Overview Life Is Work Living cells require energy from outside sources Some animals obtain energy by eating plants and some animals feed on other organisms that eat plants Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules which are used in cellular respiration aerobic Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Cells use chemical energy stored in organic molecules to regenerate ATP which powers work Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 H2O Organic O molecules 2 Cellular respiration in mitochondria ATP ATP powers most cellular work Heat energy Concept 9 1 Catabolic pathways yield energy by oxidizing organic fuels and use it to make ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 Although carbohydrates fats and proteins are all consumed as fuel it is helpful to trace cellular respiration with the sugar glucose C6H12O6 6 O2 6 CO2 6 H2O Energy ATP heat Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Redox Reactions Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP Chemical reactions that transfer electrons between reactants are called oxidation reduction reactions or redox reactions In oxidation a substance loses electrons or is oxidized In reduction a substance gains electrons or is reduced the amount of positive charge is reduced Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 UN1 Redox Reaction Oxidation and Reduction becomes oxidized loses electron becomes reduced gains electron The electron donor is called the reducing agent The electron receptor is called the oxidizing agent Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and O2 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 3 Reactants Products becomes oxidized becomes reduced Methane reducing agent Oxygen oxidizing agent Carbon dioxide Water Fig 9 UN3 Oxidation of Organic Fuel Molecules During Cellular Respiration becomes oxidized becomes reduced During cellular respiration the fuel such as glucose is oxidized and O2 is reduced Stepwise Energy Harvest via NAD and the Electron Transport Chain In cellular respiration glucose and other organic molecules are broken down in a series of steps Oxidation of glucose is carried out by enzymes called Dehydrogenases These enzymes need the help of coenzymes like NAD and FAD Electrons from organic compounds are usually first transferred to NAD a coenzyme As an electron acceptor NAD functions as an oxidizing agent during cellular respiration Each NADH the reduced form of NAD represents stored energy that is tapped to synthesize ATP Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings NADH passes the electrons to the electron transport chain Unlike an uncontrolled reaction the electron transport chain passes electrons in a series of steps instead of one explosive reaction O2 pulls electrons down the chain in an energy yielding tumble The energy yielded is used to regenerate ATP Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 5 Free energy G Explosive release of heat and light energy 2H from food via NADH Controlled release of 2H 2e energy for synthesis of ATP ATP t spor tran tron Elec chain Free energy G H2 1 2 O2 2 O2 ATP ATP 2 e 1 2H H2O a Uncontrolled reaction 1 H2O b Cellular respiration 2 O2 The Stages of Cellular Respiration A Preview Cellular respiration has three stages 1 Glycolysis breaks down glucose into two molecules of pyruvate ocurs in Cytoplasm Glycolysis can take place both in presence and absence of Oxygen 2 The citric acid cycle completes the breakdown of glucose only in presence of oxygen occurs in mitochondrial matrix 3 Oxidative phosphorylation accounts for most of the ATP synthesis occurs in inner mitochondrial memebrane Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 6 1 Electrons carried via NADH Glycolysis Pyruvate Glucose Cytosol ATP Substrate level phosphorylation Fig 9 6 2 Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Mitochondrion Cytosol ATP ATP Substrate level phosphorylation Substrate level phosphorylation Fig 9 6 3 Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Oxidative phosphorylation electron transport and chemiosmosis Mitochondrion Cytosol ATP ATP ATP Substrate level phosphorylation Substrate level phosphorylation Oxidative phosphorylation The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions Oxidative phosphorylation accounts for almost 90 of the ATP generated by cellular respiration A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate level phosphorylation Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 9 2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis splitting of sugar breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 9 8 Energy investment phase Glucose 2 ADP 2 P 2 ATP used 4 ATP formed Energy payoff phase 4 ADP 4 P 2 NAD 4 e 4 H 2 NADH 2 H 2 Pyruvate 2 H2O Net Glucose 4 ATP formed 2 ATP used 2 NAD 4 e 4 H 2 Pyruvate 2 H2O 2 ATP 2 NADH 2 H Concept 9 3 The citric acid cycle completes the energy yielding oxidation of organic molecules In the presence of O2 pyruvate enters the mitochondrion Before the citric acid cycle can begin pyruvate must be converted to acetyl CoA which links the cycle to glycolysis The citric acid cycle also called the Krebs cycle takes place within the mitochondrial matrix The