Mass Transport in Plants - Transport of Water in the Xylem

Question 1

What is a Xylem?

Answer 1

Hollow, think-walled vessels in flowering plants that transport water.

Question 2

Where is the Xylem found?

Answer 2

The stem of a plant.

Question 3

What pulls water up a xylem?

Answer 3

The force from transpiration from the evaporation of water from the leaves.

Question 4

How does water move out through the stomata?

Answer 4

• When the stomata is open, water vapour molecules diffuse out of the air spaces next to the stomata into the surrounding air.
• This is because the humidity of the atmosphere is usually less than the air spaces next to the stomata.
• The water lost by diffusion from the air spaces is replaces water evaporating from the cell walls of the surrounding mesophyll cells.

Question 5

How do plants control the rate of transpiration?

Answer 5

By changing the size of the stomatal pores.

Question 6

How does water move across the cells of a leaf?

Answer 6

• After water is lost from the mesophyll cells from their cell walls, this water is replaced by water from the xylem either though the cell walls or cytoplasm.
• In the case of the of the cytoplasmic route, mesophyll cells lose water to the air spaces by evaporation due to the heat supplied by the sun.
• These cells now have a lower water potential and so water enters by osmosis from neighbouring cells.
• The loss of water from neighbouring cells lowers their water potential.
• So in turn they take in water from their neighbouring cells.
• This establishes a water potential gradient which pulls water from the xylem, across the leaf mesophyll and finally out into the atmosphere.

Question 7

Which theory describes how water moves up the xylem?

Answer 7

Cohesion-Tension theory

Question 8

How does water move up the stem through the xylem?

Answer 8

• Water evaporates from the mesophyll cells due to heat from the sun leading to transpiration.
• Water molecules form hydrogen bonds between one another, causing cohesion to occur which means that they stick together.
• As evaporation occurs from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules are drawn up as a result of cohesion, this is the transpiration pull.