Biology 9700 · AS & A Level · Homeostasis in plants
Homeostasis in plants — practice question
If the water supply to a plant falls, the concentration of abscisic acid (ABA) in the leaves rises.
ABA attaches to receptors in the cell surface membranes of guard cells. This starts a sequence of events inside the cells, leading to stomatal closure.
Fig. 8.1 is a diagram of part of a guard cell showing some of the events that take place when ABA attaches to its receptor.
Fig. 8.1 indicates that calcium ions take part in the events within a guard cell that lead to stomatal closure.
(a)[4]
Outline the part played by calcium ions in the guard cell response to a reduced water supply and explain how this response leads to stomatal closure.
Identify X in your answer.
(b)[4]
Explain, using examples, why homeostatic control of the opening and closing of stomata is important for efficient plant functioning.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Ca2+ acts as a second messenger” …