Calvin Cycle

Introduction

The Calvin Cycle, also known as the carbon fixation stage, is a critical part of photosynthesis.
It is a series of enzyme-controlled reactions that take place in the stroma of chloroplasts.
Unlike the light-dependent reactions, the Calvin Cycle does not require direct light but is dependent on the products generated in the light-dependent reactions.

Carbon dioxide (CO2) from the atmosphere enters the Calvin Cycle.
CO2 becomes attached to a five-carbon compound called ribulose bisphosphate (RuBP) with the help of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).
This reaction produces a three-carbon compound called 3-phosphoglycerate (3PG).

Each 3PG molecule is phosphorylated using energy from adenosine triphosphate (ATP).
It then combines with hydrogen ions from nicotinamide adenine dinucleotide phosphate (NADPH), forming glyceraldehyde-3-phosphate (G3P).

Calvin Cycle Diagram

Calvin Cycle Overview

G3P has two primary fates:
- It can be used to regenerate RuBP, ensuring the continuity of the Calvin Cycle.
- Alternatively, G3P molecules can be used to synthesise sugars, such as glucose, which is a crucial energy storage molecule.

The Calvin Cycle is temperature-dependent, and its efficiency can be influenced by temperature variations.
Optimal temperatures are essential for the efficient functioning of enzymes involved in the cycle.

Calvin Cycle Temperature Dependence

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Summary

The Calvin Cycle is a series of enzyme-controlled reactions that occur in the stroma of chloroplasts.
It involves the fixation of carbon dioxide, conversion to glyceraldehyde-3-phosphate (G3P), and the potential synthesis of sugars.
G3P can be used to regenerate RuBP or for sugar production.
The Calvin Cycle is temperature-dependent and plays a vital role in converting CO2 into organic molecules, ultimately supporting plant growth and the production of glucose, which can be used for various cellular processes.