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Transcript
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) Cellular Respiration • ALL energy ultimately comes from the SUN • Catabolic pathways → Yield energy by oxidizing organic fuels • All the primary organic molecules can be consumed as fuel • We’ll only examine the most common fuel = sugar (C6H12O6) • Exergonic rxn: ∆G = -686 kcal/mol of Glucose (the energy will be used to generate ATP) Energy ultimately comes from the Sun • Energy – Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Organic + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 Heat energy 1 Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic (releases energy) • Catabolic pathways yield energy by oxidizing organic fuels Catabolic Pathways • One catabolic process, fermentation – Is a partial degradation of sugars that occurs without oxygen – Involves Glycolysis – Yields 2 ATP/Glucose molecule Catabolic Pathways • Cellular respiration – Is the most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Involves Glycolysis – Yields up to 38 ATP/Glucose molecule • To keep working – Cells must regenerate ATP 2 Cellular Respiration Redox rxns = oxidation-reduction rxns • Transfer of electrons (e-) releases energy stored in organic molecules → this energy is ultimately used to generate ATP • Oxidation = loss of e- from one substance • Reduction = addition of e- to another substance • Na + Cl → Na+ + Cl• Na is the reducing agent (donates an e- to CL) • Cl is the oxidizing agent (removes an e- from Na) Cellular Respiration Respiration is a redox rxn: • By oxidizing glucose, energy stored in glucose is liberated to make ATP – Happens in a series of enzyme-catalyzed steps – Coenzyme (NAD+) acts as e- shuttle • Electron transport chains (ETC) - breaks the energetic fall of e- into several energy-releasing steps (not one big explosive rxn), fig 9.5 – Consists of mostly proteins embedded in the inner mitochondrial membrane • Overview of Respiration: (fig 9.6) Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy – Due to the transfer of electrons 3 The Principle of Redox • Redox reactions – Transfer electrons from one reactant to another by oxidation and reduction • In oxidation – A substance loses electrons, or is oxidized • In reduction – A substance gains electrons, or is reduced Examples of redox reactions • Examples of redox reactions becomes oxidized (loses electron) Na + Na+ Cl Cl– + becomes reduced (gains electron) Some redox reactions • Do not completely exchange electrons • Change the degree of electron sharing in covalent bonds Products Reactants becomes oxidized + CH4 2O2 + Energy 2 H2O becomes reduced H O O C O H O O H H C + CO2 H H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water Figure 9.3 4 Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration – Glucose is oxidized and oxygen is reduced becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration – Oxidizes glucose in a series of steps – Allows the cell to use the energy harvested from sugar to power work rather than losing it in one explosive reaction. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electrons from organic compounds • Are usually first transferred to NAD+, a coenzyme 2 e– + 2 H+ NAD+ NH2 H C N+ CH2 O O O P O– O H H O P O– HO OH O HO CH2 H O H HO H OH Reduction of NAD+ + 2[H] (from food) Oxidation of NADH Nicotinamide (oxidized form) NADH H O C H N NH2 + Nicotinamide (reduced form) NH2 N N 2 e– + H+ Dehydrogenase O N N H Figure 9.4 5 NADH, the reduced form of NAD+ • Passes the electrons to the electron transport chain • So it is an electron shuttle and moves electrons to the ETC from both glycolysis and from the citric acid cycle. If electron transfer is not stepwise • If electron transfer is not stepwise – A large release of energy occurs – As in the reaction of hydrogen and oxygen to form water Free energy, G H2 + 1/2 O2 Figure 9.5 A Explosive release of heat and light energy (a) Uncontrolled reaction H2O The electron transport chain (ETC) • Passes electrons in a series of steps instead of in one explosive reaction • Uses the energy from the electron transfer to form ATP 6 Electron Transport Chain 2H + 1/ 2 O2 (from food via NADH) tron Elec Controlled release of energy for synthesis of ATP ATP ATP port trans Free energy, G 2 H+ + 2 e– ATP chain 2 e– 1/ 2 2 H+ O2 H2O Figure 9.5 B (b) Cellular respiration An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolsis Pyruvate Glucose Cytosol ATP Figure 9.6 Substrate-level phosphorylation Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation ATP Oxidative