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Carbon Fixation

What is Carbon Fixation

Carbon fixation, or carbon assimilation, is how plants fix inorganic atmospheric carbon (mainly CO2) into organic compounds. The compounds thus formed can be used as energy sources and for the synthesis of several biomolecules.

It is a light-independent process, similar to photosynthesis taking place in the dark. All the autotrophs, algae, bacteria, and plants fix CO2 primarily by photosynthesis. However, some organisms also use chemosynthesis in the absence of sunlight.

Where Does It Occur

In plants, carbon fixation occurs in chloroplasts of leaves, particularly in mesophyll and bundle sheath cells.

Why is Carbon Fixation Important

Carbon fixation is an essential process for the sustainability of life. As we know, plants take in CO2 from the atmosphere during photosynthesis to produce glucose and O2. On the other hand, it also releases CO2 during respiration by breaking down sugars.

The balance between carbon fixation during photosynthesis and the release of CO2 during respiration affects overall plant growth. In nature, this balance also affects the global carbon cycle.

The rising CO2 in the atmosphere from human input, leading to global warming, is a concern these days. A new study says plants can allocate more carbon for growth under warmer conditions, which effectively improves their net carbon gain. By utilizing more carbon for growth, plants are fixing more CO2 from the atmosphere. Thus, it helps in reducing the atmospheric CO2, maintaining a healthy O2-CO2 ratio.

What Happens During the Process

The carbon fixation process somewhat differs in C3, C4, and CAM plants. However, the Calvin Cycle or C3 pathway is the main biosynthetic pathway of carbon fixation. Let us explore all the processes in detail:

The Calvin Cycle

In C3 plants, carbon fixation takes place in the light-independent or dark reaction of photosynthesis. It is alternatively known as the Calvin Cycle. It occurs in all types of plants, including C3, C4, CAM, or any other. It takes place in the stroma of chloroplasts.

The first product of CO2 fixation is a 3-carbon compound known as 3-phosphoglyceric acid or PGA. In contrast, a 5-carbon compound, Ribulose biphosphate or RuBP, accepts CO2. So, carbon fixation involves the addition of carbon dioxide to RuBP.

The three critical steps of the Calvin cycle are as follows:

  1. Carboxylation: In this step, the enzyme RuBP carboxylase oxygenase or RuBisCO catalyzes the carboxylation of RuBP to produce PGA. CO2 fixation occurs in this step.
  2. Reduction: Here, the formation of carbohydrate or glucose takes place by reduction. 2 ATP and 2 NADPH formed during light reactions are used in the process per cycle.
  3. Regeneration: In this step, 1 ATP molecule is used for phosphorylation to regenerate RuBP. Hence, the cycle continues.

One glucose molecule requires six cycle repetitions. Hence, 6CO2, 12NADPH, and 18ATP are utilized in 6 cycles to form one glucose molecule.

Alternative Pathways of Carbon Fixation

Now, let us discuss some other methods of carbon fixation in plants found in dry, tropical conditions and arid, desert conditions.

Carbon Fixation in C4 Plants

Plants found in a dry tropical region, such as maize and sorghum, adapt the C4 pathway of carbon fixation. In these plants, the light-dependent reactions and the Calvin cycle are separated physically. Here, the light-dependent reactions occur in the mesophyll cells, and the Calvin cycle occurs in bundle-sheath cells.

C3 and C4 pathways differ in their first formed product of carbon fixation. As mentioned, a 3- carbon compound, 3-phosphoglyceric acid (PGA), is produced as the first product in C3 plants. In contrast, a 4-carbon compound, oxaloacetic acid (OAA), is produced in C4 plants.

The leaves of C4 plants exhibit a unique structure called Kranz anatomy to tolerate high temperatures. In these plants, the mesophyll cells cluster around the bundle-sheath, forming a wreath, where the carbon fixation occurs. Here, the CO2 acceptor is the 3-carbon compound phosphoenolpyruvate (PEP).

The steps of carbon fixation in C4 plants are as follows:

  • Initially, atmospheric CO2 gets fixed in the mesophyll cells to form 4-carbon organic acid oxaloacetate (OAA). As the mesophyll cells lack RuBisCO, this step is carried out by PEP carboxylase, which does not tend to bind O2.
  • Oxaloacetate gets converted into 4C acids like malic acid and aspartic acid, transported into bundle sheath cells, breaking down by decarboxylation and releasing a CO2 molecule. This CO2 is then fixed by rubisco and convert into sugar via the Calvin cycle, as in C3 photosynthesis.
  • The remaining 3-carbon acid is transported back to mesophyll cells by expending ATP.
  • However, as the mesophyll cells constantly pump CO2 into their neighboring bundle-sheath cells in the form of malate, a high concentration of CO2 always prevails around rubisco, thus minimizing photorespiration.

Carbon Fixation in CAM Plants

Some plants belonging to the family Crassulaceae, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. CAM plants are dominant in scorching, dry areas, like deserts.

Unlike the Calvin cycle, it separates the light-dependent reactions and the use of CO2 in time.

  • At night, these plants open their stomata, allowing CO2 to diffuse into the leaves. The diffused CO2 then gets fixed into oxaloacetate by PEP carboxylase, which is then converted to malate or any other type of organic acid.
  • The organic acid remains stored inside vacuoles until the next day. During the day, the CAM plants do not open their stomata. However, they can continue photosynthesis since the stored organic acids are transported out of the vacuole and broken down to release CO2. The CO2 then enters the Calvin cycle carrying the process further. This controlled release of organic acids maintains a high concentration of CO2 around rubisco.
  • Unlike C4 photosynthesis, the CAM pathway requires ATP at multiple steps.

However, CAM photosynthesis not only helps to avoid photorespiration but is also very water-efficient. In this process, the stomata only open at night, when humidity tends to be higher and the temperature is cooler, both factors that reduce water loss from leaves.

Why There Are Different Types of Carbon Fixation in Plants

Different plants have evolved different ways of carbon fixation as an adaptation towards varying conditions.

Plants living in arid regions need to conserve water, while plants living in more moist conditions will not need to do so. Different plants have adapted modified methods of carbon fixation to help maximize their efficiency irrespective of whatever region they belong to. Most of these adaptive methods are C₃, C₄, and CAM.


Q.1. Does carbon fixation occur in humans?

Ans.  No, carbon fixation does not take place in humans.

Q.2. What is the most common method of carbon dioxide fixation?

Ans. The most common method of CO2 fixation is the Calvin cycle.

Q.3. What is the water-conserving process of carbon fixation?

Ans. The water-conserving process of carbon fixation is the CAM pathway.

Q.4. What is the source of carbon in carbon fixation?

Ans. The source of carbon in carbon fixation is atmospheric carbon dioxide.

Q.5. Why do carbon fixation reactions require ATP?

Ans. ATP is the energy source for carbon fixation reactions.

Article was last reviewed on Tuesday, September 7, 2021

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