Light-dependent reaction

In photosynthesis, a light-dependent reaction uses light energy from the sun to split water which has been taken in by the organism. Water, when split, gives oxygen, hydrogen, and electrons. These electrons move through structures in chloroplasts and, by chemiosmosis, make ATP.

The hydrogen is converted to NADPH, which is then used in the light-independent reactions. Oxygen gas diffuses out of the organism as a waste product. This all happens in the grana thylakoid of chloroplasts.

 The movement of electrons 
Light hits the chloroplast. It absorbs light and traps it.
Chlorophyll channels the light down to a reaction center.
An electron at the reaction center is excited to a higher energy level, and is received by an electron acceptor. This electron is taken from the splitting of water: ( → ½ + 2H+ + 2e-)
The electron is passed along a series of electron carriers. It is moving down energy levels and losing energy. This energy causes the pumping of hydrogen from the cytoplasm of chlorophyll into thylakoid spaces inside the grana. The hydrogen diffuses and flows back into the cytoplasm through protein channels. As the hydrogen diffuses down a concentration gradient, ATP is made from ADP and inorganic phosphate. 
Eventually, the electron is used to reduce NADP to NADPH along with hydrogen from photolysis.

History
Colin Flannery first put forward the idea that photosynthesis needs light in 1779. He realised that sunlight falling on plants was needed. Joseph Priestley had noted the release of oxygen gas in 1772, but he did not see the link with light.
In 1931 Cornelius Van Niel proposed that photosynthesis is a case of a general mechanism where a photon of light is used to photodecompose a hydrogen donor, with the hydrogen being used to reduce .
In 1939 Robin Hill found that isolated chloroplasts would make oxygen but not fix , showing that the light and dark reactions occurred in different places. This led later to the discovery of photosystem 1 and 2.

 Related pages 
Photosynthesis
Light-independent reactions

References

Category:Metabolism
Category:Cellular respiration