Calvin Cycle

Introduction  

The  Calvin cycle is defined as the series of chemical reactions that change carbon dioxide and other compounds into glucose. It can also be renamed as the light-independent reactions, dark reactions and biosynthesis phase. The Calvin cycle exists in photosynthetic bacteria and eukaryotes and usually takes place in the stroma, which is the fluid-filled region in a plant. When light-dependent reactions produce ATP and NADPH, these reactions pick them for further processing. It involves a series of reduction and oxidation processes that conclusively produce sugar for the plants. The complete light-independent process of CO2 converting into sugar occurs in three different stages: carboxylation, reduction reaction,and ribulose regeneration.

The solar energy stored in the bonds of NADPH and ATP molecules changes into chemical energy. That’s when the cell obtains sufficient fuel to form carbohydrate molecules for long-term energy consumption. It is noteworthy that the lifespans of products of light-dependent reactions and products of light-independent reactions are entirely different.

For example, ATP and NADPH can last only up to the range of a microsecond  as these are the products of light-dependent reactions. On the contrary, carbohydrates and other products of light-independent chemical reactions are known for surviving millions of years. This remarkable resilience in carbohydrate molecules comes from carbon atoms.

The carbon dioxide molecule enters the plants through stomata to reach the mesophyll cells. It disseminates through the intercellular spaces in the leaves till it diffuses into the stroma. This is the fluid-filled area of the chloroplast where the light-independent reactions of photosynthesis take place. These reactions are divided into three following stages:

Dark Reaction of Photosynthesis Steps

1. Fixation

Other than carbon dioxide, light-independent reactions involve two essential components that include  ribulose-1, 5-bisphosphate carboxylase (RuBisCO) and ribulose-1, 5-bisphosphate (RuBP). RuBisCO is an enzyme whereas RuBP is a molecule that contains five atoms of carbon and two phosphates.

RuBisCO contributes as a catalyst that infuses the reaction between RuBP and CO2. Every RuBP molecule produces two molecules of 3-PGA after reacting with the carbon dioxide molecule. PGA stands for phosphoglycerate that consists of 3 carbon atoms and a phosphate. Every frequency of one Calvin cycle entails the reaction between one molecule of CO2 and one molecule of RuBP forming two molecules of 3-PGA.

The number of carbon atoms does not change throughout the reaction because the atoms form new bonds and result in a completely different molecule. Since the process involves the conversion of a CO2 molecule from an inorganic form to an organic molecule, it is also carbon fixation.

2. Reduction

ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate hydrogen) are two main inputs used in the light-independent reaction. It encompasses the conversion of six 3-GPA molecules into six molecules of G3P which stands for glyceraldehyde 3-phosphate. This phase is called a reduction reaction because of the gain of an electron by a molecule/atom.

Total six molecules of NADPH and ATP are used in the reaction. After the reaction, ATP turns into ADP in the formation of energy after the loss of the terminal phosphate atom. On the flip side, NADPH converts into NADP+ after losing a hydrogen atom and energy. These molecules are then transferred to light-dependent reactions for the reformation of energy.

3. Regeneration

In the final phase of the Calvin cycle, just one G3P molecule moves from the light-independent reactions. It is then transferred to the cytoplasm to partake in the production of other compounds in the plant. The G3P molecule provided by the chloroplast has three carbon atoms. Thus, it requires three frequencies of the cycle to process sufficient carbon for G3P export.

Every turn of the cycle produces two molecules of G3P. So, three different turns of the cycle make six molecules of G3P. One of these six molecules is exported for the reaction while other G3P molecules contribute to the regeneration of RuBP. It also allows the system to fix more carbon dioxide. Iit takes three molecules of ATP to enable a regeneration reaction.

Products of Light-Independent Reactions

Every turn of the Calvin cycle produces 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP and 2 NADP+. However, NADP and ADP can’t be considered ‘products’ as these are further used in the regeneration phase of the cycle. The Calvin cycle can suffice only if RuBP (ribulose 1,5-bisphosphate) is continuously regenerated. It takes 6 turns of the Calvin cycle and two G3P molecules to form one molecule of glucose. An excessive amount of G3P molecules can be used to produce cellulose, starch, sucrose and other types of carbohydrates depending on the type of plants’ requirements.

Conclusion –

Photosynthesis is the first stage of the light-independent Calvin cycle that forms energy carriers. These carriers are then used to turn carbon dioxide into carbohydrate molecules. After this occurs the fixation reaction in which RuBP and CO2 are combined to catalyze the reaction with the help of the RuBisCO enzyme. The resulting product is a 6-carbon compound that is broken down into 2 separate 3-carbon compounds. On the other hand, the energy in ATP and NADPH converts the molecules into G3P. The cycle spares some G3P molecules that later turn back into RuBP and react with carbon dioxide. It becomes a balanced energy cycle with cellular respiration.