he Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps.
Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers (ATP and NADPH) are products of light-dependent reactions.
In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO2 is fixed from an inorganic to an organic molecule.
In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP+, respectively.
In the last stage of the Calvin Cycle, RuBP is regenerated, which enables the system to prepare for more CO2 to be fixed.
Stage 1: Fixation
In the stroma, in addition to CO2,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO) and three molecules of ribulose bisphosphate (RuBP). RuBP has five atoms of carbon, flanked by two phosphates. RuBisCO catalyzes a reaction between CO2 and RuBP. For each CO2
molecule that reacts with one RuBP, two molecules of 3-phosphoglyceric
acid (3-PGA) form. 3-PGA has three carbons and one phosphate. Each turn
of the cycle involves only one RuBP and one carbon dioxide and forms two
molecules of 3-PGA. The number of carbon atoms remains the same, as the
atoms move to form new bonds during the reactions (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation because CO2 is "fixed" from an inorganic form into organic molecules.
Stage 2: Reduction
ATP
and NADPH are used to convert the six molecules of 3-PGA into six
molecules of a chemical called glyceraldehyde 3-phosphate (G3P). This is
a reduction reaction because it involves the gain of electrons
by 3-PGA. Recall that a reduction is the gain of an electron by an atom
or molecule. Six molecules of both ATP and NADPH are used. For ATP,
energy is released with the loss of the terminal phosphate atom,
converting it to ADP; for NADPH, both energy and a hydrogen atom are
lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.
Stage 3: Regeneration
At
this point, only one of the G3P molecules leaves the Calvin cycle and
is sent to the cytoplasm to contribute to the formation of other
compounds needed by the plant. Because the G3P exported from the
chloroplast has three carbon atoms, it takes three "turns" of the Calvin
cycle to fix enough net carbon to export one G3P. But each turn makes
two G3Ps, thus three turns make six G3Ps. One is exported while the
remaining five G3P molecules remain in the cycle and are used to
regenerate RuBP, which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.
