3.3.5: The transpiration stream

Question 1

What is the transpiration stream?

Answer 1

• The movement of water from the soil, through the plant, to the air surrounding the leaves.
• Main driving force is the water potential gradient between the soil and the air in the leaf spaces.

Question 2

What are root hair cells and what do they do?

Answer 2

• Cells with a long extension (root hair) that increases the surface area of the root.
• Absorb mineral ions and water from the soil.

Question 3

Where does the water move after entering the root hair cell by osmosis?

Answer 3

• Water moves across the root cortex down a water potential gradient to the endodermis of the vascular bundle.
• Water may also travel through the apoplast pathway as far as the endodermis, but must then enter the symplast pathway, as the apoplast pathway is blocked by the Casparian strip.

Question 4

What is the movement of water across the root driven by?

Answer 4

-An active process that occurs at the endodermis.

Question 5

What is the endodermis?

Answer 5

• A layer of cells surrounding the medulla and the xylem.
• This layer of cells is also known as the starch sheath, as it contains granules of starch, a sign that energy is being used.

Question 6

What does the Casparian strip do?

Answer 6

• Blocks the apoplast pathway between the cortex and the medulla.
• This ensures that water and water dissolved mineral ions (especially nitrates) have to pass into the cell cytoplasm through the plasma membranes.

Question 7

What do the plasma membranes do?

Answer 7

• They contain transporter proteins, which actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and xylem.
• This makes the water potential of the xylem and medulla more negative so that water moves from the cortex cells into the medulla and xylem by osmosis.

Question 8

Once the water has entered the medulla, why can it not pass back into the cortex?

Answer 8

Because the apoplast pathway of the endodermal cells is blocked by the Casparian strip.

Question 9

How does water move up through the xylem?

Answer 9

• By mass flow.

- A flow of water and mineral ions in the same direction.

Question 10

What are the three processes that help to move water up the stem?

Answer 10

• Root pressure
• Transpiration pull
• Capillary action

Question 11

Describe root pressure.

Answer 11

-Endodermis moves minerals into medulla and xylem by active transport.
-This draws water into the medulla by osmosis.
-Pressure in the root medulla builds up and forces water into the xylem.
(can only push water a few metres up a stem)

Question 12

Describe transpiration pull.

Answer 12

• Loss of water by evaporation from leaves must be replaced by water coming up from the xylem.
• H2O molecules attracted to each other by cohesion.
• Cohesion forces hold molecules together in a long chain/column.
• As molecules are lost from the top of the column, the whole column gets pulled up in one chain.
• This creates tension in the column of water.

Question 13

Why must xylem vessels be strengthened by lignin?

Answer 13

To prevent the column from collapsing under tension.

Question 14

Describe capillary action.

Answer 14

• Same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel.
• This is called adhesion.
• Because xylem vessels are so narrow, these forces of attraction can pull the water up the sides of the vessel.

Question 15

How does water leave the leaf and help water move up the stem?

Answer 15

• Most of the water that leaves the leaf exits as vapour through the stomata.
• Water evaporates from cells lining the cavity immediately above the guard cells.
• This lowers WP in these cells.
• Water enters these cells by osmosis from neighbouring cells.
• In turn, water is drawn from the xylem in the leaf by osmosis.