What is Carbon Fixation By Chemoautotrophs? Unit 10 – Class 12 Biology
Chemoautotrophs or chemotrophs are organisms that obtain energy by the oxidation of electron donors that are present in the environment where they survive. These electron donors can be an organic or inorganic molecule. Inorganic molecules like magnesium, hydrogen sulfide, elemental sulfur, ferrous iron, molecular hydrogen, and ammonia etc. act as electron donors. Chemoautotrophs are also able to synthesize their own organic molecules from the fixation of carbon dioxide.
Chemoautotrophs are commonly found in those environments where plants cannot survive (where the sunlight cannot reach), at the bottom of the ocean, or in acidic hot springs. Some chemoautotrophs play crucial roles in plant-based ecosystems also, where many plants rely on chemoautotroph bacteria to fix nitrogen, which is necessary to make amino acids and proteins. Though, the role of carbon fixation is performed by the plants in these ecosystems.
Chemoautotrophs are organisms that derive their energy from the oxidation or breakdown of various inorganic or organic food substances in their environment. Just like plants, they are autotrophs and are able to “fix” carbon. This means that they are able to sustain themselves without the need to consume other organisms or decomposing dead matter.
They take atoms of carbon from inorganic compounds, such as carbon dioxide, and use it to make organic compounds such as sugars, proteins, and lipids. Many ecosystems are dependent on carbon dioxide fixation by either free-living or symbiotic chemoautotrophs. CO2 fixation in the chemoautotroph occurs via the Calvin-Benson-Bassham cycle. The cycle is characterized by three unique enzymatic activities: ribulose bisphosphate carboxylase/oxygenase, phosphoribulokinase, and sedoheptulose bisphosphatase.
The cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14 in the 1950s.
In the first stage of the Calvin cycle, a CO2 molecule is incorporated into one of two three-carbon molecules (glyceraldehyde 3-phosphate or G3P), where it uses up two molecules of ATP and two molecules of NADPH. The three steps that occur in the cycle are:
This step involves the addition of one molecule of CO2 to a 5-carbon molecule, ribulose biphosphate (RuBP) which acts as an “acceptor”. This reaction is catalyzed by the enzyme RuBisCO(the most abundant protein in the biosphere). The 6-carbon product thus formed gets split into two identical 3-carbon products. These products are 3-phosphoglycerate (3-PG). In this step of the Calvin cycle, CO2is “fixed” into an organic compound but no energy has been added to the molecule.
The small black spheres represent the carbon atoms and big red spheres are phosphates
The second step in the Calvin cycle is the reduction of 3-PG. This reaction occurs in two steps:
- Phosphorylation of 3-PG by ATP to form a 1,3-biphosphoglycerate (1,3-BPG) and
- Reduction of 1, 3-BPG by NADPH to form glyceraldehyde-3-phosphate (G3P) which is a 3-carbon carbohydrate. This reaction requires both ATP and NADPH(ATP and NADPH are high energy compounds).
The final stage in the Calvin cycle is the regeneration of RuBP (which is CO2 acceptor). In this step, a series of reactions take place that converts triose phosphate first to the 5-carbon intermediate ribulose 5- phosphate (Ru5P) and then phosphorylate Ru5P to regenerate ribulose-bisphosphate (RuBP). This is an ATP requiring step.
Overall, it is a cyclic process. The three turns of the cycle take place, where, three molecules of CO2 are fixed to three molecules of Ribulose-1,5- bisphosphate (RuBP), to form six molecules of glyceraldehyde-3-phosphate (18 carbons). One molecule of glyceraldehyde-3- phosphate is used for energy production. The remaining 15 carbon atoms (5molecules of glyceraldehyde- 3- phosphates) re-enter the cycle to produce three molecules of RuBP, the starting material.
Overview of Calvin-Benson-Bassham cycle
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Tag: Calvin-Benson-Bassham cycle, Carbon Fixation By Chemoautotrophs, NCERT biology class 12