Question

Calvin cycle includes (a) Carboxylation (b) Reduction (c) Regeneration of \(\mathrm{RuBP}\) (d) All of these

Short answer

The correct answer is (d) All of these. As per the Calvin Cycle process, it does include Carboxylation, Reduction and Regeneration of RuBP, hence all of these options are included in the Calvin cycle.

Step-by-step solution

Observation

Observe all the listed options and remember the process of the Calvin cycle. The Calvin cycle is a sequence of reactions that occurs in plants during photosynthesis that results in the conversion of carbon dioxide into glucose.

Recall each step in the Calvin Cycle

There are three major steps in the Calvin cycle: (1) Carboxylation: In this step, carbon dioxide is added to \(\mathrm{RuBP}\) to make a six-carbon compound that quickly splits into two three-carbon molecules, (2) Reduction: These three-carbon molecules are reduced to two molecules of 3-phosphoglycerate, (3) Regeneration of \(\mathrm{RuBP}\): ATP and NADPH contribute to the regeneration of \(\mathrm{RuBP}\). Then the cycle restarts.

Compare the Calvin Cycle steps with the Options Given

After listing out the three major steps in the Calvin cycle, we see that Carboxylation, Reduction and Regeneration of \(\mathrm{RuBP}\) are all parts of the Calvin cycle. So comparing these with the given options, it’s clear that all of the listed processes are included in the Calvin cycle.

Key concepts

Carboxylation

Carboxylation is the first crucial step in the Calvin Cycle, and it sets the stage for carbon fixation in plants. In this process, carbon dioxide ( CO_2 ) from the atmosphere is incorporated into an organic molecule. This is mediated by an enzyme called ribulose bisphosphate carboxylase/oxygenase, often abbreviated as RuBisCO.
This enzyme facilitates the addition of CO_2 to ribulose bisphosphate (RuBP), a five-carbon sugar. As a result, a six-carbon compound is formed transiently, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This step is pivotal because it initiates the conversion of inorganic carbon into organic compounds that can be further processed into glucose and other sugars vital for plant energy needs.
Key takeaways from carboxylation include:

• This process occurs in the stroma of the chloroplast.
• RuBisCO plays a significant role, making it one of the most abundant enzymes on Earth.
• It is highly regulated since it influences the efficiency of photosynthesis significantly.

Reduction

Reduction is the second step in the Calvin cycle and involves the conversion of 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P). This step gains its name from the reduction reaction that occurs - meaning electrons are being added.
In this stage, ATP and NADPH, both generated from the light-dependent reactions of photosynthesis, provide the necessary energy and reducing power, respectively. First, ATP phosphorylates the 3-PGA, generating 1,3-bisphosphoglycerate. Next, NADPH donates electrons to reduce 1,3-bisphosphoglycerate, producing glyceraldehyde-3-phosphate (G3P).
Important points to remember about the reduction phase include:

• Reduction effectively converts energy from light into chemical energy in the form of G3P.
• This is a critical step for synthesizing glucose and other carbohydrates.
• Multiple G3P molecules must be formed for the subsequent regeneration phase to be successful.

Regeneration of RuBP

The final stage in the Calvin Cycle is the regeneration of ribulose bisphosphate (RuBP), ensuring the continuity of the cycle. The Calvin Cycle requires a fully operational RuBP as a ready receptor for carbon dioxide, necessary for the repeated generation of glucose.
Regeneration involves a complex series of reactions where some molecules of glyceraldehyde-3-phosphate (G3P) produced in the reduction phase are utilized to regenerate RuBP. This process consumes additional ATP molecules for phosphate group transfers, facilitating a series of reactions that rearrange carbon atoms to form the five-carbon RuBP.
Key aspects of RuBP regeneration include:

• Regeneration is critical for sustaining the continuous flow of the Calvin Cycle.
• The process ensures the structural stability and supply of RuBP molecules.
• With the help of the enzyme phosphoribulokinase, the cycle is completed, allowing it to start anew with another carboxylation event.