Gas exchange surfacesEdexcel A-Level Biology A: Subtopic test

1. 1
   Single-celled organisms such as Amoeba obtain oxygen by diffusion across their cell surface membrane, but large animals have specialised gas exchange surfaces such as lungs. Students use agar cubes to model how the surface area to volume ratio changes as an organism gets larger.

   (a)

   A student models a large animal as a cube with sides of 4 cm. What is the surface area to volume ratio of the cube?

   [1 mark]

   • A6 : 1
   • B0.67 : 1
   • C96 : 1
   • D1.5 : 1

   (b)

   Which feature of the mammalian lung maintains a steep concentration gradient of oxygen between the alveolar air and the blood?

   [1 mark]

   • AThe large number of alveoli
   • BVentilation of the alveoli and the flow of blood through the surrounding capillaries
   • CThe alveolar wall being one cell thick
   • DThe moist lining of the alveolar surface

   (c)

   Explain why a large mammal needs a specialised gas exchange surface, but an Amoeba does not.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Fick's law states that the rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of the exchange surface. In a healthy adult the alveolar walls have a thickness of 0.5 μm. In a disease called pulmonary fibrosis, scar tissue thickens the alveolar walls.

   (a)

   The alveolar wall in pulmonary fibrosis thickens from 0.5 μm to 1.0 μm. All other factors stay the same. How does the rate of diffusion compare with that in a healthy lung?

   [1 mark]

   • AIt is halved
   • BIt is doubled
   • CIt is unchanged
   • DIt is a quarter of the healthy rate

   (b)

   Which change would increase the rate of diffusion of oxygen from the alveoli into the blood?

   [1 mark]

   • AA decrease in the number of alveoli
   • BAn increase in the thickness of the alveolar wall
   • CAn increase in blood flow through the capillaries, removing oxygenated blood more quickly
   • DA decrease in the oxygen concentration of the air in the alveoli

   (c)

   During vigorous exercise, a person's breathing rate and heart rate both increase. Explain how this increases the rate of diffusion of oxygen into the blood.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student models diffusion across an exchange surface using Fick's law, where rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness. Surface A has an area of 12 cm², a concentration difference of 8 arbitrary units and a thickness of 0.2 mm. Surface B has an area of 12 cm², a concentration difference of 6 arbitrary units and a thickness of 0.1 mm.

   (a)

   Calculate the relative rate of diffusion for surface A and for surface B, and state how many times faster diffusion is across B than across A.

   [3 marks]

   (b)

   Explain how the structure of an alveolus and the capillaries around it provides each of the three properties in Fick's law that increase the rate of gas exchange.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Air breathed in passes down the trachea and through the bronchi and bronchioles to the alveoli. An adult lung contains about 300 million alveoli, giving a total gas exchange surface of about 70 m². Each alveolus is surrounded by a network of capillaries. A doctor is considering three situations that can reduce the uptake of oxygen: emphysema, in which alveolar walls break down so that fewer, larger air spaces form; pneumonia, in which fluid collects inside the alveoli; and climbing to high altitude, where each breath contains a lower concentration of oxygen.

   (a)

   Explain how the structure of the mammalian lung is adapted for rapid gas exchange.

   [6 marks]

   (b)

   Using Fick's law, explain how each of the three situations would reduce the uptake of oxygen, and evaluate which of them the body can most easily compensate for.

   [6 marks]

   Total for question 4: 12 marks

End of questions