The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
is a series of chemical reactions that convert
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
and hydrogen-carrier compounds into
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the
stroma, the fluid-filled region of a
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
outside the
thylakoid membranes
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...
. These reactions take the products (
ATP and
NADPH
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...
) of
light-dependent reactions
Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...
and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of to a sugar. There are three phases to the light-independent reactions, collectively called the ''Calvin cycle'': carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.
Though it is called the "dark reaction", the Calvin cycle does not actually occur in the dark or during night time. This is because the process requires NADPH, which is short-lived and comes from the light-dependent reactions. In the dark, plants instead release
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
into the
phloem
Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...
from their
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
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
carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, along with and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosph ...
,
C4 carbon fixation
carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when sup ...
, and
Crassulacean Acid Metabolism (CAM)); CAM plants store
malic acid
Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms ...
in their vacuoles every night and release it by day to make this process work.
Coupling to other metabolic pathways
The reactions of the Calvin cycle are closely coupled to the thylakoid electron transport chain as the energy required to reduce the carbon dioxide is provided by NADPH produced during the
light dependent reactions. The process of
photorespiration
Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction i ...
, also known as C2 cycle, is also coupled to the Calvin cycle, as it results from an alternative reaction of the
RuBisCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
enzyme, and its final byproduct is another glyceraldehyde-3-P molecule.
Calvin cycle
The Calvin cycle, Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate cycle (RPP cycle) or C3 cycle is a series of
biochemical
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
reactions that take place in the
stroma of
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
in
photosynthetic
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s. The cycle was discovered in 1950 by
Melvin Calvin
Melvin Ellis Calvin (April 8, 1912 – January 8, 1997) was an American biochemist known 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 hi ...
,
James Bassham
James Alan Bassham (November 26, 1922 – November 19, 2012) was an American scientist known for his work on photosynthesis.
He received a B.S. degree in chemistry in 1945 from the University of California, Berkeley, earning his Ph.D. degree ...
, and
Andrew Benson
Andrew Alm Benson (September 24, 1917 – January 16, 2015) was an American biologist and a professor of biology at the University of California, San Diego, until his retirement in 1989. He is known for his work in understanding the carbon cy ...
at the
University of California, Berkeley
The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...
by using the
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
carbon-14
Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic 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 coll ...
.
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 molecule
ATP and the moderate-energy hydrogen carrier
NADPH
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...
. The Calvin cycle uses these compounds to convert
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
and
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
into
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. 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''. The key
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
of the cycle is called
RuBisCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
. 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 most enzymes used in other metabolic pathways such as
gluconeogenesis
Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...
and the
pentose phosphate pathway
The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-pho ...
, but the enzymes in the Calvin cycle are found in the chloroplast stroma instead of the cell
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
, 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 no
net productivity.
The sum of reactions in the Calvin cycle is the following:
:3 + 6
NADPH
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...
+ 6 H
+ + 9
ATP + 5 →
glyceraldehyde-3-phosphate
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central metabolic pathway, pathways of all o ...
(G3P) + 6
NADP+ + 9
ADP + 8 P
i (P
i = inorganic
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
)
Hexose (six-carbon) sugars are not products of the Calvin cycle. Although many texts list a product of photosynthesis as , this is mainly for convenience to match the equation of
aerobic respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
, 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
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central metabolic pathway, pathways of all o ...
(G3P).
Steps
In the first stage of the Calvin cycle, a molecule is incorporated into one of two three-carbon molecules (
glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...
or G3P), where it uses up two molecules of
ATP and two molecules of
NADPH
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...
, which had been produced in the light-dependent stage. The three steps involved are:
# The enzyme
RuBisCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
catalyses the carboxylation of
ribulose-1,5-bisphosphate
Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose (ketone-containing sugar with five car ...
, 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 an unstable six-carbon compound called 2-carboxy 3-keto 1,5-biphosphoribotol (CKABP
) (or 3-keto-2-carboxyarabinitol 1,5-bisphosphate) that immediately splits into 2 molecules of
3-phosphoglycerate
3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P). This glycerate is a biochemically significant metabolic intermediate in both glycolysis and the Calvin-Benson cycle. Th ...
(also written as 3-phosphoglyceric acid, PGA, 3PGA, or 3-PGA), a 3-carbon compound.
