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BSC 2010 - Exam I Lectures and Text Pages • I. Intro to Biology (2-29) • II. Chemistry of Life – Chemistry review (30-46) – Water (47-57) – Carbon (58-67) – Macromolecules (68-91) • III. Cells and Membranes – Cell structure (92-123) – Membranes (124-140) • IV. Introductory Biochemistry – Energy and Metabolism (141-159) – Cellular Respiration (160-180) – Photosynthesis (181-200) Citric Acid Cycle • Citric acid cycle completes the energy-yielding oxidation of organic molecules • The citric acid cycle – Takes place in the matrix of the mitochondrion An overview of the citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylatio n ATP CO2 CoA NADH + H+ Acetyle CoA CoA CoA Citric acid cycle 2 CO2 3 NAD+ FADH2 FAD 3 NADH + 3 H+ ADP + P i ATP Figure 9.11 A closer look at the citric acid cycle Glycolysis Citric Oxidative acid phosphorylation cycle S CoA C O CH3 Acetyl CoA CoA SH O NADH + H+ C COO– COO– 1 CH2 COO– NAD+ 8 Oxaloacetate HO C COO– COO– CH2 COO– HO CH H2O CH2 CH2 2 HC COO– COO– Malate Figure CH2 HO Citrate 9.12 COO– Isocitrate COO– H2O COO– CH CO2 Citric acid cycle 7 3 NAD+ COO– Fumarate HC CH2 CoA SH 6 CoA SH COO– FAD CH2 CH2 COO– C O Succinate Pi S CoA GTP GDP Succinyl CoA ADP ATP 4 C O COO– CH2 5 CH2 FADH2 COO– NAD+ NADH + H+ NADH + H+ a-Ketoglutarate CH2 COO– Figure 9.12 CH CO2 Pyruvate AcetylCoA Citric Acid Cycle Yield from each 2 pyruvate molecules (from 1 glucose) • 6CO2 (2 from Pyr.AcetylCoA, 4 from CAC • 8NADH (2 from Pyr. AcetylCoA, 6 from CAC) • 2FADH2 (All from CAC) • 2 ATP (All from CAC) Two pyruvates are produced from the glycolysis of each glucose molecule resulting in a total of 2 ATP from Citric Acid Cycle, and the NADH and FADH2 go to power the Electron Transport Chain Oxidative phosphorylation • Chemiosmosis couples ETC to ATP synthesis • ETC (fig 9.13) = collection of mostly proteins embedded in the inner mitochondrial membranes (cristae = foldings that increase SA) • Each electron acceptor along the ETC is more electronegative than the previous O2 at the end, the final electron acceptor (most electronegative) • NADH transfers e- to the 1st molecule of ETC (flavoprotein) in multiprotein complex I • Ubiquinone (= small hydrophobic non-protein that’s mobile within the membrane system) transfers e- from multiprotein complex I II • FADH2 transfers e- to the 2nd multiprotein complex (provides 1/3 less E than NADH) • ATP is not directly made by the ETC Chemiosmosis couples this E release with making ATP • ETC – stores energy by pumping protons from the matrix across the inner mitochondrial membrane into the intermembrane space. (fig. 9.15) • Chemiosmosis = E stored in H+ gradient across a membrane (proton-motive force) is used to drive ATP synthesis • ATP synthase complexes in the membrane = only place H+ can freely move along concentration gradient and back into the matrix (fig. 9.14) Oxidative Phosphorylation • During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • NADH and FADH2 – Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation through chemiosmosis The Pathway of Electron Transport • In the electron transport chain – Electrons from NADH and FADH2 lose energy in several steps NADH 50 FADH2 passes electrons directly to multiprotein complex II. Electrons are passed to more electron acceptors in the remaining multiprotein complexes. Finally they are passed to oxygen, the most electronegative acceptor, forming water. FADH2 Free energy (G) relative to O2 (kcl/mol) NADH passes electrons to multiprotein complex I. They are then passed to Ubiquinone which transfers them to multiprotein complex II. 40 I Fe•S 30 20 Multiprotein complexes FAD Fe•S II O III Cyt b Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 10 0 Figure 9.13 FMN 2 H + + 12 O2 H2 O ETC stores energy in an ion gradient • At certain steps along the electron transport chain – Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space • The resulting H+ gradient – Stores energy – Drives chemiosmosis in ATP synthase – Is referred to as a proton-motive force Chemiosmosis • Chemiosmosis – Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work Chemiosmosis: The Energy-Coupling Mechanism • ATP synthase – Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ H+ H+ H+ H+ H+ H+ A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. A stator anchored in the membrane holds the knob stationary. H+ ADP + Pi Figure 9.14 MITOCHONDRIAL MATRIX ATP A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. Chemiosmosis and the electron transport chain Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP ATP H+ H+ H+ Intermembrane space Protein complex of electron carners Q I Inner mitochondrial membrane Figure 9.15 IV III ATP synthase II FADH2 NADH+ Mitochondrial matrix H+ Cyt c NAD+ FAD+ 2 H+ + 1/2 O2 H2O ADP + (Carrying electrons from, food) ATP Pi H+ Chemiosmosis Electron transport chain + ATP synthesis powered by the flow Electron transport and pumping of protons (H ), + + which create an H gradient across the membrane Of H back across the membrane Oxidative phosphorylation Energy Transfer Efficiency • Complete oxidation of 1 mole of glucose releases 686 kcal of energy • Phosphorylation of ADP ATP stores 7.3 kcal/mol • Respiration makes 38 ATP (x 7.3 kcal/mol) = 277.4 kcal (40% of 686 kcal) • Only about 40% of E stored in glucose is used to make ATP (the rest is lost as HEAT) An