Light Reaction of Photosynthesis

The process of photosynthesis take place in two steps :

Light reaction includes the following steps, i.e., absorption of light, splitting of water, release of oxygen and finally the formation of high energy chemical intermediates, i.e., ATP and NADPH.

Absorption of Light :

Photosystems : - The light harvesting complexes or photosystems are made up of hundreds of pigment molecules bounded by proteins. Each photosystem has a photocentre or reaction centre, where actual reaction takes place.

This reaction centre contains a special chlorophyll-a molecule. It is fed by hundred other pigment molecules and it forms the light harvesting system called antennae. These antennae molecule absorb light of different wavelength, but shorter then reaction centre in order to make photosynthesis more efficient.

The reaction centre is different in both the photosystems as given below

In PS-I, the reaction centre or chlorophyll-4 has peak of absorption at 700 nm, known as .

In PS-II, th reaction centre has absorption peak at 680 nm hence, called .

Electron Transport : - The photosynthetic electron transport chain initiates with the absorption of light energy by the photosystem-II. The  light of wavelength 680 nm is absorbed by the reaction centre of photosystem-II due to which a pair of electrons becomes excited and jump into an orbit away from the atomic nucleus. These electrons are then picked up by an electron acceptor, which passes them further to electron transport system consisting of cytochromes.

It is to be noted that this movement of electrons is downhill according to the redox potential scale (oxidation-reduction scale). The electrons of the electron transport chain are not used up in the chain instead they are further passed on to the pigment of  PS-I.

Now, like the PS-II, the electrons in the reaction centre of PS-I also get excited on receiving red light of wavelength 700 nm and get transferred to the another electron acceptor with higher redox potential.

But this time the electron does not move to a reaction centre or chlorophyll-a. Instead, it moves to a molecule rich in energy and . On addition of these electrons the gets reduced to .

In 1960, Bendall and Hill discovered the Z-scheme of electron transport. It is a series of reactions that we have just studied above from the whole scheme of electron transfer initiating from PS-II, uphill to the acceptor molecule, down the electron transport chain to PS-I, excitation of electrons and then their transfer to another acceptor and finally downhill to NADP in order to get reduced to .

Splitting of Water: - The electrons are continuously supplied to the photosystems-II by the available electrons, which get replaced due to the splitting of water.

In this process, the water splits into protons, electrons and oxygen. The complex for water splitting is associated with the photosystems-II that is located on the inner side of the thylakoid membrane.  and  ions also play important role in the photolysis of water molecule.

These electrons thus, obtained by the splitting of water are needed to replace those electrons which are removed from the photosytems-I thus, are provided by photosystem-II.                                                                

While all the electrons formed, are replaced, the protons accumulated in the lumen of the thylakoid and the oxygen is evolved into the atmosphere.

Photophosphorylation: -  Phosphorylation is the process through which, ATP is synthesised from ADP and inorganic phosphate (P) by the cell organelles (like mitochondria and chloroplasts). When it occurs in the presence of sunlight in chloroplast, it is called photophosphorylation.

Phosphorylation in mitochondria is not light dependent, but it uses the energy by oxidation of nutrients to produce ATP, hence it is called oxidative phosphorylation.

The process of photophosphorylation is of two types :

1. Non-cyclic Photophosphorylation : -  Non-cyclic photophosphorylation is a type of photophosphorylation in which both the photosystems (PS-I and PS-II) in cooperate light driven synthesis of ATP. During this cycle, the electron released from PS-II does not return back to it hence, it is known as non-cyclic photophosphorylation. Both NADPH and ATP are formed during this reaction, are used in light independent reaction and are called reducing power.

2. Cyclic Photophosphorylation : -  It is the type of photophosphorylation in which only PS-I is taking part and the electron released from the reaction centre .returns back to it after passing through a series of carrier, i.e., circulation takes within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.