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Encyclopedia > Calvin cycle
Overview of the Calvin cycle and carbon fixation
Overview of the Calvin cycle and carbon fixation

The Calvin cycle (or Calvin-Benson 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 and Andrew Benson at the University of California, Berkeley with James Bassham also contributing.[1] It is one of the light-independent reactions or dark reactions. Image File history File links Download high resolution version (856x742, 51 KB)This image was copied from wikipedia:en. ... Image File history File links Download high resolution version (856x742, 51 KB)This image was copied from wikipedia:en. ... Biochemistry (from Greek: , bios, life and Egyptian kēme, earth[1]) is the study of the chemical processes in living organisms. ... Stroma can refer to: The connective supportive framework of a biological cell, tissue, or organ. ... Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. ... The leaf is the primary site of photosynthesis in plants. ... Life on Earth redirects here. ... Melvin Calvin he had fun in bed Melvin Calvin (April 8, 1911 – January 8, 1997) was a chemist most famed for discovering the Calvin cycle (along with Andrew Benson), for which he was awarded the 1961 Nobel Prize in Chemistry. ... Andrew Benson was a scientist who, along with Melvin Calvin and James Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. ... Sather tower (the Campanile) looking out over the San Francisco Bay and Mount Tamalpais. ... Overview of the Calvin cycle and carbon fixation In photosynthesis, the light-independent reactions, also somewhat misleadingly called the dark reactions (they dont require darkness, but they do require the products of the light reactions), are chemical reactions that convert carbon dioxide and other compounds into glucose. ...

Contents

Overview

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. For other uses, see Light (disambiguation). ... The free energy is a measure of the amount of mechanical (or other) work that can be extracted from a system, and is helpful in engineering applications. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Benzene is the simplest of the arenes, a family of organic compounds An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. ... Carbon fixation is a process found in autotrophs, usually driven by photosynthesis, whereby carbon dioxide is converted into organic compounds. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme (EC 4. ...


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 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. Glycolysis is the sequence of reactions that converts glucose into pyruvate with the concomitant production of a relatively small amount of ATP. The word is derived from Greek γλυκύς (sweet) and λύσις (letting loose). ... Pyruvic acid Oxaloacetic acid Phosphoenolpyruvate Fructose 1,6-bisphosphate Fructose 6-phosphate Glucose-6-phosphate Glucose Gluconeogenesis is the generation of glucose from non-sugar carbon substrates like pyruvate, lactate, glycerol, and amino acids (primarily alanine and glutamine). ... Look up Respiration in Wiktionary, the free dictionary. ... Global oceanic and terrestrial photoautotroph abundance, from September 1997 to August 2000. ...


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

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 equations 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.


Steps of the Calvin cycle

  • The enzyme RuBisCO catalyses the carboxylation of Ribulose-1,5-bisphosphate, a 5 carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction [1]. Rubisco is a large, slow enzyme averaging 3 substrate per second compared to 1000/s of most other enzymes in the Calvin cycle. Two molecules of glycerate 3-phosphate, a 3-carbon compound, are created. (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA)
  • The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP (which was produced in the light-dependent stage). 1,3-bisphosphoglycerate (glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two PGAs are produced for every CO2 that enters the cycle, so this step happens twice.)
  • The enzyme G3P dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which was another product of the light-dependent stage). Glyceraldehyde 3-phosphate (also G3P, GP) is produced, and the NADPH itself was oxidized and hence becomes NADP+.

(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.) Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme (EC 4. ... Categories: Stub | Photosynthesis ... Glycerate 3-phosphate (GP) or 3-phosphoglycerate (3PG). ... ATP may refer to: Chemistry/Biochemistry Adenosine triphosphate, the universal energy currency of all known living organisms Companies Alberta Theatre Projects, a major Canadian theatre company. ... 1,3-Bisphosphoglycerate (1,3BPG), also known as PGAP, is a 3-carbon organic molecule present in most, if not all, living organisms. ... Adenosine diphosphate, abbreviated ADP, is a nucleotide. ... ed|other uses|reduction}} Illustration of a redox reaction Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. ... Nicotinamide adenine dinucleotide (NAD+) Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are two important coenzymes found in cells. ... G3P (structure) Glyceraldehyde 3-phosphate (G3P) is an intermediate in both glycolysis and gluconeogenesis. ...

  • Aldolase and Fructose-1,6-bisphosphatase convert some of these two into fructose-6-phosphate (6C). A phosphate ion is lost into solution.

