light reaction

In light reaction, ATP and NADPH2 are produced and in the dark reaction, CO2 is reduced with the help of ATP and NADPH2 to produce glucose. The light reaction is called primary photochemical reaction as it is induced by light. Light reaction is also called as Hill’s reaction as Hill proved that chloroplast produce O2 from water in the presence of light. It is also called as Arnon’s cycle because Arnon showed that the H+ ions released by the breakdown of water are used to reduce the coenzyme NADP to NADPH. 

Light reaction includesphotophosphorylation as ATP is synthesized in the presence of light. The reaction takes place only in the presence of light in grana portion of the chloroplast and it is faster than dark reaction. The chlorophyll absorbs the light energy and hence the chlorophyll is called as photosystem or pigment system. 

Chlorophylls are of different types and they absorb different wavelengths of light. Accordingly, chlorophylls exist in two photo systems, Photosystem I (PSI) and Photosystem II (PS II). Both photo systems are affected by light with wavelengths shorter than 680nm, while PS I is affected by light with wavelengths longer than 680nm.

The light reaction can be studied under the following headings.

i. Absorption of light energy by chloroplast pigments

Different chloroplast pigments absorb light in different regions of the visible part of the spectrum.

ii. Transfer of light energy from accessory pigments to chlorophyll a All the photosynthetic pigments except chlorophyll a are called as accessory or antenna pigments. The light energy absorbed by the accessory pigments is transferred by resonance to chlorophyll a which alone can take part in photochemical reaction. Chlorophyll a molecule can also absorb the light energy directly. In pigment system I, the photoreaction centre is P700 and in pigment system II, it is P680.

iii. Activation of chlorophyll molecule by photon of light

When P700 or P680 forms of chlorophyll a receives a photon (quantum) of light,

becomes an excited molecule having more energy than the ground state energy. After

passing through the unstable second singlet state and first singlet stage the chlorophyll

molecules comes to the meta stable triplet state. This excited state of chlorophyll molecule

takes part further in primary photochemical reaction i.e. the electron is expelled from the

chlorophyll a molecule.

 Light

Chlorophyll a Excited triplet state of chlorophyll a

Excited triplet state of chlorophyll a Chlorophyll a++ e-

iv. Photolysis of water and O2 evolution (oxidation of water)

These processes are associated with pigment system II and are catalyzed by Mn++ and

Cl-

 ions. When pigment system II is active i.e it receives the light, the water molecules split

into OH- and H+ ions (Photolysis of water). The OH- ions unite to form some water molecules again and release O2 and electrons.

v. Electron transport and production of assimilatory powers (NADPH2 and ATP)

It has already been observed that when chlorophyll molecule receives the photon of

light, an electron is expelled from the chlorophyll a molecule along with extra energy. This

electron after traveling through a number of electron carriers is utilized for the production of

NADPH2 from NADP and also utilized for the formation of ATP molecules from ADP and

inorganic phosphate (Pi). The transfer of electrons through a series of coenzymes is called

electron transport and the process of formation of ATP from ADP and Pi using the energy of

electron transport is called as photosynthetic phosphorylation or photophosphorylation. The

types of Phosphorylation include cyclic and non- cyclic.