Transpiration

Question 1

What is transpiration?

Answer 1

• Transpiration is the loss of water vapour from the leaves and stems of plants.
• Water diffuses out of the leaves down the water potential gradient into the air through the stomata.
• Leaves have a large SA for photosynthesis and their surfaces have a waxy cuticle that makes them waterproof.
• This prevents leaves from losing water rapidly by evaporation. But gas exchange needs to occur so photosynthesis is possible (CO2 is taken in).
• Guard cells control the movement of water vapour by changing the shape of stomatal opening.

Question 2

Transpiration stream

Answer 2

• Movement of water from its uptake in the roots to its loss in the leaves.
  1) Water molecules evaporate from surface of mesophyll cells and move through air spaces in leaf and out of stomata by diffusion.
  2) This loss of water lowers water potential so water moves from neighbouring cell into cell that lost water by osmosis.
  3) This is repeated all the way back to the xylem.
  4) Adhesion and cohesion result in water being pulled up the xylem and this is called Capillary Action where water moves against force of gravity. This creates transpiration pull.
  5) Water then moves into roots then into xylem.

Question 3

Adhesion

Answer 3

Where water molecules form bonds with the xylem vessel walls.

Question 4

Cohesion

Answer 4

Where water molecules form hydrogen bonds with other water molecules.

Question 5

Transpiration pull

Answer 5

Constant chain of water molecules moved from roots to leaves by cohesion.

Question 6

Cohesion-Tension theory

Answer 6

• It is the model of movement of water in a continuous stream.
  Evidence:
• When xylem vessel is broken air is drawn in instead of water leaking.
• When vessel is broken, air is drawn in and no water can b pulled up as chain of cohesive forces is broken.