Transport of Water in the Xylem

Question 1

What is transpiration?

Answer 1

Transpiration is the transporting and evaporation of water throughout a plant.

Question 2

How does water move through the leaf?

Answer 2

-Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
-These cells have a lower water potential and so water enters by osmosis from neighbouring cells
-The loss of water from these neighbouring cells lowers their water potential
-They take in water from their neighbours by osmosis
-This establishes a water potential gradient down which water moves

Question 3

How does water move up the xylem?

Answer 3

-Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
-Water molecules form hydrogen bonds between one another and hence tend to stick together (Cohesion)
-Water forms a continous, unbroken column across the mesophyll cells and down the xylem
-As evaporation occurs, more molecules of water are drawn up as a result of cohesion
-Transpiration pull occurs
-Negative pressure is created (Cohesion-tension theory)
-Transpiration is a passive process, so does not require metabolic energy to take place