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.
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.
Light Independent Reactions
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.
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.
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.
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
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.
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.