Calvin Cycle Dark Reaction

Definition Of Calvin Cycle

The Calvin cycle, otherwise known as the C3 cycle, is a process in biochemistry that takes place in green plants and autotrophs. It produces organic molecules from CO2. These organic molecules are rich in C–H bonds and highly reduced compared to CO2. Photosynthesis is divided into two major steps: the light-dependent reactions that require light and happen in daylight, and the light-independent reactions (also known as the dark reactions or the Calvin Cycle, C3 Cycle), which occur both in the presence and in the absence of light. This paper describes an overview of the Calvin Cycle, which pertains to its definition, stages of the cycle, products, and important points.

Explanation

Another name for the Calvin Cycle is the light-independent or dark reaction of photosynthesis. Although it occurs regardless of whether light is present or not, this cycle works more actively during a day when there is plenty of supply of NADPH and ATP. While synthesizing organic molecules, plant cells make use of raw materials the light reactions produce. This includes:

Energy:

The endergonic reactions are powered due to energy provided by ATP produced through cyclic and noncyclic photophosphorylation.

Reducing Power:

It is provided in the form of hydrogen and energy-rich electrons from NADPH created during photosystem I. These bind with carbon atoms.

• Plants store the absorbed light energy in the form of carbohydrates, mainly starch and sucrose.

• Carbon is derived from CO2, while ATP and NADPH, produced during photosynthesis, provide the energy to fix carbon.

• The process of converting CO2 to carbohydrates is called the Calvin Cycle, sometimes referred to as the C3 cycle, named after its discoverer Melvin Calvin.

• Plants that use this process to fix carbon through the Calvin Cycle are called C3 plants.

Diagram

The diagram illustrates a scheme of the Calvin Cycle, including carbon fixation, reduction, and regeneration. It indeed explains the details of how CO2 is converted into glucose.

C3 Cycle stages

The Calvin Cycle can be categorized into three major steps:

Carbon fixation

The reduction of CO2 is the most crucial step of the Calvin Cycle. The CO2 binds to RuBP in a step called carbon fixation to yield two three-carbon molecules of 3-phosphoglycerate, 3-PGA. The reactant for this is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase or RuBisCO. This is a large enzyme with four subunits and is found in the stroma. RuBisCO is said to be the most abundant protein on Earth and yet it only processes about three molecules of RuBP per second.

Reduction

In the second step of the Calvin Cycle, 3-PGA formed during carbon fixation is reduced to form glyceraldehyde-3-phosphate (G3P), a simple sugar. That step is energized by the ATP and NADPH from the light-dependent reactions. So the general role of the Calvin Cycle is to offer an avenue for the conversion of sunlight energy into long-term energy storage molecules, in this case, sugars. The reason that this step is called reduction is that electrons are donated to the 3-PGA forming G3P.

Regeneration

The third phase of the light-independent reaction is the regeneration of RuBP from G3P. This step is essential so that the cycle can be repeated as RuBP is used up in the first step of carbon fixation. Several molecules of G3P are used to synthesise glucose and some are re-circulated back to regenerate RuBP. This regeneration consumes some ATP also. RuBP is regenerated so that RuBisCo can continue to fix carbon dioxide in the cycle

Products Of The C3 Cycle

1. At each turn of the Calvin cycle, one molecule of carbon is fixed.
2. Three cycles produce a net gain of one molecule of glyceraldehyde-3-phosphate G3P.
3. One glucose molecule is produced from two molecules of G3P.
4. In the reduction of 3-PGA to G3P and the regeneration of RuBP, 3 ATP are consumed net along with 2 NADPH.
5. In the formation of one glucose molecule, a total of 18 ATP along with 12 NADPH are consumed.

Key Points On C3 Cycle

1. The C3 cycle is also the dark reaction of photosynthesis.
2. Light-independent reactions indirectly depend on light as essential energy carriers are products of light-dependent reactions.
3. The first phase initiates the light-independent reactions of the Calvin Cycle and fixes CO2.
4. In the second stage of the cycle, ATP and NADPH are reduced to ADP and NADP+ by the reduction of 3-PGA to G3P.
5. The last step is used to regenerate RuBP to repeat the process of carbon dioxide fixation.

Conclusion

The Calvin Cycle is an integral part of photosynthesis in plants, through which carbon dioxide is turned into glucose, and energy is stored within the organism in the form of carbohydrates. The stages and activities of the Calvin Cycle are, thus, a precondition for a full understanding of how plants make their food and, in turn, help the ecosystem to exist.

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