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Chapter 6 How Cells Harvest Chemical Energy PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko Introduction In eukaryotes, cellular respiration – harvests energy from food, – yields large amounts of ATP, and – Uses ATP to drive cellular work. A similar process takes place in many prokaryotic organisms. © 2012 Pearson Education, Inc. Figure 6.0_1 Chapter 6: Big Ideas Cellular Respiration: Aerobic Harvesting of Energy Fermentation: Anaerobic Harvesting of Energy Stages of Cellular Respiration Connections Between Metabolic Pathways CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY © 2012 Pearson Education, Inc. 6.1 Photosynthesis and cellular respiration provide energy for life Life requires energy. In almost all ecosystems, energy ultimately comes from the sun. In photosynthesis, – some of the energy in sunlight is captured by chloroplasts, – atoms of carbon dioxide and water are rearranged, and – glucose and oxygen are produced. © 2012 Pearson Education, Inc. 6.1 Photosynthesis and cellular respiration provide energy for life In cellular respiration – glucose is broken down to carbon dioxide and water and – the cell captures some of the released energy to make ATP. Cellular respiration takes place in the mitochondria of eukaryotic cells. © 2012 Pearson Education, Inc. Figure 6.1 Sunlight energy ECOSYSTEM Photosynthesis in chloroplasts CO2 Glucose H2O O2 Cellular respiration in mitochondria (for cellular work) ATP Heat energy 6.3 Cellular respiration banks energy in ATP molecules Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form ATP. Cellular respiration – produces up to 32 ATP molecules from each glucose molecule and – captures only about 34% of the energy originally stored in glucose. Other foods (organic molecules) can also be used as a source of energy. © 2012 Pearson Education, Inc. Figure 6.3 C6H12O6 6 Glucose Oxygen O2 6 CO2 Carbon dioxide 6 H2O ATP Water Heat 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction, – the loss of electrons from one substance is called oxidation, – the addition of electrons to another substance is called reduction, – a molecule is oxidized when it loses one or more electrons, and – reduced when it gains one or more electrons. © 2012 Pearson Education, Inc. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen A cellular respiration equation is helpful to show the changes in hydrogen atom distribution. Glucose – loses its hydrogen atoms and – becomes oxidized to CO2. Oxygen – gains hydrogen atoms and – becomes reduced to H2O. © 2012 Pearson Education, Inc. Figure 6.5A Loss of hydrogen atoms (becomes oxidized) C6H12O6 6 O2 6 CO2 6 H2O Glucose Gain of hydrogen atoms (becomes reduced) ATP Heat 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen Enzymes are necessary to oxidize glucose and other foods. NAD+ – is an important enzyme in oxidizing glucose, – accepts electrons, and – becomes reduced to NADH. © 2012 Pearson Education, Inc. Figure 6.5B Becomes oxidized 2H Becomes reduced NAD 2H 2 H NADH 2 (carries 2 electrons) H 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen There are other electron “carrier” molecules that function like NAD+. – They form a staircase where the electrons pass from one to the next down the staircase. – These electron carriers collectively are called the electron transport chain. – As electrons are transported down the chain, ATP is generated. © 2012 Pearson Education, Inc. Figure 6.5C NADH NAD ATP 2 Controlled release of energy for synthesis of ATP H 2 1 O 2 2 2 H H 2O STAGES OF CELLULAR RESPIRATION © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages Cellular respiration consists of a sequence of steps that can be divided into three stages. – Stage 1 – Glycolysis – Stage 2 – Pyruvate oxidation and citric acid cycle – Stage 3 – Oxidative phosphorylation © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages Stage 1: Glycolysis – occurs in the cytoplasm, – begins cellular respiration, and – breaks down glucose into two molecules of a threecarbon compound called pyruvate. © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages Stage 2: The citric acid cycle – takes place in mitochondria, – oxidizes pyruvate to a two-carbon compound, and – supplies the third stage with electrons. © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages Stage 3: Oxidative phosphorylation – involves electrons carried by NADH and FADH2, – shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane, – involves chemiosmosis, and – generates ATP through oxidative phosphorylation associated with chemiosmosis. © 2012 Pearson Education, Inc. Figure 6.6 CYTOPLASM NADH Electrons carried by NADH NADH Glycolysis Glucose Pyruvate Pyruvate Oxidation Citric Acid Cycle FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate In glycolysis, – a single molecule of glucose is enzymatically cut in half through a series of steps, – two molecules of pyruvate are produced, – two molecules of NAD+ are reduced to two molecules of NADH, and – a net of two molecules of ATP is produced. © 2012 Pearson Education, Inc. Figure 6.7A Glucose 2 ADP 2 NAD 2 P 2 NADH 2 ATP 2 Pyruvate 2 H 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate ATP is formed in glycolysis by substrate-level phosphorylation during which – an enzyme transfers a phosphate group from a substrate molecule to ADP and – ATP is formed. