1.2.3 - Transport in Plants (Pathway of Water) Part 15 & 16

Question 1

Inside the endodermis is a layer of cells. Some of these cells have the Casparian strip in their walls. What does this strip do?

Suggest a reason why Casparian strip does this.

Answer 1

• The Casparian strip is waterproof. It forces the water in the apoplast pathway into symplast pathway.
• The Casparian strip is there because it ensures water and nitrates enter the cytoplasm. The nitrates are taken into the xylem from the cytoplasm of the cortex via active transport. This lowers the water potential so water can move from the cortex into the xylem by osmosis.
• The Casparian strip also stops water from escaping the xylem into the cortex.

Question 2

Describe how root pressure helps the movement of water.

Answer 2

• The endodermis moves minerals into the xylem to lower its water potential.
• This forces water into the xylem by osmosis and pushes the water up the xylem.
• This, by itself, can only move the water up by a few metres.

Question 2

Describe how capillary action helps the movement of water.

Answer 2

• Water is attracted to the sides of the xylem vessel. This is known as adhesion.
• The xylem vessels are very narrow, maintaining the forces of attraction so it can be pulled upwards.

Question 3

Name and describe the three possible routes water can take when moving between plant cells.

Answer 3

• Apoplast pathway - Cellulose walls have water-filled spaces between the cellulose molecules. Water and dissolved minerals pass through these spaces, but not through any plasma membranes.
• Symplast pathway - Water enters the cell cytoplasm, moving from cell to cell via the plasmodesmata.
• Vacuolar pathway - Water is not confined to the cytoplasm. It is able to enter and pass through the vacuoles or cells.

Question 4

State the three processes that help water move up the plant.

Answer 4

• Root pressure
• Transpiration pull
• Capillary action.

Question 6

Describe how transpiration pull can help the movement of water.

Answer 6

• Loss of water must be replaced by water in the xylem.
• Water molecules are attracted to each other. These forces are known as the forces of cohesion.
• This makes a long chain of water molecules.
• As water molecules are lost from the top, the whole column is pulled up, creating a transpiration stream.
• If one stream breaks, it can still be maintained in another xylem vessel via pits.

Question 7

Describe how water in the soil is transported into the root cortex.

Answer 7

• The endodermis moves minerals from the cortex into the xylem by active transport.
• This decreases the water potential in the xylem so water moves into the xylem via osmosis.
• Now the water potential outside endodermis is low. This creates a gradient across the whole cortex.