Calvin Cycle Notes Class 11

Calvin Cycle is a series of biochemical reactions that occur in the chloroplasts of plants during photosynthesis. Its stages include carbon fixation, reduction, and regeneration of Ribulose bisphosphate. During carbon fixation, CO₂ is incorporated into a five-carbon sugar molecule, which undergoes reduction using ATP and NADPH to form glucose precursors. The cycle completes with the regeneration of RuBP to restart the cycle.

Calvin Cycle includes the use of ATP and NADPH to convert carbon dioxide into glucose and contributes to the synthesis of organic molecules in plants. In this article, we will study the Calvin cycle, Stages, and Key points of CalvinCycle in detail.

Definition Calvin Cycle

The Calvin Cycle is the set of chemical reactions in photosynthesis that converts carbon dioxide into glucose in the chloroplasts of plants.

What is Calvin Cycle?

The Calvin Cycle, also known as the C3 cycle, is the light-independent reactions as they are not directly driven by light. It takes place during the day when ATP and NADPH, products of the light-dependent reactions, are available. It begins with carbon fixation, where carbon dioxide is combined with ribulose bisphosphate (RuBP) by the enzyme rubisco, forming an unstable six-carbon compound. Reduction follows, utilizing ATP and NADPH to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).

For every three turns of the cycle, one G3P molecule is produced, and the cycle continues. The regeneration phase rearranges molecules to reform ribulose bisphosphate, preparing for more carbon fixation. The Calvin Cycle stores light energy in carbohydrates, mainly starch and sucrose.

C3 Cycle Diagram

The C3 cycle diagram below shows the different stages of Calvin Cycle.  

Stages of C3 Cycle

The Calvin cycle, also known as the C3 cycle, is divided into three stages:

1. Carbon Sequestration

• The primary stage involves carbon fixation, where carbon dioxide combines with ribulose bisphosphate (RuBP) to form phosphoglycerate.
• The enzyme involved in the reaction is ribulose bisphosphate carboxylase/oxygenase (RuBisCO). It is a protein present in the stroma of chloroplast and is a relatively slow enzyme.

2. Reduction

• In the second stage, 3-PGA molecules produced from carbon fixation are transformed into simple sugars like glucose.
• ATP and NADPH, products of the light-dependent reactions, provide the energy for this stage, converting solar energy into long-term storage components.

3. Regeneration

• The third stage involves a complex process that requires ATP.
• Some glyceraldehyde 3-phosphate molecules are recycled to regenerate the RuBP acceptor. The other converts into glucose and the Calvin Cycle gets complete.

Calvin Cycle or C3 Cycle Final Products

The final products of the calvin cycle are:

• At each turn of the Calvin Cycle, one carbon molecule is fixed.
• Every three rotations of the Calvin Cycle result in the production of one molecule of glyceraldehyde-3 phosphate.
• Two glyceraldehyde-3 phosphate molecules combine to generate a single glucose molecule.
• During the reduction of 3-phosphoglyceric acid to glyceraldehyde-3 phosphate and the regeneration of RuBP, 3 ATP and 2 NADPH molecules are consumed.
• The synthesis of one glucose molecule involves the consumption of 18 ATP and 12 NADPH.

Key Points on C3 Cycle

Following are the key points in C3 cycle are:

• The dark reaction of photosynthesis is referred known as the C3 cycle.
• The C3 cycle, also known as the Calvin Cycle, begins with the fixation of carbon dioxide by ribulose bisphosphate (RuBP) using the enzyme RuBisCO.
• ATP and NADPH reduces 3Phospoglycerate to Glyceraldehyde3-phosphate in the second stage of the C3 cycle. After that, ATP and NADPH are transformed to ATP and NADP+.
• Some G3P molecules are used to regenerate RuBP, ensuring the continuity of the cycle and two G3P molecules combine to produce one glucose molecule.
• The ultimate goal of the C3 cycle is to convert carbon dioxide into organic molecules, particularly carbohydrates, which serve as energy storage for the plant.

FAQs on Calvin Cycle

1. What is Calvin Cycle?

The Calvin Cycle is the light-independent phase of photosynthesis where carbon dioxide is converted into organic molecules, facilitating the synthesis of carbohydrates in plants.

2. What are the Different Steps Involved in the Calvin Cycle?

The different steps involved in the Calvin cycle are:

• Carbon fixation
• Reduction
• Regeneration

3. What are the End Products of C3 Cycle?

The end products of C3 cycle are ADP, NADP, and glucose. In the first stage of C3 cycle ADP and NADP are produced and in the second stage glucose is produced.

4. What is Carbon Fixation in the Calvin Cycle?

Carbon fixation in the Calvin Cycle involves the conversion of carbon dioxide into an organic compound, usually ribulose bisphosphate, catalyzed by the enzyme RuBisCO. It produces 3-phosphoglycerate and starting the next stages of photosynthetic carbon assimilation.

5. Why is the Third Step of the Calvin Cycle Called the Regeneration Step?

The third step of the Calvin Cycle is called the regeneration step as it involves the regeneration of ribulose bisphosphate (RuBP). This molecule essential for initiating carbon fixation.