mass transport in plants EQ

Question 1

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 the phloem tube. (3)

Answer 1

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 2

phloem pressure is reduced during the hottest part of the day. use information in the graph above along with your understanding of transpiration and mass flow to explain why. (3)

Answer 2

1. high rate of transpiration/evaporation;
2. water lost through stomata;
   OR
   (high) tension in xylem;
3. (causes) less water movement from xylem to phloem
   OR
   insufficient water potential in phloem to draw water from xylem;

Question 3

the student measured the time taken for water movement.
give two other measurements he made to calculate the rate of water movement. (2)

Answer 3

1. initial and final mass (of beaker and all contents);
2. number of (groups of) xylem vessels;

Question 4

give the reason for adding a layer of oil to the water in the beaker. (1)

Answer 4

prevent evaporation/water loss;

Question 5

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

Answer 5

1. cut away from body;
2. against hard/non-slip/flat surface;

Question 6

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

Answer 6

1. 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 7

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

Answer 7

1. 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;