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Calvin Cycle

All living things on Earth, including humans, are made of carbon. The magnificent human body's intricate molecules are composed of carbon-based building blocks. You might already be aware of your carbon-based makeup, but have you ever given the origin of all that carbon any thought?
Your body contains carbon atoms formerly part of the atmosphere's carbon dioxide (CO2) molecules. The Calvin cycle, the second step of photosynthesis, is where carbon atoms enter both you and other living things.

What is a Calvin Cycle?
Carbon dioxide enters plants through stomata or internal leaf holes and travels to the chloroplast stroma, where sugar is created during the Calvin cycle. Since light is not the primary driver of these processes, they are often called light-independent reactions. Carbon atoms are fixed (integrated into organic molecules) during the Calvin cycle, allowing the creation of sugars with three carbon atoms. The light reactions' byproducts of ATP and NADPH are essential to this process and help it along. Calvin cycle reactions take place in the stroma or inside of chloroplasts, as opposed to light reactions that take place in the thylakoid membrane (Calvin cycle diagram below)

Reactions of the Calvin Cycle
Carbon fixation: A carbon dioxide molecule is coupled with ribulose-1,5-bisphosphate (RuBP), a carbon acceptor molecule with five atoms. In this stage, a complex with six carbon atoms is created, which breaks down into two molecules of a chemical with three carbons, known as 3-phosphoglyceric acid (3-PGA). The RuBP carboxylase enzyme, often known as rubisco, catalyzes this process.


Reduction: In the second stage of the Calvin cycle, 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a sugar molecule with three carbon atoms, using ATP and NADPH molecules. The name of this step comes from the fact that NADPH forms G3P by donating electrons to a three-carbon intermediate.


Regeneration: While certain G3P molecules may renew the RuBP acceptor by forming glucose, other G3P molecules must be regenerated. Regeneration necessitates ATP and entails a convoluted chain of processes known as the "carbohydrate scramble."

Three CO2 molecules must enter the cycle to provide the three additional firmly linked carbon atoms needed for one G3P molecule to exit the cycle and contribute to glucose production. Six molecules of G3P are created when three carbon dioxide molecules enter the cycle. The other five must be recycled to replenish three molecules of the RuBP acceptor, while one exits the cycle and becomes a component of glucose.

Photosynthesis of Calvin Cycle
Photosynthesis is a vital biochemical process in which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose and other organic molecules. The Calvin cycle, also known as the Calvin-Benson cycle or the dark reaction, is one of the key phases of photosynthesis in the chloroplasts' stroma. It involves a series of enzyme-catalyzed reactions that ultimately lead to the production of glucose.

Carbon Fixation (RuBisCO Reaction): The Calvin cycle starts with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), catalyzing the fixation of carbon dioxide (CO2) onto a five-carbon compound, ribulose-1,5-bisphosphate (RuBP). This reaction produces two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound.

Reduction Phase: In this phase, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) generated during the light-dependent reactions of photosynthesis provide the energy and reducing power needed to convert 3-PGA molecules into another three-carbon compound, glyceraldehyde-3-phosphate (G3P). This conversion occurs through a series of enzyme-catalyzed reactions, consuming ATP and NADPH.

Regeneration of RuBP: Some of the G3P molecules produced in the reduction phase are used to regenerate RuBP through additional enzyme-catalyzed reactions. These reactions ensure that the Calvin cycle can continue, as RuBP is essential for the initial carbon fixation step.

Glucose Formation: From every three molecules of CO2 that enter the Calvin cycle, one molecule of G3P is ultimately converted into glucose or other carbohydrates. This occurs when two G3P molecules combine and undergo further reactions to form glucose and other sugars.

Release of One Molecule: Out of the six G3P molecules generated in the cycle, five are used for RuBP regeneration, and one is used to produce glucose or other carbohydrates. Therefore, it takes six turns of the Calvin cycle to produce one glucose molecule.

(edited 6 months ago)

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