Transport of water

Question 1

Describe the structure of xylem

Answer 1

Xylem vessels consist of dead cells. They have a thick cellulose wall stiffened by lignin and a hollow lumen.

Question 2

Explain how xylem tissue is adapted for its function.

Answer 2

long cells / tubes with no end walls;
continuous water columns; no cytoplasm / no organelles / named organelle
; to impede / obstruct flow / allows easier water flow; thickening / lignin; support / withstand tension / waterproof / keeps water in cells; pits in walls;
allow lateral movement / get round blocked vessels;

Question 3

Define transpiration

Answer 3

Transpiration is the evaporation of water vapour from aerial parts of the plant (e.g. leaves).

Question 4

Describe the process of transpiration

Answer 4

• Water vapour diffuses out through the stomata.
• There is a water potential gradient between the atmosphere and the air spaces next to the stomata.
• Consequently, water will diffuse out of the air spaces in the leaf to the surrounding air.
• This water will be replaced with water from surrounding mesophyll cells – this causes water to move through the leaf

Question 5

State the factors hat increase the rate of transpiration

Answer 5

• Increase light intensity
• Increase temperature
• Increase wind intensity
• Decrease humidity

Question 6

Describe and explain what happens to the rate of transpiration as we increase the light intensity

Answer 6

As the light intensity increases, the rate of transpiration increases, as the stomata opens wider to allow more carbon dioxide into the leaf for photosynthesis.

Question 7

Describe and explain what happens to the rate of transpiration as we increase the temperature

Answer 7

Transpiration is faster in higher temperatures as evaporation and diffusion are faster at higher temperatures because the particles have more kinetic energy and so move quicker.

Question 8

Describe and explain what happens to the rate of transpiration as we increase the wind intensity

Answer 8

Transpiration is faster in windy conditions as water vapour is removed quickly by air movement, speeding up diffusion by increasing the difference in concentration between the outside and inside of the leaf.

Question 9

Describe and explain what happens to the rate of transpiration as we increase the humidity

Answer 9

Transpiration is slower in humid conditions as diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air because the concentration gradient is less.

Question 10

Explain the cohesion tension theory of water transport\

Answer 10

• Water lost from leaf because of transpiration
• Lowers water potential of mesophyll/leaf cells;
• Water pulled up xylem (creating tension);
• Water molecules cohere /’stick together by hydrogen bonds;
  (forming continuous) water column;
• Adhesion of water (molecules) to walls of xylem;