Accounting of ATP Production by Cellular Respiration • During respiration, most energy flows in this sequence – Glucose to NADH to electron transport chain to proton-motive force to ATP Three main processes in this metabolic enterprise About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making approximately 38 ATP Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA + 2 ATP by substrate-level phosphorylation Maximum per glucose: Figure 9.16 6 NADH Citric acid cycle + 2 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 32 or 34 ATP by substrate-level by oxidative phosphorylation, depending on which shuttle transports electrons phosphorylation from NADH in cytosol About 36 or 38 ATP Fermentation Fermentation = extension of glycolysis that can generate ATP solely by substratelevel phosphorylation (as long as there’s plenty of NAD+) by transferring e- from NADH to pyruvate (or its derivatives). Does not use oxygen. 2 common types of fermentation: 1. Alcohol fermentation (fig 9.17a) • • Pyruvate ethanol (2 steps) – 1st step: releases CO2 from pyruvate acetaldehyde (2-C) – 2nd step: acetaldehyde reduced by NADH ethanol – NAD+ continues glycolysis Used to make beer/wine and in baking 2. Lactic acid fermentation (fig 9.17b) • Pyruvate lactate • Used to make cheese & yogurt (bacteria and fungi) • Your muscles also make ATP this way when O2 is scarce Facultative anaerobes = organisms that can either use pyruvate in fermentation OR in respiration (depending of O2 availability) To make the same amt of ATP an organism w/o O2 would have to consume sugar at a much faster rate Fermentation enables some cells to produce ATP w/o oxygen: • In aerobic respiration, O2 pulls e- through the ETC – Yields up to 19 times more ATP • w/o O2, other e- acceptors can be used • Glycolysis = exergonic process that uses NAD+ (not O2) as an e- acceptor Fermentation • Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration – Relies on oxygen to produce ATP • In the absence of oxygen – Cells can still produce ATP through fermentation Glycolysis • Glycolysis – Can produce ATP with or without oxygen, in aerobic or anaerobic conditions – Under anaerobic conditions, it couples with fermentation to produce ATP Types of Fermentation • Fermentation consists of – Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis Alcohol Fermentation • In alcohol fermentation 2 ADP + 2 – Pyruvate is converted to ethanol in two steps, one of which releases CO2 P1 2 ATP O– C O Glucose Glycolysis C O CH3 2 Pyruvate 2 NADH 2 NAD+ H 2 CO2 H H C OH C O CH3 CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Glucose P1 2 ATP Glycolysis O– C O C O O 2 NAD+ C O H C OH CH3 2 Lactate Figure 9.17 (b) Lactic acid fermentation 2 NADH CH3 2 pyruvate Lactic Acid Fermentation • During lactic acid fermentation 2 ADP + 2 P1 2 ATP O– C O Glucose Glycolysis C O CH3 2 Pyruvate – Pyruvate is reduced directly by NADH to form lactate as a waste product 2 NADH 2 NAD+ H 2 CO2 H H C OH C O CH3 CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Glucose P1 2 ATP Glycolysis O– C O C O O 2 NAD+ C O H C OH CH3 2 Lactate Figure 9.17 (b) Lactic acid fermentation 2 NADH CH3 2 pyruvate Fermentation and Cellular Respiration Compared • Both fermentation and cellular respiration – Use glycolysis to oxidize glucose and other organic fuels to pyruvate • Fermentation and cellular respiration – Differ in their final electron acceptor • Cellular respiration – Produces more ATP Pyruvate • Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Figure 9.18 Acetyl CoA Citric acid cycle The Evolutionary Significance of Glycolysis • Glycolysis – Occurs in nearly all organisms – Probably evolved in ancient prokaryotes (3.5 bya) before there was oxygen in the atmosphere – They did not have oxygen or mitochondria Versatility in Catabolism • Glycolysis and the citric acid cycle connect to many other metabolic pathways • Our bodies generally use many sources of energy in respiration (fig 9.19) regulated by feedback inhibition (fig 9.20) • Carbohydrates simple sugars, enter glycolysis • Proteins amino acids (used to build new proteins) – Excess amino acids are deaminated intermediates of glycolysis or citric acid cycle, or form acetyl CoA • Fats gylcerol + fatty acids (where most E stored) – Gylcerol intermediate of glycolysis – Beta oxidation breaks fatty acids down to 2-C fragments citric acid cycle as acetyl CoA The Versatility of Catabolism • Glycolysis and the citric acid cycle connect to many other metabolic pathways • Catabolic pathways – Funnel electrons from many kinds of organic molecules into cellular respiration The catabolism of various molecules from food Proteins Carbohydrates Amino acids Sugars Fats Glycerol Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Figure 9.19 Oxidative phosphorylation Fatty acids Biosynthesis (Anabolic Pathways) • The body – Uses small molecules to build other substances • These small molecules – May come directly from food or through glycolysis or the citric acid cycle Regulation of Cellular Respiration via Feedback Mechanisms • Cellular respiration Glucose Glycolysis Fructose-6-phosphate – Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle – Releases energy, but does not produce it. – Inhibits AMP Stimulates + Phosphofructokinase – Fructose-1,6-bisphosphate Inhibits Pyruvate Citrate ATP Acetyl CoA Citric acid cycle Figure 9.20 Oxidative phosphorylation