Carbon Fixation

Carbon Fixation Meaning

Carbon fixation means assimilation of inorganic carbon and conversion to organic compounds, which can be used as an energy store and for the synthesis of biomolecules.

All the autotrophs, bacteria, algae and plants fix atmospheric carbon dioxide by the process of photosynthesis or chemosynthesis.

Carbon Fixation Process

Photosynthesis is the main process of carbon fixation. Carbon fixation occurs in the dark reaction or light-independent reaction of the photosynthesis process.

The process of carbon fixation slightly differs in C3, C4, and CAM plants but the Calvin Cycle or C3 pathway is the main biosynthetic pathway of carbon fixation.

Carbon fixation by Photosynthesis

Carbon Fixation in C3 Plants

Carbon fixation in C3 plants occurs in the dark reaction or light-independent reaction of photosynthesis. It is also known as the Calvin Cycle.

Calvin cycle occurs in all the plants, be it C3, C4, CAM or any other plants.

  • It occurs in the stroma of chloroplasts
  • The first product of carbon dioxide fixation is 3 carbon compound known as 3-phosphoglyceric acid or PGA
  • CO2 acceptor is a 5 carbon compound ribulose biphosphate or RUBP
  • Calvin cycle has three main steps:
    1. Carboxylation – In this process CO2 fixation takes place. Enzyme RUBP carboxylase oxygenase or RuBisCO catalyses the carboxylation of RUBP to form PGA
    2. Reduction – Formation of carbohydrate or glucose takes place by reduction. ATP and NADPH formed during light reaction are used in the process. 2 ATP and 2 NADPH are used per cycle
    3. Regeneration – Regeneration of RUBP is an important step for the cycle to continue, 1 ATP molecule is used for phosphorylation
  • One molecule of glucose requires 6 cycle repetition, hence in total 6CO2, 18ATP and 12NADPH are utilized in 6 Calvin cycles to form one glucose.

Carbon Fixation in C4 Plants

C4 pathway of carbon fixation is adapted by plants found in a dry tropical region, e.g. maize, sorghum, etc.

C3 and C4 pathway differ in the first product of carbon fixation. In C3 plants, 3 carbon compound 3-phosphogyceric acid (PGA) is produced, whereas, in C4 plants, 4 carbon compound oxaloacetic acid (OAA) is produced.

In C4 plants also C3 pathway is used in the formation of a glucose molecule

  • C4 plants have Kranz anatomy in leaves to tolerate high temperature. Large bundle sheath cells are present around vascular bundles of leaves
  • Bundle sheath cells have thick walls, no intercellular spaces and have large chloroplasts
  • Carbon fixation occurs in mesophyll cells
  • CO2 acceptor is a 3 carbon compound phosphoenolpyruvate (PEP)
  • The enzyme PEP carboxylase (PEPcase) catalyses the reaction. Mesophyll cell lack RuBisCO
  • The first product of carbon dioxide fixation is 4 carbon compound OAA
  • OAA is then converted to other 4C acids like malic acid and aspartic acid. They are transported to bundle sheath cells
  • By decarboxylation in bundle sheath cells, CO2 is released, which enters Calvin cycle
  • The 3-carbon acid is transported back to mesophyll cells
  • The bundle sheath cells have RuBisCO but lack PEPcase

Carbon Fixation in CAM Plants

CAM pathway of carbon fixation or Crassulacean acid metabolism is present in plants present in arid conditions, e.g. cactus.

In the CAM pathway, plants take CO2 during the night through the stomatal opening. It is converted to malic acid (4 carbon compound) and stored in vacuoles. During the daytime, malic acid is transported to chloroplast and CO2 is released, which enters the Calvin cycle.

To summarize the C3 cycle or Calvin cycle is the main pathway of carbon fixation in plants.

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