Carbon fixation or ? arbon assimilation is the process of converting inorganic carbon (carbon dioxide) into organic compounds by living organisms. The most prominent example is photosynthesis, though chemosynthesis is another form of carbon fixation that can occur in the absence of sunlight. Organisms that grow by fixing carbon are called autotrophs. Autotrophs include photoautotrophs, which synthesize organic compounds using solar energy, and lithoautotrophs, which synthesize organic compounds using inorganic oxidation energy. Heterotrophs are the organisms that grow using the carbon set by autotrophs. Organic compounds are used by heterotrophs to generate energy and to build up body structures. "Carbon remains", "reduces carbon", and "organic carbon" are equivalent terms for different organic compounds.
Video Carbon fixation
Fiksasi net vs gross CO 2
It is estimated that approximately 258 billion tons of carbon dioxide are converted with photosynthesis each year. Majority of fixation occurs in the marine environment, especially areas with high nutrients. The gross amount of carbon dioxide remains much larger because about 40% is consumed by respiration after photosynthesis. Given the scale of this process, it is understandable that RuBisCO is the most abundant protein on Earth.
Maps Carbon fixation
Path overview
The six paths of autotrophic carbon fixation are known in 2011. The Calvin cycle improves carbon in plant chloroplasts and algae, and in cyanobacteria. It also improves carbon in photosynthetic anoxygenics in one type of proteobacteria called purple bacteria, and in some non-phototrophic proteobacteria.
Oxygenic photosynthesis
In photosynthesis, energy from sunlight pushes the path of carbon fixation. [Photosynthesis oxygen] is used by major producers - plants, algae, and cyanobacteria. They contain chlorophyll pigments, and use the Calvin cycle to repair carbon autotrophically. The process works like this:
- 2H 2 O -> 4e - 4H O 2
In the first step, water is separated into free electrons, protons, and oxygen. This allows the use of water, one of the most abundant substances on Earth, as an electron donor - as a power-reducing source. The release of free oxygen is a side effect of enormous consequences. The first step uses the energy of sunlight to oxidize water to O 2 , and, finally, to generate ATP
- ADP P i ? ATP H 2 O
and reductant, NADPH
- NADP 2e - 2H ? NADPH H
In the second step, called the Calvin cycle, the actual carbon dioxide fixation is done. This process consumes ATP and NADPH. The Calvin cycle in plants contributes more to the carbon fixation on land. In algae and cyanobacteria, it accounts for the dominant carbon fixation in the oceans. The Calvin cycle converts carbon dioxide into sugars, such as triose phosphate (TP), which is a 3-phosphate glyceraldehyde (GAP) together with dihydroxyacetone phosphate (DHAP):
- 3 CO 2 12 e - 12 H P i -> TP 4 H < sub> 2 O
Alternative perspective accounts for NADPH (source e - ) and ATP:
- 3 CO 2 6 NADPH 6 H 9 ATP 5 H 2 O -> TP 6 NADP 9 ADP 8 P i
The formula for inorganic phosphate (P i ) is HOPO 3 2 - 2H . The formula for triose and TP is C 2 H 3 2 OH and C 2 2 2H
Evolutionary considerations
Between 3.5 and 2.3 billion years ago, the ancestors of cyanobacteria evolved oxygenic photosynthesis, allowing the use of the heavily oxidized but relatively subdued O molecules as electron donors to the electron-transport chain of light-catalyst proton-pumping is responsible for efficient ATP synthesis. When this evolutionary breakthrough occurs, autotrophy (growth using inorganic carbon as a single carbon source) is believed to have been developed. However, the proliferation of cyanobacteria, due to their new ability to exploit water as a source of electrons, radically alters the global environment by oxidizing the atmosphere and by achieving substantial flux consumption of CO 2 .
The carbon concentration mechanism
Many photosynthetic organisms do not have an inorganic carbon concentration mechanism (CCM), which increases the concentration of carbon dioxide available to the early carboxylase of Calvin cycle, RuBisCO enzyme. The benefits of CCM include increased tolerance to inorganic low-carbon external concentrations, and reduced photorespiratory loss. CCM can make plants more tolerant to heat and water pressure.
The carbon concentrate mechanism uses an anhydrase carbonate (CA) enzyme, which catalyzes both the dehydration of bicarbonate to carbon dioxide and the hydration of carbon dioxide into bicarbonate
- HCO 3 - H ? CO 2 H 2 O
The lipid membrane is much easier to absorb bicarbonate than carbon dioxide. To capture inorganic carbon more effectively, some plants have adapted to anaplerotic reactions
- HCO 3 - H PEP -> OAA P i
catalyzed by PEP carboxylase (PEPC), to carboxylate phosphoenolpyruvate (PEP) to oxaloacetate (OAA) which is a C 4 dicarboxylic acid.
