The light-dependent reactions, or photoreduction, is the first stage of photosynthesis, is a process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH. In the light-independent reactions, the formed NADPH and ATP drive the reduction of CO
2 to more useful organic compounds, such as glucose. However, although light-independent reactions are, by convention, also called dark reactions, they are not independent of the need of light, for they are driven by ATP and NADPH, products of light. They are often called the Calvin Cycle or C3 Cycle.
The light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.
The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.
Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle by night. They, instead, release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work.