Mass Transport

Question 1

Explain how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta. (5 marks)
Start anywhere in sequence, but events must be in the correct order.

Answer 1

1. Atrium has higher pressure than ventricle (due to filling / contraction) causing atrioventricular valves to open;
   (Accept bicuspid, reject tricuspid)
2. Allow: blood passes through the valve = valve open / blood stopped from passing through the valve = valve closed
3. Ventricle has higher pressure than atrium (due to filling / contraction) causing atrioventricular valves to close;
4. Ventricle has higher pressure than aorta causing semilunar valve to open;
   (Allow aortic valve)
5. Higher pressure in aorta than ventricle (as heart relaxes) causing semilunar valve to close;
   (Allow aortic valve)
6. (Muscle / atrial / ventricular) contraction causes increase in pressure

Question 2

Describe the cohesion-tension theory of water transport in the xylem. [5 marks]

Answer 2

1. Water lost from leaf because of transpiration/evaporation of water from mesophyll/leaf cells
2. Lowers water potential of mesophyll/leaf cells
3. Water pulled up xylem (creating tension)
4. Water molecules cohere/’stick’ together by hydrogen bonds
5. (forming continuous) water column
6. Adhesion of water (molecules) to walls of xylem