Chapter 9 Cellular Respiration Harvesting Chemical Energy Overview Life Is Work Living cells require transfusions of energy from outside sources to perform their many Energy enters ecosystems as sunlight and leaves as heat Photosynthesis generates oxygen and organic molecules that the mitochondria of tasks eukaryotes use as fuel for cellular respiration Respiration has three key pathways glycolysis the citric acid cycle and oxidative phosphorylation Concept 9 1 Catabolic pathways yield energy by oxidizing organic fuels Catabolic Pathways and Production of ATP Some of the released energy is used to do work the rest is dissipated as heat Enzymes catalyze the systematic degradation of organic molecules that are rich in energy The arrangement of atoms of organic molecules represents potential energy to simpler waste products with less energy Catabolic pathways are metabolic pathways that release stored energy by breaking down complex molecules One type of catabolic process fermentation is a partial degradation of sugars that occurs in the absence of oxygen A more efficient and prevalent catabolic process cellular respiration consumes oxygen as a reactant along with organic molecules In eukaryotic cells mitochondria are the site of most of the processes of cellular respiration The overall process is organic compounds O2 CO2 H2O energy ATP heat Carbohydrates fats and proteins can all be used as fuel but cells use glucose most often C6H12O6 6O2 6CO2 6H2O Energy ATP heat The catabolism of glucose is exergonic with a G of 686 kcal per mole of glucose G indicates that the products of the chemical process store less energy than the reactants and that the reaction happens spontaneously Some of this energy is used to produce ATP which can perform cellular work The Principle of Redox The loss of electrons is called oxidation The addition of electrons is called reduction The formation of table salt from sodium and chloride is a redox reaction Catabolic pathways transfer the electrons stored in food molecules releasing energy that is used to synthesize ATP Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation reduction reactions or redox reactions Na Cl Na Cl Here sodium is oxidized and chlorine is reduced its charge drops from 0 to 1 More generally Xe Y X Ye X the electron donor is the reducing agent and reduces Y Y the electron recipient is the oxidizing agent and oxidizes X Redox reactions require both a donor and acceptor Redox reactions also occur when the transfer of electrons is not complete but involves a change in the degree of electron sharing in covalent bonds from it Energy must be added to pull an electron away from an atom The more electronegative the atom the more energy is required to take an electron away Lecture Outline for Campbell Reece Biology 7th Edition Pearson Education Inc 9 1 An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one A redox reaction that relocates electrons closer to oxygen such as the burning of methane releases chemical energy that can do work Oxidation of Organic Fuel Molecules during Cellular Respiration C6H12O6 6O2 6CO2 6H2O Energy Glucose is oxidized and oxygen is reduced In general organic molecules that have an abundance of hydrogen are excellent fuels becayse their bonds are a source of electrons whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen By oxidizing glucose respiration liberates stored energy from glucose and makes it available for ATP synthesis with hydrogen The main energy foods carbohydrates and fats are reservoirs of electrons associated Stepwise Energy Harvest via NAD and the Electron Transport Chain Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time because energy can t be harvested if released all at once Rather glucose and other organic fuels are broken down in a series of steps each one catalyzed by a specific enzyme At key steps electrons are stripped from the glucose coenzyme called NAD nicotinamide adenine dinucleotide As an electron acceptor NAD functions as an oxidizing agent during respiration How does NAD trap electrons from glucose and other organic molecules In many oxidation reactions each electron travels with a proton a hydrogen atom The hydrogen atoms are not transferred directly to oxygen but are passed first to a Enzymes called dehydrogenase remove two hydrogen atoms 2 protons and electrons from the substrate ex sugar thereby oxidizing it The enzyme passes two electrons and one proton to its coenzyme NAD The other proton is released as H to the surrounding solution By receiving two electrons and only one proton NAD has its charge neutralized when it is reduced to NADH NAD is the most versatile electron acceptor in cellular respiration and functions in several of the redox steps during the breakdown of sugar Electrons lose very little of their potential energy when they are transferred from food to Each NADH molecule formed during respiration represents stored energy that can be NAD tapped to synthesize ATP as electrons fall down an electron gradient from NADH to oxygen How are electrons extracted from food and stored by NADH finally reach oxygen The hydrogen that reacts with oxygen is derived from organic molecules rather than H2 Cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several energy releasing steps The electron transport chain consists of several molecules primarily proteins built into Electrons removed from food are shuttled by NADH to the top higher energy end of the inner membrane of a mitochondrion the chain At the bottom lower energy end oxygen captures these electrons along with H forming water Lecture Outline for Campbell Reece Biology 7th Edition Pearson Education Inc 9 2 Electron transfer from NADH to oxygen is an exergonic reaction with a free energy change of 53 kcal mol Electrons cascade down the chain from one carrier to the next losing only a small amount of energy with each step until they reach oxygen the terminal electron acceptor which has a great affinity for electrons Oxygen pulls electrons down the chain in an energy yielding tumble route food NADH electron transport chain oxygen In summary during cellular respiration most electrons travel the following downhill The Stages of Cellular