The entire process of photosynthesis takes place inside the chloroplast. The structure of chloroplast is such that the light dependent (light reaction) and light independent (dark reaction) reactions take place at different sites in the same organelle.
The thylakoids have the pigments and other necessary components to absorb light and transfer electrons to carry out the light reaction or Electron Transport Chain (ETC). In ETC upon absorption of light, the electrons from PSII and PSI are excited to a higher energy level i.e. the electrons acquire excitation energy.
As the electrons gain this energy, they are accepted by the electron acceptor which in turn is reduced, leaving the reaction centres of PSII and PSI i.e. P680 and P700 molecules in an oxidized state. This represents the conversion of light energy into chemical energy. The electrons then travel downhill in energy terms, from one electron acceptor to another in a series of oxidation-reduction reaction.
This electron flow is coupled to the formation of ATP. In addition, NADP is reduced to NADPH2. The product of light reaction is called the reducing power or assimilatory power (ATP and NADPH2) which move out of the thylakoid into the stroma of the chloroplast.
In the stroma, the second step called as dark reaction or biosynthetic pathway occurs, where CO2 is reduced by the reducing power generated in the first step and carbohydrates are produced.
After receiving light PSII absorbs light energy and passes it on to its reaction centre, P680. When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule (Primary electron acceptor i.e. pheophytin) and it itself comes to the ground state. However by losing an electron P680 is oxidized and in turn it splits water molecule to release O2.
This light dependent splitting of water is called photolysis. With the breakdown of water electrons are generated, which are then passed on to the electron deficient P680 (which had transferred its electrons earlier). Thus the oxidized P680 regains its lost electrons from water molecules. The reduced primary acceptor now donates electrons to the down stream components of the electron transport chain. The electrons are finally passed onto the reaction centre P700 or PSI.
During this process, energy is released and stored in the form of ATP. Similarly, PSI also gets excited when it absorbs light and P700 (Reaction centre of PSI) gets oxidized as it transfers its electrons to another primary acceptor molecule. While the oxidized P700 draws its electrons from PSII, the reduced primary acceptors molecule of PSI transfers its electrons via other electron carrier to NADP (Nicotinamide Adenine Dinucleotide Phosphate) to produce NADPH2 a strong reducing agent.
Reduction of CO2 to carbohydrate also requires ATP, which too are generated via electron transport chain. As the energy rich electrons pass down the electron transport system, it releases energy which is sufficient to bind inorganic phosphate (Pi) with ADP to form ATP. This process is called photo-phosphorylation. Since this takes place in presence of light it is called Photo-phosphorylation. It occurs in chloroplast in two ways:
(a) Non-cyclic photophosphorylation where electrons flow from water molecule to PSII and then to PSI and ultimately reduce NADP to NADPH2. Since the electron flow is unidirectional and the electrons released from one molecule do not return to the same molecule, it is called non-cyclic photosphorylation.
(b) Cyclic photophosphorylation occurs in photosynthetic bacteria which lack PS-II, and it involves PSI only. During this process electrons from PSI are not passed on to NADP. Instead the same electrons are returned to the oxidized P700 molecule. During this downhill movement of electrons ATP formation takes place. Thus this is termed as cyclic photophosphorylation.
In higher photosynthetic plants, extra ATP can be made via cyclic photophosphorylation if cyclic and non-cyclic photophosphorylaiton occur side by side. The efficiency of energy conversion in the light reactions of photosynthesis is high and estimated at about 39%.