TheCalvin cycle,light-independent reactions,bio synthetic phase,dark reactions, orphotosynthetic carbon reduction (PCR)cycle[1] ofphotosynthesis is a series of chemical reactions that convertcarbon dioxide and hydrogen-carrier compounds intoglucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in thestroma, the fluid-filled region of achloroplast outside thethylakoid membranes. These reactions take the products (ATP andNADPH) oflight-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and thereducing power of NADPH from the light-dependent reactions to producesugars for the plant to use. These substrates are used in a series of reduction-oxidation (redox) reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle:carboxylation, reduction reactions, andribulose 1,5-bisphosphate (RuBP) regeneration.
Though it is also called the "dark reaction", the Calvin cycle does not occur in the dark or during nighttime. This is because the process requiresNADPH, which is short-lived and comes fromlight-dependent reactions. In the dark, plants instead releasesucrose into thephloem from theirstarch reserves to provide energy for the plant. The Calvin cycle thus happens when light is available independent of the kind of photosynthesis (C3 carbon fixation,C4 carbon fixation, andcrassulacean acid metabolism (CAM)); CAM plants storemalic acid in their vacuoles every night and release it by day to make this process work.[2]
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The reactions of the Calvin cycle are closely coupled to thethylakoid electron transport chain,[3] as the energy required to reduce the carbon dioxide is provided byNADPH produced during thelight dependent reactions. The process ofphotorespiration, also known as C2 cycle, is also coupled to the Calvin cycle, as it results from an alternative reaction of theRuBisCO enzyme, and its final byproduct is anotherglyceraldehyde-3-P molecule.
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TheCalvin cycle,Calvin–Benson–Bassham (CBB) cycle,reductive pentose phosphate cycle (RPP cycle) orC3 cycle is a series ofbiochemicalredox reactions that take place in thestroma ofchloroplast inphotosyntheticorganisms. The cycle was discovered in 1950 byMelvin Calvin,James Bassham, andAndrew Benson at theUniversity of California, Berkeley by using theradioactiveisotopecarbon-14.[4]
Photosynthesis occurs in two stages in a cell. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage moleculeATP and the moderate-energy hydrogen carrierNADPH. The Calvin cycle uses these compounds to convertcarbon dioxide andwater intoorganic compounds[5] that can be used by the organism (and by animals that feed on it). This set of reactions is also calledcarbon fixation. The keyenzyme of the cycle is calledRuBisCO. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by thepH.[citation needed]
The enzymes in the Calvin cycle are functionally equivalent to most enzymes used in other metabolic pathways such asgluconeogenesis and thepentose phosphate pathway, but the enzymes in the Calvin cycle are found in the chloroplast stroma instead of the cellcytosol, 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 being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions when they have nonet productivity.[citation needed]
The sum of reactions in the Calvin cycle is the following:[citation needed]
Hexose (six-carbon) sugars are not products of the Calvin cycle. Although many texts list a product of photosynthesis asC
6H
12O
6, this is mainly for convenience to match the equation ofaerobic 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", namely,glyceraldehyde-3-phosphate (G3P).[citation needed]
In the first stage of the Calvin cycle, a CO2 molecule is incorporated into one of two three-carbon molecules (glyceraldehyde 3-phosphate or G3P), where it uses up two molecules ofATP and two molecules ofNADPH, which had been produced in the light-dependent stage. The three steps involved are:[citation needed]
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The next stage in the Calvin cycle is to regenerateRuBP. Five G3P molecules produce threeRuBP molecules, using up three molecules of ATP. Since each CO2 molecule produces two G3P molecules, three CO2 molecules produce six G3P molecules, of which five are used to regenerateRuBP, leaving a net gain of one G3P molecule per three CO2 molecules (as would be expected from the number of carbon atoms involved).[citation needed]
.svg)
The regeneration stage can be broken down into a series of steps.
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. This requires nine ATP molecules and six NADPH molecules per threeCO
2 molecules. The equation of the overall Calvin cycle is shown diagrammatically below.[citation needed]
RuBisCO also reacts competitively withO
2 instead ofCO
2 inphotorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine plusCO
2. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3-PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixingCO
2, this process leads to loss ofCO
2.C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Furthermore, RuBisCOs catalyzing the light-independent reactions of photosynthesis generally exhibit an improved specificity for CO2 relative to O2, in order to minimize the oxygenation reaction. This improved specificity evolved after RuBisCO incorporated a new protein subunit.[9]
The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP+. (ADP and NADP+ are not really "products". They are regenerated and later used again in thelight-dependent reactions). Each G3P molecule is composed of 3 carbons. For the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose. Therefore, there is only 1 net carbon produced to play with for each turn. To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle. To make one glucose molecule (which can be created from 2 G3P molecules) would require 6 turns of the Calvin cycle. Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.[10]
These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH.[11]
There are two regulation systems at work when the cycle must be turned on or off: thethioredoxin/ferredoxin activation system, which activates some of the cycle enzymes; and theRuBisCo enzyme activation, active in the Calvin cycle, which involves its own activase.[12]
The thioredoxin/ferredoxin system activates the enzymes glyceraldehyde-3-P dehydrogenase, glyceraldehyde-3-P phosphatase, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinase, which are key points of the process. This happens when light is available, as the ferredoxin protein is reduced in thephotosystem I complex of the thylakoid electron chain when electrons are circulating through it.[13] Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing acystine bond found in all these enzymes. This is a dynamic process as the same bond is formed again by other proteins that deactivate the enzymes. 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.[citation needed]
The enzyme RuBisCo has its own, more complex activation process. It requires that a specificlysine amino acid becarbamylated to activate the enzyme. This lysine binds toRuBP and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, calledRuBisCo activase, helps this carbamylation process by removing one proton from the lysine and making the binding of the carbon dioxide molecule possible. Even then the RuBisCo enzyme is not yet functional, as it needs a magnesium ion bound to the lysine to function. 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 itsphosphorylation.[14]
The results suggested that the integrity of thylakoid membranes may be essential for the organisation of sequential enzymes of the Calvin cycle in vivo and facilitate their functioning.
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