phosphorylation Three Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle (Kreb’s Cycle) – Oxidative phosphorylation (driven by the ETC) 7 Stages of Cellular Respiration 1. Glycolysis – Breaks down glucose into two molecules of pyruvate – Produces net 2 ATP and 2 NADH Conversion of pyruvate to acetyl CoA yields 2NADH 2. The citric acid cycle – Completes the breakdown of glucose – Produces net 2 ATP, 6 NADH and 2 FADH2 from 2 Acetyl CoA Stages of Cellular Respiration 3. Oxidative phosphorylation – Is driven by the electron transport chain (receives electrons from NADH and FADH2) – Generates 32 – 34 ATP An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolsis Pyruvate Glucose Cytosol ATP Figure 9.6 Substrate-level phosphorylation Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation ATP Oxidative phosphorylation 8 Cellular Respiration • Glycolysis & Citric Acid Cycle = catabolic pathways that breakdown glucose • Glycolysis → pyruvate + coenzymes + ATP • CAC Æ coenzymes + ATP • ATP formed by substrate-level phosphorylation (fig 9.7) = enzyme transfers a phosphate group from an organic substrate to ADP to make ATP • Oxidative Phosphorylation = ATP synthesis powered by ETC. Makes 90% of the 38 ATPs Both glycolysis and the citric acid cycle • Can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP Substrate P Figure 9.7 + ATP Product Glycolysis • Glycolysis harvests chemical E by oxidizing glucose to pyruvate • Glucose → Two 3-C sugars → oxidized & rearranged → Two pyruvates • Two Major Phases of Glycolysis • 1. E-investment phase (fig 9.9) • Rearrange glucose + add phosphate groups (uses 2 ATP) • Split 6-C sugar → two 3-C sugar isomers • Glyceraldehyde-3-phosphate form → next phase • 2. E-payoff phase (fig 9.9) • 2 NAD+ → 2 NADH & a phosphate group added to each of 2 3-C sugars • 4 ATP produced by substrate-level phosphorylation • Rearrangement of remaining phosphate group and the 3-C substrate Final Products from 1 Glucose = 2 ATP + 2 pyruvate + 2NADH 9 Glycolysis • Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis – Means “splitting of sugar” – Breaks down glucose into pyruvate – Occurs in the cytoplasm of the cell Glycolysis • Glycolysis consists of two major phases – Energy investment phase Glycolysis – Energy payoff phase Citric acid cycle Oxidative phosphorylation ATP ATP ATP Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 2NAD+ + 4 e- + 4H + 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used Figure 9.8 2NAD+ + 4e– + 4H + 2 Pyruvate + 2 H2O 2 ATP 2NADH + 2H+ A closer look at the energy investment phase CH2OH HH H HO H OH H OH Glycolysis HO Citric Oxidative acid cycle phosphorylation Glucose ATP ADP 1 Hexokinase CH2OH P HH OH OH H HO H OH Glucose-6-phosphate 2 Phosphoglucoisomerase CH2O P O CH2OH H HO HO H HO H Fructose-6-phosphate ATP 3 Phosphofructokinase ADP P O CH2 O CH2 O P HO H OH HO H Fructose1, 6-bisphosphate Aldolase 4 5 H P O CH2 Isomerase C O C O CHOH CH2OH CH2 O P Figure 9.9 A Dihydroxyacetone phosphate Uses 2 ATP. Produces 2 Glyceraldehyde-3-phosphates to feed into energy payoff phase. Glyceraldehyde3-phosphate 10 A closer look at the energy payoff phase 6 Triose phosphate dehydrogenase 2 NAD+ 2 Pi 2 NADH + 2 H+ 2 P O C O CHOH CH2 O P 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP O– 2 C CHOH Produces 4 ATP, 2 NADH (for ETC), and 2 pyruvates to be converted to Acetyl-CoA and fed into Citric Acid Cycle. CH2 O P 3-Phosphoglycerate 8 Phosphoglyceromutase 2 O– C O H C O P CH2OH 2-Phosphoglycerate 9 Enolase 2H O 2 2 O– So, net of glycolysis is 2 ATP, 2 NADH, and 2 pyruvate. C O C O P CH2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 O– C O C O CH3 Pyruvate Figure 9.8 B Citric acid cycle • Citric acid cycle completes the E-yielding oxidation of organic molecules • Pyruvate enters mitochondrion via active transport → converted to acetyl coenzyme A (acetyl CoA) – Happens in 3 rxns catalyzed by a multienzyme complex • Citric acid cycle (also = Krebs cycle) • Citrate (ionized form of citric acid) = 1st molecule produced • Acetyl CoA brings two C atoms to cycle → recycles oxaloacetate → C atoms leave cycle as CO2 (completely oxidized) • Ultimately get CO2, NADH, FADH2, and ATP from the CAC. Before the citric acid cycle can begin • Pyruvate must first be converted to acetyl CoA, which links the citric acid cycle to glycolysis • Happens in 3 rxns catalyzed by a multienzyme complex. CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA C O 2 C Process yields 2 NADH (for ETC) from 2 pyruvate C O O 1 3 CH3 Pyruvate CH3 Acetyle CoA CO2 Coenzyme A Transport protein Figure 9.10 11