Photosynthesis in Higher Plants

| 199



Single chlorophyll a acts as reaction centre.



In PS I, the reaction centre absorbs light at 700 nm, and so called P700.



In PS II, the reaction centre absorbs light at 680 nm, and so called P680.

LÏGHT RËÅÇTÏØÑ (PHØTØÇHËMÏÇÅL PHÅSË)

Photon

Reaction

centre

Pigment

molecules

Primary acceptor

Electron

transport

system

-

-

e acceptor

e acceptor

Light

Photosystem II

Photosystem I

NADPH

NADP ⁺

LHC

LHC

H O 2e + 2H + [O]

2

-

+

ADP+iP

ATP



Light reactions include light absorption, water splitting, oxygen release

and formation of ATP & NADPH (high-energy chemical intermediates).

(NEET 2018)

1. THË ËLËÇTRØÑ TRÅÑSPØRT

When PS II absorbs red light of 680 nm wavelength, electrons are

excited and transferred to an electron acceptor.

The electron acceptor passes them through a chain of electron

transport system consisting of cytochromes.

This movement of electrons is downhill, in terms of redox potential

scale.

Then electrons are transferred to the pigments of PS I.

Simultaneously, electrons in PS I are also excited when they receive

red light of 700 nm and are transferred to another acceptor

molecule having a greater redox potential.

These electrons are moved downhill to a molecule of NADP⁺. As a

result, NADP⁺ is reduced to NADPH + H⁺.

Transfer of electrons from PS II to PS I and finally downhill to NADP⁺ is

called the Z scheme, due to its zigzag shape. This shape is formed when

all the carriers are placed in a sequence on a redox potential scale.