Chapter 9: Cellular Respiration: Harvesting Chemical EnergyOverview: Life Is Work- Living cells require transfusions of energy from outside sources to perform their many tasks- Energy enters ecosystems as sunlight and leaves as heat.- Photosynthesis generates oxygen and organic molecules that the mitochondria of 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 fuelsCatabolic Pathways and Production of ATP- The arrangement of atoms of organic molecules represents potential energy.- Enzymes catalyze the systematic degradation of organic molecules that are rich in energyto simpler waste products with less energy.- Some of the released energy is used to do work; the rest is dissipated as heat.- 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- 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. 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. 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.- 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 from it.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. - The main energy foods (carbohydrates and fats) are reservoirs of electrons associated with hydrogen 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 atonce- 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. 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 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? 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 NAD+- Each NADH molecule formed during respiration represents stored energy that can be 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 the inner membrane of a mitochondrion.- Electrons removed from food are shuttled by NADH to the “top,” higher-energy end of 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