Mechanism of Photosynthesis

1.0There are two types of reactions in photosynthesis

1. Light reaction

2. Dark reaction

Photosynthesis is a redox reaction during which oxidation of H₂O occurs (as it provides H⁺ and e^–) during light reaction and reduction of CO₂ occurs (as it accepts H⁺ & e^–) during dark reaction (biosynthetic phase).

Photosynthesis is an anabolic (synthesising) & endergonic (Energy absorbing) process.

2.0What is Light Reaction?

Light reactions or the 'Photochemical' phase include light absorption, water splitting, oxygen release and formation of high energy chemical intermediates like ATP and NADPH.

Several protein complexes are involved in the process. The groups of photosynthetic pigments in thylakoid membranes are known as photosystems. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II(PS II)  . These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae or LHC. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700 while in PS II it has absorption maxima at 680 nm and is called P680.

Photosystem = Reaction centre + LHC               

In every photosystem, there is a reaction centre (molecule of chlorophyll –a) surrounded by accessory pigments. The PS II is located in the appressed region of granal thylakoids and PS I in non-appressed region of grana and in stroma thylakoids. (In the other way, we can say that granal thylakoids have both PS I and PS II whereas stroma thylakoids have only PS I).

3.0Difference Between PS-I & PS-II