Which of the following statements is incorrect ?

Correct Answer:
"Cyclic photophosphorylation involves both PS I and PS II." is the incorrect statement.

Step-by-step Explanation:

To determine which statement is incorrect, let us analyze the processes of light reactions in photosynthesis:

1. Non-Cyclic Photophosphorylation (Z-scheme):
This process involves both Photosystem I (PS I) and Photosystem II (PS II) working in series. During this pathway, electrons expelled from PS II do not return to it but are ultimately transferred to NADP⁺ to form NADPH + H⁺. Along the electron transport chain, a proton gradient is established, driving the synthesis of ATP. Therefore, both ATP and NADPH + H⁺ are synthesized during non-cyclic photophosphorylation. This makes the first statement correct.

2. Stroma Lamellae Membranes:
The stroma lamellae membranes (stroma thylakoids) lack PS II and the enzyme NADP reductase. They possess only PS I. Because they lack PS II and NADP reductase, they cannot perform non-cyclic electron flow or reduce NADP⁺ to NADPH. Instead, they perform cyclic photophosphorylation to produce ATP. This makes the second statement correct.

3. Grana Lamellae Membranes:
The grana lamellae (appressed regions of thylakoids) contain both Photosystem I (PS I) and Photosystem II (PS II). This makes the third statement correct.

4. Cyclic Photophosphorylation:
Cyclic photophosphorylation occurs when only PS I is functional. The electron expelled from the reaction center (P700) of PS I passes through a series of electron carriers and eventually cycles back to the same reaction center (P700). It does not involve PS II. Because the electrons cycle back, there is no photolysis of water (which is associated with PS II) and no synthesis of NADPH (as NADP reductase is absent or bypassed in this cycle). It results only in the synthesis of ATP.
Therefore, the statement "Cyclic photophosphorylation involves both PS I and PS II" is scientifically incorrect, as it only involves PS I.