Q.(a) Differentiate Cyclic Photophosphorylation and Non-cyclic Photophosphorylation. OR

Non-cyclic photophosphorylation uses both photosystems to generate ATP, NADPH, and O2, whereas cyclic photophosphorylation uses only Photosystem I in a closed loop to generate ATP alone, without NADPH or O2 evolution.

During the light reactions of photosynthesis, light energy absorbed by the photosystems is used to move electrons and generate ATP (photophosphorylation). This happens in two ways:

Non-cyclic Photophosphorylation:

• Involves both Photosystem II (PS II) and Photosystem I (PS I) working together in series.
• Begins with the splitting (photolysis) of water at PS II, which releases electrons, protons (H+), and oxygen (O2) as a by-product.
• Electrons flow: Water → PS II → electron transport chain → PS I → further electron transport → NADP+ reductase → NADPH.
• The electron flow is linear/one-directional (non-cyclic), replacing the electrons lost from PS I with new electrons ultimately derived from water.
• Produces both ATP (via the proton gradient generated across the thylakoid membrane) and NADPH, and releases O2.
• Occurs mainly in the grana (appressed) region of the thylakoid membrane.

Cyclic Photophosphorylation:

• Involves only Photosystem I (PS I).
• The electron ejected from excited PS I, instead of being passed on to NADP+, is cycled back to PS I itself through a chain of electron carriers (like cytochrome b6f complex). …