Question

Give a graphical representation of ${C_3}$ cycle.

Answer 1

Calvin cycle is also known as the ${C_3}$ cycle or light-independent or dark reaction of photosynthesis. It is a set of light independent redox reactions that occur during photosynthesis and carbon fixation to convert carbon dioxide into sugar glucose.

Complete answer:
Calvin Cycle or ${C_3}$ cycle takes place in stroma. It is named after Melvin Calvin who discovered it. The plants that undergo Calvin cycle for carbon fixation are known as ${C_3}$ plants. The Calvin cycle is a part of photosynthesis that completes in two stages. In the first stage, chemical reactions use energy from light to produce ATP and NADPH. In the second stage of Calvin cycle or dark reactions, carbon dioxide and water are converted into organic molecules, such as glucose.
Calvin cycle is divided into three main phases:

Carboxylation: Three molecules of additive 6C compound immediately split up in the presence of water (by hydrolysis) into six molecules of 3C compound called 3-phosphoglyceric acid (3-PGA). It is the first stable product of photosynthesis. The first stable product is 3C compound, the plants performing the Calvin cycle are called C3 plants.

Reduction: In this phase, one molecule of 3-PGAL, a triose sugar can be directly used for metabolism of the plant or it can be converted into hexose sugar and is ultimately stored in the form of starch.

Regeneration: It is the third stage of the Calvin cycle and is a complex process which requires ATP. In this stage, some of the G3P molecules are used to produce glucose, while others are recycled to regenerate the RuBP acceptor.

Thus, for the formation of one molecule of glucose, 18 molecules of ATP and 12 molecules of NADPH2 are required; and totally 6 Calvin cycles are necessary.

Calvin Cycle Chemical Equation
The complete chemical equation for the Calvin cycle is:

$$3{\text{ }}C{O_2}\; + {\text{ }}6{\text{ }}NADPH{\text{ }} + {\text{ }}5{\text{ }}{H_2}O{\text{ }} + {\text{ }}9{\text{ }}ATP{\text{ }} \to {\text{ }}glyceraldehyde - 3 - phosphate{\text{ }}\left( {G3 - P} \right){\text{ }} + {\text{ }}2{\text{ }}{H^ + }\; + {\text{ }}6{\text{ }}NAD{P^ + }\; \\\ \+ {\text{ }}9{\text{ }}ADP{\text{ }} + {\text{ }}8{\text{ }}Pi{\text{ }}\left( {iP{\text{ }} = {\text{ }}inorganic{\text{ }}phosphate} \right) \\\$$

Six complete 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: Calvin cycle or Photosynthetic Carbon Reduction (PCR) cycle occurs in all photosynthetic plants whether they have ${C_3}$ or ${C_4}$ pathways. Although the Calvin cycle may be called the "dark reactions," these reactions don't actually occur in the dark or during night time. The reactions require reduced NADP, which comes from a light-dependent reaction.