Overview
This lecture explains the light-independent stage of photosynthesis (the Calvin cycle), focusing on carbon fixation, formation of organic molecules, and the regeneration of ribulose bisphosphate (RuBP).
Location and Purpose of the Light-Independent Stage
- The light-independent stage (Calvin cycle) is the second stage of photosynthesis and occurs in the stroma of chloroplasts.
- It uses ATP and reduced NADP from the light-dependent stage along with atmospheric carbon dioxide to synthesize organic molecules, mainly glucose.
- Although it does not require light directly, it depends on products formed during the light-dependent stage.
Calvin Cycle: Steps and Reactions
- Carbon dioxide enters the leaf via stomata, diffuses into plant cells, and reaches the stroma.
- Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP, 5 carbons), catalyzed by rubisco, forming an unstable 6-carbon compound.
- The unstable 6-carbon compound splits into two molecules of glycerate-3-phosphate (GP), each with 3 carbons.
- GP is reduced to triose phosphate (TP) using hydrogen from reduced NADP and energy from ATP (both from the light-dependent stage).
- Two TP molecules combine to form one glucose molecule (6 carbons).
- TP can also be used to synthesize other organic molecules like amino acids, fatty acids, glycerol, sucrose, starch, and cellulose.
Regeneration of RuBP
- Most TP is used not for glucose formation but to regenerate RuBP and keep the cycle running.
- Out of every 12 TP molecules made (from 6 CO₂), 10 are used to regenerate 6 RuBP molecules.
- The remaining 2 TP molecules are used to synthesize glucose and other organic compounds.
Optimum Conditions & Enzyme Activity
- Proton pumping during the light-dependent stage increases the pH of the stroma to around 8, ideal for Calvin cycle enzymes.
Key Terms & Definitions
- Stroma — Fluid-filled space in chloroplasts where the Calvin cycle occurs.
- Calvin cycle — Series of reactions in the light-independent stage that fix CO₂ into organic molecules.
- RuBP (Ribulose bisphosphate) — 5-carbon compound that combines with CO₂ at the start of the Calvin cycle.
- Rubisco — Enzyme catalyzing the fixation of CO₂ to RuBP.
- GP (Glycerate-3-phosphate) — 3-carbon compound formed in the Calvin cycle.
- TP (Triose phosphate) — 3-carbon sugar produced by reduction of GP; precursor to glucose.
- ATP/ADP — Energy currency molecules involved in converting GP to TP.
- Reduced NADP/NADP — Electron carrier providing reducing power for the conversion of GP to TP.
Action Items / Next Steps
- Review detailed steps of the Calvin cycle and related enzyme functions.
- Prepare for questions on carbon fixation and the roles of ATP/NADP in photosynthesis.