Calvin cycle
Encyclopedia
The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical
redox
reactions that take place in the stroma
of chloroplast
s in photosynthetic
organism
s. It is also known as the "dark reactions".
The cycle was discovered by Melvin Calvin
, James Bassham
, and Andrew Benson
at the University of California, Berkeley
by using the radioactive isotope
carbon-14
. It is one of the light-independent (dark) reactions
, used for carbon fixation
.
and NADPH. The light-independent Calvin cycle 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 that feed on it). This set of reactions is also called carbon fixation
. The key enzyme
of the cycle is called RuBisCO
. In the following biochemical 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 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:
Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as , 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," namely, glyceraldehyde-3-phosphate (G3P).
(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.)
Thus, of 6 G3P produced, three RuBP (5C) are made, totaling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 .
RuBisCO
also reacts competitively with instead of in photorespiration
. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3PGA 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 +. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing , this process leads to loss of . C4 carbon fixation
evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.
). Each G3P molecule is composed of 3 carbons. In order for the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5/6 carbon 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.
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reactions that take place in the stroma
Stroma (fluid)
Stroma, in botany, refers to the colourless fluid surrounding the grana within the Plastid, chloroplast.Within the stroma are grana, stacks of thylakoids, the sub-organelles, where photosynthesis is commenced before the chemical changes are completed in the stroma.Photosynthesis occurs in two...
of chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s in photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s. It is also known as the "dark reactions".
The cycle was discovered by Melvin Calvin
Melvin Calvin
Melvin Ellis Calvin was an American chemist most famed for discovering the Calvin cycle along with Andrew Benson and James Bassham, for which he was awarded the 1961 Nobel Prize in Chemistry. He spent most of his five-decade career at the University of California, Berkeley.- Life :Calvin was born...
, James Bassham
James Bassham
James Alan Bassham James Alan Bassham James Alan Bassham (born November 26, 1922 in Sacramento, California is an American scientist known for his work on photosynthesis.He received a B.S. degree in chemistry in 1945 from the University of California and his Ph.D. degree in 1949...
, and Andrew Benson
Andrew Benson
Andrew Alm Benson is an American biologist and a professor of biology at the University of California, San Diego until his retirement in 1989...
at the University of California, Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
by using the radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...
. It is one of the light-independent (dark) reactions
Light-independent reaction
The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions...
, used for carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
.
Overview
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecules ATPAdenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
into organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
. The key enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
of the cycle is called RuBisCO
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
.
The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as gluconeogenesis
Gluconeogenesis
Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....
and the pentose phosphate pathway
Pentose phosphate pathway
The pentose phosphate pathway is a process that generates NADPH and pentoses . There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars...
, 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 being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
.
The sum of reactions in the Calvin cycle is the following:
- 3 + 6 NADPHNicotinamide adenine dinucleotide phosphateNicotinamide adenine dinucleotide phosphate, abbreviated NADP or TPN in older notation , is a coenzyme used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 5 + 9 ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
→ glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+Nicotinamide adenine dinucleotide phosphateNicotinamide adenine dinucleotide phosphate, abbreviated NADP or TPN in older notation , is a coenzyme used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 9 ADPAdenosine diphosphateAdenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
+ 8 Pi- or
- 3 + 6 + 5 + 9 → - + 2 + 6 NADP+Nicotinamide adenine dinucleotide phosphateNicotinamide adenine dinucleotide phosphate, abbreviated NADP or TPN in older notation , is a coenzyme used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 9 + 8 Pi
Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as , 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," namely, glyceraldehyde-3-phosphate (G3P).
Steps of the Calvin cycle
- The enzyme RuBisCORuBisCORibulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
catalyses the carboxylation of ribulose-1,5-bisphosphateRibulose-1,5-bisphosphateRibulose-1,5-bisphosphate is an organic substance that is involved in photosynthesis. The anion is a double phosphate ester of the ketose called ribulose. Salts of this species can be isolated, but its crucial biological function involves this colourless anion in solution...
, RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction. The product of the first step is enediol-enzyme complex that can capture or . Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of 3-phosphoglycerate, or PGA, a 3-carbon compound (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA). - The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(which was produced in the light-dependent stage). 1,3-Bisphosphoglycerate1,3-Bisphosphoglycerate1,3-Bisphosphoglyceric acid is a 3-carbon organic molecule present in most, if not all, living organisms. It primarily exists as a metabolic intermediate in both glycolysis during respiration and the Calvin cycle during photosynthesis...
(glycerate-1,3-bisphosphate) and ADPAdenosine diphosphateAdenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
are the products. (However, note that two PGAs are produced for every that enters the cycle, so this step utilizes two ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
per fixed.) - The enzyme G3P dehydrogenase catalyses the reductionRedoxRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphateGlyceraldehyde 3-phosphateGlyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GADP, GAP, TP, GALP or PGAL, is a chemical compound that occurs as an intermediate in several central metabolic pathways of all organisms...
(also G3P, GP, TP, PGAL) is produced, and the NADPH itself was oxidized and becomes NADP+. Again, two NADPH are utilized per fixed.
(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.)
- Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
- AldolaseAldolaseAldolase A is an enzyme that catalyses a reverse aldol reaction: The substrate, fructose 1,6-bisphosphate is broken down into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate . This reaction is a part of glycolysis. Three aldolase isozymes , encoded by three different genes, are...
and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphateFructose 6-phosphateFructose 6-phosphate is fructose sugar phosphorylated on carbon 6 . The β-D-form of this compound is very common in cells. The vast majority of glucose and fructose entering a cell will become converted to this at some point...
(6C). A phosphate ion is lost into solution. - Then fixation of another generates two more G3P.
- F6P has two carbons removed by transketolaseTransketolaseTransketolase, an enzyme of both the pentose phosphate pathway in animals and the Calvin cycle of photosynthesis, catalyzes two important reactions, which operate in opposite directions in these two pathways...
, giving erythrose-4-phosphate. The two carbons on transketolaseTransketolaseTransketolase, an enzyme of both the pentose phosphate pathway in animals and the Calvin cycle of photosynthesis, catalyzes two important reactions, which operate in opposite directions in these two pathways...
are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P). - E4P and a DHAP (formed from one of the G3P from the second fixation) are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
- Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique to plants) cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.
- Fixation of a third generates two more G3P. The ketose S7P has two carbons removed by transketolaseTransketolaseTransketolase, an enzyme of both the pentose phosphate pathway in animals and the Calvin cycle of photosynthesis, catalyzes two important reactions, which operate in opposite directions in these two pathways...
, giving ribose-5-phosphate (R5P), and the two carbons remaining on transketolaseTransketolaseTransketolase, an enzyme of both the pentose phosphate pathway in animals and the Calvin cycle of photosynthesis, catalyzes two important reactions, which operate in opposite directions in these two pathways...
are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 , with generation of three pentoses that can be converted to Ru5P. - R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerasePhosphopentose epimerasePhosphopentose epimerase is an enzyme which intraconverts ribulose 5-phosphate and xylulose 5-phosphate.It plays a role in the Calvin cycle and the pentose phosphate pathway.-External links:* *...
. - Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.
Thus, of 6 G3P produced, three RuBP (5C) are made, totaling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 .
RuBisCO
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
also reacts competitively with instead of in photorespiration
Photorespiration
Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...
. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3PGA 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 +. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing , this process leads to loss of . C4 carbon fixation
C4 carbon fixation
C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.
Products of the Calvin cycle
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 the Light-dependent reactionsLight-dependent reactions
The 'light-dependent reactions', or light reactions, are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH...
). Each G3P molecule is composed of 3 carbons. In order for the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5/6 carbon 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.
See also
- Light-independent reactionLight-independent reactionThe light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions...
- Light-dependent reactionsLight-dependent reactionsThe 'light-dependent reactions', or light reactions, are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH...
- Citric Acid CycleCitric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
- PhotorespirationPhotorespirationPhotorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...
- C4 carbon fixationC4 carbon fixationC4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
- Nitrogen FixationNitrogen fixationNitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...