C4 carbon fixation is one of three biochemical mechanisms, along with
C3 carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...
and CAM photosynthesis, functioning in land plants to "fix"
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(binding the gaseous molecules to dissolved compounds inside the plant) for
sugarSugar 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...
production through
photosynthesisPhotosynthesis is a 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...
. C
4 fixation is an elaboration of C
3 carbon fixation (which operates in most plants), and is believed to have evolved more recently. C
4 and CAM overcome the tendency of
RuBisCORibulose-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...
(the first enzyme in the
Calvin cycleThe Calvin cycle or Calvin-Benson-Bassham cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham and Andrew Benson at the University of California, Berkeley by using the radioactive...
) to fix oxygen rather than carbon dioxide, which leads to a loss of energy and carbon in a process called
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
. This is achieved by using a more efficient enzyme to fix CO
2 in mesophyll cells and shuttling the fixed carbon via malate or oxaloacetate to bundle-sheath cells, where Rubisco is sequestered from atmospheric oxygen and can be saturated with CO
2 released by decarboxylation of the malate or oxaloacetate. However, these additional steps require energy in the form of
ATPAdenosine-5'-triphosphate is a multifunctional nucleotide that plays an important role in cell biology as a coenzyme, that is, the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
. Because of these tradeoffs, no one of these three photosynthetic pathways is considered superior to the others -- rather, each is best suited to a different set of conditions. The name "C
4" comes from the fact that the first product of CO
2 fixation in these plants has four carbon atoms, rather than three, as is the case in C
3 plants.
The pathway
The C
4 pathway was discovered by M. D. Hatch and C. R. Slack, in Australia, in 1966, so it is sometimes called the Hatch-Slack pathway.
In C
3 plants, the first step in the
light-independent reactionPhotosynthesis, the light-independent reactions, are chemical reactions that convert carbon dioxide and other compounds into glucose. They occur in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes. These reactions take the products of the light-dependent...
s of photosynthesis involves the fixation of CO
2 by the enzyme
RuBisCORibulose-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...
into 3-phosphoglycerate. However, due to the dual carboxylase /
oxygenaseAn oxygenase is any enzyme that oxidizes a substrate by transferring the oxygen from molecular oxygen O2 to it. The oxygenases form a class of oxidoreductases; their EC number is EC 1.13 or EC 1.14....
activity of
RuBisCoRibulose-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...
, an amount of the substrate is oxidized rather than carboxylated resulting in loss of substrate and consumption of energy, in what is known as
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
.
In order to bypass the
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
pathway, C
4 plants have developed a mechanism to efficiently deliver CO
2 to the
RuBisCORibulose-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...
enzyme. They utilize their specific leaf anatomy where chloroplasts exist not only in the mesophyll cells in the outer part of their leaves but in the bundle sheath cells as well. Instead of direct fixation in the
Calvin cycleThe Calvin cycle or Calvin-Benson-Bassham cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham and Andrew Benson at the University of California, Berkeley by using the radioactive...
, CO
2 is converted to a 4-carbon
organic acidAn organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO
2OH, are relatively stronger acids. The relative stability of the...
which has the ability to regenerate CO
2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO
2 to generate carbohydrates by the conventional C
3 pathway.
The first step in the pathway is the conversion of pyruvate to PEP by the enzyme pyruvate-phosphate dikinase (pyruvate, orthophosphate dikinase); this reaction requires inorganic phosphate and
ATPAdenosine-5'-triphosphate is a multifunctional nucleotide that plays an important role in cell biology as a coenzyme, that is, the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
plus pyruvate, giving
phosphoenolpyruvatePhosphoenolpyruvic acid , or phosphoenolpyruvate as the anion, is an important chemical compound in biochemistry. It has the high-energy phosphate bond found in living organisms, and is involved in glycolysis and gluconeogenesis...
,
AMPAdenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is found in RNA. It is an ester of phosphoric acid and the nucleoside adenosine...
, and PPi (inorganic pyrophosphate) as products. The next step is the fixation of CO
2 by the enzyme
phosphoenolpyruvate carboxylasePhosphoenolpyruvate carboxylase is an enzyme in the family of carboxy-lyases that catalyzes the addition of CO2 to phosphoenolpyruvate to form the four-carbon compound oxaloacetate:This reaction is used for carbon fixation in so-called "CAM" and "C4" plants where it plays a key role in...
. Both of these steps occur in the mesophyll cells:
- pyruvate + Pi + ATP → PEP + AMP + PPi
- PEP carboxylase + PEP + CO2 → oxaloacetate
PEP carboxylase has a lower
KmMichaelis–Menten kinetics approximately describes the kinetics of many enzymes. It is named after Leonor Michaelis and Maud Menten. This kinetic model is relevant to situations where very simple kinetics can be assumed, Michaelis–Menten kinetics (occasionally also referred to as...
for CO
2—and hence higher affinity—than Rubisco. Furthermore, O
2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO
2, most CO
2 will be fixed by this pathway.
The product is usually converted to malate, a simple
organic compoundAn 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...
that is transported to the bundle-sheath cells surrounding a nearby
veinIn the circulatory system, veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood...
, where it is decarboxylated to release CO
2, which enters the
Calvin cycleThe Calvin cycle or Calvin-Benson-Bassham cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham and Andrew Benson at the University of California, Berkeley by using the radioactive...
. The decarboxylation leaves pyruvate, which is transported back to the mesophyll cell.
