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Chapter 6 How Cells Harvest Chemical Energy PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN © 2015 Pearson Education, Inc. Lecture by Edward J. Zalisko Introduction • Oxygen is a reactant in cellular respiration, the process that breaks down sugar and other food molecules and generates ATP, the energy currency in cells, and heat. • Brown fat has a “short circuit” in its cellular respiration, which generates only heat, not ATP. • Brown fat is important for heat production in small mammals, including humans. © 2015 Pearson Education, Inc. CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY © 2015 Pearson Education, Inc. 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 • sugar and oxygen are produced. © 2015 Pearson Education, Inc. Photosynthesis and cellular respiration provide energy for life • In cellular respiration, • sugar 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. • In these energy conversions, some energy is lost as heat. © 2015 Pearson Education, Inc. Figure 6.1 Sunlight energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Organic Cellular respiration in mitochondria ATP Heat energy © 2015 Pearson Education, Inc. molecules + O2 ATP powers most cellular work Breathing supplies O2 for use in cellular respiration and removes CO2 • Respiration, as it relates to breathing, and cellular respiration are not the same. • Respiration, in the breathing sense, refers to an exchange of gases. Usually an organism brings in oxygen from the environment and releases waste CO2. • Cellular respiration is the aerobic (oxygen-requiring) harvesting of energy from food molecules by cells. © 2015 Pearson Education, Inc. Figure 6.2-0 O2 Breathing CO2 Lungs O2 Transported in bloodstream CO2 Muscle cells carrying out Cellular Respiration Glucose + O2 ➞ CO2 + H2O + ATP © 2015 Pearson Education, Inc. Cellular respiration banks energy in ATP molecules • Cellular respiration is an exergonic (energyreleasing) process that transfers energy from the bonds in glucose to form ATP. © 2015 Pearson Education, Inc. Cellular respiration banks energy in ATP molecules • Cellular respiration • can produce up to 32 ATP molecules for each glucose molecule, • uses about 34% of the energy originally stored in glucose, and • releases the other 66% as heat. • This energy conversion efficiency is better than most energy conversion systems. • Only about 25% of the energy in gasoline produces the kinetic energy of movement. © 2015 Pearson Education, Inc. C6H12O6 6 O2 6 CO2 Glucose Oxygen Carbon dioxide © 2015 Pearson Education, Inc. 6 H2O Water ATP Heat The human body uses energy from ATP for all its activities • Your body requires a continuous supply of energy just to stay alive—to keep your heart pumping and you breathing. © 2015 Pearson Education, Inc. • A kilocalorie (kcal) is • the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1C, • the same as a food Calorie, and • used to measure the nutritional values indicated on food labels. © 2015 Pearson Education, Inc. • The average adult human needs about 2,200 kcal of energy per day. • About 75% of these calories is used to maintain a healthy body. • The remaining 25% is used to power physical activities. • A balance of energy intake and expenditure is required to maintain a healthy weight. © 2015 Pearson Education, Inc. Activity kcal consumed per hour by a 67.5-kg (150-lb) person* Running (8–9 mph) 979 Dancing (fast) 510 Bicycling (10 mph) 490 Swimming (2 mph) 408 Walking (4 mph) 341 Walking (3 mph) 245 Dancing (slow) Driving a car Sitting (writing) 204 61 28 *Not including kcal needed for body maintenance © 2015 Pearson Education, Inc. Cells capture energy from electrons “falling” from organic fuels to oxygen • How do your cells extract energy from glucose? • The answer involves the transfer of electrons during chemical reactions. Cells capture energy from electrons “falling” from organic fuels to oxygen • During cellular respiration, • electrons are transferred from glucose to oxygen and • energy is released. • Oxygen attracts electrons very strongly. • An electron loses potential energy when it is transferred to oxygen. 