Mass transport in plants Flashcards
Humidity on rate of transpiration:
- 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.
Wind on rate of transpiration: (5)
- 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.
Temperature on rate of transpiration:
- 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
Light intensity on rate of transpiration:
- 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
Mass transport definition:
- bulk movement of substances
- over large distances,
- at the same speed and in
- the same direction.
- movement is due to differences in pressure
transpiration definition.
The process of water movement through a plant AND its evaporation from the leaves.
Flowchart to describe movement of water in a leaf: (5)
- atmosphere less humid than air in leaf’s air spaces
- water vapour in air spaces diffuse out of open stomata into the air
- water evaporates from the cell wall of the mesophyll cells into the air spaces (to replace lost water)
- water potential in mesophyll cells decrease
- water moves from xylem and into mesophyll cells by osmosis
Describe the cohesion-tension theory: (6)
- water lost from leaf due to transpiration
- lower WP of mesophyll
- water leaves xylem by osmosis into mesophyll
- water pulled up xylem
- creating tension
- in a continuous column
- cohesion due to h bonding
- adhesion of water molecules to walls of xylem
Why is water movement in the xylem an example of mass transport?
- 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
From the root, water is transported upward through the stem. Explain how evaporation from the leaves can cause the water to move upwards. (6)
- water potential in mesophyll cells decrease
- water moves out of xylem into mesophyll by osmosis
- creates tension on the water in the xylem
- which is in a continuous column
- cohesion betw/ water molecules due to h-bonding
- column doesn’t break due to adhesion w/ xylem walls
Explain how xerophytes reduce water loss (6)
- thick cuticle;
waterproof; - sunken stomata;
reduces water potential gradient; - shape of leaf / rounded;
small surface area: vol ratio;
Describe the mass flow hypothesis for the mechanism of translocation in plants: (5)
- In source / leaf sugars actively transported into phloem;
- By companion cells;
- Lowers water potential of sieve cell / tube and water enters by osmosis;
- Increase in pressure causes mass movement (towards sink / root);
- Sugars used / converted in root for respiration for storage.
Other than the distribution of stomata, suggest and explain two xerophytic features the leaves of this plant might have. (2)
- Hairs so ‘trap’ water vapour and water potential gradient decreased;
- Stomata in pits so ‘trap’ water vapour and water potential gradient decreased;
- Thick cuticle layer so increases diffusion distance;
- Waxy cuticle so reduces evaporation/transpiration.
- Curled leaves so ‘trap’ water vapour and water potential gradient decreased;
- Spines so reduces surface area to volume ratio;
Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration. (2)
- Water used for support/turgidity;
- Water used in photosynthesis;
Explain why trans rate inc. when temp. inc: (2)
- Molecules have more kinetic energy;
- Faster / more evaporation of water
