carbon fixation is the most common of three
metabolic pathway
In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...
s for
carbon fixation in
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 ...
, along with
and
CAM
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...
. This process converts
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
ribulose 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 sugar) into two molecules of
3-phosphoglycerate through the following reaction:
:CO
2 + H
2O + RuBP → (2) 3-phosphoglycerate
This reaction was first discovered 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 h ...
,
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 ...
and
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. degr ...
in 1950. C
3 carbon fixation occurs in all plants as the first step of the
Calvin–Benson cycle
The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...
. (In and CAM plants, carbon dioxide is drawn out of
malate
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 (L ...
and into this reaction rather than directly from the
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
.)
![Cross section of Arabidopsis thaliana, a C3 plant](https://upload.wikimedia.org/wikipedia/commons/e/ec/Cross_section_of_Arabidopsis_thaliana%2C_a_C3_plant..jpg)
Plants that survive solely on fixation ( plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate,
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 ...
concentrations are around 200
ppm or higher, and
groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
is plentiful. The plants, originating during
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
and
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
eras, predate the
plants and still represent approximately 95% of Earth's plant biomass, including important food crops such as rice, wheat, soybeans and barley.
plants cannot grow in very hot areas at today's athmospheric CO
2 level (significantly depleted during hundreds of millions of years from above 5000 ppm) because
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 ...
incorporates more oxygen into
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 ...
as temperatures increase. This leads to
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 ...
(also known as the oxidative photosynthetic
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
, or
C2 photosynthesis
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 ...
), which leads to a net loss of carbon and nitrogen from the plant and can therefore limit growth.
plants lose up to 97% of the water taken up through their roots by transpiration.
In dry areas, plants shut their
stomata to reduce water loss, but this stops from entering the leaves and therefore reduces the concentration of in the leaves. This lowers the :O
2 ratio and therefore also increases photorespiration. and
CAM
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...
plants have adaptations that allow them to survive in hot and dry areas, and they can therefore out-compete plants in these areas.
The
isotopic signature
An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic ' stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample ...
of plants shows higher degree of
13C depletion than the plants, due to variation in
fractionation of carbon isotopes in oxygenic photosynthesis Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another c ...
across plant types. Specifically, plants do not have PEP carboxylase like plants, allowing them to only utilize ribulose-1,5-bisphosphate carboxylase (Rubisco) to fix through the Calvin cycle. The enzyme Rubisco largely discriminates against carbon isotopes, evolving to only bind to
12C isotope compared to
13C (the heavier isotope), attributing to why there's a low
13C depletion seen in plants compared to plants especially since the pathway uses PEP carboxylase in addition to Rubisco.
Variations
Not all C3 carbon fixation pathways operate at the same efficiency.
Refixation
Bamboo
Bamboos are a diverse group of evergreen perennial flowering plants making up the subfamily Bambusoideae of the grass family Poaceae. Giant bamboos are the largest members of the grass family. The origin of the word "bamboo" is uncertain, bu ...
s and the related
rice
Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima
''Oryza glaberrima'', commonly known as African rice, is one of the two domesticated rice species. It was first domesticated and grown i ...
have an improved C3 efficiency. This improvement might be due to its ability to recapture CO
2 produced during photorespiration, a behavior termed "carbon refixation". These plants achieve refixation by growing chloroplast extensions called "stromules" around the stroma in mesophyll cells, so that any photorespired CO
2 from the mitochondria has to pass through the RuBisCO-filled chloroplast.
Refixation is also performed by a wide variety of plants. The common approach involving growing a bigger
bundle sheath
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will includ ...
leads down to
C2 photosynthesis
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 ...
.
Synthetic glycolate pathway
C3 carbon fixation is prone to
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 ...
(PR) during dehydration, accumulating toxic
glycolate
Glycolic acid (or hydroxyacetic acid; chemical formula HOCH2CO2H) is a colorless, odorless and hygroscopic crystalline solid, highly soluble in water. It is used in various skin-care products. Glycolic acid is widespread in nature. A glycolate (so ...
products. In the 2000s scientists used computer simulation combined with an
optimization algorithm
Mathematical optimization (alternatively spelled ''optimisation'') or mathematical programming is the selection of a best element, with regard to some criterion, from some set of available alternatives. It is generally divided into two subfi ...
to figure out what parts of the metabolic pathway may be tuned to improve photosynthesis. According to simulation, improving
glycolate
Glycolic acid (or hydroxyacetic acid; chemical formula HOCH2CO2H) is a colorless, odorless and hygroscopic crystalline solid, highly soluble in water. It is used in various skin-care products. Glycolic acid is widespread in nature. A glycolate (so ...
metabolism would help significantly to reduce photorespiration.
Instead of optimizing specific enzymes on the PR pathway for glycolate degradation, South et al. decided to bypass PR altogether. In 2019, they transferred ''
Chlamydomonas reinhardtii
''Chlamydomonas reinhardtii'' is a single-cell green alga about 10 micrometres in diameter that swims with two flagella. It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an ...
''
glycolate dehydrogenase
In enzymology, a glycolate dehydrogenase () is an enzyme that catalyzes the chemical reaction
:glycolate + acceptor \rightleftharpoons glyoxylate + reduced acceptor
Thus, the two substrates of this enzyme are glycolate and acceptor, whereas i ...
and ''
Cucurbita maxima
''Cucurbita maxima'', one of at least five species of cultivated squash, is one of the most diverse domesticated species. This species originated in South America from the wild subspecies ''Cucurbita maxima subsp. andreana'' over 4,000 years ago. ...
''
malate synthase
In enzymology, a malate synthase () is an enzyme that catalyzes the chemical reaction
:acetyl-CoA + H2O + glyoxylate \rightarrow (''S'')-malate + CoA
The 3 substrates of this enzyme are acetyl-CoA, H2O, and glyoxylate, whereas its two products ...
into the chloroplast of
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
(a
model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
). These enzymes, plus the chloroplast's own, create a catabolic cycle:
acetyl-CoA combines with glyoxylate to form
malate
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 (L ...
, which is then split into
pyruvate and CO
2; the former in turn splits into acetyl-CoA and CO
2. By forgoing all transport among organelles, all the CO
2 released will go into increasing the CO
2 concentration in the chloroplast, helping with refixation. The end result is 24% more biomass. An alternative using ''E. coli''
glycerate pathway produced a smaller improvement of 13%. They are now working on moving this optimization into other crops like wheat.
References
{{reflist
Photosynthesis
Metabolic pathways
Carbon