Calvin Cycle: The Calvin (Benson-Bassham) (CBB) cycle, also named reductive pentose phosphate pathway or dark reactions, is a group of biochemical reactions in photoautotrophs. 

These reactions form the light-independent stage of photosynthesis, where the energy converted from light is used to assimilate carbon dioxide from the atmosphere. 

Calvin cycle is also known as the C3 cycle or light-independent or dark reaction of photosynthesis. However, it is most active during the day when NADPH and ATP 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 is the source of hydrogen and the energetic electrons required to bind them to carbon atoms. Much of the light energy captured during photosynthesis ends up in the energy-rich C—H bonds of sugars.

Importance of Calvin Cycle

• The Calvin cycle is important to perform chemical reactions by plants to fix carbon from CO₂ into three-carbon sugars. 
• Afterward, plants and creatures can transform these three-carbon compounds into amino acids, nucleotides, and more complex sugars like starches. 
• This course of carbon fixation is the means by which most new natural matter is made. 
• The sugars made in the Calvin cycle are additionally utilized by plants for long-haul energy capacity, not at all like ATP, which is spent rapidly after it is made. 
• These plant sugars can likewise turn into a wellspring of energy for creatures who eat the plants, and hunters who eat those herbivores. 
• The Calvin cycle is controlled by ATP and NADPH, which are made by outfitting the energy from photons in the light-reliant responses.

Steps or stages of Calvin cycle

There are three main stages of the Calvin cycle: 

• Carbon fixation
• Reduction and 
• Regeneration.

Step 1

In the initial stage of the Calvin cycle, a CO₂ molecule is incorporated. The catalyst Rubisco (the most plentiful enzyme on the planet) catalyzes the carboxylation of a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP) with carbon dioxide molecule, coming about of a sum of 6 carbons. Then it is parted into 2 molecules of 3-PGA (3-phosphoglycerate), a 3-carbon compound.

It includes a two-step reaction:

• The result of the initial step is an enediol-enzyme that can catch CO₂ or O₂. Hence, the enediol enzyme complex is the correct carboxylase or oxygenase.
• The CO₂ that is caught by enediol in the second step delivers an unsteady six-carbon compound called 2-carboxy 3-keto 1,5-biphosphoribotol that quickly parts into 2 particles of 3-phosphoglycerate, a 3-carbon compound.

Step 2

In the next stage of the Calvin cycle, the 3-PGA molecules created through carbon fixation are changed over into molecules of simple sugar – glyceraldehyde-3 phosphate.

This stage utilizes energy from ATP and NADPH created in the light-dependent reactions of photosynthesis. The manner by which plants convert energy from sunlight into long-term storage molecules like sugars. The energy from the ATP and NADPH is moved to the sugars.

• Phosphoglycerate kinase catalyzes the phosphorylation of 3-PGA by ATP. 1,3-Bisphosphoglycerate and ADP are the items. Two ATPs are used.
• Glyceraldehyde 3-phosphate dehydrogenase catalyzes the reduction of 1,3BPGA by NADPH. Glyceraldehyde 3-phosphate is created, and NADPH changes to NADP+. Two NADPH are used.

Step 3

This is the final stage of the Calvin cycle. The other G3P should be reused to recover or regenerate the five-carbon RuBP compound that is utilized to acknowledge new carbon molecules during some glyceraldehyde-3 phosphate molecules go to make glucose. The recovery process requires ATP. It is a complex process including many steps. Since it takes six carbon particles to make glucose, this cycle should be rehashed multiple times to make a single molecule of glucose.

For one G3P to leave the cycle, three CO₂ molecules should enter the cycle, giving three new atoms of fixed carbon. At the point when three CO₂ molecules enter the cycle, six G3P molecules are made. One leaves the cycle and is utilized to make glucose, while the other five should be reused to recover three atoms of the RuBP acceptor.

6 NADPH + 9 ATP + 3CO₂  + 5 H₂O → G3P + 2H⁺ + 6NADP⁺ + 9ADP + 8Pi

Regulation of each step in Calvin Cycle

In the initial step, RuBisCO is the primary enzyme used during the carbon fixation and its enzymatic movement is profoundly regulated. RuBisCO enzymatic movement is controlled by various factors including- ion particles, ATP, ADP, CO₂, reduction, oxidation states, and phosphate. The different elements impacting RuBisCO action straightforwardly influence the first stage of the Calvin cycle.

In the following stage or step, one of the two G3P atoms shaped is additionally changed over completely to dihydroxyacetone phosphate (DHAP) and the catalyst aldolase is utilized to consolidate G3P and DHAP to frame fructose-1,6-bisphosphate. The enzyme aldolase is normally portrayed as a glycolytic catalyst with the capacity to part fructose 1,6-bisphosphate into DHAP and G3P.

The third step or phase of the Calvin cycle involves the regeneration of RuBP. This particular stage includes a progression of responses where there are various catalysts or enzymes for regulation. They are:

• Trio phosphate isomerase changes over all Glyceraldehyde 3-phosphate molecules into DHAP (dihydroxyacetone phosphate).
• Enzyme aldolase and fructose 1, 6 bisphosphate changes over Glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate into fructose-6-phosphate.
• The transketolase enzyme eliminates two carbon atoms in fructose 6-phosphate to create erythrose 4-phosphate (E4P).
• The two eliminated carbons from fructose 6-phosphate are added to Glyceraldehyde 3-phosphate (G3P) to deliver xylulose-5-phosphate (Xu5P).
• Aldolase enzyme changes over erythrose 4-phosphate (E4P) and a DHAP (dihydroxyacetone phosphate) to sedoheptulose-1,7-bisphosphate.
• Sedoheptulose-1,7-bisphosphatase severs the sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate (S7P) by removal of a phosphate group.
• Transketolase enzymes eliminate two carbons from S7P and two carbons are moved to one of the Glyceraldehyde 3-phosphate (G3P) molecules creating ribose-5-phosphate (R5P) and another xylulose-5-phosphate.
• Phosphopentose isomerase (or Ribose-5-phosphate encoded by RPIA gene) is an enzyme that changes over the ribose-5-phosphate R5P into ribulose-5-phosphate (Ru5P)
• Phosphopentose epimerase (encoded by the RPE gene) catalyzes or changes over from xylulose-5-phosphate (Xu5P) into ribulose-5-phosphate (Ru5P).
• Phosphoribulokinase (an important photosynthetic enzyme) catalyzes to phosphorylate of ribulose-5-phosphate (Ru5P) into ribulose-1,5-bisphosphate.

After this last enzyme plays out this change, the Calvin cycle is thought of as complete. The last phase of the Calvin cycle is viewed as the most complicated and regulated of the cycle.

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By Team Learning Mantras