Calvin cycle

In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes.

The Calvin cycle uses the chemical energy of ATP and the reducing power of NADPH from the light-dependent reactions to produce sugars for the plant to use.

The Calvin cycle thus happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and crassulacean acid metabolism (CAM)); CAM plants store malic acid in their vacuoles every night and release it by day to make this process work.

The cycle was discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14.

In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecule ATP and the moderate-energy hydrogen carrier NADPH.

In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.

Although many texts list a product of photosynthesis as C6H12O6, this is mainly for convenience to match the equation of aerobic respiration, where six-carbon sugars are oxidized in mitochondria.

The three steps involved are:[citation needed] The next stage in the Calvin cycle is to regenerate RuBP.

Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose.

C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example.

Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.

The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available.

This magnesium ion is released from the thylakoid lumen when the inner pH drops due to the active pumping of protons from the electron flow.

RuBisCo activase itself is activated by increased concentrations of ATP in the stroma caused by its phosphorylation.