Dark Reaction The Calvin Cycle

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

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

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)

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

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)

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.

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

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 CO2 enters and O2 leaves (when stomata are closed CO2 levels decrease and O2 levels increase)

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

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

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

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

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

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

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