Biology 1020 -- Chapter 9
Biology 1020 -- Chapter 9 BIOL 1020 (Dr. Overturf)
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This 5 page Class Notes was uploaded by Brittney Champagne on Tuesday October 4, 2016. The Class Notes belongs to BIOL 1020 (Dr. Overturf) at University of Louisiana at Monroe taught by Dr. Overturf in Fall 2016. Since its upload, it has received 6 views. For similar materials see Principles of Biology (Dr. Overturf) in Biology at University of Louisiana at Monroe.
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Date Created: 10/04/16
Chapter 9 — Cellular Respiration and Fermentation Life is Work • Living cells require energy from outside sources • Energy flows into an ecosystem as sunlight and leaves as heat. • Cells use chemical energy stored in organic molecules to generateATP Catabolic Pathways — release stored energy by breaking down complex molecules • Electron transfer plays a major role in these pathways • These processes are central to cellular respiration Catabolic pathways and production ofATP • The breakdown of organic molecules is exergonic • Fermentation — partial degradation of sugars that occurs without oxygen Aerobic respiration — consumes organic molecules and oxygen, yieldsATP • • Anaerobic respiration — consumes compounds other than oxygen • Cellular respiration — includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Redox Reactions: Oxidation and Reduction • The transfer of electrons during chemical reactions releasees energy stored in organic molecules This released energy is ultimately used to synthesizeATP • The Principle of Redox • Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions • In oxidation, a substance loses electrons • Reduction, a substance gains electrons • Reducing agent — loses electrons and potential energy oxidizing agent — gains electrons and potential energy • • Example: When a glucose molecule loses a hydrogen atom as the result of a redox reaction, the molecule becomes oxidized. Example: When a molecule of NAD+ gains a hydrogen atom, the molecule becomes • reduced. Oxidation of organic fuel molecules During cellular respiration, the fuel (such as glucose) is oxidized, and oxygen is reduced Energy Harvest via NAD+ • Electrons from organic compounds are usually transferred to NAD+, a coenzyme • NAD+ is reduced to NADH during glycolysis, pyruvate oxidation, and the citric acid cycle. Each NADH (the reduced form of NAD+) represents stored energy that is tapped to • synthesizeATP Energy Harvest via NAD+ and Electron Transport Chain NADH passes the electrons to the electron transport chain — a series of redox reactions • • Oxygen pulls electrons down the chain in an energy-yielding tumble • The energy yielded is used to regenerateATP The Stages of Cellular Respiration:APreview • Harvesting of energy from glucose has three stages: • Glycolysis — breaks down glucose into two molecules of pyruvate • the CitricAcid Cycle — completes the breakdown of glucose Oxidative phosphorylation — accounts for most of theATP synthesis • • Process of an endergonic reaction coupled to an exergonic reaction • The process that generates the mostATP is called oxidative phosphorylation because it is powered by redox reactions Asmaller amount ofATP is formed in glycolysis and the citric acid cycle by substrate-level • phosphorylation • Substrate-level phosphorylation occurs in both glycolysis and the citric acid cycle. • For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules ofATP Glycolysis — “sugar splitting,” breaks down glucose into two molecules of pyruvate • Occurs in the cytosol Occurs whether or not O2 is present • • The energy-containing products of glycolysis are 2NADH, 2 pyruvate, and 2ATP. • Glycolysis uses 2ATP to produce 4ATP. CitricAcid Cycle — in the presence of oxygen, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed • The citric acid cycle, also called the krebs cycle, completes the breakdown of pyruvate into carbon dioxide If you remove a carbon from a pyruvate, you get acetyl CoA. • • Electron carriers that function in the citric acid cycle: NADH and FADH2 • Most of the carbon dioxide from the catabolism of glucose is released during the citric acid cycle. Oxidation of Pyruvate toAcetyl CoA • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoenzymeA (acetyl CoA) which links glycolysis to the citric acid cycle Oxidative Phosphorylation • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation The Pathway of Electron Transport • The electron transport chain is made of proteins in the inner membrane (cristae) of the mitochondrion • The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to oxygen, • forming water Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes to O2 • • The electron transport chain generates noATP directly • During aerobic respiration, FADH2 directly donates electrons to the electron transport chain at the lowest energy level. The extraction of energy from high-energy electrons remaining from glycolysis and the citric • acid cycle takes place in the electron transport chain. Chemiosmosis — the Energy-coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H+ from the • mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through the protein complex,ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation ofATP • • The most direct source of energy used to convertADP+i toATP is energy released from movement of protons throughATP synthase, down their electrochemical gradient Accounting ofATPproduction • During cellular respiration, most energy flows in this sequence: • glucose → NADH → electron transport chain → proton-motive force →ATP • About 34% of the energy in a glucose molecule is transferred toATP during cellular respiration, making about 32ATP Fermentation andAnaerobic Metabolism • Most cellular respiration requires oxygen to produceATP. Without oxygen, the electron transport chain will not work. • Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generateATP. • Substrate level phosphorylation accounts for 100% of theATP formed by the reactions of glycolysis. Glycolysis and TCAcycle Connect Many Other Metabolic Pathways • Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Versatility of Catabolism • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Carbohydrates and fats are considered high-energy foods because they have a lot of electrons associated with hydrogen. • Fatty acids are broken down by beta oxidation and yield acetyl CoA Regulation of Cellular Respiration • Feedback inhibition is the most common mechanism for metabolic control IfATP concentration begins to drop, respiration speeds up; when there is plenty ofATP, • respiration slows down • Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
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