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates. © 2012 Pearson Education, Inc. Figure 6.7B Enzyme P Enzyme ADP ATP P P Substrate Product 6.8 Pyruvate is oxidized prior to the citric acid cycle The pyruvate formed in glycolysis is transported from the cytoplasm into a mitochondrion where – the citric acid cycle and – oxidative phosphorylation will occur. © 2012 Pearson Education, Inc. 6.8 Pyruvate is oxidized prior to the citric acid cycle Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which – a carboxyl group is removed and given off as CO2, – the two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH, – coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and – acetyl CoA enters the citric acid cycle. © 2012 Pearson Education, Inc. Figure 6.8 NAD NADH H 2 CoA Pyruvate Acetyl coenzyme A 1 CO2 3 Coenzyme A 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules The citric acid cycle – is also called the Krebs cycle (after the German-British researcher Hans Krebs, who worked out much of this pathway in the 1930s), – completes the oxidation of organic molecules, and – generates many NADH and FADH2 molecules. © 2012 Pearson Education, Inc. Figure 6.9A Acetyl CoA CoA CoA 2 CO2 Citric Acid Cycle 3 NAD FADH2 3 NADH FAD 3 H ATP ADP P 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules During the citric acid cycle – the two-carbon group of acetyl CoA is added to a fourcarbon compound, forming citrate, – citrate is degraded back to the four-carbon compound, – two CO2 are released, and – 1 ATP, 3 NADH, and 1 FADH2 are produced. © 2012 Pearson Education, Inc. 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose. Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is – 2 ATP, – 6 NADH, and – 2 FADH2. © 2012 Pearson Education, Inc. 6.10 Most ATP production occurs by oxidative phosphorylation Oxidative phosphorylation – involves electron transport and chemiosmosis and – requires an adequate supply of oxygen. © 2012 Pearson Education, Inc. 6.10 Most ATP production occurs by oxidative phosphorylation Electrons from NADH and FADH2 travel down the electron transport chain to O2. Oxygen picks up H+ to form water. Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space. In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP. © 2012 Pearson Education, Inc. Figure 6.10 H Intermembrane space H H H H Mobile electron carriers Protein complex of electron carriers H ATP synthase IV I II FADH2 Electron flow NADH Mitochondrial matrix H H III Inner mitochondrial membrane H NAD FAD 2 H 1 2 O2 H2O H ADP P ATP H Electron Transport Chain Oxidative Phosphorylation Chemiosmosis 6.12 Review: Each molecule of glucose yields many molecules of ATP Recall that the energy payoff of cellular respiration involves 1. glycolysis, 2. alteration of pyruvate, 3. the citric acid cycle, and 4. oxidative phosphorylation. © 2012 Pearson Education, Inc. 6.12 Review: Each molecule of glucose yields many molecules of ATP The total yield is about 32 ATP molecules per glucose molecule. This is about 34% of the potential energy of a glucose molecule. In addition, water and CO2 are produced. © 2012 Pearson Education, Inc. Figure 6.12 CYTOPLASM Electron shuttles across membrane 2 NADH Mitochondrion 2 NADH or 2 FADH2 6 NADH 2 NADH Glycolysis 2 Pyruvate Glucose Pyruvate Oxidation 2 Acetyl CoA Citric Acid Cycle 2 FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) Maximum per glucose: 2 ATP by substrate-level phosphorylation 2 ATP by substrate-level phosphorylation about 28 ATP by oxidative phosphorylation About 32 ATP FERMENTATION: ANAEROBIC HARVESTING OF ENERGY © 2012 Pearson Education, Inc. 6.13 Fermentation enables cells to produce ATP without oxygen Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation – takes advantage of glycolysis, – produces two ATP molecules per glucose, and – reduces NAD+ to NADH. The trick of fermentation is to provide an anaerobic path for recycling NADH back to NAD+. © 2012 Pearson Education, Inc. 6.13 Fermentation enables cells to produce ATP without oxygen Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which – NADH is oxidized to NAD+ and – pyruvate is reduced to lactate. Animation: Fermentation Overview © 2012 Pearson Education, Inc. Figure 6.13A 2 ADP 2 P 2 ATP Glycolysis Glucose 2 NAD 2 NADH 2 Pyruvate 2 NADH 2 NAD 2 Lactate 6.13 Fermentation enables cells to produce ATP without oxygen The baking and winemaking industries have used alcohol fermentation for thousands of years. In this process yeasts (single-celled fungi) – oxidize NADH back to NAD+ and – convert pyruvate to CO2 and ethanol. © 2012 Pearson Education, Inc. Figure 6.13B Glucose 2 NAD Glycolysis 2 ADP 2 P 2 ATP 2 NADH 2 Pyruvate 2 NADH 2 CO2 2 NAD 2 Ethanol CONNECTIONS BETWEEN METABOLIC PATHWAYS © 2012 Pearson Education, Inc. 6.15 Cells use many kinds of organic molecules as fuel for cellular respiration Although glucose is considered to be the primary source of sugar for respiration and fermentation, ATP is generated using – carbohydrates, – fats, and – proteins. © 2012 Pearson Education, Inc. 6.15 Cells use many kinds of organic molecules as fuel for cellular respiration Fats make excellent cellular fuel because they – contain many hydrogen atoms and thus many energyrich electrons and – yield more than twice as much ATP per gram than a gram of carbohydrate or protein. © 2012 Pearson Education, Inc. Figure 6.15 Food, such as peanuts Carbohydrates Sugars Fats Proteins Glycerol Fatty acids Amino acids Amino groups Glucose G3P Pyruvate Glycolysis Pyruvate Oxidation Acetyl CoA ATP Citric Acid Cycle Oxidative Phosphorylation Figure 6.16 ATP needed to drive biosynthesis Citric Acid Cycle ATP Pyruvate Oxidation Acetyl CoA Glucose Synthesis Pyruvate G3P Glucose Amino groups Amino acids Proteins Fatty acids Glycerol Fats Cells, tissues, organisms Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules. Sugars Carbohydrates Figure 6.UN02 Cellular respiration has three stages generates oxidizes uses produce some produces many energy for glucose and organic fuels (a) (b) (d) to pull electrons down (c) cellular work (f) by a process called chemiosmosis uses (g) H diffuse through ATP synthase (e) uses pumps H to create H gradient to