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OVERVIEW During photosynthesis, Light Energy is used to generate chemical Free Energy , stored in ATP and NADPH . 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 Compound s 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 Glycolysis and Gluconeogenesis , but they are to be found in the chlorophlast 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: :6 CO2 + 12 NADPH + 12 H+ + 18 ATP → C6H12O6 + 6 H2O + 12 NADP+ + 18 ADP + 18 Pi STEPS OF THE CALVIN CYCLE
(Simplified versions of the Calvin cycle integrate the remaining steps, except for the last one, into one general step - the regeneration of RuBP - also, one G3P would exit here.)
Up to this point, as per the overall equation given above, 6 carbon dioxide molecules would have been converted, with the use of 6 RuBP, 12 ATP and 12 NADPH, to 12 G3P molecules. One F6P, (= 2 G3P) then exits the cycle, while 10 of these G3P molecules continue, giving a ratio of 1:5 G3P. Obviously, the ratio of carbon dioxide entering the cycle to RuBP already present is also 1:5.
All the G3P produced earlier is converted into RuBP (5C), so 10 G3Ps (30C, 10 phosphates) were needed to produce 6 RuBPs (30C, 6 phosphates). 6 ATPs were also needed in the last step, giving a total of 18 ATPs used up per 6 CO2s. However, four phosphate ions are lost and these also form ATP. The energy in those ATPs is used to drive some of the reactions. At high temperatures, RuBisCO will react with O2 instead of CO2 in '' Photorespiration ''. This turns RuBP into 3PGA and 2-phosphoglycolate, a 2-carbon molecule which can be converted into 3PGA, some of which will exit the Calvin cycle. However, if this continues the RuBP will eventually be depleted, which slows down the cycle if electrons are entering from the light-dependent reaction too quickly. PRODUCTS OF THE CALVIN CYCLE The two G3P molecules (or one F6P molecule) which have exited the cycle are used to make carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P after exit, but this conversion is also part of the cycle. Hexose isomerase converts about half of the F6P molecules into Glucose-6-phosphate . These are dephosphorylated and the Glucose can be used to form Starch , which is stored in, for example, Potato es, or Cellulose used to build up Cell Walls . Other glucose, with Fructose , forms Sucrose , the plant sugar. REFERENCES
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