9.2 Water Transport

Question 1

Describe the effect of low external water potential on a plant cell.

Answer 1

• Water moves out of the cell
• Cell membrane shrinks away from the cell wall, enabling plants to survive low EWP for short periods of time
• Cell is said to be plasmolysed

Question 2

How is water absorbed into root hair cells?

Answer 2

• Water and mineral ions are contained within small air spaces in the soil.
• There is a higher concentration of solutes in the plant than the soil.
• When the roots make contact with the soil, water moves into the roots via osmosis.

Question 3

What are the adaptations of root hair cells and their purposes?

Answer 3

1. Microscopic size - Can penetrate easily between soil particles
2. Large quantity of microscopic hairs - Increases SA:V ratio
3. Each hair has a thin surface layer (only cell wall and cell-surface membrane) - Reduces diffusion distance
4. Concentration of solutes in cytoplasm of root hair cells is maintained - Ensures that there is a water potential gradient between soil water and cell

Question 4

What are the 3 pathways through which water moves from the root hairs to the xylem?

Answer 4

• Symplast pathway
• Vacuolar pathway
• Apoplast pathway

Question 5

What is the purpose of multiple pathways for water to move from the root hairs to the xylem?

Answer 5

Allows water to travel as fast as possible.

Question 6

Describe the symplast pathway.

Answer 6

1. Water enters the cytoplasm across the plasma membrane.
2. Moves through the cytoplasm of different cells.
3. Moves from cell to cell through plasmodesmata.

Question 7

Describe the vacuolar pathway.

Answer 7

1. Water enters the cytoplasm across the plasma membrane.
2. Water moves into the cytoplasm.
3. Can move into the sap in the vacuole, through the tonoplast.
4. Moves from cell to cell through plasmodesmata

Question 8

Describe the Apoplast pathway.

Answer 8

1. Water enters the cell wall of the root cortex cells.
2. Moves through the cell wall - cohesive and tension forces act on the cell walls, pulling water up
3. May move from cell wall to cell wall, either directly or across apoplast (extracellular space)

Question 9

Which pathway for the movement of water is fastest and slowest?

Answer 9

Fastest - Apoplast
Slowest - Vacuolar

Question 10

Why is the symplast pathway slower than the apoplast pathway?

Answer 10

Water must move through cells, taking a longer route. Organelles also obstruct the flow of water.

Question 11

What is the Casparian strip and where is it found?

Answer 11

• An impermeable layer of Suberin - waxy material
• Found in the endodermis
• Forces all water in apoplast pathway into symplast pathways

Question 12

What is the purpose of the Casparian strip?

Answer 12

• In the apoplast pathways, water and dissolved substances bypass the cell membrane which is selectively permeable and ensures that harmful substances (e.g. toxins) do not enter
• Casparian strip forces water to go through membranes of endodermal cells
• Also helps to maintain water potential gradient between cortex and xylem

Question 13

What causes root pressure?

Answer 13

• Cells in the endodermis use active transport to pump mineral ions into the xylem
• Lowers water potential of xylem
• Water diffuses into xylem by osmosis

Question 14

Explain the evidence for active transport in root pressure

Answer 14

1. Effect of cyanide - stops mitochondria from working, so root pressure decreases
2. Effect of temperature - Root pressure changes with temperature (increase/decrease) suggesting an enzyme controlled chemical reaction
3. Reactant availability - If oxygen/respiratory substrate levels drop, root pressure decreases
4. Guttation - Sap and water move out of cut stems, suggesting that they are actively pumped out, not drawn up by transpiration