# The enzyme
phosphoglycerate kinase
Phosphoglycerate kinase () (PGK 1) is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate (1,3-BPG) to ADP producing 3-phosphoglycerate (3-PG) and ATP :
:1,3-bisphosphoglycerate + ADP glycerat ...
catalyses the phosphorylation of 3-PGA by ATP (which was produced in the light-dependent stage).
1,3-Bisphosphoglycerate
1,3-Bisphosphoglyceric acid (1,3-Bisphosphoglycerate or 1,3BPG) 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 cycl ...
(glycerate-1,3-bisphosphate) and
ADP are the products. (However, note that two 3-PGAs are produced for every that enters the cycle, so this step utilizes two ATP per fixed.)
# The enzyme
glyceraldehyde 3-phosphate dehydrogenase
Glyceraldehyde 3-phosphate dehydrogenase (abbreviated GAPDH) () is an enzyme of about 37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules. In addition to this long establishe ...
catalyses the
reduction of 1,3BPGA by NADPH (which is another product of the light-dependent stage).
Glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...
(also called G3P, GP, TP, PGAL, GAP) is produced, and the NADPH itself is oxidized and becomes NADP
+. Again, two NADPH are utilized per fixed.
The next stage in the Calvin cycle is to regenerate RuBP. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP. Since each molecule produces two G3P molecules, three molecules produce six G3P molecules, of which five are used to regenerate RuBP, leaving a net gain of one G3P molecule per three molecules (as would be expected from the number of carbon atoms involved).
![Calvin cycle](https://upload.wikimedia.org/wikipedia/commons/8/82/Calvin_cycle.svg)
The regeneration stage can be broken down into a series of steps.
#
Triose phosphate isomerase
Triose-phosphate isomerase (TPI or TIM) is an enzyme () that catalyzes the reversible interconversion of the triose phosphate isomers dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate.
TPI plays an important role in glycolysis and ...
converts all of the G3P reversibly into
dihydroxyacetone phosphate
Dihydroxyacetone phosphate (DHAP, also glycerone phosphate in older texts) is the anion with the formula HOCH2C(O)CH2OPO32-. This anion is involved in many metabolic pathways, including the Calvin cycle in plants and glycolysis.Nelson, D. L.; ...
(DHAP), also a 3-carbon molecule.
#
Aldolase
Fructose-bisphosphate aldolase (), often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate ( ...
and
fructose-1,6-bisphosphatase
The enzyme fructose bisphosphatase (EC 3.1.3.11; systematic name D-fructose-1,6-bisphosphate 1-phosphohydrolase) catalyses the conversion of fructose-1,6-bisphosphate to fructose 6-phosphate in gluconeogenesis and the Calvin cycle, which ar ...
convert a G3P and a DHAP into
fructose 6-phosphate
Fructose 6-phosphate (sometimes called the Neuberg ester) is a derivative of fructose, which has been phosphorylated at the 6-hydroxy group. It is one of several possible fructosephosphates. The β-D-form of this compound is very common in cells ...
(6C). A phosphate ion is lost into solution.
# Then fixation of another generates two more G3P.
# F6P has two carbons removed by
transketolase
Transketolase (abbreviated as TK) is an enzyme that is encoded by the TKT gene. It participates in both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. Transketolase catalyzes two important reactions, which ...
, giving
erythrose-4-phosphate
Erythrose 4-phosphate is a phosphate of the simple sugar erythrose. It is an intermediate in the pentose phosphate pathway and the Calvin cycle.
In addition, it serves as a precursor in the biosynthesis of the aromatic amino acids tyrosine, pheny ...
(E4P). The two carbons on
transketolase
Transketolase (abbreviated as TK) is an enzyme that is encoded by the TKT gene. It participates in both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. Transketolase catalyzes two important reactions, which ...
are added to a G3P, giving the ketose
xylulose-5-phosphate
D-Xylulose 5-phosphate (D-xylulose-5-P) is an intermediate in the pentose phosphate pathway. It is a ketose sugar formed from ribulose-5-phosphate. Although previously thought of mainly as an intermediary in the pentose phosphate pathway, recent r ...
(Xu5P).
