3.3.5: Mass Transport In Plants

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

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. Allow a named
   example of these reactions
2. Water used in photosynthesis;
3. Water produced in respiration;
4. Apparatus not sealed / ‘leaks’;

Question 3

Give two precaution
s 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

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;
   Accept pressure gradient?
   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

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

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;