The reactions of photosynthesis can be divided into two main stages: the light reactions, which convert light energy into chemical energy the Calvin cycle (sometimes called the dark or carbon reactions), which uses the products of the light reactions to produce sugar In this tutorial, you will identify the inputs and outputs of each stage, describe the oxidation-reduction (redox) reactions in the light reactions and Calvin cycle, and identify the cellular compartments in which these reactions occur. Before beginning this tutorial, watch the Photosynthesis animation. Pay close attention to the role of light, the formation of NADPH and ATP, and the use of NADPH and ATP in sugar production.

Photosynthesis occurs in two stages: light-dependent and light-independent stages. In the light stage which occurs in the thylakoid of the chloroplast, chlorophyll molecule (consisting of Photosystem I, P680 and II, P700) reacts with light (from sun) to yield electron and becomes activated (positively charged). The electron, proton (H+) and NADP+ reacts to produce NADPH, an hydrogen-carrier which enters the dark stage. Electrons from the activated Chlorophyll molecule splits water molecules into protons (builds up in the thylakoid), oxygen and electron. These electrons return to Chlorophyll molecule and deactivates it. As protons move along the thylakoid, a proton pump is formed, generating energy used to synthesize ATP from ADP and Pi (inorganic phosphate).

N.B: The light stage produces NADPH and ATP, which are needed for the synthesis of glucose in the dark stage. The light stage starts with light, water, NADP+, ADP. Oxygen is released as a kind of exhaust gas.

In the dark stage, so called because it does not require light to occur, CO2 (enters the the stomata) is assimilated by a CO2 acceptor molecule called Ribulosephosphate (RUBP), catalyzed by Ribulosephosphate carboxylase (RUBISCO) enzyme. This forms an unstable 6-carbon molecule which is soon broken down into a 3-carbon compound called 3-phosphoglycerate (PGA). The formation of this compound is the reason for naming this stage C3 cycle. This PGA undergoes further reduction and phosphorylation using NADPH and ATP as sources of hydrogen and energy respectively in a series of reaction to form Glyceraldehyde-3-phosphate (G3P) which is utterly used to produce one molecule of glucose for every 6 molecules of CO2 absorbed by RUBP. RUBP must be regenerated in order to proceed with the cycle.

This Calvin or C3 cycle which occurs in the stroma of the CHLOROPLAST starts with CO2, NADPH, ATP and C5 (RuBP). It ends with glucose (C6H12O6), NADP+, ADP, Pi and the same RUBP which returns to accept CO2 for the cycle to continue.