Photosynthesis occurs in 3 main stages: energy is captured from sunlight, then its converted into chemical energy and stored in organic molecules.Photosystems are light harvesting units of the thylakoid membrane. Each photosystem has four hundred pigment molecules.

A photon hits the pigment molecules and travels through each molecule and then reaches the reaction center as shown in figure 1-1.The thylakoid membrane has two photosystems, photosystem 1 & 2. The reaction center in photosystem 1 is called P700 because this pigment is best at absorbing 700 mm light. Photosystem 2s reaction center is called P680 because it is best at absorbing 680 mm light.

The Calvin cycle begins by incorporating Co2 from the air into molecules in the chloroplast. This action is called carbon fixation. The next thing it does is add electrons to the fixed carbon to make it carbohydrate. The power of the Calvin cycle comes from NADPH. The Calvin cycle also needs energy from ATP, which is generated by light reactions.

This is reaction is called dark reactions because it does not directly need light to work. So basically all the Calvin cycle does is make sugar with out light but with help from ATP & NADPH.