Y1: Mass Transport

Question 1

Describe how water is moved through a plant according to the cohesion-tension hypothesis.

6

Answer 1

1. water evaporates / transpires from leaves;
2. reduces water potential in cell / water potential / osmotic gradient across
   cells
3. water is drawn out of xylem;
4. creates tension
5. cohesive forces between water molecules;
6. water pulled up as a column;

Question 2

Describe the mass flow hypothesis for the mechanism of translocation in plants.

5

Answer 2

1. In source / leaf sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell / tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink / root);
5. Sugars used / converted in root for respiration for storage.

Question 3

Mass flow?

9

Answer 3

1. at source, Sucrose is actively transported from the companion cells into the sieve tubes
2. lowers water potential of sieve tubes
3. water moves in via osmosis
4. increases hydrostatic pressure
5. at sink, sucrose actively transported out to companion cells
6. lowers water potential of companion cells
7. water moves out via osmosis
8. lowers hydrostatic pressure
9. t4 HS pressure gradient generated that transports sucrose down