Mass Transport In Plants

Question 1

What is the function of the xylem?

Answer 1

Transports water and mineral ions from the roots to the leaves of the plant.

Question 2

How is the xylem adapted to it’s function?

Answer 2

1. The cells are joined together with no end walls forming a long continuous tube. This allows water to flow as a continuous column.
2. The cells contain no organelles and nuclei, so there are no obstructions to the flow of water.
3. The cells have thick cell walls which contain lignin. This provides support.
4. There are pits on the side of the walls. This allows substances to move in and out of the xylem.

Question 3

Explain the cohesion-tension theory of water transport in the xylem?

Answer 3

1. Water is lost from the lead through transpiration.
2. This reduces the water potential of the mesophyll cells.
3. This causes water to be drawn out of the xylem via osmosis.
4. This creates tension in the xylem.
5. Hydrogen bonds result in cohesion between the water molecules, so water is pulled up as a continuous column.
6. The water molecules also adhere to the walls of the xylem.
7. Water enter the roots via osmosis.

Question 4

Describe how transpiration occurs?

Answer 4

1. Water evaporates from the mesophyll cells into air spaces.
2. Water vapour then diffuses through the open stomata.

Question 5

How does light intensity affect the rate of transpiration?

Answer 5

1. As the light intensity increases the rate of transpiration increases.
2. This is because the stomata open when there is light to let carbon dioxide in for photosynthesis.
3. This allows more water to evaporate through the open stomata.

Question 6

How does temperature affect the rate of transpiration?

Answer 6

1. As temperature increases the rate of transpiration increases.
2. This is because the particles have more kinetic energy.
3. This means the water evaporates faster.

Question 7

How does wind intensity affect the rate of transpiration?

Answer 7

1. As the wind intensity increases the rate of transpiration increases.
2. This is because the wind blows away the water molecules surrounding the stomata.
3. This decreases the water potential of the air surrounding the stomata.
4. This increases the water potential gradient and causes the water to evaporate faster.

Question 8

How does humidity affect the rate of transpiration?

Answer 8

1. As humidity increases the rate of transpiration decreases.
2. This is because there will be more water molecules surrounding the stomata.
3. This decreases the water potential gradient and decreases the evaporation of water.

Question 9

What is the function of the phloem?

Answer 9

It transports solutes up and down the plant.

Question 10

How is the phloem adapted to its function?

Answer 10

1. The sieve tube elements have no nucleus, which makes it easier for the substances to float.
2. The sieve tube elements contain sieve plates which allow the substances to move through them.
3. They contain companion cells that have many mitochondria that produce more ATP for active transport via aerobic respiration.

Question 11

What is translocation?

Answer 11

The movement of solutes from the sources to the sinks.

Question 12

Explain the mass flow hypothesis for translocation in pants.

Answer 12

1. At the sources sucrose is actively transported into the phloem.
2. By companion cells.
3. This decreases the water potential inside the phloem and causes water to move into the phloem by osmosis.
4. This increases the hydrostatic pressure inside the phloem.
5. This causes the mass flow of solutes to the sinks.
6. At the sinks sucrose is unloaded and used for respiration.

Question 13

What is the evidence that supports the mass flow hypothesis?

Answer 13

1. When the leaves are punctured with a needle sap flows out. This shows that there is a high pressure at the source tissues.
2. Phloem tissue at the sources has a higher sugar concentration compared to phloem tissue at the sinks. This shows that sucrose is moved into the phloem at the sources, and it is removed from the phloem at the sinks.

Question 14

What is the evidence that goes against the mass flow hypothesis?

Answer 14

1. The presence of sieve plates means that the pressure would have to be really high to create a pressure gradient.
2. The solutes need to travel to multiple sinks not just the ones with the lowest hydrostatic pressure.
3. The solutes need to travel in multiple directions and not just one.