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Light-independent Reaction
OTHER NAMES: Dark Reaction & Calvin Cycle
OCCURENCE: stroma (a fluid-filled compartment where sugars are made from carbon dioxide; located in the chloroplast)
CONDITION FOR OCCURENCE: absence (or presence) of light
MAJOR CONSEQUENCES: the chemical bond energy generated in the Light reaction is released and stored in a more stable form - bonds of sugars and other nutrients.
MAJOR STEPS IN LIGHT-INDEPENDENT REACTION:
1. The enzyme rubisco (which is the most abundant protein on earth and which makes up about 25% of the protein present in the chloroplast) combines carbon dioxide with RuBP (Ribulose biphosphate), a five-carbon sugar molecule. This process is called CO2 fixation.
2. Three molecules of carbon dioxide combine with 3 molecules of RuBP to make 6 molecules of PGA (phosphoglycerate) with 3 carbons each.
3. In two steps, energy from ATP (Adenosine Triphosphate) & NADPH is used to make 6 high-energy molecules of PGAL (phosphoglyceraldehyde) with 3 carbons each.
4. In further reactions, two molecules of PGAL combine to form 1 molecule of glucose and other sugars.
5. Only one molecule of PGAL is used to make glucose. The other 5 are used to generate RuBP.
6. Much of the ATP made during the light-dependent reactions is used to recycle the C in PGAL into RuBP.
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The Calvin cycle (or Calvin-Benson-Bassham cycle or carbon fixation) is a series of biochemical reactions that takes place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham and Andrew Benson at the University of California, Berkeley .It is one of the light-independent reactions or dark reactions.
During photosynthesis, light energy is used to generate chemical free energy, stored in glucose. The light-independent Calvin cycle, also (misleadingly) known as the "dark reaction" or "dark stage", uses the energy from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals which feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.
The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast stroma instead of the cell cytoplasm, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from operating in reverse to respiration, which would create a continuous cycle of carbon dioxide being reduced to carbohydrates, and carbohydrates being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.
The sum of reactions in the Calvin cycle is the following:
3 CO2 + 6 NADPH + 5 H2O + 9 ATP ? C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi
OR
3 CO2 + 6 C21H29N7O17P3 + 5 H2O + 9 C10H16N5O13P3 ? C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 C10H15N5O10P2 + 8 Pi
It should be noted that hexose (six carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C6H12O6, this is mainly a convenience to counter the equation of respiration, where six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin Cycle are three-carbon sugar phosphate molecules, or "triose phosphates," specifically, glyceraldehyde-3-phosphate.
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