Calvin Cycle Overview

Overview

This section explains how the Calvin cycle uses energy from light-dependent reactions to fix carbon dioxide and make organic molecules, focusing on the three stages of the cycle and its role in the energy flow of living things.

The Calvin Cycle Overview

• The Calvin cycle is the set of light-independent reactions in photosynthesis that create carbohydrates using energy from ATP and NADPH.
• CO₂ enters leaves via stomata, diffuses to mesophyll cells, and then into the chloroplast stroma where the Calvin cycle occurs.
• Alternative names include the Calvin-Benson cycle; the term "dark reactions" is outdated and misleading.

Stages of the Calvin Cycle

Stage 1: Carbon Fixation

• The enzyme RuBisCO catalyzes the addition of CO₂ to RuBP, forming two 3-PGA molecules per CO₂.
• This process "fixes" inorganic carbon from CO₂ into an organic molecule.

Stage 2: Reduction

• ATP and NADPH are used to reduce 3-PGA into glyceraldehyde 3-phosphate (G3P), a three-carbon sugar.
• Each reduction reaction uses six ATP and six NADPH molecules for six 3-PGA.

Stage 3: Regeneration

• One G3P molecule exits the cycle after three turns to contribute to sugar synthesis.
• The remaining five G3P molecules regenerate RuBP, using three more ATP to allow the cycle to repeat.

Photosynthesis, Water Conservation, and Evolution

• Photosynthesis evolved from bacterial, anoxygenic forms with one photosystem to modern oxygenic forms with two photosystems.
• Desert plants, like cacti, open stomata at night to minimize water loss and fix CO₂ for use during daytime photosynthesis.

Energy Flow and the Carbon Cycle

• All organisms access energy by breaking down carbohydrates made via photosynthesis.
• Cellular respiration uses oxygen to break down sugars, releasing CO₂, whereas photosynthesis uses CO₂ and releases O₂.
• Matter and energy are endlessly recycled in nature, with no true waste.

Key Terms & Definitions

• Calvin cycle — Series of light-independent reactions fixing CO₂ to make carbohydrates.
• RuBisCO — Enzyme catalyzing the fixation of CO₂ to RuBP.
• RuBP (Ribulose bisphosphate) — Five-carbon molecule starting the Calvin cycle.
• 3-PGA (3-phosphoglyceric acid) — Initial three-carbon molecule formed after carbon fixation.
• G3P (Glyceraldehyde 3-phosphate) — Three-carbon sugar produced by the Calvin cycle.
• Carbon fixation — Conversion of inorganic CO₂ into organic compounds.

Action Items / Next Steps

• Review the animation of the Calvin cycle linked in the section for visual reinforcement.
• Study the flow of energy and carbon between photosynthesis and respiration, emphasizing the cyclical nature.