Question

For easy understanding, the Calvin cycle is divided into how many stages? (a) 1 (b) 2 (c) 3 (d) 4

Short answer

The Calvin Cycle is divided into three stages.

Step-by-step solution

Recall the stages of the Calvin cycle

Remembering that the Calvin cycle has three different stages: The Carbon Fixation, The Reduction and The Regeneration of the starting molecule.

Key concepts

Carbon Fixation

Carbon fixation is the first crucial step of the Calvin cycle. This step is all about taking carbon dioxide from the atmosphere and converting it into an organic molecule that can be used by plants.
The molecule that plays a pivotal role in this process is ribulose-1,5-bisphosphate (RuBP). It is a five-carbon sugar that acts like a magnet, attracting carbon dioxide. An enzyme called ribulose bisphosphate carboxylase/oxygenase (commonly known as RuBisCO) facilitates the joining of carbon dioxide with RuBP.
When carbon dioxide is fixed, a six-carbon molecule is formed. This molecule is unstable and immediately splits into two three-carbon molecules called 3-phosphoglycerate (3-PGA).

• RuBP combines with carbon dioxide.
• RuBisCO helps in this reaction.
• The result is two molecules of 3-PGA.

This process is called fixation because it captures atmospheric carbon dioxide and incorporates it into an organic molecule.

Reduction

In the next stage of the Calvin cycle, called reduction, the energy captured from the sun is used to convert 3-PGA into the sugar glyceraldehyde 3-phosphate (G3P). This phase requires energy and electrons, which are provided by molecules known as ATP and NADPH.
Here's a breakdown of what's happening:

• ATP provides energy, helping to transform 3-PGA into an intermediate compound.
• NADPH donates electrons, converting this intermediate into G3P.
• For every three carbon dioxide molecules that enter the cycle, six G3P molecules are produced.

It's important to note that while six G3P molecules are generated, only one of these will be used to build glucose. The remaining five G3P molecules are needed for the next stage, which is regeneration.
Ultimately, the reduction phase is about transforming 3-PGA into G3P using energy-rich molecules.

Regeneration

Regeneration is the final stage of the Calvin cycle, and it ensures that the cycle can continue to work efficiently. During this phase, the remaining G3P molecules are used to regenerate RuBP, the molecule essential for capturing carbon dioxide.
This is how it works:

• Five G3P molecules (from the reduction stage) are rearranged and converted back into three molecules of RuBP.
• ATP is used to provide the energy required for this conversion.
• With RuBP regenerated, the cycle can capture new carbon dioxide molecules and start the process again.

The regeneration step is like resetting the stage. Without it, there would be no RuBP available to combine with carbon dioxide and keep the Calvin cycle moving. This is what makes the Calvin cycle a true cycle—continuously moving forward to sustain plant life.
By anchoring the entire cycle, regeneration keeps plants functioning smoothly and efficiently, allowing them to produce the necessary carbohydrates for growth.