Component 3.1 - Stomatal Mechanism

Question 1

Describe the Malate cycle

Answer 1

1) chloroplasts photosynthesise producing ATP
2) ATP gives energy for K+ ions to enter the guard cells by active transport from surrounding epidermal cells
3) Stored starch is converted into malate
4) K+ and malate lower the water potential, so water enters the guard cells by osmosis
5) Guard cells expand where walls are thinner. Thicker walls are opposite each other on the inside creating a pore

Reverse happens at night

Question 2

What is the function of guard cells?

Answer 2

Guard cells control the opening and closing of the stomata

Question 3

Why are stomata on the lower surface of the leaf?

Answer 3

Plants lose water by evaporation through their stomata in a process called transpiration. Sunlight on the upper surface of the leaf would increase evaporation and cause plants to wilt. So having them on the lower surface minimises water loss.

Question 4

Why do stomata close and in what situations do they close?

Answer 4

Gas exchange and water lose both happen through Stomata and the plants must balance the conflicting needs of gas exchange and water loss, so they close :

1) At night when there is insufficient light for photosynthesis
2) In bright light as this is usually accompanied by intense heat which increases evaporation
3) If there is excessive water loss