Exchange of substances T2

Question 1

Use Fick’s law of diffusion to explain the adaptations of mammalian gas exchange surfaces.
(5)

Answer 1

• Rate of diffusion is proportional to surface area. Alveoli have large surface area.
• Rate of diffusion is proportional to difference in concentration. Breathing maintains a difference in gas concentrations. As does blood flow.
• Rate of diffusion is inversely (one increases the other decreases) proportional to diffusion distance. Walls of alveoli and capillaries one cell thick.
• Diffusion distance is reduced due to flattened cells forming alveoli and capillary walls.
• Rate of diffusion is proportional to diffusion constant. Cell membranes are permeable to non-polar gas molecules.

Question 2

What is Fick’s law?

Answer 2

Rate of diffusion= surface area x difference in concentration/ thickness of gas exchange surface

Question 3

Explain why animals need a heart and a circulatory system.

4

Answer 3

• Heart needed to pump blood around the body.
• Mass flow
• Many animals have a small surface area to volume ratio.
• Circulatory system needed to overcome limitations of diffusion.
• Transport molecules in blood. (Plasma, white blood cells, red blood cells.)
• Many animals have a high metabolic rate.

Question 4

Describe and explain how the lungs of a mammal are adapted for rapid gas exchanged.
(5)

Answer 4

• Alveoli one cell thick.
• Endothelium walls of capillaries one cell thick.
• Alveoli covered with capillaries.
• Short diffusion distance.
• Alveoli and capillaries provide large surface area.
• Concentration gradient maintained by breathing/ventilation.
• Large no. of red blood cells, haemoglobin binds to O2.
• Concentration gradient maintained by blood flow.
• Fick’s law.

Question 5

Suggest how the heart of a daphnia enables organs to carry out effective gas exchange.
(2)

Answer 5

• Blood carries oxygen.
• Blood moving maintains concentration gradient.
• Massflow.
• Organs have a large surface area to volume ratio.

Question 6

In mammals, blood passes through the heart twice for each circulation of the body. Suggest how this type of circulation enables mammals to carry out effective gas exchange.
(3)

Answer 6

• One side of the heart transports blood to lungs, other the body.
• Sep. of oxygenated and deoxygenated blood.
• Maintaining concentration gradient.
• Blood pressure lower to lungs, higher to body.
• Mass flow.
• Need good supply of oxygen as mammals have high metabolic rate.

Question 7

Explain how the partial permeability of the surface membrane of the cells lining the bronchi allows osmosis to take place.
(2)

Answer 7

• Partial permeability is a barrier to some solutes but not water.
• Enables a concentration gradient of water/solutes.

Question 8

Histamine increases the permeability of capillary walls so that large molecules can pass through. Explain how this change in permeability results in swelling around the bite.
(4)

Answer 8

• Proteins can move into tissue fluid.
• Lowers water potential of tissue fluid.
• Increases tendency to draw water out.
• Reduces tendency to reabsorb water.