Question

Proton gradient helps in:

• A. ATP synthesis
• B. NADPH synthesis
• C. Providing electrons to PS-ll
• D. Providing oxygen to PS-ll

Answer 1

The correct option is A ATP synthesis

During photosynthetic electron transport and also due to splitting of water, protons accumulate inside the thylakoid lumen. The number of protons is high inside the lumen of thylakoids when compared to the stroma of chloroplast. This creates a proton gradient and the breakdown of this gradient generates ATP. The gradient is broken down due to the movement of protons across the membrane into the stroma through ATP synthase. The protons pass through a channel of the CF${}_0$-CF${}_1$ of ATP synthase enzyme. This movement of protons from a region of high concentration (thylakoid lumen) to low concentration (stroma) activates the enzyme and helps in the synthesis of ATP.


NADPH is synthesised in the stroma of chloroplast from NADP+ with the help of NADP reductase enzyme. NADP+ accepts electrons from PS-I and utilising protons from stroma gets reduced to NADPH. This is one of the assimilatory powers produced during light reaction and the other one is ATP.

Electrons lost by photosystem II are regained by electrons released due to photolysis of water (splitting of water).

Splitting of water leads to the evolution of oxygen which diffuses out through stomata. Oxygen is not required by PS-II.