Topic 3G: 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 from mesophyll cells;
2. Lowers water potential of mesophyll cells;
3. Water pulled up xylem (creating tension);
4. Water molecules cohere by hydrogen bonds;
5. (forming continuous) water molecule columns;
6. Adhesion of water to walls of xylem.

Question 2

Suggest precautions students should take when setting up a photometer to obtain reliable measurements of water uptake by a plant shoot. [5]

Answer 2

1. Ensure airtight (to prevent water loss);
2. Cut shoot under water (stops air entering shoot to ensure continuous water columns in xylem);
3. Cut shoot at a slant (increases surface area for water uptake;
4. Dry off leaves (ensures stomata are not blocked by water droplets);
5. Insert into apparatus under water (stops air bubbles getting in);
6. Ensure no air bubbles are present (other than the single air bubbles you are tracking);
7. Shut tap (to stop air bubble moving backwards);
8. Note where bubble is at start (to then measure how far the bubble goes)

Question 3

Explain why water moves up stalks. [3]

Answer 3

1. Water evaporates (from leaves);
2. Lower water potential creates tension;
3. Hydrogen bonds maintain column

Question 4

Describe how you should handle a scalpel safely during a procedure. [2]

Answer 4

1. Cut away from body;
2. Against hard surface

Question 5

Explain the relationship between the number of leaves removed from a plant shoot and the rate of water uptake. [3]

Answer 5

As number of leaves are reduced:
1. Fewer stomata;
2. Less evaporation;
3. Less tension

Question 6

Explain the relationship between stomata opening and photosynthesis. [2]

Answer 6

1. Stomata allow uptake of carbon dioxide;
2. Carbon dioxide used in photosynthesis

Question 7

Give three environmental variables that need to be controlled when investigating growth of plants. [3]

Answer 7

1. Temperature;
2. Light;
3. Carbon dioxide;
4. Type of soil;
5. Minerals/nutrients;
6. Water (in soil);
7. Humidity (of air);
8. pH (of soil)

Question 8

Explain why the transpiration rate increases when the temperature increases. [2]

Answer 8

1. Molecules have more kinetic energy;
2. More evaporation of water

Question 9

Explain why the values for the pressure in the xylem are negative. [1]

Answer 9

1. (Water is under) tension

Question 10

Explain why the diameter of a trunk is smallest at midday. [4]

Answer 10

1. Stomata open in light so greater water loss;
2. Water evaporates more when warm;
3. Hydrogen-bonding between water molecules;
4. Adhesion between water molecules and walls of xylem vessels;
5. (Xylem) pulled inwards by tension

Question 11

Describe the transport of carbohydrates in plants. [5]

Answer 11

1. Sucrose actively transported into phloem;
2. (by) companion cells;
3. Lowers water potential (in phloem) and water enters (from xylem) by osmosis;
4. (Produces higher hydrostatic) pressure;
5. Mass flow to respiring cells;
6. Removed (from phloem) by active transport

Question 12

Explain how pressure is generated inside the phloem. [3]

Answer 12

1. Sucrose actively transported (into phloem);
2. Reduces water potential;
3. Water moves (into phloem) by osmosis (from xylem)

Question 13

Explain why phloem pressure is reduced during the hottest part of the day. [3]

Answer 13

1. High (rate of) transpiration;
2. High tension in xylem;
3. (Causes) less water movement from xylem to phloem

Question 14

Explain how phloem-sap-feeders get sap to leave the phloem. [3]

Answer 14

1. Contents of phloem vessel pushed into insect’s mouth by high pressure;
2. caused by loading of sugars into phloem in leaf;
3. and entry of water by osmosis