The two stages of photosynthesis are light reactions and the Calvin cycle; light reactions take place first, forming the photo portion of photosynthesis, while the Calvin cycle follows, completing the cycle with several steps involving photosynthesis. Photosynthesis is defined as one process, but contains two distinct stages, which in turn break down into a series of steps. During the steps in the light reaction stage, energy from the sun converts into chemical energy, while the Calvin cycle brings carbon dioxide fixation.
The process of light reactions begins when chlorophyll in plants absorbs light. Electrons and hydrogen from molecules then translate to NADP, a receptor for charged electrons. During this process, water separates from gases, which in turn produces oxygen as a by-product of the light reaction stage.
Successive steps in the light reaction phase see the conversion of NADP molecules into NADPH compounds. This transformation stems from the addition of multiple electrons to existing NADP molecules, ultimately changing their chemical structures. Light reactions also produce the substance ATP from molecules of ADP by adding a second phosphate group.
The Calvin cycle first fixes carbon, then reduces amounts of carbon in cells through conversion. This conversion includes the exchange of carbon into carbohydrates. Plants then use carbohydrates, or sugars, immediately or store them for future use.