2.1.1 Gas exchange surfaces and Fick's Law Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 2

The 20 questions

1. Which three properties of a gas exchange surface are needed for efficient diffusion?

   • Large surface area, thin surface and a large concentration difference across the surface
   • Small surface area, thick surface and a small concentration difference across the surface
   • Large surface area, thick surface and a small concentration difference across the surface
   • Small surface area, thin surface and a large concentration difference across the surface

2. According to Fick's Law, what happens to the rate of diffusion if the thickness of the exchange surface is halved?

   • The rate of diffusion doubles
   • The rate of diffusion halves
   • The rate of diffusion is unchanged
   • The rate of diffusion quadruples

3. Fick's Law states that the rate of diffusion is proportional to which quantities?

   • Surface area and concentration difference, and inversely proportional to thickness
   • Concentration difference and thickness, and inversely proportional to surface area
   • Surface area and thickness, and inversely proportional to concentration difference
   • Volume and thickness, and inversely proportional to surface area

4. An exchange surface has an area of 2.0 m^2, a concentration difference of 10 units and a thickness of 0.5 units. A second surface has an area of 4.0 m^2, the same concentration difference and a thickness of 1.0 unit. How does the rate on the second surface compare with the first?

   • It is four times the rate
   • It is half the rate
   • It is the same
   • It is double the rate

5. Why does the mammalian lung have a very large total surface area for gas exchange?

   • It contains a single large alveolus, which has a high surface area to volume ratio
   • It contains millions of alveoli, which together give a very large surface area
   • It contains red blood cells that increase the surface area of the alveolar walls
   • It is covered by many thick muscular layers, which create folds for gas exchange

6. Which feature of an alveolus makes diffusion of oxygen rapid into the blood?

   • Its wall is thick and muscular, which increases the pressure of air entering the blood
   • Its wall is coated with mucus, which dissolves oxygen before it enters the blood
   • It is connected to the bronchi by a single long tube, which slows the flow of air
   • Its wall is only one cell thick, and it is surrounded by a dense capillary network

7. Why are the lungs ventilated and the blood flowed through the capillaries continuously?

   • To maintain a steep concentration gradient for oxygen and carbon dioxide across the alveolar surface
   • To keep the alveoli filled with water, which allows oxygen to dissolve before diffusing
   • To remove carbon monoxide from the blood, which is the only gas that diffuses across the alveoli
   • To prevent the alveoli from collapsing, which otherwise would stop all gas exchange in the lungs

8. A cube-shaped gas exchange model has a surface area to volume ratio of 6 per cm. Which change would most increase the rate of diffusion according to Fick's Law?

   • Increasing the thickness of the membrane to keep the cell structurally strong
   • Reducing the concentration difference across the membrane by stopping ventilation
   • Keeping the surface still so that no diffusion gradient is maintained
   • Increasing the surface area to volume ratio, for example by folding the surface

9. Which of these explains why small animals such as insects can use simple diffusion through their body surfaces for gas exchange?

   • They have a small surface area to volume ratio and thick walls, which slows diffusion appropriately
   • They have a large surface area to volume ratio and short diffusion distances
   • They have a large volume and thick cell membranes, which speed up the diffusion of oxygen
   • They have a circulatory system that carries oxygen through the body surfaces by pumping

10. Why does the rate of diffusion across a surface slow down as the concentration difference decreases?

    • The rate increases as the concentration difference decreases, because particles have more space to move
    • A smaller concentration difference increases the thickness of the surface, which then slows diffusion
    • The concentration difference has no effect on the rate, because diffusion depends only on temperature
    • The concentration gradient is the driving force, so a smaller gradient gives a slower net rate

11. Which gas exchange feature in the alveoli is linked with the maintenance of the concentration gradient in the lungs?

    • The large number of red blood cells that prevents oxygen from moving across the alveolar walls
    • The high pressure of air that forces oxygen to move against its concentration gradient into blood
    • The thick mucus layer that traps gases and stops them leaving the alveolar space in the lungs
    • Blood flow through the capillaries removes oxygen and brings carbon dioxide, which keeps the gradient steep

12. A fish's gills have a large surface area but thin walls, with water flowing in the opposite direction to blood. What is the advantage of the counter-current flow?

    • It reduces the surface area so that the blood does not lose too much oxygen to the water
    • It stops water and blood mixing, which prevents the blood from becoming diluted during exchange
    • It maintains a concentration gradient along the whole length of the gill, so more oxygen diffuses into the blood
    • It increases the thickness of the gill to protect the blood from damage by the moving water

13. Which of these is a property of gas exchange surfaces in living organisms?

    • They have no concentration gradient, because the exchange is by active transport
    • They have a large surface area to volume ratio and a thin surface
    • They have a small surface area to volume ratio and a thick surface
    • They have a large volume and a small surface area for maximum storage

14. Why does the alveolar epithelium need to be kept moist?

    • Moisture increases the thickness of the epithelium, which protects the alveoli from infection
    • Water prevents the alveoli from expanding, which keeps the surface area constant
    • Water is needed to produce surfactant, which is an enzyme that converts oxygen to carbon dioxide
    • Gases must dissolve in a thin film of water before they can diffuse across the surface

15. Which statement about Fick's Law is correct?

    • It describes the rate of osmosis across a partially permeable membrane in terms of water potential
    • It describes the rate of diffusion as proportional to surface area, concentration difference and inverse thickness
    • It describes the rate of diffusion as proportional to thickness and inversely proportional to concentration difference
    • It describes the rate of active transport, which requires ATP and carrier proteins

16. A student doubles the surface area of a model exchange surface but also doubles its thickness. What is the net effect on the rate of diffusion?

    • The rate halves, because thickness has a larger effect on rate than surface area
    • The rate quadruples, because area and thickness each affect rate by the same factor
    • The rate is unchanged, because the doubled area is offset by the doubled thickness
    • The rate doubles, because the area increase is larger than the thickness increase

17. Which statement about the lung surface is correct?

    • Alveolar walls are several cells thick, which provides protection against the high pressure of blood
    • Alveolar walls are one cell thick, which is an adaptation that shortens the diffusion distance
    • Alveolar walls are made of smooth muscle, which controls the rate of diffusion of oxygen
    • Alveolar walls are made of cartilage, which keeps the surface rigid for gas exchange

18. Why is a large difference in oxygen concentration across the alveolar wall important?

    • It makes the alveolar wall impermeable to oxygen so that gas is trapped in the lungs
    • It causes the red blood cells to burst, which releases oxygen directly into the plasma
    • It stops carbon dioxide from diffusing out of the blood into the alveoli
    • It provides the driving force for net diffusion of oxygen into the blood

19. A gas exchange surface has an area of 1.0 m^2 and a thickness of 1.0 unit. What would be the effect of increasing the area to 3.0 m^2 and reducing the thickness to 0.5 unit, keeping the concentration difference constant?

    • The rate of diffusion halves
    • The rate of diffusion is unchanged
    • The rate of diffusion increases by a factor of six
    • The rate of diffusion increases by a factor of three

20. Which of these would reduce the rate of diffusion across a gas exchange surface?

    • Increasing the surface area by folding the epithelium
    • Thickening the surface by adding extra layers of cells
    • Increasing the concentration difference by ventilating the lungs
    • Shortening the diffusion distance by making the surface thinner

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