6.5 Cells capture energy from electrons “falling” from organic fuels to oxygen • Energy can be released from glucose by simply burning it. • This electron “fall” happens very rapidly. • This energy is dissipated as heat and light and is not available to living organisms. © 2015 Pearson Education, Inc. • Cellular respiration is a more controlled descent of electrons and like rolling down an energy hill. • Energy is released in small amounts and can be stored in the chemical bonds of ATP. • 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 • a molecule is reduced when it gains one or more electrons. Oxidation States of Carbon -4 +4 Highest Energy Least Stable Lowest Energy Most Stable In Respiration, Carbon Carbon is Oxidized from its highest energy to a lower one. The energy coming out is eventually trapped and held in the cells as ATP. ATP provides this energy to run all of life’s processes. In Fats, most of the carbon atoms are at the -4 level. In Sugars and starches, they are in the -2 or 0 level. Cells capture 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. Loss of hydrogen atoms (becomes oxidized) C6H12O6 + 6 O2 (Glucose) 6 CO2 + 6 H2O + ATP + Heat Gain of hydrogen atoms (becomes reduced) © 2015 Pearson Education, Inc. • An important player in the process of oxidizing glucose is a coenzyme called NAD+, which • accepts electrons and • becomes reduced to NADH. Becomes oxidized +2H NAD+ + 2H Becomes reduced 2 H+ + 2 © 2015 Pearson Education, Inc. NADH (carries) 2 electrons) H+ • NADH delivers electrons to a string of electron carrier molecules, which moves electrons down a hill. • These carrier molecules constitute an electron transport chain. • At the bottom of the hill is oxygen (1/2 O2), which • accepts two electrons, • picks up two H+, and • becomes reduced to water. NAD+ NADH 2 Energy released and available for making ATP 2 1 − 2 O2 H2O 2 H+ © 2015 Pearson Education, Inc. STAGES OF CELLULAR RESPIRATION 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 the citric acid cycle • Stage 3: Oxidative phosphorylation Cellular respiration occurs in three main stages • Stage 1: Glycolysis • occurs in the cytosol, • begins cellular respiration, and • breaks down glucose into two molecules of a threecarbon compound called pyruvate. Cellular respiration occurs in three main stages • Stage 2: Pyruvate oxidation and the citric acid cycle • take place in mitochondria, • oxidize pyruvate to a two-carbon compound, and • supply the third stage with electrons. • The cell makes a small amount of ATP during glycolysis and the citric acid cycle. Cellular respiration occurs in three main stages • Stage 3: Oxidative phosphorylation • NADH and a related electron carrier, FADH2, shuttle electrons to an electron transport chain embedded in the inner mitochondrial membrane. • Most ATP produced by cellular respiration is generated by oxidative phosphorylation, which uses the energy released by the downhill fall of electrons from NADH and FADH2 to oxygen to phosphorylate ADP. • Stage 3: Oxidative phosphorylation • As the electron transport chain passes electrons down the energy hill, it also pumps hydrogen ions (H+) across the inner mitochondrial membrane, into the narrow intermembrane space, and produces a concentration gradient of H+ across the membrane. • In chemiosmosis, the potential energy of this concentration gradient is used to make ATP. Figure 6.6-1 Electrons carried by NADH Glycolysis Glucose Pyruvate CYTOSOL ATP © 2015 Pearson Education, Inc. Pyruvate Oxidation Citric Acid Cycle FADH2 Oxidative Phosphorylation (Electron transport and chemiosmosis) MITOCHONDRION Substrate-level phosphorylation Substrate-level ATP phosphorylation ATP Oxidative phosphorylation 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 • there is a net gain of two molecules of ATP. © 2015 Pearson Education, Inc. Glucose 2 ADP 2 NAD+ +2 P 2 NADH 2 ATP 2 Pyruvate © 2015 Pearson Education, Inc. +2 H+ 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 known as intermediates. © 2015 Pearson Education, Inc. Enzyme P Enzyme ADP ATP P Substrate © 2015 Pearson