The Calvin cycle presented in simplified form

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You know that plants do photosynthesis. You are also familiar with the fact that oxygen is created from CO2 in this way. But what the Calvin cycle is supposed to have to do with it is still unclear to you? It's a shame, because the phenomenon is actually an interesting, biochemical process.

The Calvin cycle is part of photosynthesis, in which CO2 turns into oxygen.

Metabolic processes of plants and bacteria

Glyceraldehyde-3-phosphate is formed in the synthesis phase of the dark reaction. © Sima Moussavian

The Calvin cycle, often referred to as the Calvin-Benson cycle or ribulose bisphosphate cycle, goes back to the biochemists Calvin and Benson. It affects all photosynthetic organisms. It is part of the plant and bacterial metabolism and corresponds to the cyclical sequence of individual chemical reactions. The conversion of CO₂ in glucose is the main goal of the cycle.

While humans and animals absorb organic substances for metabolism, plants and bacteria use inorganic substances such as CO₂ return. Their metabolic rate is therefore higher than in humans or animals.

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Unlike animals and humans, plants are not heterotrophic but autotrophic organisms. That is, they make their own during photosynthesis energy and produce your own nutrients by converting light energy.
The metabolism of bacteria is very different. For example, some are chemoautotrophic and convert carbon dioxide to carbon.
The metabolic process of autotrophic organisms requires water, CO as the starting material₂and minerals, for example via a soil type of certain composition. Since temperatures affect the rate and product of reactions, they also require certain temperatures.

Biology - the dark reaction

No biology student can avoid the subject matter of photosynthesis. A …

Light and dark reactions are the two basic stages of the metabolism of autotrophic organisms, i.e. photosynthesis. Both reactions serve to produce nutrients, with oxygen being produced as waste.

The Calvin cycle is another name for the first metabolic stage of the dark reaction. It affects the spectrum of all plants, right down to some groups of algae and certain types of bacteria. But what exactly happens during the dark reaction?

A simplified explanation of the dark reaction in photosynthesis

Consider plant metabolism:

For photosynthesis, the organisms assimilate carbon dioxide with the help of ATP and NADPH to oxygen and carbon. That is the dark reaction.
In the course of the dark reaction, carbon dioxide is initially reduced by several oxidation levels. In simple terms, this means that one of the two outgoing O atoms is split off. At the same time, two H atoms are added, creating glucose.
This glucose consists of six of the units H-C-OH. C-C connections connect the six individual units with one another.
This cycle is followed by the cyclical light reaction, during which, in addition to oxygen, the starting materials ATP and NADPH are restored. So the game can start over.

Fructose-1,6-diphosphate is the product of the first reorganization phase step. © Sima Moussavian

This description only reproduces the dark reaction in the context of photosynthesis in an extremely simplified manner. If you are interested in the detailed individual processes, the following section may help you.

The Calvin cycle in detail

The Calvin cycle consists of a synthesis and a reorganization phase.

the summary: At the beginning of the cycle, carbon dioxide binds to a C5 molecule, the so-called ribulose-1,5-diphosphate. This creates a C6 body, which then divides six times.
This division results in two unstable C3 bodies, i.e. a total of twelve C3 bodies, called phosphoglycerate molecules. With the help of the oxidants ATP and NADH, the organism reduces these twelve C3 bodies. These twelve molecules are glyceraldehyde-3-phosphate molecules.
The organism uses two of them to produce glucose. Finally, the remaining C3 bodies are converted back into C5 bodies by ATPs, which corresponds to the reorganization phase.
This reorganization phase is the most complex part of the cycle. From four C3 bodies there are initially two C6 bodies, so-called fructose-1,6-diphosphate. With two additional C3 bodies, this results in two C4 bodies each, which are called erythrose-4-phosphate. At the same time, two C5 bodies, called ribulose-5-phosphate, are formed.
The C4 bodies then each react with two additional C3 bodies to form two C7 bodies, which are called sedoheptulose-1,7-diphosphate. These two bodies react with two C3 bodies again to form four C5 bodies. Ribulose-5-phosphate regenerates four times.
There are now no more C3 bodies, but six ribulose-5-phosphate are left. These correspond to a precursor of ribulose-1,5-diphosphate, which serves as a docking point for carbon dioxide at the beginning of the Calvin cycle. The ribulose-1,5-diphosphate molecules are produced from this preliminary stage in a phospholization step with the help of six ATP. The cycle now starts all over again.

Ribulose-5-phosphate is produced in the second step of the reorganization. © Sima Moussavian

This process is always interesting. Your knowledge of the photosynthesis of plants has thus expanded significantly. The sketches will help you understand the complex process even better.