Carbon Fixation

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

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.

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.

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.