What happens during the carbon fixation stage of the Calvin Cycle (light-indepedent reactions)? Energy from ATP and electrons from NADPH are used to reduce 3-phosphoglycerate to form G3P molecules (which can be used to form other organic molecules such as glucose) Oxygen is produced through this process The remaining G3P molecules that have exited the cycle are used to re-form RuBP through a series of chemical reactions, so that the Calvin cycle can continue. The enzyme Rubisco attaches to RuBP, causing the formation of molecules of 3-phosphoglycerate
The Correct Answer and Explanation is :
The carbon fixation stage of the Calvin Cycle (which is part of the light-independent reactions in photosynthesis) involves the incorporation of carbon dioxide (CO₂) into an organic molecule. This process is critical for building sugars and other organic molecules that plants need for growth and energy.
Here’s how the process works in more detail:
- RuBP and Rubisco Interaction:
The cycle begins with a molecule of ribulose bisphosphate (RuBP), a 5-carbon compound. The enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) facilitates the reaction by attaching CO₂ to RuBP. This step is crucial for incorporating atmospheric carbon into the cycle. - Formation of 3-Phosphoglycerate:
The addition of CO₂ to RuBP forms an unstable 6-carbon intermediate, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. These molecules are now the starting point for further processing in the Calvin Cycle. - Energy and Electron Inputs:
To convert 3-PGA into sugars, energy is required. This energy is provided by ATP (produced in the light reactions of photosynthesis) and NADPH (also produced during the light-dependent reactions). ATP provides the energy needed for phosphorylation (adding a phosphate group), while NADPH supplies high-energy electrons. - Reduction to G3P:
ATP and NADPH are used to reduce 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. G3P is a versatile molecule that can be used to form glucose and other carbohydrates that plants use for energy and structure. - Regeneration of RuBP:
While some G3P molecules exit the cycle to form glucose, most are used to regenerate RuBP. This step requires ATP and involves a series of reactions that convert G3P back into RuBP, ensuring that the cycle can continue.
In summary, carbon fixation captures CO₂ from the air, attaches it to RuBP with the help of Rubisco, and begins the process of transforming inorganic carbon into organic molecules, eventually leading to the production of sugars and other vital plant compounds. This cycle is essential for the energy and structural needs of plants and indirectly supports life on Earth by producing oxygen and organic materials that form the basis of food chains.