2. Photosynthesis 2: Light-Independent Reactions (The Calvin Cycle) Page 166 in your text for a good diagram to refer to. Light-Independent Reactions (The Calvin Cycle) Page 166 in your text for a good diagram to refer to.

3. Light Independent Reactions: Overview  Reactions that can take place in the presence or absence of light.  CO2 assimilation: converting CO2 to organic molecules such as glucose.  Pathway called the Calvin Cycle  It directly makes a three-carbon molecule called glyceraldehyde-3-phosphate (G3P)  G3P is then used as a starting substrate in many other metabolic pathways.  Reactions that can take place in the presence or absence of light.  CO2 assimilation: converting CO2 to organic molecules such as glucose.  Pathway called the Calvin Cycle  It directly makes a three-carbon molecule called glyceraldehyde-3-phosphate (G3P)  G3P is then used as a starting substrate in many other metabolic pathways.

4. Light Independent Reactions

5. Light Independent Reactions: Phase 1  Called Carbon Dioxide Fixation  A 5-carbon molecule called ribulose-1,5- bisphosphate (RuBP) that is found in the stroma bonds with a CO2 (this will happen for a total of 6 molecules of CO2)  This forms a brief 6 carbon molecule that is so unstable it immediately breaks down to two molecules of 3-phosphoglycerate (PGA) per CO2 (a total of 12 molecules of PGA)  The enzyme that catalyzes this reaction is called rubisco.  Called Carbon Dioxide Fixation  A 5-carbon molecule called ribulose-1,5- bisphosphate (RuBP) that is found in the stroma bonds with a CO2 (this will happen for a total of 6 molecules of CO2)  This forms a brief 6 carbon molecule that is so unstable it immediately breaks down to two molecules of 3-phosphoglycerate (PGA) per CO2 (a total of 12 molecules of PGA)  The enzyme that catalyzes this reaction is called rubisco.

6. Light Independent Reactions: Phase 2  Called Reduction Phase  The 12 PGA molecules are low energy, so they are activated by 12 ATP to form 12 molecules of 1, 3-biphosphoglycerate  Which are then reduced by 12 NADPH molecules to form 12 energized glyceraldehyde 3-phosphate (G3P)  Two G3P molecules then exit the cycle and form glucose and other sugars  Ten G3P molecules will stay in the cycle and continue to phase 3.  Called Reduction Phase  The 12 PGA molecules are low energy, so they are activated by 12 ATP to form 12 molecules of 1, 3-biphosphoglycerate  Which are then reduced by 12 NADPH molecules to form 12 energized glyceraldehyde 3-phosphate (G3P)  Two G3P molecules then exit the cycle and form glucose and other sugars  Ten G3P molecules will stay in the cycle and continue to phase 3.

7. Light Independent Reactions: Phase 3  Called the Regenerating RuBP Phase  The G3P molecules are broken down by 6 ATP molecules to form 6RuBP molecules  The cycle is ready to start again.  Called the Regenerating RuBP Phase  The G3P molecules are broken down by 6 ATP molecules to form 6RuBP molecules  The cycle is ready to start again.

8. Adaptations to Photosynthesis  Rubisco is a critical enzyme, but it has an undesirable property: it can also use oxygen as a substrate.  When O2 is the substrate, it reacts with RuBP in a process called photorespiration and makes a phosphoglycolate and a 3-phosphoglycerate  This reduces the efficiency of photosynthesis  In really hot and dry environments, leaves close their stomata so water doesn’t escape, but this does not allow for oxygen to escape either, increasing the photorespiration  Rubisco is a critical enzyme, but it has an undesirable property: it can also use oxygen as a substrate.  When O2 is the substrate, it reacts with RuBP in a process called photorespiration and makes a phosphoglycolate and a 3-phosphoglycerate  This reduces the efficiency of photosynthesis  In really hot and dry environments, leaves close their stomata so water doesn’t escape, but this does not allow for oxygen to escape either, increasing the photorespiration

9. C4 Plants  Have adapted the Calvin Cycle to perform better at higher temperatures.  CO2 is fixed by a 3-carbon molecule called PEP to form oxaloacetate (4- carbon molecule)  They use energy to pump CO2 into the bundle-sheath cells to a very high concentration of CO2.  Examples: corn, sugarcane, some grasses.  Have adapted the Calvin Cycle to perform better at higher temperatures.  CO2 is fixed by a 3-carbon molecule called PEP to form oxaloacetate (4- carbon molecule)  They use energy to pump CO2 into the bundle-sheath cells to a very high concentration of CO2.  Examples: corn, sugarcane, some grasses.

11. CAM Plants  Use the same reactions as C4 plants, but all reactions happen in the same cell  CO2 fixation is separated by the time of day.  Stomata are closed during the day and the CO2 is fixed at night.  Examples: cacti, pineapples.  Use the same reactions as C4 plants, but all reactions happen in the same cell  CO2 fixation is separated by the time of day.  Stomata are closed during the day and the CO2 is fixed at night.  Examples: cacti, pineapples.