The Calvin cycle is not light dependent but is stimulated by light. Which mechanism best explains this stimulation?

Prepare for the Foundations of Biology Exam 1. Dive into key biological concepts with multiple choice questions, hints, and explanations. Ace the exam with our efficient study methods!

Multiple Choice

The Calvin cycle is not light dependent but is stimulated by light. Which mechanism best explains this stimulation?

Light doesn’t power the Calvin cycle directly; it enables a redox mechanism that turns on the cycle’s enzymes. Electrons from Photosystem I travel through ferredoxin to thioredoxin, which reduces disulfide bonds on key Calvin cycle enzymes. This reduction relieves the inhibition and activates enzymes such as fructose-1,6-bisphosphatase, allowing the cycle to run using the ATP and NADPH produced by the light reactions. So the main way light stimulates the cycle is through this thioredoxin-mediated redox activation of enzymes. Other ideas are less central: changes in stromal pH happen with light but don’t directly activate the enzymes; NADPH is used to reduce 3-PGA in later steps rather than directly driving Rubisco; stomata opening affects CO2 supply but doesn’t explain the enzyme activation inside the chloroplast.

Light doesn’t power the Calvin cycle directly; it enables a redox mechanism that turns on the cycle’s enzymes. Electrons from Photosystem I travel through ferredoxin to thioredoxin, which reduces disulfide bonds on key Calvin cycle enzymes. This reduction relieves the inhibition and activates enzymes such as fructose-1,6-bisphosphatase, allowing the cycle to run using the ATP and NADPH produced by the light reactions. So the main way light stimulates the cycle is through this thioredoxin-mediated redox activation of enzymes.

Other ideas are less central: changes in stromal pH happen with light but don’t directly activate the enzymes; NADPH is used to reduce 3-PGA in later steps rather than directly driving Rubisco; stomata opening affects CO2 supply but doesn’t explain the enzyme activation inside the chloroplast.

More practice questions