Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
10 Abscisic acid (ABA) is a weak acid. Its structure can be represented as ABA-H. It dissociates into positively
charged H + ions (protons) and negatively charged ABA − ions as shown:
ABA-H ABA − + H +
The following observations have been made by scientists:
• light stimulates proton (H + ion) uptake into the grana of chloroplasts
• ABA-H can diffuse into and out of chloroplasts, but ABA − cannot.
This information is summarised in the diagram below.
light
chloroplast
envelope
granum
ABA − + H +
328
ABA–H
diffusion
ABA–H
stroma
ABA − + H +
a Using all the information provided, predict what happens to the pH of the stroma in the light. [1]
b i When light shines on the chloroplast, dissociation of ABA-H is stimulated. Explain why this happens. [2]
ii Explain the effect that this will have on diffusion of ABA-H into or out of the chloroplast. [2]
When the mesophyll cells of leaves become dehydrated, some of the ABA stored in the chloroplasts is released
into the transpiration stream in the apoplast.
c ABA travels in the apoplast pathway to the guard cells. Explain why this is an advantage when the leaf is dehydrated. [2]
[Total: 7]
11 a Describe the structure of a kidney, including its associated blood vessels. [6]
b Describe the mechanisms involved in reabsorption in the proximal convoluted tubule and describe
how the epithelial cells of the proximal convoluted tubule are adapted to carry out this process. [9]
[Total: 15]
Cambridge International AS and A Level Biology 9700 Paper 41, Question 10, May/June 2012