Mass Transport In Plants

Question 1

Water - 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 by 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

Water - T A potometer measures the rate of water
uptake rather than the rate of transpiration.
Give two reasons why the potometer does not truly
measure the rate of transpiration
(2)

Answer 2

1. Water used for support / turgidity;
   Accept: water used in (the cell’s) hydrolysis or
   condensation (reactions) for one mark.
2. Water used in photosynthesis;
3. Water produced in respiration;
4. Apparatus not sealed / ‘leaks’;

Question 3

Water - Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot. (2)

Answer 3

1. Seal joints / ensure airtight / ensure watertight;
   Answer must refer to precautions when setting up the apparatus
   Ignore: references to keeping other factors constant
2. Cut shoot under water;
3. Cut shoot at a slant;
4. Dry off leaves;
5. Insert into apparatus under water;
6. Ensure no air bubbles are present;
7. Shut tap;
8. Note where bubble is at start / move bubble to the
   start position;

Question 4

Sucrose - Describe the transport of carbohydrate in plants. (5)

Answer 4

1. (At source) sucrose is transported into the
   phloem/sieve element/tube;
2. By active transport
   OR
   By co-transport with H+;
   Accept co-transport with hydrogen/H ions
3. By companion/transfer cells;
4. Lowers water potential in phloem and water enters by osmosis;
   For ‘phloem’ accept ‘sieve element/tube’.
5. (Produces) high (hydrostatic) pressure;
6. Mass flow;
7. Transport from site of photosynthesis to respiring
   cells
   OR
   Transport from site of photosynthesis to storage organ
   OR
   Transport from storage organ to respiring cells;

Question 5

Sucrose - 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

Sucrose - Phloem pressure is reduced during the hottest part of the day. Use your understanding
of transpiration and mass flow to explain why (3)

Answer 6

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;