Education, Inc. P Product • The steps of glycolysis have two main phases. • In steps 1–4, the energy investment phase, energy is consumed as two ATP molecules are used to energize a glucose molecule, which is then split into two small sugars. • In steps 5–9, the energy payoff phase, two NADH molecules are produced for each initial glucose molecule and four ATP molecules are generated. • There is a net gain of two ATP molecules for each glucose molecule that enters glycolysis. Glucose ATP Steps 1 – 3 Glucose is energized, using ATP. Step ENERGY INVESTMENT PHASE 1 ADP P Glucose 6-phosphate P Fructose 6-phosphate P Fructose 1,6-bisphosphate 2 ATP 3 ADP Step 4 A six-carbon intermediate splits into two three-carbon intermediates. P 4 P © 2015 Pearson Education, Inc. P Glyceraldehyde 3-phosphate (G3P) P Step 5 A redox reaction generates NADH. P NAD+ NAD+ 5 P NADH 5 P NADH + H+ ENERGY PAYOFF PHASE + H+ P P ADP Steps 6 – 9 ATP and pyruvate are produced. Glyceraldehyde 3-phosphate (G3P) P P 1,3-Bisphosphoglycerate ADP 6 6 ATP ATP P P 7 3-Phosphoglycerate 7 P P 2-Phosphoglycerate 8 H2O P P ADP Phosphoenolpyruvate (PEP) ADP 9 9 ATP 8 H2O ATP Pyruvate © 2015 Pearson Education, Inc. 6.8 Pyruvate is oxidized in preparation for the citric acid cycle • The pyruvate formed in glycolysis is transported from the cytosol into a mitochondrion where the citric acid cycle and oxidative phosphorylation will occur. • Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. © 2015 Pearson Education, Inc. • 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, and • coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA. • Then two molecules of acetyl CoA enter the citric acid cycle. NAD+ NADH + H+ 2 CoA Pyruvate Acetyl coenzyme A 1 CO2 © 2015 Pearson Education, Inc. 3 Coenzyme A 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 GermanBritish 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. Acetyl CoA CoA CoA Citric Acid Cycle 2 CO2 3 NAD+ FADH2 3 NADH FAD + 3 H+ ATP © 2015 Pearson Education, Inc. ADP + P 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 joined to a four-carbon compound, forming citrate, • citrate is degraded back to the four-carbon compound, • two CO2 are released, and • one ATP, three NADH, and one FADH2 are produced. 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. © 2015 Pearson Education, Inc. The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules • Thus, after glycolysis and the citric acid cycle, the cell has gained • 4 ATP, • 10 NADH, and • 2 FADH2. • To harvest the energy banked in NADH and FADH2, these molecules must shuttle their highenergy electrons to an electron transport chain. CoA Acetyl CoA CoA 2 carbons enter cycle Oxaloacetate Citric Acid Cycle Step 1 Acetyl CoA stokes the furnace. © 2015 Pearson Education, Inc. 1 CoA Acetyl CoA CoA 2 carbons enter cycle 1 Oxaloacetate Citrate NAD+ NADH + H+ 2 Citric Acid Cycle CO2 leaves cycle Alpha-ketoglutarate CO2 3 NAD+ Succinate ADP + P ATP Step 1 Acetyl CoA stokes the furnace. © 2015 Pearson Education, Inc. Steps 2 – 3 NADH, ATP, and CO2 are generated during redox reactions. NADH + H+ leaves cycle Figure 6.9b-3 CoA Acetyl CoA CoA 2 carbons enter cycle 1 Oxaloacetate Citrate NADH + H+ NAD+ 6 NAD+ NADH + H+ 2 Malate Citric Acid Cycle CO2 H2O leaves cycle 5 Alpha-ketoglutarate Fumarate FADH2 CO2 4 3 FAD leaves cycle NAD+ Succinate ADP + P NADH + H+ ATP Step 1 Acetyl CoA stokes the furnace. © 2015 Pearson Education, Inc. Steps 2 – 3 NADH, ATP, and CO2 are generated during redox reactions. Steps 4 – 6 Further redox reactions generate FADH2 and more NADH. Most ATP production occurs by oxidative phosphorylation • The final stage of cellular respiration is oxidative phosphorylation, which • involves electron transport and chemiosmosis and • requires an adequate supply of oxygen. • The arrangement of electron carriers built into a membrane makes it possible to • create an H+ concentration gradient across the membrane and then • use the energy of that gradient to drive ATP synthesis. 