Lecture Notes on the Calvin Cycle

Overview

• Calvin cycle is a cyclic electron flow using ATP and NADPH to reduce carbon dioxide into sugar (G3P).
• Requires three cycles to net one G3P molecule.
• Steps: Carbon fixation, reduction, regeneration of RuBP.

Light Reactions Recap

• Inputs: Water, NADP+, ADP
• Outputs: ATP, NADPH, O2
• ATP and NADPH are used in the Calvin cycle.

Calvin Cycle Steps

Phase 1: Carbon Fixation

• CO2 is incorporated one at a time into RuBP, catalyzed by enzyme rubisco.
• Forms 3-phosphoglycerate.
• RuBP is a 5-carbon sugar.

Phase 2: Reduction

• 3-phosphoglycerate is phosphorylated by ATP into 1,3-bisphosphoglycerate.
• NADPH donates electrons to reduce it to G3P.
• Outputs: 6 G3P molecules (only 1 net gain).

Phase 3: Regeneration of RuBP

• 5 G3P molecules regenerate RuBP.
• Uses 3 ATP.
• Prepares for further CO2 intake.

Inputs and Outputs

• Inputs: CO2, ATP, NADPH (from light-dependent reactions)
• Outputs: 1 net G3P, 9 ADP, 6 NADP+

Importance of Photosynthesis

• Produces G3P and many other organic molecules necessary for plant function and survival.

Photorespiration

• Occurs on hot days when stomata close to conserve water.
• Results in less CO2 and more O2 inside cells.
• Rubisco binds to O2, wasting energy and producing CO2 without sugar.

Plant Adaptations to Hot Climates

C4 Plants

• Spatial separation of steps.
• Mesophyll cells fix CO2 into a 4-carbon molecule, transferred to bundle sheath cells.
• Examples: Corn, grasses, sugar cane.

CAM Plants

• Temporal separation of steps.
• Stomata open at night, CO2 stored in organic acids.
• Examples: Pineapples, cacti, succulents.

FRQ Discussion

• Experiment: Effect of light wavelength on plant growth.
  • Least growth: 550 nm (green) wavelength due to chlorophyll reflecting green light.
• Photorespiration Claim: Early Earth had no oxygen, supporting the claim that it's an evolutionary relic.

Next Topics

• Cellular respiration and fitness.