topic 3 mass transport in plants exam questions

Question 1

Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 1

1. Water lost from leaf because of transpiration / evaporation of water
   (molecules) / diffusion from mesophyll / leaf cells;
   OR
   Transpiration / evaporation / diffusion of water (molecules)
   through stomata / from leaves;
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;

Question 2

The student wanted to determine the rate of water loss per mm2 of surface
area of the leaves of the shoot in Figure 1.
Outline a method she could have used to find this rate. You should assume
that all water loss from the shoot is from the leaves.
(3)

Answer 2

1. Method for measuring area;
   e.g. draw round (each) leaf on graph paper and count squares;
2. Of both sides of (each) leaf;
3. Divide rate (of water loss / uptake from potometer) by (total)
   surface area (of leaves);

Question 3

The rate of water movement through a shoot in a potometer may not be the
same as the rate of water movement through the shoot of a whole plant.
Suggest one reason why. (1)

Answer 3

Plant has roots
OR
xylem cells very narrow;

Question 4

Describe the mass flow hypothesis for the mechanism of translocation in
plants. (4)

Answer 4

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 5

The mass flow hypothesis is used to explain the movement of substances
through phloem.
Use your understanding of the mass flow hypothesis to explain how
pressure is generated inside this phloem tube. (3)

Answer 5

1. Sucrose actively transported (into phloem);
2. Lowering/reducing water potential
   OR
   More negative water potential;
3. Water moves (into phloem) by osmosis (from xylem);

Question 6

The student used a sharp scalpel to cut the celery. Describe how she
should ensure she handled the scalpel safely during this procedure. (2)

Answer 6

1. Cut away from body;
   Accept description of cutting technique to avoid cutting
   fingers
2. Against hard/non-slip/flat surface;