6.10 Most ATP production occurs by oxidative phosphorylation • Electrons from NADH and FADH2 travel down the electron transport chain to O2, the final electron acceptor. • Oxygen picks up H+, which forms water. • Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space. © 2015 Pearson Education, Inc. • In chemiosmosis, the H+ diffuses back across the inner membrane, through ATP synthase complexes, driving the synthesis of ATP. Figure 6.10a OUTER MITOCHONDRIAL MEMBRANE Intermembrane space H+ Protein complex of electron carriers + Mobile H electron H+ carriers III H+ H+ Inner mitochondrial membrane H+ H+ H+ Cyt c Q I H+ ATP synthase IV II Electron flow FADH2 NADH Mitochondrial matrix FAD 1 − O2 + 2 H+ 2 NAD+ H+ H2O Electron Transport Chain Oxidative Phosphorylation © 2015 Pearson Education, Inc. ADP + P ATP H+ Chemiosmosis Figure 6.10b INTERMEMBRANE SPACE H+ Rotor Internal rod Catalytic knob ADP + P MITOCHONDRIAL MATRIX © 2015 Pearson Education, Inc. ATP Scientists have discovered heat-producing, calorie-burning brown fat in adults • Mitochondria in brown fat can burn fuel and produce heat without making ATP. • Ion channels spanning the inner mitochondrial membrane • allow H + to flow freely across the membrane and • dissipate the H+ gradient that the electron transport chain produced, which does not allow ATP synthase to make ATP. • Scientific studies of humans indicate that • brown fat may be present in most people and • when activated by cold environments, the brown fat of lean individuals is more active. Review: Each molecule of glucose yields many molecules of ATP • Recall that the energy payoff of cellular respiration involves 1. 2. 3. 4. glycolysis, alteration of pyruvate, the citric acid cycle, and oxidative phosphorylation. © 2015 Pearson Education, Inc. Review: Each molecule of glucose yields many molecules of ATP • The total yield is about 32 ATP molecules per glucose molecule. • The number of ATP molecules cannot be stated exactly for several reasons. • The NADH produced in glycolysis passes its electrons across the mitochondrial membrane to either NAD+ or FAD. Because FADH2 adds its electrons farther along the electron transport chain, it contributes less to the H+ gradient and thus generates less ATP. • Some of the energy of the H+ gradient may be used for work other than ATP production, such as the active transport of pyruvate into the mitochondrion. © 2015 Pearson Education, Inc. CYTOSOL MITOCHONDRION 2 NADH Glycolysis 2 Pyruvate Glucose 6 NADH + 2 FADH2 2 NADH Pyruvate Oxidation 2 Acetyl CoA Oxidative Phosphorylation (electron transport and chemiosmosis) Citric Acid Cycle O2 Maximum per glucose: H2O +2 ATP by substrate-level phosphorylation © 2015 Pearson Education, Inc. CO2 +2 ATP by substrate-level phosphorylation + about 28 ATP About 32 ATP by oxidative phosphorylation FERMENTATION: ANAEROBIC HARVESTING OF ENERGY Fermentation enables cells to produce ATP without oxygen • Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation • uses glycolysis, • produces two ATP molecules per glucose, and • reduces NAD+ to NADH. • Fermentation also provides an anaerobic path for recycling NADH back to NAD+. © 2015 Pearson Education, Inc. Fermentation enables cells to produce ATP without oxygen • Your muscle cells and certain bacteria can regenerate NAD+ through lactic acid fermentation, in which • NADH is oxidized back to NAD+ and • pyruvate is reduced to lactate. 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 NADH 2 NAD+ 2 Lactate © 2015 Pearson Education, Inc. Fermentation enables cells to produce ATP without oxygen • Lactate is carried by the blood to the liver, where it is converted back to pyruvate and oxidized in the mitochondria of liver cells. • The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt. • Other types of microbial fermentation turn soybeans into soy sauce and cabbage into sauerkraut. • The baking and winemaking industries have used alcohol fermentation for thousands of years. • In this process, yeast (single-celled fungi) • oxidize NADH back to NAD+ and • convert pyruvate to CO2 and ethanol. 