light dependent reactions and light independent or dark reactions.
The light reaction takes place in the thylakoid discs.
We'll look at a simpler example of photosynthesis first, and use it as an introduction to photosynthesis in plants and cyanobacteria (blue-green algae). Although the primary reactions of photosynthesis take place at "photosynthetic reaction centers," the first level of interaction of light with an organism that carries out photosynthesis is at an assembly of chlorophyll molecules that "harvest" light (the "light-harvesting complex"). Such an assembly results in a greater chance that photons will be captured and, because of the strategic arrangement of the individual chlorophyll and other accessory light-absorbing molecules, the transfer of energy to the photosynthetic reaction center is very fast (-10 s) and very efficient (>90%).
PHOTOSYNTHESIS Light and Dark Reaction song song.
Both the LHC and the reaction centers are membrane bound structures but there are no chloroplasts in the purple photosynthetic bacteria. The electron transfer processes occur within the cell membrane and the overall process is a cyclic one (i.e., there is no net oxidation-reduction). Protons are transferred across the membrane, from the cytoplasmic side to the outside, establishing a proton gradient whose dissipation drives ATP synthesis. A similar situation holds for the cyanobacteria and plants, but in these organisms, the process occurs in chloroplasts and the overall reaction is not a cyclic one.
high light intensity increases the rate of photosynthesis
Light reactions need light to produce organic energy molecules (ATP and NADPH). They are initiated by colored pigments, mainly green colored chlorophylls.
Photosynthesis - Light-Dependent Reactions
It turns out that the overall reaction of photosynthesis takes place in two steps. In the first step, the oxygen in water is oxidized by the light energy: