Also known as the dark reaction The Calvin Cycle Also known as the dark reaction

The Dark Reaction Calvin cycle – a cycle that uses ATP and NADPH, from the light reaction, to make glucose, without the use of light energy.

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

Carbon dioxide diffuses into the stroma from the surrounding cytosol 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

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

So RuBP actually begins and ends the Calvin Cycle. 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.

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.

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.

Equations 6CO2 + 6H2O + light energy C6H12O6 + 6O2

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

Rate of Photosynthesis Rate is affected by the environment As light intensity 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

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-