Question

What causes the diurnal changes in pH in the stroma of chloroplasts? Be as complete as possible

Answer 1

During light reactions of photosynthesis, a water molecule is split and protons are actively transported into the thylakoid lumen. Energy from the sunlight is required for the splitting of a water molecule and active transport of protons.

The proton transport occurs against a concentration gradient by an active transport mechanism.

pH = -log[H+]

High [H+] = Low pH

Low [H+] = High pH

Since the lumen has relatively high proton concentration, it has low pH compared to the stroma.