Stage 3 - The Calvin Cycle and Carbon Fixation Melvin Calvin won the Novel Prize in Chemistry in 1961 for his work in discovering this pathway He used a radioactive isotope of carbon as a tracer to discover the sequence of reactions that results in carbon fixation

Carbon fixation is the process of forming high-energy molecules from CO2. These reactions are light independent The Calvin cycle occurs in the stroma The cycle uses materials generated by the light dependent reactions - ATP molecules and high energy electrons carried by NADPH

1. Fixing Carbon Dioxide Carbon dioxide is bonded to a molecule of ribulose bisphosphate (RuBP) - a 5-carbon compound which exists in the stroma The new 6-carbon compound is very unstable Immediately breaks down into two identical 3-carbon molecules CO2 + RuBP  unstable C6  2 C3

2. Energy Transfer 3 ATP and 2 NADPH are used for EVERY CO2 that enters the cycle Each turn of the Calvin cycle produces 2 high-energy C3 molecules

3. Replacing RuBP Most of the C3 molecules are used to generate more RuBP in the stroma The Calvin Cycle must be completed six times in order to synthesize one molecule of glucose Each cycle produces 2 C3 molecules,  at the end of 6 cycles, there are 12 C3 molecules. 10 of these are used to regenerate RuBP, and 2 are used to make glucose.

Summary of reactants and products 1. Light-dependent: Reactants: Products: Carbon-fixation:

Why do plants require H2O? H2O undergoes photolysis and provides electrons to PSII

Why do plants require CO2? To be “fixed” into C3 and glucose molecules by the Calvin cycle

When and where is O2 produced? When water undergoes photolysis, O2 is released. Photolysis occurs in the lumen.

What happens to the hydrogen of H2O? Contributes to the electrochemical gradient that drives chemiosmosis

18 ATP molecules and 12 NADPH molecules How many ATP and NADPH are required to produce one molecule of glucose? 18 ATP molecules and 12 NADPH molecules