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. Triose phosphate isomerase is an enzyme which, during glycolysis, converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate. ... DHAP (or Dihydroxyacetonephosphate) is a biochemical compound involved in many reactions, from the Calvin Cycle in plants to the ether-lipid biosynthesis process in Leishmania mexicana. ... Fructosephosphates are sugar phosphates based upon fructose. ...

  • F6P is then combined with another G3P (total 9C) and then cleaved into xylulose-5-phosphate (X5P) and erythrose-4-phosphate by transketolase.
  • E4P and DHAP are converted into sedoheptulose-1,7-bisphosphate (7C) by a transaldolase enzyme.
  • S17BPase cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.
  • S7P is then combined with another G3P (total 10C) and then cleaved into another X5P and ribose-5-phosphate (R5P) again by transketolase.
  • X5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose epimerase. R5P is also converted into RuP by ribose isomerase.
  • Finally, phosphoribulokinase phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.

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. Xylulose 5-phosphate is an intermediate in the pentose phosphate pathway. ... Erythrose 4-phosphate is an intermediate in the pentose phosphate pathway. ... Transketolase, an enzyme that transfer a 2 Carbon unit(the critical part that determine it is a ketone)from a ketose to an aldose. ... Sedoheptulose 7-phosphate is an intermediate in the pentose phosphate pathway. ... Ribose 5-phosphate is an intermediate in the pentose phosphate pathway. ... Ribulose 5-phosphate is one of the end products of the pentose phosphate pathway. ... Phosphopentose epimerase is an enzyme which intraconverts ribulose 5-phosphate and xylulose 5-phosphate. ...


RuBisCO also reacts competatively with O2 instead of CO2 in photorespiration. The rate of photorespiration is higher at high temperatures. "photorespiration" 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. Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme (EC 4. ... Photorespiration refers to the alternate pathway for production of Glyceraldehyde 3-phosphate (G3P) by Rubisco, the main enzyme of the light-independent reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). ... Photorespiration refers to the alternate pathway for production of Glyceraldehyde 3-phosphate (G3P) by Rubisco, the main enzyme of the light-independent reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). ...


The cycle has to be repeated for six times, because each time the cycle is done one atom of carbon is produced, so six carbon atoms are needed for the production of fructose and other similar plant compounds that are consisted of exactly six carbon atoms. This Process is also referred to as a DARK Cycle Reaction. For other uses, see Carbon (disambiguation). ... Fructose (or levulose) is a simple sugar (monosaccharide) found in many foods and is one of the three most important blood sugars along with glucose and galactose. ...


Products of the Calvin cycle

The immediate product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P) and water. Two G3P molecules (or one F6P molecule) that have exited the cycle are used to make larger carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P or F5P 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, potatoes, or cellulose used to build up cell walls. Other glucose, with fructose, forms sucrose, the plant sugar. Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. ... Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. ... Starch (CAS# 9005-25-8, chemical formula (C6H10O5)n,[1]) is a mixture of amylose and amylopectin (usually in 20:80 or 30:70 ratios). ... For other uses, see Potato (disambiguation). ... Cellulose as polymer of β-D-glucose Cellulose in 3D Cellulose (C6H10O5)n is a polysaccharide of beta-glucose. ... A cell wall is a more or less solid layer surrounding a cell. ... Fructose (or levulose) is a simple sugar (monosaccharide) found in many foods and is one of the three most important blood sugars along with glucose and galactose. ... Flash point N/A Except where noted otherwise, data are given for materials in their standard state (at 25 Â°C, 100 kPa) Infobox disclaimer and references Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C12H22O11. ... This article is about sugar as food and as an important and widely traded commodity. ...


See also

The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. ... Photorespiration refers to the alternate pathway for production of Glyceraldehyde 3-phosphate (G3P) by Rubisco, the main enzyme of the light-independent reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). ... Overview of C4 carbon fixation C4 carbon fixation is one of three methods, along with C3 and CAM photosynthesis, used by land plants to fix carbon dioxide (binding the gaseous molecules to dissolved compounds inside the plant) for sugar production through photosynthesis. ...

References

  1. ^ Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis". J Biol Chem 185 (2): 781-7. PMID 14774424. 
  • Bassham, J.A. (2003). Mapping the carbon reduction cycle: a personal retrospective. Photosynthesis Research, volume 76, pages 25-52 (see: Entrez PubMed 16228564).Mario Otmman (1998)
  • Diwan, Joyce J. (2005). Photosynthetic Dark Reaction at [2]

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