What is the primary function of the Calvin cycle?
a. use ATP to release carbon dioxide
b. use NADPH to release carbon dioxide
c. split water and release oxygen
d. transport RuBP out of the chloroplast
e. synthesize simple sugars from carbon dioxide
The correct answer and explanation is:
The correct answer is:
e. synthesize simple sugars from carbon dioxide
Explanation:
The Calvin cycle is a critical part of photosynthesis that occurs in the stroma of chloroplasts. Its primary function is to convert carbon dioxide (CO₂) from the atmosphere into organic compounds, such as simple sugars. This process is essential for plant growth and energy production and is often referred to as the “light-independent reactions” or the “dark reactions” of photosynthesis. Although it doesn’t directly require light, it relies on the products of the light-dependent reactions (ATP and NADPH) to drive the synthesis of sugars.
Here’s a breakdown of how the Calvin cycle works:
- Carbon Fixation: The cycle begins when the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) incorporates CO₂ from the atmosphere into a five-carbon sugar called ribulose bisphosphate (RuBP). This forms an unstable six-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound.
- Reduction: The ATP and NADPH produced during the light-dependent reactions are used in this phase to convert 3-PGA into a higher energy molecule, glyceraldehyde-3-phosphate (G3P). In this step, ATP provides the energy for phosphorylation, and NADPH donates electrons for the reduction of 3-PGA.
- Regeneration of RuBP: For the cycle to continue, the enzyme RuBisCO must regenerate RuBP. This occurs in a series of steps, using ATP to rearrange the G3P molecules. Some of the G3P molecules exit the cycle to form glucose and other carbohydrates, which the plant can use for energy, growth, or storage. The rest is used to regenerate RuBP, enabling the cycle to repeat.
Ultimately, the Calvin cycle helps produce glucose (C₆H₁₂O₆) from CO₂, which is vital for plant metabolism and energy storage. This glucose can be further converted into starch, cellulose, or other essential biomolecules.