The Light-Dependent Reaction

Chlorophyll molcule absorbs light energy > boosting the energy of a pair of electrons within the chlorophyll molecule to a higher energy level. These electrons become so energetic they leave the chlorophyll molecule altogether and taken up by an electron carrier. Therefore the chlorophyll molecule has been oxidised and the elctron carrier reduced.

A series of oxidation-reduction reactions take place to pass the elctrons along the electron carriers. The electron carriers form a transfer chain located in the membranes of the thylakoids. Each new carrier is at a slightly lower energy level to the previous one, so electrons lose energy at each stage.This energy is used to combine an inorganic phosphate molecule to ADP, forming ATP.

When light hits the chlorophyll molecule and elctrons are lost, it becomes short of elctrons which need to be replaced if it is to go on absorbing light energy. The replacement electrons are supplied by water molecules that are split using light energy.

The photolysis of water also yields hydrogen ions (protons)

2H2O --> 4H+  +  4e-  +  O2

These hydrogen ions are taken up by an electron carrier called NADP. By taking up H ions the NADP is reduced. The reduced NADP then enters the LIR, along with elctrons from the chlorophyll molecules. Reduced NADP is a potential source of chemical energy to the plant. The oxygen by-product  is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis.

The LDR takes place in the thylakoids of the chloroplasts. Four of the ways in which chloroplasts are structurally adapted to their function of capturing sunlight for the LDR of photosynthesis are listed below:

      > Thylakoid memebranes form a large SA for the attachment of chlorophyll, electron                 carriers and enzymes that carry out the LDR.

      > Network of proteins in the grana hold chlorophyll in a very precise manner allowing               maximum absorption of light.

      > Granal memebranes have enzymes attached to them, which help manufacture ATP

      > Chloroplasts contain DNA and ribosomes so they can quickly & easily manufacture                 some of the proteins needed for the LDR.