Fiksacija ugljenika

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Modrozelene bakterije poput ovih izvode fotosintezu. Njihova pojava je nagovestila evoluciju mnogih fotosintetskih biljaka, što je dovelo do oksigenacije zemljine atmosfere.

Fiksacija ugljenika ili ugljenična asimilacija je proces konverzije neorganskog ugljenika (ugljen dioksida) do organskih jedinjenja posredstvom živih organizama. Najprominentniji primer je fotosinteza. Hemosinteza je još jedna forma ugljenične fiksacije, do koje može doći u odsustvu sunčevog svetla. Organizmi koji rastu putem fiksacije ugljenika se nazivaju autotrofima. Autotrofi obuhvataju fotoautotrofe, koji sintetišu organska jedinjenja koristeći energiju sunčevog svetla, and lithoautotrophs, which synthesize organic compounds using the energy of inorganic oxidation. Heterotrophs are organisms that grow using the carbon fixed by autotrophs. The organic compounds are used by heterotrophs to produce energy and to build body structures. "Fixed carbon", "reduced carbon", and "organic carbon" are equivalent terms for various organic compounds.

Neto vs bruto CO₂ fiksacija

Graphic showing net annual amounts of CO₂ fixation by land and sea-based organisms.

It is estimated that approximately 258 billion tons of carbon dioxide are converted by photosynthesis annually. The majority of the fixation occurs in marine environments, especially areas of high nutrients. The gross amount of carbon dioxide fixed is much larger since approximately 40% is consumed by respiration following photosynthesis. Given the scale of this process, it is understandable that RuBisCO is the most abundant protein on Earth.

Pregled puteva

Six autotrophic carbon fixation pathways are known as of 2011. The Calvin cycle fixes carbon in the chloroplasts of plants and algae, and in the cyanobacteria. It also fixes carbon in the anoxygenic photosynthetic in one type of proteobacteria called purple bacteria, and in some non-phototrophic proteobacteria.

Kiseonična fotosinteza

In photosynthesis, energy from sunlight drives the carbon fixation pathway. Oxygenic photosynthesis is used by the primary producers—plants, algae, and cyanobacteria. They contain the pigment chlorophyll, and use the Calvin cycle to fix carbon autotrophically. The process works like this:

2H₂O → 4e⁻ + 4H⁺ + O₂

CO₂ + 4e⁻ + 4H⁺ → CH₂O + H₂O

In the first step, water is dissociated into electrons, protons, and free oxygen. This allows the use of water, one of the most abundant substances on Earth, as an electron donor—as a source of reducing power. The release of free oxygen is a side-effect of enormous consequence. The first step uses the energy of sunlight to oxidize water to O₂, and, ultimately, to produce ATP

ADP + P_i ⇌ ATP + H₂O

and the reductant, NADPH

NADP⁺ + 2e⁻ + 2H⁺ ⇌ NADPH + H⁺

In the second step, called the Calvin cycle, the actual fixation of carbon dioxide is carried out. This process consumes ATP and NADPH. The Calvin cycle in plants accounts for the preponderance of carbon fixation on land. In algae and cyanobacteria, it accounts for the preponderance of carbon fixation in the oceans. The Calvin cycle converts carbon dioxide into sugar, as triose phosphate (TP), which is glyceraldehyde 3-phosphate (GAP) together with dihydroxyacetone phosphate (DHAP):

3 CO₂ + 12 e⁻ + 12 H⁺ + P_i → TP + 4 H₂O

An alternative perspective accounts for NADPH (source of e⁻) and ATP:

3 CO₂ + 6 NADPH + 6 H⁺ + 9 ATP + 5 H₂O → TP + 6 NADP⁺ + 9 ADP + 8 P_i

The formula for inorganic phosphate (P_i) is HOPO₃²⁻ + 2H⁺. Formulas for triose and TP are C₂H₃O₂-CH₂OH and C₂H₃O₂-CH₂OPO₃²⁻ + 2H⁺

Literatura

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