Module 3 - Mass transport in plants

Question 1

5 marks

Why is the diameter of the trunksmallest at midday

Answer 1

1. Midday= more sunlight
2. Stomata open in light= more water loss
3. Water evaporates more when warm (fall in WP- water is drawn out of xylem)
4. Hydrogen bonds between water molecules= cohesive= creates a column of water within xylem
5. Adhesion (water molecules stick to wall of xylem) = helps to pull the water column upwards
6. Xylem pulled inwards by tension/ faster flow of water

Question 2

3 marks

Explain the importance of the xylem being kept open as a continuous tube

Answer 2

1. There are no barriers to the movement of water
2. Due to cohesion between the H bonds
3. As water molecules evaporate, tension is created (cohesion-tension theory)

Question 3

2 marks

Explain one way sieve tube cells are adapted for mass transport

Answer 3

• Cytoplasm only at edges of the cell/ fewer organelles
• Easy flow of the sugars

Question 4

1 mark

Explain why values for the pressure in the xylem are negative

Answer 4

(Inside xylem) lower than atmospheric pressure / (water is under) tension;

Question 5

2 marks

One way in which seive cells are adapted for mass transport of sugars

Answer 5

1. Cytoplasm only at edge of the cell
2. Easy flow of the sugars

Question 6

2 marks

One way in which companion cells are adapted for mass transport of sugars

Answer 6

1. Lots of mitochondria
2. Release energy for active transport of sucrose into the phloem

Question 7

5 marks

Describe mass flow hypothesis

Answer 7

1. Sucrose actively transported into phloem using companion cells
2. This lower water potential of phloem
3. Water enters phloem via osmosis from xylem
4. Increase in water volume = increase in hydrostatic pressure= more liquid forced to sink cell
5. Sucrose actively transported to sink cell–> used in respiration as insoluble starch

Question 8

5 marks

Describe the transport of carbohydrate in plants.

Answer 8

1. Sucrose actively transported into phloem cell
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. Unloaded/removed from phloem by active transport

Question 9

2 marks

Advantage of sunken stomata on xerophytic plants

Answer 9

• Reduces transpiration/ evaporation
• Fall in water potential gradient

Question 10

2 marks

Use knowledge of structure of leaf to explain why less water is loss through the upper surface of leaves than is lost through the lower surface

Answer 10

1. More stomata on the lower surface
2. Waxy cuticle on upper surface

Question 11

4 marks

Explain how xylem tissue is adapted for its function

Answer 11

1. Tubes with no end walls- continuous water columns
2. No cytoplasm / no organelles - allows easier water flow
3. Lignin- Withstand tension
4. Pits in walls- allow lateral movement / get round blocked vessels