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The Calvin Cycle

The Calvin Cycle. Also known as the dark reaction. Carbon Fixation by the Calvin Cycle. From the light reaction, NADPH, ATP and oxygen was produced. Carbon fixation : C is bonded, or fixed, into an organic compound Calvin cycle has 3 major steps occurring in the stroma. Step One.

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The Calvin Cycle

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  1. The Calvin Cycle Also known as the dark reaction

  2. Carbon Fixation by the Calvin Cycle • From the light reaction, NADPH, ATP and oxygen was produced. • Carbon fixation: C is bonded, or fixed, into an organic compound • Calvin cycle has 3 major steps occurring in the stroma

  3. Step One • Carbon dioxide diffuses into the stroma from the surrounding cytosol • An enzyme combines CO2 with RuBP, a 5 carbon molecule • The 6 carbon molecules splits into two 3-C molecules called PGA

  4. Step Two • PGA is converted into another 3-C molecule called PGAL • Each PGA gains a phosphate from a molecule of ATP • This compound receives a proton (H+) from NADPH, then releases the phosphate group. • The resulting NADP+,ADP and phosphate go back to the light reaction

  5. Step Three • Most of the PGAL is converted back to RuBP by using up a phosphate from ATP, making ADP • So RuBP actually begins and ends the Calvin Cycle.

  6. Where’d that other Carbon go • Some PGAL and other molecules made in the Calvin cycle are used to make amino acids, lipids, and carbohydrates. • Carbohydrates include glucose, fructose, sucrose, glycogen, starch, and cellulose.

  7. How Many? • PGAL is a 3-C molecule, and one turn of the Calvin cycle fixes one carbon, so it takes 3 turns of the cycle to make 1 PGAL. • For 3 turns, 9 molecules of ATP and 6 molecules of NADPH are used.

  8. Equations • CO2 + H2O + light energy  (CH2O) + O2 • 6CO2 + 6H2O + light energy C6H12O6 + 6O2

  9. light-independent reactions in stroma of chloroplast light-dependent reactions at thylakoids of chloroplast light LIGHT-DEPENDENT REACTIONS 6O2 CO2 into leaf O2 out 12H2O ADP + Pi ATP NADP+ NADPH PGA CALVIN-BENSON CYCLE PGAL 6CO2 6H2O RuBP P C6H12O6 (phosphorylated glucose) end product (e.g. sucrose, starch, cellulose) Fig. 6.17, p. 127

  10. Rate of Photosynthesis • Rate is affected by the environment • As light increases, the rate increases then levels off. • As temperature increases, the rate increases, hits a peak, then declines. • As CO2 levels increase, the rate increases then levels off

  11. Conclusion and Dark Reaction Chart • Location – Stroma, fluid in the chloroplast surrounding the Thylakoid membranes • Function – produce carbohydrates, (glucose) • Reactants – Carbon Dioxide, RuBP, ATP and NADPH+ • Products – Glucose, ADP and NADP-

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