© 2011 Pearson Education, Inc. Chapter 9 – Cellular Respiration & Fermentation© 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. file:///Users/ericarowe/Documents/UTK%20Biology/UTK%20Bio140%20Fall%202011/Chapter_09/A_PowerPoint_Lecture_Tools/09_Lecture_Outline/RedoxReactions.html© 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 water dioxide 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. 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 fructose-1,6-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