Question 7b (i)(ii)
(i) When chloroplasts are exposed to light, the dissociation of ABA-H into ABA− and H+ increases. Explain why this occurs.
(ii) Describe how this increase in ABA-H dissociation affects the movement of ABA-H into or out of the chloroplast.
(i) Light exposure stimulates the uptake of H+ ions into the grana of chloroplasts. As protons are drawn into the grana, the H+ concentration in the stroma decreases, causing a shift in the equilibrium to favor the dissociation of ABA-H into ABA− and H+ to balance the proton levels.
(ii) With more ABA-H dissociating into ABA− and H+, there is a reduction in the concentration of ABA-H within the chloroplast. This concentration gradient favors the diffusion of ABA-H from outside the chloroplast into the stroma to maintain equilibrium.