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 NADH 2 CO2 2 NAD+ 2 Ethanol © 2015 Pearson Education, Inc. • Obligate anaerobes • require anaerobic conditions, • are poisoned by oxygen, and • live in stagnant ponds and deep soils. • Facultative anaerobes • can make ATP by fermentation or oxidative phosphorylation and • include yeasts and many bacteria. Figure 6.13c-1 © 2015 Pearson Education, Inc. Glycolysis evolved early in the history of life on Earth • Glycolysis is the universal energy-harvesting process of life. • The role of glycolysis in fermentation and respiration dates back to life long before oxygen was present, when only prokaryotes inhabited the Earth, about 3.5 billion years ago. © 2015 Pearson Education, Inc. Glycolysis evolved early in the history of life on Earth • The ancient history of glycolysis is supported by its • occurrence in all the domains of life and • location within the cell, using pathways that do not involve any membrane-enclosed organelles of the eukaryotic cell. © 2015 Pearson Education, Inc. CONNECTIONS BETWEEN METABOLIC PATHWAYS 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. 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 energy-rich electrons and • yield more than twice as much ATP per gram as a gram of carbohydrate. • Proteins can also be used for fuel, although your body preferentially burns sugars and fats first. © 2015 Pearson Education, Inc. Food, such as peanuts Fats Carbohydrates Sugars Proteins Glycerol Fatty acids Amino acids Amino groups Glucose G3P Glycolysis Pyruvate Acetyl CoA ATP © 2015 Pearson Education, Inc. Citric Acid Cycle Oxidative Phosphorylation Organic molecules from food provide raw materials for biosynthesis • A cell must be able to make its own molecules to • build its structures and • perform its functions. • Food provides the raw materials your cells use for biosynthesis, the production of organic molecules, using energy-requiring metabolic pathways. Figure 6.16-1 ATP needed to drive biosynthesis Citric Acid Cycle ATP Acetyl CoA Glucose Synthesis Pyruvate G3P Glucose Amino groups Amino acids Proteins Fatty Glycerol acids Fats Cells, tissues, organisms © 2015 Pearson Education, Inc. Sugars Carbohydrates Organic molecules from food provide raw materials for biosynthesis • Metabolic pathways are often regulated by feedback inhibition in which an accumulation of product suppresses the process that produces the product. REVIEW You should now be able to 1. Compare the processes and locations of cellular respiration and photosynthesis. 2. Explain how breathing and cellular respiration are related. 3. Provide the overall chemical equation for cellular respiration. 4. Explain how the human body uses its daily supply of ATP. © 2015 Pearson Education, Inc. You should now be able to 5. Explain how the energy in a glucose molecule is released during cellular respiration. 6. Explain how redox reactions are used in cellular respiration. 7. Describe the general roles of dehydrogenase, NADH, and the electron transport chain in cellular respiration. 8. Compare the reactants, products, and energy yield of the three stages of cellular respiration. © 2015 Pearson Education, Inc. You should now be able to 9. Describe the special function of brown fat. 10. Compare the reactants, products, and energy yield of alcohol and lactic acid fermentation. 11. Distinguish between strict anaerobes and facultative anaerobes. 12. Explain how carbohydrates, fats, and proteins are used as fuel for cellular respiration. © 2015 Pearson Education, Inc. Figure 6.UN01 C6H12O6 6 O2 6 CO2 Glucose Oxygen Carbon dioxide © 2015 Pearson Education, Inc. 6 H2O Water ATP + Heat Figure 6.UN02 Electrons carried by NADH Glycolysis Glucose Pyruvate Pyruvate Oxidation Citric Acid Cycle FADH2 Oxidative Phosphorylation (Electron transport and chemiosmosis) CYTOSOL MITOCHONDRION ATP © 2015 Pearson Education, Inc. Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation Figure 6.UN03 Cellular respiration generates has three stages oxidizes uses ATP energy for produce some produces many glucose and organic fuels (a) C6H12O6 (b) (d) to pull electrons down (c) cellular work by a process called chemiosmosis (f) uses H+ diffuse through (e) ATP synthase uses pumps H+ to create H+ gradient © 2015 Pearson Education, Inc. (g) to