The Calvin cycle is a series of reactions which takes place in the stroma of chloroplasts in a plant cell. The carbon dioxide taken up by the plant cell is reduced to glucose with the help of ATP and NADPH which is formed in the dark reaction of photosynthesis. The relatively stable compound that is formed in this cycle is a 3-carbon sugar.
The pathway was first elucidated by an American biochemist Melvin Calvin and his co-workers and the cycle involves the fixation of carbon dioxide and its reduction to carbohydrate.
Plants cell produce organic molecules using the products of the light reactions such as ATP and NADPH.
ATP is used as the source of energy driving the endergonic reactions whilst the reducing power of NADPH is used as a source of hydrogen and electrons required to bind them to carbon atoms.
The light energy captured during photosynthesis is used in the C-H bonds of sugar.
A major component of the Calvin cycle is the enzyme ribulose-1, 5- biphosphate carboxylase also known as RUBISCO. It generates a trio of products in the C3 cycle which are 3-Phosphoglycerate (3-PGA), glyceraldehyde 3-P (GAP) and dihydroxyacetone phosphate or DHAP. All these products are used to synthesise fructose-1, 6 biphosphate and fructose-6 phosphate.
Three complex events take place during the Calvin cycle and these events are carried out in the dark reaction phase of photosynthesis. The Calvin cycle steps include
We will discuss these steps in the following.
Carbon Fixation: This is the first key step of the Calvin cycle where carbon dioxide is reduced and attached to an organic molecule. The molecule is formed by reassembling the bonds of two intermediate glycolysis products: fructose 6-phosphate, and glyceraldehyde 3-phosphate to produce an energy-rich 5-C compound ribulose 1, 5-bisphosphate (RuBP) and a 4-C sugar.
Carbon dioxide binds to RuBP in a process known as carbon fixation and forms two molecules 3-PGA. The enzymes which are used to catalyse this reaction is ribulose bisphosphate carboxylase/oxygenase. This is a large 4-subunit enzyme found in the stroma and it works slowly, processing only molecules of RuBP per second. The process is called carbon fixation because CO2 is fixed from an inorganic form into organic molecules.
Reduction: After the two molecules of 3-PGA are formed, they are converted to a simple sugar- glyceraldehyde-3 phosphate (G3P). ATP and NADPH are utilised as energy sources in this step and the energy is transferred to the sugar molecules to be reserved as long-term storage. This step is known as reduction because electrons are transferred to 3-PGA to form G3P.
Regeneration: The regeneration of RuBP takes place in this step. It starts with one of the G3P molecules leaving the Calvin cycle and is transported to the cytoplasm to contribute to the formation of products needed by a plant body. This leaves the chloroplast with 3 carbon atoms and it takes three turns of the Calvin cycle to fix net carbon to export one molecule of G3P.
However, each turn forms 2 molecules of G3Ps so in total 6 molecules of the compound are formed. Whilst one is exported the other remaining G3P molecules are used to regenerate RuBP. Three extra molecules of ATP are used in the regeneration reactions and it enables the system to fix more CO2.
6 NADPH + 9 ATP + 3CO2 + + 5 H2O → G3P + 2H+ + 6NADP+ + 9ADP + 8Pi (Pi = inorganic phosphate)
Pi= Inorganic phosphate
The products of the Calvin cycle are as follows:
One molecule of carbon is fixed in every turn of the cycle
One molecule of G3P is formed in 3 turns of the cycle
2 molecules of G3P combine to form one molecule of glucose
3 molecules of ATP and 2 molecules of NADPH are used in the reduction phase to convert 3-PGA to G3P and the regeneration of RuBP.
18 molecules of ATP and 12 molecules of NADPH are used to form one molecule of glucose.
The Calvin or the C3 cycle is also known as the Calvin–Benson–Bassham (CBB) cycle and the Reductive pentose phosphate cycle.
The C3 cycle is partially dependent on light and utilises ATP and NADPH produced in the light reactions to operate the cycle.
Regenerated RuBP in the last stage assists in more carbon fixation.
The produced sugars are used as energy storage units.
1. Why is the C3 cycle important to most ecosystems?
Through photosynthesis and the Calvin cycle (C3 cycle) plant cells turn the carbon dioxide in the air into carbon that can be used to make sugars, proteins, nucleotides and lipids. It helps in storing light energy as sugars for long term storage. This sugar can be used by plants and also eaten by animals forming the basis of the food chain. It also assists in removing CO2 which is a greenhouse gas from the environment. That is why the C3 cycle is important to most ecosystems.
2. What is the source of NADPH and ATP in the Calvin cycle?
The Calvin cycle is powered by light energy which is harnessed by the chloroplasts. During light phase reactions of photosynthesis, this light energy is converted to ATP and NADPH molecules which are used in the C3 cycle.
3. What are the different steps involved in the Calvin cycle?
The Calvin cycle consists of three steps:
Step 1: Carbon fixation: Carbon dioxide is fixed to a 3-carbon sugar
Step 2: Reduction: The 3-carbon sugar (3-PGA) is reduced to Glyceraldehyde-3 Phosphate with the help of donor electrons from ATP and NADPH molecules from the light from reactions.
Step 3: Regeneration: RuBP is regenerated for more carbon fixation. One G3P molecule is exported to the cytoplasm to be used in cellular activities and help in the production of glucose.
4. Why does the Calvin cycle take place 6 times?
One G3P molecule leaves the C3 cycle to contribute to the formation of glucose which is a 6-carbon molecule. That's why it takes 6 turns of the cycle to form one molecule of glucose (one for each CO3 molecule fixed). The remaining G3P molecules assist in the regeneration of RuBP.