Question

Which of the following statement is true with regard to the light reaction of photosynthetic mechanism in plants?

• A. Chlorophyll a occurs with peak absorption at 680 nm in photosystem I and at 700 nm in photosystem II.
• B. Magnesium and sodium ions are associated with photolysis of water molecules.
• C. $O_2$ is evolved during cyclic photophosphorylation.
• D. Photosystems I and II are both involved in non-cyclic photophosphorylation.
• E. Both ATP and $NADP{H}_2$ are formed during cyclic photophosphorylation.

Answer 1

The correct option is D Photosystems I and II are both involved in non-cyclic photophosphorylation.

PSII have molecules of chlorophyll a arranged with a geometry to facilitate absorption of shorter wavelength, higher energy photons then photosystem I. Photosystem II have absorption peak of approximately 680 nanometers while PS I reaction center has absorption peak of 700 nm. Photosystem II has P680 reaction center with associated Mn atoms. These Mn atoms are associated with an enzyme that splits water under sunlight (photolysis). Cyclic photophosphorylation is non-oxygenic photosynthesis in which electrons are passed from P700 of PS-I, via electron transfer chain, to back to P700 with the formation of ATP. Electrons are not passed to NADP reductase; no $NADP{H}_2$ is formed. Inorganic compounds, not water, serve as energy/electron source and oxygen is not released. Noncyclic photophosphorylation includes the flow of electron from PSII through quinone, cyt $b_6$ f complex and plastocyanin to NADPH via PSI; it does not return to PSII and includes both PSI and PSII. Thus, the correct answer is option D.