7.7 transport of water in the xylem

Question 1

What is transpiration?

Answer 1

the process by which water is evaporated from the leaves of a plant.

• this causes water to be drawn up through the xylem
• energy is provided by the sun, so it is a passive process.

Question 2

Describe the movement of water out of the stomata.

Answer 2

• the humidity of the atmosphere is less than around the stomata, so a WP gradient is created between the air spaces through the stomata and the air
• when the stomata are open, water vapour molecules diffuse out
• water lost from the air spaces by diffusion is replaced by water evaporating from mesophyll cells
• plants change their stomata size to control the transpiration rate

Question 3

Describe the movement of water across the cells of a leaf.

Answer 3

• water is evaporated from the cell walls of mesophyll cells into the air spaces in the leaf
• this is replaced by water reaching the mesophyll cells from the xylem via cell walls/cytoplasm

Question 4

Why water replaces by water reaching the mesophyll cells through the cytoplasm?

Answer 4

• mesophyll cells lose water due to sunlight
• cells then have a lower WP so water enters by osmosis from surrounding cells
• loss of water from surrounding cells lowers their water potential so water is taken in by osmosis.