Calvin cycle

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Calvin cycle

The Calvin cycle (or Calvin-Benson-Bassham cycle, light-independent reaction, or carbon fixation) is a series of biochemical

Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
reactions that take place in the stroma

Stroma (animal tissue)

In animal tissue, stroma refers to the connective, non-functional supportive framework of a biological cell , biological tissue, or Organ ....
of chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
s in photosynthetic

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
organism

Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
s. It was discovered by Melvin Calvin

Melvin Calvin

Melvin Ellis Calvin was an United States 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....
, James Bassham

James Bassham

James Alan Bassham was born in Sacramento, California, November 26, 1922. He received a B.S. degree in chemistry in 1945 from the University of California and his Ph.D....
and Andrew Benson

Andrew Benson

Andrew Benson is a scientist who, along with Melvin Calvin and James Bassham, elucidated the path of carbon assimilation in plants. The carbon reduction cycle is known as the Calvin cycle, which inappropriately ignores the contribution of Bassham and Benson....
at the University of California, Berkeley

University of California, Berkeley

The University of California, Berkeley is a public university research university located in Berkeley, California, California, United States. The oldest of the ten major campuses affiliated with the University of California, Berkeley offers some 300 undergraduate and graduate degree programs in a wide range of disciplines....
. It is one of the light-independent reaction

Light-independent reaction

Photosynthesis, the light-independent reactions, are chemical reactions that convert carbon dioxide and other Chemical compounds into glucose. They occur in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes....
s or dark reactions.

ng photosynthesis, light

Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
energy

Energy

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Encyclopedia

The Calvin cycle (or Calvin-Benson-Bassham cycle, light-independent reaction, or carbon fixation) is a series of biochemical

Biochemistry

Stroma (animal tissue)

In animal tissue, stroma refers to the connective, non-functional supportive framework of a biological cell , biological tissue, or Organ ....
of chloroplast

Chloroplast

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
organism

Organism

Melvin Calvin

James Bassham

James Alan Bassham was born in Sacramento, California, November 26, 1922. He received a B.S. degree in chemistry in 1945 from the University of California and his Ph.D....
and Andrew Benson

Andrew Benson

University of California, Berkeley

Light-independent reaction

Overview

During photosynthesis, light

Light

Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
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

Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....
and water

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
into organic compound

Organic compound

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered Inorganic compound....
sthat can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation
Carbon fixation

Carbon fixation is a process found in autotrophs , usually driven by photosynthesis, whereby carbon dioxide is changed into organic materials. Carbon fixation can also be carried out by the process of calcification in marine, calcifying organisms such as Emiliania huxleyi....
. The key enzyme

Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
of the cycle is called RuBisCO

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of fuel molecules such as sucrose....
. 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

Gluconeogenesis

Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactic acid, glycerol, and glucogenic amino acids....
and the pentose phosphate pathway

Pentose phosphate pathway

The pentose phosphate pathway is a process that serves to generate NADPH and the synthesis of pentose sugars. There are two distinct phases in the 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

Primary production

Primary production is the production of organic compounds from atmospheric or aquatic carbon dioxide, principally through the process of photosynthesis, with chemosynthesis being much less important....
.

The sum of reactions in the Calvin cycle is the following:

3 CO₂ + 6 NADPH

Nicotinamide adenine dinucleotide phosphate

Nicotinamide adenine dinucleotide phosphate is used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 5 H₂O + 9 ATP

Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
? C₃H₅O₃-PO₃^2- + 2 H⁺ + 6 NADP⁺

Nicotinamide adenine dinucleotide phosphate

Nicotinamide adenine dinucleotide phosphate is used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 9 ADP

Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleotide. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate Functional group, the pentose sugar ribose, and the nucleobase adenine....
+ 8 P_i

3 CO₂ + 6 C₂₁H₂₉N₇O₁₇P₃ + 5 H₂O + 9 C₁₀H₁₆N₅O₁₃P₃ ? C₃H₅O₃-PO₃^2- + 2 H⁺ + 6 NADP⁺

Nicotinamide adenine dinucleotide phosphate

Nicotinamide adenine dinucleotide phosphate is used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
+ 9 C₁₀H₁₅N₅O₁₀P₂ + 8 P_i

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," to be specific, glyceraldehyde-3-phosphate (G3P).

