Light-Independant Reactions (Calvin Cycle)

Q: What is the second phase of photosynthesis called?

A: The Light-Independent Reactions (Calvin Cycle).

Q: Does the Calvin Cycle require direct sunlight?

A: No, it happens without direct use of sunlight.

Q: What does the Calvin Cycle use instead of sunlight?

A: It uses the byproducts from light-dependent reactions.

Q: What type of reaction occurs during this phase?

A: An endergonic reaction where CO₂ is reduced to sugar using NADPH.

Q: How many catalyzed reactions occur in this phase?

A: A series of 11 catalyzed reactions.

Q: What are the three phases of the Calvin Cycle?

A:

1. Fixation

2. Reduction

3. Regeneration

Q: What happens during the Fixation phase?

A: Each CO₂ is fixated in a 6-carbon compound, which is then broken down to 3-phosphoglycerate.

Q: What happens during the Reduction phase?

A: The 6-carbon compound is reduced to G3P.

Q: What happens during the Regeneration phase?

A: RuBP (ribulose-1,5-bisphosphate) is produced from G3P.

Q: How many times does the Calvin Cycle occur?

A: The cycle occurs three times.

Q: What happens during Carbon Fixation in the Calvin Cycle?

A: 3 CO₂ react with 3 RuBP to form a 6-carbon molecule.

Q: What happens to the 6-carbon molecule?

A: It breaks down into six 3-phosphoglycerate molecules.

Q: What enzyme catalyzes this reaction?

A: Rubisco.

Q: Why is this step important?

A: It is considered the most important step in the Calvin Cycle.

Calvin Cycle – Carbon Fixation

• 3 CO₂ react with 3 RuBP

• Forms a 6-carbon molecule

• The 6-C molecule breaks down

• into six 3-phosphoglycerate

• Catalyzed by Rubisco (enzyme)

• Considered the most important step in the Calvin Cycle

Q: What occurs during the Reduction phase of the Calvin Cycle?

A: ATP hydrolysis occurs and a Pi attaches to each 3-phosphoglycerate, forming 1,3-bisphosphoglycerate.

Q: What happens to NADPH during this phase?

A: NADPH is oxidized to NADP⁺.

Q: What is produced as a result?

A: 6 glyceraldehyde-3-phosphate (G3P) molecules.

Calvin Cycle – Reduction

• ATP hydrolysis occurs
• A Pi attaches to each 3-phosphoglycerate
→ Produces 1,3-bisphosphoglycerate
• NADPH is oxidized to NADP⁺
→ Produces 6 glyceraldehyde-3-phosphate (G3P)

Q: What happens during the Regeneration phase of the Calvin Cycle?

A: 5 G3P’s are rearranged to produce Ribulose-5-phosphate, while 1 G3P stays the same.

Q: What attaches to the molecule to regenerate RuBP?

A: A free Pi attaches to regenerate 3 RuBP’s.

Q: What process helps this step occur?

A: ATP hydrolysis.

Calvin Cycle – Regeneration

• 5 G3P’s are rearranged to make Ribulose-5-phosphate
- 1 G3P stays the same (used for sugar)
• A free Pi attaches to regenerate 3 RuBP’s
- Process uses ATP hydrolysis

Q: How many G3P molecules remain at the end of the Calvin Cycle?

A: 1 G3P remains.

Q: What is G3P?

A: A high-energy carbon molecule.

Q: What is required to make one molecule of G3P?

A: 9 ATP and 6 NADPH.

Q: How many G3P molecules are needed to make one glucose molecule?

A: 2 G3P’s are needed to make glucose.