Mass Transport in Plants

Question 1

Answer 1

14/14.02/14.024

Question 2

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.

Answer 2

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

Question 3

Describe the relationship between the phloem pressure and the rate of water movement.

Answer 3

Phloem pressure falls as (rate of) water movement (in xylem) increases
OR
Inversely proportional

Question 4

Phloem pressure is reduced during the hottest part of the day. Use the info on Figure 6 along with your understanding of transpiration and mass flow to explain why.

Answer 4

1. High (rate of) transpiration/evaporation; 2. Water lost through stomata
   OR
   (High) tension in xylem;
2. (Causes) less water movement from xylem to phloem
   OR
   Insufficient water potential in phloem to draw water from xylem;

Question 5

The student measured the time taken for water movement.

Give two other measurements he made to calculate the rate of water movement.

Answer 5

1. Initial and final mass (of beaker and all contents);

2. Number of (groups of) xylem vessels;

Question 6

Give the reason for adding a layer of oil to the water in the beaker.

Answer 6

Prevent evaporation/water loss

Question 7

Explain why coloured water moved up the stalk.

Answer 7

1. Water evaporates/is transpired (from leaves/ stalk/celery/plant);
2. Water potential gradient/lower water potential creates tension/pulls up water
   OR
   Osmosis creates tension/pulls up water;
3. Hydrogen bonds/cohesion/adhesion maintains column;

Question 8

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

Answer 8

1. Cut away from body;

2. Against hard/non-slip/flat surface;

Question 9

The student had to choose whether to summarise her measurements by calculating the mean, median and mode.

Choose the most appropriate term from the 3 and give a reason for your choice and find the value using the measurements from the table.

Answer 9

Median (no mark)
1. (Presence of) outliers/80/70
OR
Small sample size/8 (measurements); 2. 41;
Accept for 1 mark, Mean of 47
OR
Mode of 35

Question 10

Treatment D is a control. Explain how the measurement obtained from this control is used by the scientist.

Answer 10

Used to compare effect of other treatments / as a baseline;

2. Shows / Measures effect of substance (X);

Question 11

Use Figure 4 and Table 3, what can you conclude from treatments D and E about root growth?

Answer 11

1. (D shows) substance (X) is not required for (some) root growth / production of roots; OR
   Substances (already) present in stem cause (some) root growth;
2. Substance X moves through plant;
3. (E shows) substance (X) causes / increases /
   doubles number of roots / root growth;

Question 12

The mass flow hypothesis is used to explain the movement of substances through phloem.
Evaluate whether the information from this investigation supports this hypothesis.
Do not consider statistical analysis in the answer.

Answer 12

In support of mass flow hypothesis
1. (F shows) phloem is involved;
2. (G shows) respiration / active transport is
involved (in flow / movement);
3. Because 4 C / cooling reduces / slows / stops
flow / movement;
4. The agar block is the source;
5. Roots are the sink;
Against the mass flow hypothesis
6. No bulge above ringing (in F);
7. No (role for) osmosis / hydrostatic pressure /
water movement;
8. Movement could be due to gravity;
9. Roots still grow without (intact/functioning)
phloem;
10. No leaves / sugars / photosynthesis to act
as a source;

Question 13

The scientists concluded that this heat treatment damaged the phloem. Explain how the results in Figure 9 support this conclusion.
A-level paper

Answer 13

EITHER
1. The radioactively labelled carbon is converted into sugar/organic substances during photosynthesis;
2. Mass flow/translocation in the phloem throughout the plant only in plants that were untreated/B/control
OR
Movement of sugar/organic substances in the phloem throughout the plant only in plants that were untreated/B/control;
OR
3. Movement in phloem requires living cells/respiration/active transport/ATP;
4. Heat treatment damages living cells so transport in the phloem throughout the plant only in plants that were untreated/B/control
OR
Heat treatment stops respiration/active transport/ATP production so transport in the phloem throughout the plant only in plants that were untreated/B/control;

Question 14

The scientists also included that this heat treatmebt did not affect the xylem.
Explain how the results in Table 4 support this conclusion.
A-level paper

Answer 14

1. (The water content of the leaves was) not different because (means ± 2) standard deviations overlap;
2. Water is (therefore) still being transported in the xylem (to the leaf)
   OR
   Movement in xylem is passive so unaffected by heat treatment;

Question 15

What can you conclude about the movement of Fe3+ in barley plants?
A-level paper

Answer 15

1. Heat treatment has a greater effect on young leaves than old;
2. Heat treatment damages the phloem;
3. Fe3+ moves up the leaf/plant;
4. (Suggests) Fe3+ is transported in the xylem in older leaf;
5. In young leaf, some in xylem, as some still reaches top part of leaf;
6. (Suggests) Fe3+ is (mostly) transported in phloem in young leaf
   OR
   Xylem is damaged in young leaf OR
   Xylem is alive in young leaf;
7. Higher ratio of Fe3+ in (all/untreated) old leaves than (all/untreated) young;
8. All ratios show there is less Fe3+ in the top than the lower part of leaves;
9. (But) no statistical test to show if the difference(s) is significant;

Question 16

Describe the cohesion-tension theory.

Answer 16

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;