The Calvin cycle or C3 cycle is a part of photosynthesis, the process plants and other autotrophs use to create nutrients from sunlight and carbon dioxide. The process was first identified by American biochemist Dr. Melvin Calvin in 1957. The C3 is a process that plants and algae use to turn carbon dioxide from the air into sugar, the food autotrophs need to grow.
Every living thing on Earth depends on the C3 cycle. Plants depend on the Calvin cycle for energy and food. Other organisms, including herbivores, also depend on it indirectly because they depend on plants for food. Even organisms that eat other organisms, such as carnivores, depend on the Calvin cycle. Without it, they wouldn’t have the food, energy, and nutrients they need to survive.
Stages of Calvin Cycle
C3 cycle can be divided into three main stages:
- Carbon fixation: The key step in the Calvin cycle is the event that reduces CO2. CO2 binds to RuBP in the key process called carbon fixation, forming two-three carbon molecules of phosphoglycerate. The enzyme that carries out this reaction is ribulose bisphosphate carboxylase, which is very large with a four-subunit and present in the chloroplast stroma. This enzyme works very sluggishly, processing only about three molecules of RuBP per second (a typical enzyme process about 1000 substrate molecules per second). In a typical leaf, over 50% of all the protein is RuBisCO. It is thought to be the most abundant protein on the earth.
- Reduction: This stage obtains energy from ATP and NADPH formed during the light-dependent reactions of photosynthesis. In this way, the Calvin cycle becomes a pathway in which plants convert sunlight energy into long-term storage molecules, such as sugars. The energy from the ATP and NADPH is transferred to the sugars. This step is known as reduction since electrons are transferred to 3-PGA molecules to form glyceraldehyde-3 phosphate.
- Regeneration: In this stage, some of the G3P molecules are used to produce glucose, while others are recycled to regenerate the RuBP acceptor.