Question

When does evolution of $O_2$ occur?

• A. Cyclic photophosphorylation
• B. Non-cyclic photophosphorylation
• C. Splitting of water
• D. Both b and c

Answer 1

The correct option is D Both b and c

Non-cyclic Photophosphorylation invoves both the photosystems. When PS II receives a light of wavelength 680nm, its outermost electron gets excited and is released from the reaction centre. PSII is now unstable and needs to acquire electrons to compensate for the lost one, to be stable.
Splitting of $H_{2}O$ occurs in the membrane of the thylakoids. When the chloroplasts (chlorophyll pigment containing plastids) are illuminated with light, 2 molecules of $H_{2}O$ are split into 4$H^{+}$, 4$e^{-}$ and 1 $O_2$ is released. These electrons are used to replenish the electrons lost from PS II.
The reaction is as follows
$2H_{2}O\underset{\mathrm{chlorophyll}}{\overset{\mathrm{sunlight}}{\to }}4H^{+}+4e^{-}+O_2$
Cyclic Photophosphorylation:- This involves PS I only. Cyclic photophosphorylation occurs when plants have enough of NADPH and need only ATP without using NADPH to yield ATP. There is no splitting of water involved.