Calvin Cycle – Concept, Phases, Function and Importance

We explain what the Calvin Cycle is, its stages, its function and its products. Also, its importance for autotrophic organisms.

photosynthesis autotrophic calvin cycle
The Calvin cycle is the “dark phase” of photosynthesis.

What is the Calvin Cycle?

It is known as the Calvin Cycle, the Calvin-Benson Cycle or the Cycle of carbon fixation in photosynthesis, at a set of biochemical processes that take place in the stomata of plant chloroplasts and other autotrophic organisms whose nutrition is carried out through photosynthesis.

The reactions that make up this cycle belong to the so-called dark phase of the photosynthetic process or light-independent phase, during which carbon dioxide (CO2) taken from the atmosphere, incorporating it into the body in the form of glucose (C6H12OR6) thanks to the action of the enzyme RuBisCo (ribulose-1,5-bisphosphate carboxylase / oxygenase).

The Calvin Cycle owes its name to its discoverer, the American Melvin Calvin, earning him the Nobel Prize in Chemistry in 1961. His other important collaborators on the study were James Bassham and Andrew Benson, all from the University of California, Berkeley.

Stages of the Calvin cycle

calvin cycle autotroph
Each Calvin cycle goes through the stages of fixation, reduction and regeneration.

The Calvin cycle is made up of three distinct stages:

  • Stage 1. CO fixation2, the enzyme RuBisCo catalyzes the carboxylation of ribulose diphosphate, that is, the fixation of carbon dioxide to form PGA (3-Phosphoglyceric Acid).
  • Stage 2. Reduction of PGA to a sugar (CH2O) through the formation of glycealdehyde-3-phosphate (GAP) with NADPH (Nicotidamide adenine dinucleotide phosphate) and ATP (Adenosine Triphosphate) produced in light-dependent reactions.
  • Stage 3. Ribulose diphosphate regeneration, which requires ATP as well.

Function of the Calvin Cycle

The Calvin Cycle has a fundamental role in the life of plants: generate glucose, one of the main sugars (of six carbon atoms) of biochemical use as energy source and structural or storage material.

The cycle uses six CO molecules2 to obtain glucose, attaching them to various receptors in a repetitive circuit of energy-consuming chemical reactions (ATP). Six turns of the cycle are necessary to compose a glucose molecule. In addition, every 3 turns of the cycle a molecule of triose phosphate is produced, used in other processes such as the synthesis of starch.

Importance of the Calvin Cycle

The Calvin cycle is the only metabolic pathway that use autotrophic organisms to incorporate the inorganic matter on which they feed, just like CO2 atmospheric, which breathing organisms expel from their organisms. This occurs in both photosynthetic and chemosynthetic organisms.

At the same time, this process is of tremendous ecological importance, since in this cycle in plant tissues energy is stored and transmitted upward in the trophic pyramid, serving as food for herbivorous animals, which in turn serve as food for their predators.

On the other hand, this process of fixation of the carbon contained in the CO2, a known greenhouse gas, contributes to the cooling of the atmosphere and with the reduction of the gases responsible for global warming and climate change. That is why today is more important than ever.

Calvin cycle products

The Calvin Cycle produces a fixed carbon atom in each of its turns, so it takes three rounds of the cycle (and the entry of three molecules of CO2, one at a time) to form a molecule of Glyceraldehyde-3-phosphate (3GP). Thus, part of the material produced can be recycled to reactivate the RuBisCo and another part can be used for the production of glucose.