What is Calvin Cycle?

All living organisms on Earth have carbon present in their systems. Carbon is an essential component that makes up the complex molecules of various organisms, humans included. The presence of carbon in living organisms distinguishes them from inorganic elements whose compounds lack the said element. Without carbon, biomolecules such as carbohydrates would never be completed. Carbohydrate is essential because its serves as the energy needed to fuel the cells in our bodies.

Additionally, this element is essential because it is incorporated in carbon dioxide, a gas that plants need in order to proceed with their life processes. Animals exhale this with each of their breath. Indeed, this transfer of carbon through carbon dioxide between animals and plants distributes carbon across the atmosphere. Upon knowing this, you should already be wondering how does carbon come to form? Everything boils down to the Calvin Cycle, the second stage of photosynthesis. In this article, we are going to be acquainted with the Calvin Cycle, the processes involved on this cycle, and the products which are formed.

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The Calvin Cycle, also known as the Calvin-Benson Cycle, refers to the set of light independent redox reactions that takes place in the chloroplasts during photosynthesis and carbon fixation that would convert carbon dioxide into the sugar glucose. Furthermore, the cycle also refers to the reactions involved in photosynthesis that use the energy that is stored by the light-dependent reactions in order to form glucose and other carbohydrate molecules. These reactions take place in the stroma of the chloroplast, a fluid-filled region that is found between the inner membrane of the chloroplast and the thylakoid membrane.

There are other names for Calvin Cycle. It is also referred to as the dark reactions, C3 cycle, or the reductive pentose phosphate cycle. Moreover, it is also known as the Calvin-Benson-Bassham (CBB) Cycle, an attribution to its discoverers: Melvin Calvin, James Bassham, and Andrew Benson.

Calvin, Bassham, and Benson discovered the cycle in 1960 at the University of California, Berkeley. They used the radioactive carbon-14 in order to trace the path of the carbon atoms in carbon fixation. They were able to trace the carbon-14 from soaking up its atmospheric Carbon Dioxide to its conversion into organic compounds such as carbohydrates.

The Calvin group exhibited results showing that the sunlight acts on the chlorophyll into a plant in order to fuel the production of organic compounds, not directly on carbon dioxide as it was formerly believed. Because of this discovery, Melvin Calvin won a Nobel Prize in Chemistry in 1961.

Calvin Cycle Steps

Before identifying the different processes involved in Calvin Cycle, it would be essential to identify the stages of photosynthesis where the cycle is a part of. Photosynthesis is defined as the process where plants and other organisms convert light energy into chemical energy which could be used produce energy for the activities of the plants. It involves two stages: the light reaction phase and the dark reaction phase.

Under the first stage, chemical reactions would use energy from the light in order to produce NADPH and ATP.

The second stage is the dark reaction phase, where water and carbon dioxide are converted into organic molecules. The second stage is where the Calvin cycle comes in.

The reactions of the Calvin cycle could be divided into three primary stages: the carbon fixation stage, the reduction stage, and the regeneration of the starting molecule. Even though the cycle is called the dark reaction phase, the aforementioned reaction do not really occur in the dark. Instead, they require the reduction of NADP which comes from the first stage.

1. Carbon Fixation

The first stage in the cycle involves incorporating carbon from carbon dioxide into an organic molecule. Under carbon fixation, a carbon dioxide molecule would combine with ribulose-1,5-bisphosphate (RuBP), a five-carbon acceptor molecule.

Such carbon dioxide would enter the mesophyll layer of the leaves by entering through the stomata. An enzyme called RuBP carboxylase/oxygenase or rubisco would catalyze the attachment of carbon dioxide to the RuBP. This process would make a six-carbon compound.

But because the said compound is unstable, it would quickly split into two molecules of a three-carbon compound which is called as the 3-phosphoglyceric acid or 3-PGA. Hence, for each of carbon dioxide that would enter the cycle, two 3-PGA molecules are formed.

2. Reduction

The reduction stage or second stage of the Calvin cycle requires ATP and NADPH. These compounds are used to convert the 3-PGA molecules (which were taken from the carbon fixation stage) into a three carbon sugar known as the glyceraldehyde-3-phosphate or G3P.

The process takes place in two major steps. In the first step, each molecule of 3-PGA would receive a phosphate group from ATP, turning into a 1,3-bisphosphoglycerate, a double phosphorylated molecule. This would leave ADP as a by-product. Under the second step, the 1,3-bisphosphoglycerate molecules are reduced by gaining electrons. Each of the molecules would receive two electrons from NADPH and loses one of its phosphate groups. After which, the glyceraldehyde 3-phosphate or G3P, a three-carbon sugar, is produced.

The second step of the reduction stage produces phosphate and NADP+ as by-products. It should be noted that the reduction stage received its name because NADPH donates or reduces electrons to a three-carbon intermediate in order to make G3P.

3. Regeneration

Under the regeneration stage, some G3P molecules would produce glucose while the others would be recycled in order to regenerate the RuBP acceptor. This stage would requires ATP and involve a complex set of reactions.

Three molecules of carbon dioxide must enter the cycle in order for one G3P to leave the cycle and go towards the glucose synthesis, and provide three new atoms of fixed carbon. Six G3P molecules will be produced when three carbon dioxide molecules will enter the cycle. One would leave the cycle to be used to produce glucose while the rest would be recycled in order to regenerate three molecules of the RuBP acceptor.

Products of Calvin Cycle

Generally, the carbohydrate products of the Calvin cycle are the three carbon sugar phosphate molecules or the triose phosphates (G3P). The products formed after a single turn of the Calvin cycle are 3 ADP, 2 glyceraldehyde-3-phosphate (G3P) molecules, and 2 NADP+.

It should be noted, however, that NADP+ and ADP are not really technically products but they are regenerated and are later used again during the light-dependent reactions. Each of the G3P molecules consists of three carbons. In order for the cycle to continue, the RuBP or the ribulose 1,5-bisphosphate must be regenerated. Hence, five of the six carbons from the two G3P molecules are used. From this, only one net carbon produced would play with for each turn.

In order to create a surplus G3P, three carbons are required, allowing three turns of the Calvin cycle. Six turns of the cycle are required in order to make a glucose molecule which could be created from two G3P molecules. Surplus G3P could also be used to form other carbohydrates such as cellulose, sucrose, and starch depending on what the plant would need.

Conclusion

To sum the processes and the products of Calvin cycle, the overall chemical equation of the phase is the following:

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → G3P + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi stands for inorganic phosphate)

Six runs of the cycle are needed in order to come up with one glucose molecule. As mentioned earlier, the surplus G3P which is produced by the reactions could be used to form other carbohydrates depending on the necessities of the plants.

Reference:
https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/a/calvin-cycle
Photo by: “The Calvin cycle: Figure 1,”

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