Calvin Cycle

Photosynthesis is the biological process which occurs in all green plants or autotrophs producing organic molecules from carbon dioxide (CO2). These organic molecules contain many carbon-hydrogen (C–H ) bonds and are highly reduced compared with CO2.

Also, read more about Photosynthesis 

There are two stages of Photosynthesis –

Light-dependent reactions – As the name suggests, it requires light and mainly occurs during the day time.

Light-independent reactions – It also called the dark reaction or Calvin cycle. This reaction occurs both in the presence and absence of sunlight.

Also, read about Light reaction And Dark reaction

Here, in this article let us learn more about Calvin Cycle along with its stages

Calvin Cycle Definition

The Calvin cycle is defined as the cycle of chemical reactions usually performed by plants to fix carbon from CO2 into three-carbon sugars.

The Calvin Cycle is also called as the light-independent or Dark Reaction of photosynthesis, but itis the most active during the daylight hours when ATP and NADPH are abundant. To build organic molecules, the plant cells use raw materials provided by the light reactions:

1. Energy: ATP  – provided by cyclic and noncyclic photophosphorylation, which drives the endergonic reactions.

2. Reducing power: NADPH – provided by photosystem I,  provides a source of hydrogen and the energetic electrons required to bind them to carbon atoms. Much of the light energy captured in photosynthesis ends up invested in the energy-rich C—H bonds of sugars.

Plants store light energy in the form of carbohydrate, primarily starch and sucrose. The carbon and oxygen for this process are obtained from CO2, and the energy for carbon fixation is derived from the ATP and NADPH produced during the photosynthesis process. The conversion of CO2 to carbohydrate is called the Calvin Cycle and is named after Melvin Calvin who discovered it. Those plants that utilize the Calvin cycle for carbon fixation are known as C3plants.

The Calvin Cycle requires the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase commonly called rubisco. The Calvin cycle generates the triose phosphates 3-phosphoglycerate (3-PGA), glyceraldehyde-3P (GAP) and dihydroxyacetone phosphate (DHAP), all of which are used to synthesize the hexose phosphates fructose-1,6-bisphosphate and fructose 6-phosphate.

Calvin Cycle


Stages of the Calvin Cycle or C3 Cycle

The Calvin cycle reactions can be divided into three main stages:

  • Carbon fixation 

The key step in the Calvin cycle is the event that makes the reduction of CO2 possible and is the attachment of CO2 to a very special organic molecule. Photosynthetic cells produce this molecule by reassembling the bonds of two intermediates in glycolysis – fructose 6-phosphate, and glyceraldehyde 3-phosphate, to form the energy-rich five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), and a four-carbon sugar.

CO2 binds to RuBP in the key process called carbon fixation, forming two three-carbon molecules of phosphoglycerate. The enzymes that carry out this reaction are ribulose bisphosphate carboxylase/oxygenase, which is very large with a four-subunit enzyme present in the chloroplast stroma. This enzyme works very sluggishly, processing only about three molecules of RuBP per second (a typical enzyme process about 1000 substrate molecules per second). This is mainly because of the enzymes works slowly, and many molecules of Rubisco are needed. In a typical leaf, over 50% of all the protein is rubisco. It is thought to be the most abundant protein on earth.

  • Reduction

It is the second stage of the Calvin cycle, the 3-PGA molecules created through carbon fixation that are converted into molecules of simple sugar – glyceraldehyde-3 phosphate (G3P).

This stage uses energy from ATP and NADPH created in the light-dependent reactions of photosynthesis. In this way, the Calvin cycle becomes the way in which plants convert energy from sunlight into long-term storage molecules, such as sugars. The energy from the ATP and NADPH is transferred to the sugars.

This step is called as the reduction because NADPH donates electrons to the 3-phosphoglyceric acid molecules to create glyceraldehyde-3 phosphate. The reduction in the process of donating one electron and is the opposite of the oxidation reaction.

  • Regeneration

It is the third stage of the Calvin cycle. It is a complex process, which requires ATP. In this stage, some of the G3P molecules are used to produce glucose, while others are recycled to regenerate the RuBP acceptor.

Discover more about Photosynthesis, Calvin cycle, its process, types, and other topics only @ BYJU’S Biology

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