What is Light Reaction?

Light reactions, also called the 'photochemical' phase, include several events: light absorption, water splitting, oxygen release, and the formation of the high-energy chemical intermediates ATP and NADPH. Several protein complexes are involved in these processes.

The pigments are organised into two discrete photochemical light harvesting complexes (LHC), one within Photosystem I (PS I) and the other within Photosystem II (PS II). Importantly, these photosystems are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction.

Each light harvesting complex is built as follows:

• The LHC is made up of hundreds of pigment molecules bound to proteins.
• Each photosystem holds all the pigments except one molecule of chlorophyll a, forming a light harvesting system also called the antennae.
• These antenna pigments help make photosynthesis more efficient by absorbing different wavelengths of light and passing the energy inward.

The single chlorophyll a molecule that is left forms the reaction centre. The reaction centre is different in the two photosystems:

• In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, and is therefore called P700. …