# E4P and a DHAP (formed from one of the G3P from the second fixation) are converted into
sedoheptulose-1,7-bisphosphate
Sedoheptulose-bisphosphatase (also sedoheptulose-1,7-bisphosphatase or SBPase, EC number 3.1.3.37; systematic name sedoheptulose-1,7-bisphosphate 1-phosphohydrolase) is an enzyme that catalyzes the removal of a phosphate group from sedoheptulose 1, ...
(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
Sedoheptulose-bisphosphatase (also sedoheptulose-1,7-bisphosphatase or SBPase, EC number 3.1.3.37; systematic name sedoheptulose-1,7-bisphosphate 1-phosphohydrolase) is an enzyme that catalyzes the removal of a phosphate group from sedoheptulose 1, ...
into
sedoheptulose-7-phosphate
Sedoheptulose 7-phosphate is an intermediate in the pentose phosphate pathway.
It is formed by transketolase and acted upon by transaldolase.
Sedoheptulokinase is an enzyme that uses sedoheptulose and ATP to produce ADP and sedoheptulose 7-phos ...
, releasing an inorganic phosphate ion into solution.
# Fixation of a third generates two more G3P. The ketose S7P has two carbons removed by
transketolase
Transketolase (abbreviated as TK) is an enzyme that is encoded by the TKT gene. It participates in both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. Transketolase catalyzes two important reactions, which ...
, giving
ribose-5-phosphate (R5P), and the two carbons remaining on
transketolase
Transketolase (abbreviated as TK) is an enzyme that is encoded by the TKT gene. It participates in both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. Transketolase catalyzes two important reactions, which ...
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
Ribulose 5-phosphate is one of the end-products of the pentose phosphate pathway. It is also an intermediate in the Calvin cycle.
It is formed by phosphogluconate dehydrogenase, and it can be acted upon by phosphopentose isomerase and phosphopen ...
(Ru5P, RuP) by
phosphopentose isomerase. Xu5P is converted into RuP by
phosphopentose epimerase.
# Finally,
phosphoribulokinase Phosphoribulokinase (PRK) () is an essential photosynthetic enzyme that catalyzes the ATP-dependent phosphorylation of ribulose 5-phosphate (RuP) into ribulose 1,5-bisphosphate (RuBP), both intermediates in the Calvin Cycle. Its main function is ...
(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 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 three molecules. The equation of the overall Calvin cycle is shown diagrammatically below.
RuBisCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
also reacts competitively with instead of in
photorespiration
Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction i ...
. 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 plus . 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 fixing , this process leads to loss of .
C4 carbon fixation
carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when sup ...
evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Furthermore, RuBisCO's catalyzing the light-independent reactions of photosynthesis generally exhibit an improved specificity for CO
2 relative to O
2, in order to minimize the oxygenation reaction. This improved specificity evolved after RuBisCO incorporated a new protein subunit.
Products
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 reactions
Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...
). 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.
Light-dependent regulation
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.
There are two regulation systems at work when the cycle must be turned on or off: the
thioredoxin
Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. In humans, thioredoxins are encoded by ''TXN'' and ''TXN2'' genes. Loss-of-func ...
/
ferredoxin
Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...
activation system, which activates some of the cycle enzymes; and the
RuBisCo
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
enzyme activation, active in the Calvin cycle, which involves its own activase.
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 the
photosystem I
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses ...
complex of the thylakoid electron chain when electrons are circulating through it.
Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing a
cystine
Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a me ...
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.
The enzyme RuBisCo has its own, more complex activation process. It requires that a specific
lysine
Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...
amino acid be carbamylated to activate the enzyme. This lysine binds to
RuBP
Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose ( ketone-containing sugar with five c ...
and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, called
RuBisCo activase
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
, 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 its
phosphorylation
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
.
References
;Citations
;Bibliography
*
*
*
Further reading
Rubisco Activase, from the Plant Physiology Online websiteThioredoxins, from the Plant Physiology Online website
External links
*
ttps://www.ncbi.nlm.nih.gov/books/NBK22344/ The Calvin Cycle and the Pentose Phosphate Pathwayfrom ''Biochemistry'', Fifth Edition by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer. Published by W. H. Freeman and Company (2002).
{{BranchesofChemistry
Photosynthesis
Metabolism