Category: PLANTS

 

The Calvin cycle is a set of light independent redox reactions that occur during photosynthesis and carbon fixation to convert carbon dioxide into the sugar glucose. These reactions occur in the stroma of the chloroplast, which is the fluid-filled region between the thylakoid membrane and inner membrane of the organelle.

Here is a look at the redox reactions that occur during the Calvin cycle.
Other Names for the Calvin Cycle include dark reaction. The set of reactions also is known as the dark reactions, C3 cycle, Calvin-Benson-Bassham (CBB) cycle, or reductive pentose phosphate cycle. The cycle was discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley. They used radioactive carbon-14 to trace the path of carbon atoms in carbon fixation. From these findings, they were able to demonstrate the cycle.

Overview of Calvin Cycle


The Calvin cycle is part of photosynthesis, which occurs in two stages. In the first stage, chemical reactions use energy from light to produce ATP and NADPH. This reaction is popularly known as light reaction due to photolysis. Photolysis simply means the splitting of water molecules by light energy. In the second stage (Calvin cycle or dark reactions), carbon dioxide and water are converted into organic molecules, such as glucose.

Although the Calvin cycle may be called the “dark reactions,” these reactions don’t actually occur in the dark or during night time. It simply depicts that it’s independent of light presence.

The reactions require reduced NADP, which comes from a light-dependent reaction. The Calvin cycle consists of:
Carbon fixation – Carbon dioxide (CO2) is reacted to produce glyceraldehyde 3-phosphate (G3P). The enzyme RuBisCO catalyzes the carboxylation of a 5-carbon compound to make a 6-carbon compound that splits in half to form two 3-phosphoglycerate (3-PGA) molecules. RubisCO is a short form of ribulose-1,5-bisphospho carboxylase oxygenase.

The enzyme phosphoglycerate kinase catalyzes phosphorylation of 3-PGA to form 1,3-biphosphoglycerate (1,3BPGA).Reduction reactions – The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyzes reduction of 1,3BPGA by NADPH.

Ribulose 1,5-bisphosphate (RuBP) regeneration – At the end of the regeneration, the net gain of the set of reactions is one G3P molecule per 3 carbon dioxide molecules.

Calvin Cycle Chemical Equation

The overall chemical equation for the Calvin cycle is:
3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi

(Pi = inorganic phosphate)S

Seven runs of the cycle are required to produce one glucose molecule. Surplus G3P produced by the reactions can be used to form a variety of carbohydrates, depending on the needs of the plant.

Note About Light Independence

Although the steps of the Calvin cycle don’t require light, the process only occurs when light is available (daytime). Why? Because it’s a waste of energy because there is no electron flow without light.

The enzymes that power the Calvin cycle are therefore regulated to be light dependent even though the chemical reactions themselves don’t require photons.


At night, plants convert starch into sucrose and release it into the phloem. CAM plants store malic acid at night and release it during the day. These reactions are also known as “dark reactions.”

Function of the Calvin Cycle?

 

The Calvin cycle is responsible for completion of Photosynthesis

The Calvin cycle is the final step of photosynthesis. Here is an explanation of the primary function of this important step:

Converting Carbon Dioxide and Water Into Glucose

In the most general sense, the primary function of the cycle is to make organic products that plants need using the products from the light reactions of photosynthesis (ATP and NADPH). These organic products include glucose, the sugar made using carbon dioxide and water, plus protein (using nitrogen fixed from the soil) and lipids (e.g., fats and oils).This is carbon fixation, or fixing inorganic carbon into organic molecules that the plant can use:

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

(Pi = inorganic phosphate)

The key enzyme for the reaction is RuBisCO. Although most texts simply say the cycle makes glucose, the Calvin cycle actually produces 3-carbon molecules, which are eventually converted into the hexose (C6) sugar, glucose.

It is a set of light-independent chemical reactions, so you might also hear it referred to as the dark reactions. This doesn’t mean the Calvin cycle occurs only in the dark; it just doesn’t require energy from light for the reactions to occur.

Summary

The primary function of the Calvin cycle is carbon fixation, which is making simple sugars from carbon dioxide and water.

 

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