Since every CO
2 molecule has to be fixed twice, the C
4 pathway is more energy-consuming than the C
3 pathway. The C
3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C
4 pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
, the C
4 pathway is an adaptive mechanism for minimizing the loss.
There are several variants of this pathway:
- The 4-carbon acid transported from mesophyll cells may be malate as above, or may be aspartate.
- The 3-carbon acid transported back from bundle-sheath cells may be pyruvate as above, or alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 proteinogenic amino acids, i.e. the building blocks of proteins. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...
.
- The enzyme which catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme, in millet, it is NAD-malic enzyme, and in Panicum
Panicum is a large genus of about 450 species of grasses native throughout the tropical regions of the world, with a few species extending into the northern temperate zone...
maximum it is PEP carboxykinase.
C4 leaf anatomy
The C
4 plants possess a characteristic
leafIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called bundle sheath cells, contain
starchStarch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.Starch is produced by all green plants as an energy store and is a major food source for humans....
-rich
chloroplastChloroplasts 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.The word chloroplast is...
s lacking grana which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called kranz anatomy (kranz, German for "wreath"). The primary function of kranz anatomy is to provide a site in which carbon dioxide can be concentrated around RuBisCO, thereby reducing
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
. In order to facilitate the maintenance of a significantly higher carbon dioxide concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to carbon dioxide, a property which may be enhanced by the presence of
suberinSuberin is a waxy substance found in higher plants. Suberin is a main constituent of cork, and is named after the Cork Oak, Quercus suber.- Anatomy and physiology :...
.
Although most C
4 plants exhibit kranz anatomy, there are a number of species which operate a limited C
4 cycle without any distinct bundle sheath tissue.
SuaedaSuaeda is a genus of plants containing species known generally as seepweeds and seablites. Many are halophytes, and many can tolerate very alkaline soils. Some species have thick, juicy leaves that resemble those of ice-plant...
aralocaspica (formerly known as
Borszczowia aralocaspica),
Bienertia cycloptera and
Bienertia sinuspersici (all
chenopodsThe Chenopodioideae is a subfamily of the flowering plant family Amaranthaceae, formerly treated as a distinct family, Chenopodiaceae, and comprising all of the genera formerly included in this family except for those transferred to the subfamilies Salicornioideae and Salsoloideae...
) are terrestrial plants which inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell C
4 carbon dioxide concentrating mechanisms which are unique amongst the known C
4 mechanisms. Although the cytology of both species differ slightly, the basic principle is that fluid filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol can therefore be kept separate from decarboxylase enzymes and RuBisCo in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath type area and a mesophyll type area to be established within a single cell. Although this does allow a limited C
3 cycle to operate, it is relatively inefficient, with much leakage of CO
2 from around RuBisCO occurring. There is also evidence for the non-kranz aquatic macrophyte
Hydrilla verticillata exhibiting inducible C
4 photosynthesis under warm conditions, although the mechanism by which CO
2 leakage from around RuBisCO is minimised is currently uncertain.
The evolution and advantages of the C4 pathway
C
4 plants have a competitive advantage over plants possessing the more common
C3 carbon fixation carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...
pathway under conditions of
droughtA drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
, high
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
s and
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
or
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
limitation. 97% of the water taken up by plants is lost through transpiration, compared to a much lower proportion in plants, demonstrating their advantage in a dry environment.
C
4 carbon fixation has
evolvedIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
on up to 40 independent occasions in different groups of plants, making it an example of
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
. C
4 plants arose around during the
OligoceneThe Oligocene is a geologic epoch of the Paleogene period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
(precisely when is difficult to determine) and did not become ecologically significant until around , in the
Miocene PeriodThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
. C
4 metabolism originated in open habitats, where the high sunlight gave it an advantage over the pathway. Drought was not necessary for its innovation - rather, the increased resistance to water stress was a by-product of the pathway and allowed plants to more readily colonise arid environments.
Today, plants represent about 5% of Earth's plant biomass and 1% of its known plant species. Despite this scarcity, they account for around 30% of terrestrial carbon fixation. Increasing the proportion of C4 plants on earth could assist
biosequestrationBiosequestration is the capture and storage of the atmospheric greenhouse gas carbon dioxide by an increased volume or quality of photosynthesis , as well as enhanced soil carbon in agriculture...
of CO
2 and represent an important
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
strategy. Present-day plants are concentrated in the tropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by
RuBisCORibulose-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...
, which increases rates of photorespiration in C
3 plants.
Examples of plants that use carbon fixation
- Many grass (Poaceae
Poaceae or Gramineae is a family in the Class Liliopsida of the flowering plants. Plants of this family are usually called grasses, or, to distinguish them from other graminoids, true grasses; the shrub- or tree-like plants in this family are called bamboo...
) species including: MiscanthusMiscanthus is a genus of about 15 species of perennial grasses native to subtropical and tropical regions of Africa and southern Asia, with one species Miscanthus is a genus of about 15 species of perennial grasses native to subtropical and tropical regions of Africa and southern Asia, with one...
, Maize, Sugar cane, SorghumSorghum is a genus of numerous species of grasses, some of which are raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...
, MilletThe millets are a group of small-seeded species of cereal crops or grains, widely grown around the world for food and fodder. They do not form a taxonomic group, but rather a functional or agronomic one. Their essential similarities are that they are small-seeded grasses grown in difficult...
See also
- C3 carbon fixation
carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...
- CAM Photosynthesis
Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some plants. These plants fix carbon dioxide during the night, storing it as the four carbon acid malate. The is released during the day, where it is concentrated around the enzyme...