These electrons are used to produce NADPH as well as ATP in a series of reactions called the light reactions because they require light. NADPH and ATP formed by the action of light then reduce carbon dioxide and convert it into 3-phosphoglycerate by a series of reactions called the Calvin cycle or the dark reactions.

The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

Light Reactions: Light-dependent reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where the Calvin cycle takes place.

In the Calvin cycle, carbon atoms from CO2​start text, C, O, end text, start subscript, 2, end subscript are fixed (incorporated into organic molecules) and used to build three-carbon sugars. This process is fueled by, and dependent on, ATP and NADPH from the light reactions.

RUBISCO

Photorespiration depends on relative concentration of oxygen and carbon dioxide gases. These plants maintain high concentration of carbon dioxide in the bundle sheath cells and absence of grana ensures no release of oxygen due to photolysis. Thus photorespiration is absent in C4 plants.

Photorespiration wastes energy and steals carbon Two molecules are produced: a three-carbon compound, 3-PGA, and a two-carbon compound, phosphoglycolate. Photorespiration results in a loss of 3 fixed carbon atoms under these conditions, while the Calvin cycle results in a gain of 6 fixed carbon atoms.