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Dark Reaction

Dark Reaction. The Calvin Cycle. 6CO 2 +6H 2 O C 6 H 12 O 6 +6O 2. The dark reactions use the energy stored in ATP and NADPH the light reactions. Carbon atoms from CO 2 are bonded, or fixed, into organic compounds = carbon fixation. THIS OCCURS IN THE STROMA. STEP 1.

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Dark Reaction

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  1. Dark Reaction The Calvin Cycle

  2. 6CO2 +6H2O C6H12O6 +6O2 • The dark reactions use the energy stored in ATP and NADPH the light reactions. • Carbon atoms from CO2 are bonded, or fixed, into organic compounds = carbon fixation. • THIS OCCURS IN THE STROMA

  3. STEP 1 • An enzyme (rubisco), combines CO2 with a 5-carbon sugar RuBP • The product, 6-C sugar, immediately splits into 2, -3C molecules (PGA) • PGA– Phosphoglyceric Acid

  4. Step 2 • PGA is converted to another 3- Carbon molecule PGAL in a 2 part process: • Each PGA receives a P group from ATP • The resulting compound receives a proton from NADPH and releases the P, producing PGAL ( ADP & NADP+ return to light rxn., to make ATP and NADPH)

  5. Step 3 • Most of the PGAL is converted back to RuBP • Requires a P from another ATP • Some PGAL leave and used by plants create organic compounds

  6. Balance Sheet for Photosynthesis • How much ATP & NADH are required to make 1 molecule of PGA from carbon dioxide? • Each turn fixes one CO2 • PGAL is a 3-C molecule (takes 3 turns to make each molecule) • Each turn of the cycle: • 3 ATP ( 2 in step 2 & 1 in step 3) • 2 NADPH (step 3)

  7. About 50% made to fuel cell Respiration • Some of the PGAL is used to make amino acids, lipids, carbohydrates like glucose and fructose, glycogen, starch, and cellulose.

  8. Alternative Pathways • Plants that fix carbon exclusively through the Calvin Cycle- C3 Plants • Because of the 3-C compound PGA, that is initially formed. Example: Rice, wheat, oats, and soybeans

  9. Alternative Pathways • Plants in hot, dry climates use alternative pathways • Plant lose H2O to the air-through small pores called stomata (underside of leaf) • Can be partially closed to prevent water loss • Stomata are the major passageways through which CO2enters and O2leaves (when stomata are closed CO2 levels decrease and O2 levels increase)

  10. Transpiration-Evaporation of water from leaves • Rate of transpiration related to the degree of stomata opening and evaporation demand of environment

  11. C4 Plants • Fix CO4 into 4-C compounds • Partially close stomata during hottest part of the day • Enzymes fix CO2 into 4-C compounds and transport them to cells where CO2 is released and enters calvin cycle ( lose ½ as much H2O as C3) • Examples • Corn, sugar cane, crabgrass

  12. CAM Pathways • Open stomata at night; close during the day • Take in CO2 at night and fix into compounds • Release O2 during the day and enter the Calvin cycle • Example: cacti, pineapples

  13. Rate of Photosynthesis • Light Intensity: • Increase rate of photosynthesis, then levels off ( max. rate of photosynthesis) • Higher intensity, excites more electrons in cholorphyll • @ same intensity, all available electrons are excited

  14. Amount CO2 • Amount of CO2: • Increases rate of photosynthesis to a point, then levels off

  15. Temperature • Higher temperature accelerates the chemical rxns. • Peaks @ certain temp. because the enzymes becomes ineffective and unstable • Stomata closes-limiting H2O loss and CO2 entry into the leaves

  16. Concentration of O2 • Higher O2 will decrease the rate of photosynthesis and increase the rate of photorespiration • Rubisco will bind with oxygen • Will send PGA into respiration, instead of finishing photosynthesis • Decreasing amount of organic compound produced

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