Meaning

Process

Carbon Fixation in C3 Plants

Carbon Fixation in C4 Plants

Carbon Fixation in CAM Plants

Carbon fixation refers to the process of assimilating inorganic carbon and converting it into organic compounds, which can then be used as an energy source and for the creation of biomolecules.

Carbon Fixation Definition:

Carbon fixation is a process by which inorganic carbon is converted into organic compounds by living organisms. It is an essential part of the carbon cycle and is important for the growth of plants and other photosynthetic organisms.

“Plants use carbon fixation to convert atmospheric carbon into organic compounds.”

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All autotrophs, such as bacteria, algae and plants, fix atmospheric carbon dioxide through 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 in C3 Plants

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

All plants, including C3, C4, CAM, and any other type, undergo the Calvin cycle.

It occurs in the stroma of chloroplasts

The first product of carbon dioxide fixation is a three carbon compound known as 3-phosphoglyceric acid (PGA)

RUBP (Ribulose Biphosphate) is a 5 carbon compound that acts as a CO2 acceptor.

The Three Main Steps of the Calvin Cycle Are:

1. Carboxylation - In this process, CO2 fixation takes place, catalyzed by the enzyme RuBisCO (RUBP carboxylase oxygenase). This results in the carboxylation of RUBP to form PGA.

2. Reduction – The formation of carbohydrate or glucose occurs through reduction. The ATP and NADPH produced during the light reaction are used in the process, with 2 ATP and 2 NADPH used per cycle.

3. Regeneration of RUBP - Regeneration of Ribulose-1,5-bisphosphate (RUBP) is an important step for the cycle to continue, as it requires 1 ATP molecule for phosphorylation.

One molecule of glucose requires 6 cycles of the Calvin cycle to form, and thus 6CO2, 18ATP, and 12NADPH are utilized in the 6 cycles.

Carbon Fixation in C4 Plants

The C4 pathway of carbon fixation is adapted by plants, such as maize and sorghum, found in dry tropical regions.

In C3 plants, the first product of carbon fixation is 3-phosphoglyceric acid (PGA), while in C4 plants, the first product of carbon fixation is oxaloacetic acid (OAA).

In C4 plants, the C3 pathway is also utilized in the formation of a glucose molecule.

C4 plants have Kranz anatomy in their leaves, which allows them to withstand high temperatures. Large bundle sheath cells are located around the vascular bundles of leaves.

Bundle sheath cells have thick walls, lack intercellular spaces, and possess large chloroplasts.

Carbon fixation occurs in mesophyll cells

Phosphoenolpyruvate (PEP) is a 3 carbon compound which acts as a CO2 acceptor.

Mesophyll cells lack RuBisCO, but the enzyme PEP carboxylase (PEPcase) catalyses the reaction.

The first product of carbon dioxide fixation is a 4-carbon compound, Oxaloacetate (OAA).

The other 4C acids, such as malic acid and aspartic acid, are produced from OAA and then transported to bundle sheath cells.

CO2 released through decarboxylation in bundle sheath cells enters the Calvin cycle.

The mesophyll cells transport the 3-carbon acid back.

The bundle sheath cells have RuBisCO but do not possess PEPcase

Carbon Fixation in CAM Plants

The Crassulacean Acid Metabolism (CAM) pathway of carbon fixation is present in plants present in arid conditions, such as cacti.

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.

See Also: CAM Plants

The C3 cycle, or Calvin cycle, is the primary pathway of carbon fixation in plants.

Frequently Asked Questions

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What is carbon fixation and why is it important?

Carbon fixation is the process by which carbon dioxide is converted into organic compounds, such as sugar and other carbohydrates. It is an important process in the global carbon cycle, as it helps to regulate the amount of carbon dioxide in the atmosphere. Carbon fixation is essential for all living organisms, as it provides the carbon necessary for growth and development.

Carbon fixation is a process in which inorganic carbon from the atmosphere is taken up by living organisms and converted into organic compounds. These compounds are used to store chemical energy, making it an essential process for the sustainability of life. Through this process, the energy in the biosphere is made available to living organisms, allowing them to carry out various metabolic processes. This energy is then transferred to the various trophic levels and to all the living organisms.

What is carbon fixation?

Carbon fixation is a process by which atmospheric carbon is converted into organic compounds that can be used for metabolism. During photosynthesis, carbon fixation in plants relies on ATP and NADPH to take in carbon dioxide and create carbohydrates.

Carbon fixation is a process in the Calvin cycle in which carbon dioxide is converted into energy-rich organic compounds, such as glucose.

The Calvin cycle is the dark reaction of photosynthesis. It is the biosynthetic phase where CO2 is converted into sugar. It utilises ATP and NADPH produced during the light reaction of photosynthesis. Carbon fixation is the first step in the Calvin cycle where carboxylation of RUBP results in the fixation of CO2 to a stable organic intermediate.

What are the alternative pathways for carbon fixation?

The alternative pathways of carbon fixation to the Calvin cycle, which is the main pathway of carbon fixation in plants, algae and cyanobacteria, are:

Reductive Citric Acid Cycle - In Bacteria

3-hydroxypropionate Cycle – in Bacteria and Archaea

Reductive Acetyl CoA Pathway - in Bacteria and Archaea

The 3 stages of Calvin Cycle are:

1. Carbon fixation
2. Reduction reactions
3. Regeneration of RuBP

The three stages of Calvin cycle are:

1. Carbon Fixation
2. Reduction
3. Regeneration

Carboxylation - It is the first step in which RuBisCO catalyses the carboxylation of RUBP, resulting in the formation of two molecules of PGA.

Reduction – ATP and NADPH are utilized to form glucose. Phosphoglyceric acid (3PGA) is reduced to form glyceraldehyde 3-phosphate (G3P).

Regeneration – The primary acceptor of CO2, i.e. RUBP, is regenerated using ATP for phosphorylation.

Yes, carbon fixation requires light.

Carbon fixation is a dark reaction or light-independent reaction of photosynthesis. It does not require light directly, but depends on the products of the light reaction of photosynthesis, i.e. ATP and NADPH.

The enzyme RuBisCO is responsible for carbon fixation.

Enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is responsible for carbon fixation by the Calvin cycle. It catalyses the carboxylation of RUBP to form 2 molecules of 3PGA.