Steps of the Calvin cycle

The enzyme RuBisCO
RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of fuel molecules such as sucrose....
catalyses the carboxylation of Ribulose-1,5-bisphosphate
Ribulose-1,5-bisphosphate

Ribulose-1,5-bisphosphate is an important substrate involved in carbon fixation. The enzyme ribulose bisphosphate carboxylase catalyzes RuBP with carbon dioxide in order to synthesize a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, which decays virtually instantaneously into two molecules of...
, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction . The initial product of the reaction is a six-carbon intermediate so unstable that it immediately splits in half, forming two molecules of glycerate 3-phosphate
Glycerate 3-phosphate

3-Phosphoglyceric acid , or glycerate 3-phosphate , is a biochemistry significant 3-carbon molecule that is a metabolic intermediate in both glycolysis and the Calvin cycle....
, a 3-carbon compound. (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA)
The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP
Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
(which was produced in the light-dependent stage). 1,3-bisphosphoglycerate
1,3-Bisphosphoglycerate

1,3-Bisphosphoglycerate , also known as PGAP, 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 Cellular respiration and the Calvin cycle during photosynthesis....
(glycerate-1,3-bisphosphate) and ADP
Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleotide. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate Functional group, the pentose sugar ribose, and the nucleobase adenine....
are the products. (However, note that two PGAs are produced for every CO₂ that enters the cycle, so this step utilizes 2ATP
Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
per CO₂ fixed.
The enzyme G3P dehydrogenase catalyses the reduction
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate

Glyceraldehyde 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 CO₂ 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 some G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
Aldolase
Aldolase

Aldolase A is an enzyme which catalyses one of the aldol reactions: The substrate , fructose 1,6-bisphosphate is broken down into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate ....
and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate
Fructose 6-phosphate

Fructose 6-phosphate is fructose sugar phosphorylated on carbon 6 . The ?-D-form of this compound is very common in cell . 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 CO₂ generates two more G3P.
F6P has two carbons removed by transketolase
Transketolase

Transketolase, 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 transketolase
Transketolase

Transketolase, 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 CO₂ 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 CO₂ generates two more G3P. The ketose S7P has two carbons removed by transketolase
Transketolase

Transketolase, 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 transketolase
Transketolase

Transketolase, 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 CO₂, with generation of three pentoses which can be converted to Ru5P.
R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase
Phosphopentose isomerase

Phosphopentose isomerase is an enzyme which converts ribulose 5-phosphate to ribose 5-phosphate .In the pentose phosphate pathway, it converts the aldose to the ketose....
. Xu5P is converted into RuP by phosphopentose epimerase
Phosphopentose epimerase

Phosphopentose 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....
.
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 totalling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO₂.

RuBisCO

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of fuel molecules such as sucrose....
also reacts competitively with O₂ instead of CO₂ in photorespiration
Photorespiration

Photorespiration is the alternate pathway for production of glyceraldehyde 3-phosphate by RuBisCO, the main enzyme of the light-independent reactions of photosynthesis ....
. The rate of photorespiration is higher at high temperatures. "photorespiration

Photorespiration

Photorespiration is the alternate pathway for production of glyceraldehyde 3-phosphate by RuBisCO, the main enzyme of the light-independent reactions of photosynthesis ....
" 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 +CO₂. 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 CO₂, this process leads to loss of CO₂. C4 carbon fixation

C4 carbon fixation

C4 carbon fixation is one of three biochemical mechanisms, along with C3 carbon fixation and CAM photosynthesis, functioning in land plants to "fix" carbon dioxide for sugar production through photosynthesis....
evolved to circumvent photorespiration, but can occur only in certain plants living in very warm or tropical climates.

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 in to glucose-6-phosphate

Glucose-6-phosphate

Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cell as the vast majority of glucose entering a cell will become phosphorylated in this way....
. These are dephosphorylated and the glucose

Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
can be used to form starch

Starch

File:Amylose2.svgFile:Amylopektin Sessel.svgStarch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds....
, which is stored in, for example, potato

Potato

The potato is a starchy, tuberous crop from the perennial plant Solanum tuberosum of the Solanaceae family. The word potato may refer to the plant itself as well....
es, or cellulose

Cellulose

File:Cellulose Sessel.svgCellulose is an organic compound with the chemical formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand ? linked D-glucose units....
used to build up cell walls. Glucose, with fructose

Fructose

Fructose is a simple Reducing sugar sugar found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose....
, forms sucrose

Sucrose

Sucrose is a disaccharide of glucose and fructose, with the molecular formula C12H22O11. Its systematic name is a-D-glucopyranosyl- -?-D-fructofuranoside ....
, a non-reducing sugar that, unlike glucose, is a stable storage sugar

Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
.

Calvin cycle

Overview

Steps of the Calvin cycle

Products of the Calvin cycle

See also