CAM crop
CAM plants that use Crassulacean acid metabolism as an adaptation for dry conditions. CO 2 enters through the stomata at night and is converted into a 4-carbon, malic acid, which releases CO 2 for use in the Calvin cycle during the day, when the stomata is closed. Fertilizer factory ( Crassula ovata ) and cactus are CAM plants. Sixteen thousand species of plants use CAM. This plant has carbon isotope marks from -20 to -10 ?. C 4 plants
C 4 The plant begins the Calvin cycle with a reaction combining CO 2 into one of 4-carbon, malic acid or aspartic acid. C 4 plants have a distinctive internal leaf anatomy. Tropical grasses, such as sugarcane and corn are C 4 plants, but there are many broad-leaved plants that are C 4 . Overall, 7600 species of terrestrial plants use carbon fixation C 4 , representing about 3% of all species. This plant has carbon isotope marks from -16 to -10 ?.
C 3 plants
Most of the plants are C 3 . They are called to distinguish them from CAM and C 4 plants, and because the carboxylation product of the Calvin cycle is a 3-carbon compound. They do not have a dicarboxylic acid cycle of C 4 , and therefore have higher carbon dioxide compensation points than CAM or C 4 . C 3 plants have carbon isotope marks from -24 to -33 ?.
Bacteria
Some bacteria use carboxysomes as a carbon concentration mechanism.
Other autotropic paths
Of the five other autotrophic pathways, two are known only in bacteria, two only in archaea, and one in both bacteria and archaea.
Reductive citric acid cycles
Reductive citric acid cycle is a cycle of oxidative citrate acid that runs in reverse. It has been found in anaerobic and microaerobic bacteria. It was proposed in 1966 by Evans, Buchanan and Arnon who worked with the anoxygenic photosynthetic green sulfur bacteria they called Chlorobium thiosulfatophilum . Reductive citric acid cycles are sometimes called Arnon-Buchanan cycles.
Reductive CoA acetyl coating
Reductive acetylene coene lines are strictly operated on anaerobic (acetogen) and archaea (methanogen) bacteria. The line was proposed in 1965 by Ljungdahl and Wood. They worked with gram-positive acetate-producing bacteria Clostridium thermoaceticum , now called Moorella thermoacetica . Hydrogenotrophic methanogenesis, which is found only in certain archaea and accounts for 80% of global methanogenesis, is also based on the reductive acetyl Coene pathway. This path is often referred to as the Wooden-Ljungdahl path.
The 3-hydroxypropionate cycle is used only by green nonsulfur bacteria. It was proposed in 2002 for photosynthetic anoxygenic Chloroflexus aurantiacus . None of the enzymes that participate in the 3-hydroxypropionate cycle are highly sensitive to oxygen.
A variant of the 3-hydroxypropionate pathway was found to operate in extreme aerobic thermoacidophile archaeology Metallosphaera sedula . This path, called 3-hydroxypropionate/4-hydroxybutyrate cycle. And another variant of the 3-hydroxypropionate path is the dicarboxylic/4-hydroxybutyric cycle. It is found in anaerobic archaea. It was proposed in 2008 for the arche hypertermofil Ignicoccus hospitalis .
Chemosynthesis
Chemosynthesis is a carbon fixation driven by the oxidation of inorganic substances (eg, hydrogen gas or hydrogen sulphide). Sulfur and hydrogen-oxidizing bacteria often use the Calvin cycle or reductive citrate acid cycle.
Non-autotrophic path
Although almost all heterotrophs can not synthesize complete organic molecules from carbon dioxide, some carbon dioxide is included in their metabolism. Especially pyruvate carboxylase consumes carbon dioxide (as bicarbonate ions) as part of gluconeogenesis, and carbon dioxide is consumed in various anaplerotic reactions.
Discrimination of carbon isotopes
Some carboxylases, in particular RuBisCO, preferably bind carbon to lighter carbon-12 isotope of carbon-13. This is known as carbon isotope discrimination and results in a carbon-12 to carbon-13 ratio in higher plants than in free air. Measurement of these ratios is important in evaluating the efficiency of water use at the plant, as well as in assessing the likelihood or possibility of carbon resources in global carbon cycle studies.
References
Further reading
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