Mass transport in plants

Question 1

Humidity on rate of transpiration:

Answer 1

• Increased air humidity
• increased number of water molecules in the air.
• Water potential gradient between air spaces and atmosphere inside the leaf is reduced.
• Decreased transpiration.

Question 2

Wind on rate of transpiration: (5)

Answer 2

• Water vapour diffuses through stomata (transpiration) and accumulates around the outside of the leaf.
• Faster air movement quickly removes/ disperses the thick layer of humid/ moist air.
• Water potential outside the stomata decreases
• Water potential gradient between the moist air in air space in leaf and drier air outside is increased.
• transpiration increases.

Question 3

Temperature on rate of transpiration:

Answer 3

• increased temp
• inc. KE (speed) of H2O molecules
• inc. rate of evaporation of water from leaves AND inc. rate of diffusion
• inc. rate of transpiration

Question 4

Light intensity on rate of transpiration:

Answer 4

• inc. light intensity
• inc. photosynthesis
• more stomata open to let CO2 diffuse in for photosynthesis
• more water moves out of leaf into atmosphere/ transpiration increases

Question 5

Mass transport definition:

Answer 5

• bulk movement of substances
• over large distances,
• at the same speed and in
• the same direction.
• movement is due to differences in pressure

Question 6

transpiration definition.

Answer 6

The process of water movement through a plant AND its evaporation from the leaves.

Question 7

Flowchart to describe movement of water in a leaf: (5)

Answer 7

1. atmosphere less humid than air in leaf’s air spaces
2. water vapour in air spaces diffuse out of open stomata into the air
3. water evaporates from the cell wall of the mesophyll cells into the air spaces (to replace lost water)
4. water potential in mesophyll cells decrease
5. water moves from xylem and into mesophyll cells by osmosis

Question 8

Describe the cohesion-tension theory: (6)

Answer 8

1. water lost from leaf due to transpiration
2. lower WP of mesophyll
3. water leaves xylem by osmosis into mesophyll
4. water pulled up xylem
5. creating tension
6. in a continuous column
7. cohesion due to h bonding
8. adhesion of water molecules to walls of xylem

Question 9

Why is water movement in the xylem an example of mass transport?

Answer 9

• movement over large distances
• bulk movement of all substances
• at the same speed (quicker than diffusion)
• in the same direction
• movement due to differences in pressure

Question 10

From the root, water is transported upward through the stem. Explain how evaporation from the leaves can cause the water to move upwards. (6)

Answer 10

1. water potential in mesophyll cells decrease
2. water moves out of xylem into mesophyll by osmosis
3. creates tension on the water in the xylem
4. which is in a continuous column
5. cohesion betw/ water molecules due to h-bonding
6. column doesn’t break due to adhesion w/ xylem walls

Question 11

Explain how xerophytes reduce water loss (6)

Answer 11

• thick cuticle;
  waterproof;
• sunken stomata;
  reduces water potential gradient;
• shape of leaf / rounded;
  small surface area: vol ratio;

Question 12

Describe the mass flow hypothesis for the mechanism of translocation in plants: (5)

Answer 12

1. In source / leaf sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell / tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink / root);
5. Sugars used / converted in root for respiration for storage.

Question 13

Other than the distribution of stomata, suggest and explain two xerophytic features the leaves of this plant might have. (2)

Answer 13

1. Hairs so ‘trap’ water vapour and water potential gradient decreased;
2. Stomata in pits so ‘trap’ water vapour and water potential gradient decreased;
3. Thick cuticle layer so increases diffusion distance;
4. Waxy cuticle so reduces evaporation/transpiration.
5. Curled leaves so ‘trap’ water vapour and water potential gradient decreased;
6. Spines so reduces surface area to volume ratio;

Question 14

Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration. (2)

Answer 14

1. Water used for support/turgidity;
2. Water used in photosynthesis;

Question 15

Explain why trans rate inc. when temp. inc: (2)

Answer 15

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