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:

The following observations have been made by scientists:

• Light stimulates proton (H⁺ ion) uptake into the grana of chloroplasts

• ABA-H can diff use into and out of chloroplasts, but ABA⁻ cannot.

This information is summarised in the diagram below.

a. Using all the information provided, predict what happens to the pH of the stroma in the light.

b. i. When light shines on the chloroplast, dissociation of ABA-H is stimulated. Explain why this happens.

ii. Explain the effect that this will have on diffusion of ABA-H into or out of the chloroplast.

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.

Transcribed Image Text:

АВА-Н АВА + Н* ABA-+H+