

   (c)

   Use Fick's law to explain why the rate of diffusion of oxygen into the blood is reduced in the first patient.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The wall of each alveolus in a healthy lung is only one cell thick, and each alveolus is surrounded by a dense network of capillaries. Blood entering these capillaries has a low oxygen concentration and a high carbon dioxide concentration, while the air in the alveoli is repeatedly replaced by breathing.

   (a)

   Describe how the structure of the alveoli and the surrounding capillaries is adapted for rapid gas exchange.

   [3 marks]

   (b)

   Explain how breathing and blood flow help to maintain the concentration gradients for oxygen and carbon dioxide between the alveolar air and the blood.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A healthy climber ascends to high altitude, where the percentage of oxygen in the air is unchanged but the pressure is lower, so the concentration of oxygen in her alveolar air is much lower than at sea level. A second climber has emphysema, a condition in which many alveolar walls break down so that the lungs contain fewer, larger air spaces.

   (a)

   Use Fick's law to explain why gas exchange in the healthy climber is slower at high altitude, and suggest how her breathing and circulation could limit the effect.

   [6 marks]

   (b)

   Evaluate the statement that the climber with emphysema will be affected more by high altitude than the healthy climber.

   [6 marks]

   Total for question 4: 12 marks

End of questions