Key Concepts In cells the endergonic reactions needed for life are paired with exergonic reactions requiring ATP Cellular respiration produces ATP from molecules with high potential energy often glucose Cellular respiration has four components 1 Glycolysis 2 Pyruvate processing 3 The citric acid cycle 4 Electron transport and chemiosmosis 2011 Pearson Education Inc Key Concepts Respiration and fermentation are carefully regulated Fermentation pathways allow glycolysis to continue when the lack of an electron acceptor shuts down electron transport chains 2011 Pearson Education Inc Introducing ATP ATP adenosine triphosphate is the cellular currency for energy it provides the fuel for most cellular activities ATP has high potential energy and allows cells to do work ATP works by phosphorylating transferring a phosphate group target molecules 2011 Pearson Education Inc The Nature of Chemical Energy and Redox Reactions In cells electrons are the most important source of chemical potential energy The amount of potential energy in an electron is based on its position relative to positive and negative charges Electrons closer to negative charges from other electrons and farther from positive charges in nuclei of nearby atoms have higher potential energy In general a molecule s potential energy is a function of its electrons configuration and position 2011 Pearson Education Inc Structure and Function of ATP The electrons in ATP have high potential energy because the four negative charges in its three phosphate groups repel each other Hydrolysis of the bond between the two outermost phosphate groups results in formation of ADP and Pi inorganic phosphate H2PO4 in a highly exergonic reaction The released phosphate group is transferred to a protein 2011 Pearson Education Inc 2011 Pearson Education Inc ATP Hydrolysis and Protein Phosphorylation Hydrolysis of ATP is exergonic because the entropy of the product molecules is much higher than that of the reactants Energy released during ATP hydrolysis is transferred to a protein during phosphorylation This phosphorylation usually causes a change in the protein s shape 2011 Pearson Education Inc 2011 Pearson Education Inc How Does ATP Drive Endergonic Reactions When a protein is phosphorylated the exergonic phosphorylation reaction is paired with an endergonic reaction in a process called energetic coupling In cells endergonic reactions become exergonic when the substrates or enzymes involved are phosphorylated 2011 Pearson Education Inc 2011 Pearson Education Inc What Is a Redox Reaction Reduction oxidation reactions redox reactions are chemical reactions that involve electron transfer Redox reactions drive ATP formation When an atom or molecule gains an electron it is reduced When an atom or molecule loses an electron it is oxidized Oxidation and reduction events are always coupled if one atom loses an electron another has to gain it Electron donors are always paired with electron acceptors 2011 Pearson Education Inc The Gain or Loss of an Electron Can Be Relative During a redox reaction electrons can be transferred completely from one atom to another or they can simply shift their position in covalent bonds 2011 Pearson Education Inc 2011 Pearson Education Inc Electrons Are Usually Accompanied by Protons Each electron transferred from one molecule to another during a redox reaction is usually accompanied by a proton H The reduced molecule gains a proton and has higher potential energy The oxidized molecule loses a proton and has lower potential energy Nicotinamide adenine dinucleotide NAD is reduced to form NADH NADH readily donates electrons to other molecules and is thus called an electron carrier and has reducing power 2011 Pearson Education Inc 2011 Pearson Education Inc What Happens When Glucose Is Oxidized The carbon atoms of glucose are oxidized to form carbon dioxide and the oxygen atoms in oxygen are reduced to form water C6H12O6 6 O2 6 CO2 6 H2O energy glucose oxygen carbon dioxide water In cells glucose is oxidized through a long series of carefully controlled redox reactions The resulting change in free energy is used to synthesize ATP from ADP and Pi Together these reactions comprise cellular respiration 2011 Pearson Education Inc 2011 Pearson Education Inc An Overview of Cellular Respiration All organisms use glucose to build fats carbohydrates and other compounds cells recover glucose by breaking down these molecules Glucose is used to make ATP through either cellular respiration or fermentation Cellular respiration produces ATP from a molecule with high potential energy usually glucose Each of the four steps of cellular respiration consists of a series of chemical reactions and a distinctive starting molecule and characteristic set of products 2011 Pearson Education Inc 2011 Pearson Education Inc The Steps of Cellular Respiration Cellular respiration is any suite of reactions that produces ATP in an electron transport chain Cellular respiration has four steps 1 Glycolysis glucose is broken down to pyruvate 2 Pyruvate processing pyruvate is oxidized to form acetyl CoA 3 Citric acid cycle acetyl CoA is oxidized to CO2 4 Electron transport and chemiosmosis compounds that were reduced in steps 1 3 are oxidized in reactions leading to ATP production 2011 Pearson Education Inc 2011 Pearson Education Inc Glycolysis Processing Glucose to Pyruvate Glycolysis a series of 10 chemical reactions is the first step in glucose oxidation All of the enzymes needed for glycolysis are found in the cytosol In glycolysis glucose is broken down into two 3 carbon molecules of pyruvate and the potential energy released is used to phosphorylate ADP to form ATP 2011 Pearson Education Inc The Glycolysis Reactions Glycolysis consists of an energy investment phase and an energy payoff phase In the energy investment phase two molecules of ATP are consumed and glucose is phosphorylated twice forming fructose1 6 bisphosphate In the energy payoff phase Sugar is split to form two pyruvate molecules Two molecules of NAD are reduced to NADH Four molecules of ATP are formed by substrate level phosphorylation net gain of 2 ATP 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Methods of Producing ATP Substrate level phosphorylation occurs when ATP is produced by the enzyme catalyzed transfer of a phosphate group from an intermediate substrate to ADP This is how ATP is produced in glycolysis and the citric acid cycle In an electron