Bios 208 1st Edition Lecture 25 Outline of Last Lecture I. Excitation of Chlorophyll by LightII. Linear Electron FlowOutline of Current Lecture Cyclic Electron Flow, Chemiosmosis, Chloroplasts, Mitochondria,C4 Plants,CAM Plants, PhotosynthesisCurrent LectureI. Cyclic Electron FlowA. Cyclic electron flow uses only photosystem I and produces ATP, but not NADPHB. No oxygen is releasedC. Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycleD. Some organisms such as purple sulfur bacteria have PS I but not PS IIE. Cyclic electron flow is thought to have evolved before linear electron flowF. Cyclic electron flow may protect cells from light-induced damageII. A Comparison of Chemiosmosis in Chloroplasts and MitochondriaA. In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrixB. In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stromaC. ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place.D. In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPHE. The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugarF. The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycleG. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPHThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.H. Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)I. For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2J. The Calvin cycle has three phasesK. Carbon fixation (catalyzed by rubisco)L. ReductionM. Regeneration of the CO2 acceptor (RuBP)N. Alternative mechanisms of carbon fixation have evolved in hot, arid climatesO. Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesisP. On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesisQ. The closing of stomata reduces access to CO2 and causes O2 to build upR. These conditions favor an apparently wasteful process called photorespirationS. Photorespiration: An Evolutionary Relic?T. In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)U. In photorespiration, Rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compoundV. Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugarW. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2X. Photorespiration limits damaging products of light reactions that build up in the absenceof the Calvin cycleY. In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycleIII. C4 PlantsA. C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cellsB. This step requires the enzyme PEP carboxylaseC. PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are lowD. These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycleE. In the last 150 years since the Industrial Revolution, CO2 levels have risen greatlyF. Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these speciesG. The effects of such changes are unpredictable and a cause for concernIV. CAM PlantsA. Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbonB. CAM plants open their stomata at night, incorporating CO2 into organic acidsC. Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycleV. The Importance of Photosynthesis: A ReviewA. The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compoundsB. Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cellsC. Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruitsD. In addition to food production, photosynthesis produces the O2 in our