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Light of wavelength beyond 680 nm alone excites

(a) PS I mainly Correct
(b) PS II only Your answer
(c) both photosystems equally Your answer
(d) neither photosystem Your answer

Your answer: Correct answer: (a)

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Explanation

Why (a) is correct

Light beyond 680 nm is absorbed mainly by PS I (P700), not PS II. This is why only cyclic flow can run in such light.

Why the other options are wrong

(b) · your answer PS II absorbs up to 680 nm and works poorly beyond it.
(c) · your answer PS II is hardly excited beyond 680 nm.
(d) · your answer PS I is excited by light up to about 700 nm.

NCERT point: Cyclic photophosphorylation occurs when only light of wavelength beyond 680 nm is available for excitation.

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