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Chapter 6 How Cells Harvest Chemical Energy 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. PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Lecture by Edward J. Zalisko © 2012 Pearson Education, Inc. © 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 CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY Connections Between Metabolic Pathways © 2012 Pearson Education, Inc. 6.1 Photosynthesis and cellular respiration provide energy for life 6.1 Photosynthesis and cellular respiration provide energy for life Life requires energy. In cellular respiration In almost all ecosystems, energy ultimately comes from the sun. – glucose is broken down to carbon dioxide and water and In photosynthesis, – the cell captures some of the released energy to make ATP. – some of the energy in sunlight is captured by chloroplasts, Cellular respiration takes place in the mitochondria of eukaryotic cells. – atoms of carbon dioxide and water are rearranged, and – glucose and oxygen are produced. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 1 Figure 6.1 Figure 6.1_1 Sunlight energy Sunlight energy ECOSYSTEM ECOSYSTEM Photosynthesis in chloroplasts CO2 Photosynthesis in chloroplasts Glucose CO2 O2 H 2O H 2O Cellular respiration in mitochondria Glucose O2 Cellular respiration in mitochondria (for cellular ATP work) (for cellular ATP work) Heat energy 6.2 Breathing supplies O2 for use in cellular respiration and removes CO2 Heat energy Figure 6.2 O2 Breathing Respiration, as it relates to breathing, and cellular respiration are not the same. CO2 Lungs – Respiration, in the breathing sense, refers to an exchange of gases. Usually an organism brings in oxygen from the environment and releases waste CO2. CO2 O2 Bloodstream – Cellular respiration is the aerobic (oxygen requiring) harvesting of energy from food molecules by cells. Muscle cells carrying out Cellular Respiration Glucose + O2 CO2 + H2O + ATP © 2012 Pearson Education, Inc. 6.3 Cellular respiration banks energy in ATP molecules Figure 6.3 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 C6H12O6 6 Glucose Oxygen O2 6 CO2 Carbon dioxide 6 H 2O ATP Water + Heat – 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. 2 6.4 CONNECTION: The human body uses energy from ATP for all its activities 6.4 CONNECTION: The human body uses energy from ATP for all its activities The average adult human needs about 2,200 kcal of energy per day. A kilocalorie (kcal) is – About 75% of these calories are used to maintain a healthy body. – The remaining 25% is used to power physical activities. © 2012 Pearson Education, Inc. 979 Dancing (fast) 510 Bicycling (10 mph) 490 Swimming (2 mph) 408 Walking (4 mph) 341 Walking (3 mph) 245 Dancing (slow) Sitting (writing) – used to measure the nutritional values indicated on food labels. 6.5 Cells tap energy from electrons falling from organic fuels to oxygen kcal consumed per hour by a 67.5-kg (150-lb) person* Running (8–9 mph) Driving a car – the same as a food Calorie, and © 2012 Pearson Education, Inc. Figure 6.4 Activity – the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC, 204 The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules. An important question is how do cells extract this energy? 61 28 *Not including kcal needed for body maintenance © 2012 Pearson Education, Inc. 6.5 Cells tap energy from electrons falling from organic fuels to oxygen 6.5 Cells tap energy from electrons falling from organic fuels to oxygen When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen. Energy can be released from glucose by simply burning it. – Oxygen has a strong tendency to attract electrons. The energy is dissipated as heat and light and is not available to living organisms. – An electron loses potential energy when it “falls” to oxygen. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 3 6.5 Cells tap energy from electrons falling from organic fuels to oxygen 6.5 Cells tap energy from electrons falling from organic fuels to oxygen On the other hand, cellular respiration is the controlled breakdown of organic molecules. The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction, Energy is – gradually released in small amounts, – captured by a biological system, and – stored in ATP. – 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 © 2012 Pearson Education, Inc. Figure 6.5A A cellular respiration equation is helpful to show the changes in hydrogen atom distribution. Loss of hydrogen atoms (becomes oxidized) Glucose – loses its hydrogen atoms and C6H12O6 – becomes oxidized to CO2. Glucose 6 O2 6 CO2 6 H 2O + Heat Gain of hydrogen atoms (becomes reduced) Oxygen ATP – gains hydrogen atoms and – becomes reduced to H2O. © 2012 Pearson Education, Inc. 6.5 Cells tap energy from electrons falling from organic fuels to oxygen Figure 6.5B Enzymes are necessary to oxidize glucose and other foods. Becomes oxidized 2H NAD+ – is an important enzyme in oxidizing glucose, – accepts electrons, and – becomes reduced to NADH. 2H NAD+ 2 H+ Becomes reduced 2 NADH H+ (carries 2 electrons) © 2012 Pearson Education, Inc. 4 6.5 Cells tap energy from electrons falling from organic fuels to oxygen There are other electron carrier molecules that function like NAD+. Figure 6.5C NADH ATP NAD+ 2 Controlled release of energy for synthesis of ATP H+ El ec – They form a staircase where the electrons pass from one to the next down the staircase. tr o n tr an – These electron carriers collectively are called the electron transport chain. sp or tc ha in – As electrons are transported down the chain, ATP is generated. 2 1 O 2 2 2 H+ H 2O © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages STAGES OF CELLULAR RESPIRATION 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. © 2012 Pearson Education, Inc. 6.6 Overview: Cellular respiration occurs in three main stages 6.6 Overview: Cellular respiration occurs in three main stages Stage 1: Glycolysis Stage 2: The citric acid cycle – occurs in the cytoplasm, – takes place in mitochondria, – begins cellular respiration, and – oxidizes pyruvate to a two-carbon compound, and – breaks down glucose into two molecules of a threecarbon compound called pyruvate. – supplies the third stage with electrons. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 5 6.6 Overview: Cellular respiration occurs in three main stages Figure 6.6 Stage 3: Oxidative phosphorylation – involves electrons carried by NADH and FADH2, CYTOPLASM NADH – shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane, Electrons carried by NADH Glycolysis – involves chemiosmosis, and Glucose Pyruvate Pyruvate Oxidation NADH Citric Acid Cycle FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) – generates ATP through oxidative phosphorylation associated with chemiosmosis. Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation © 2012 Pearson Education, Inc. Figure 6.6_1 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate CYTOPLASM NADH Electrons carried by NADH Glycolysis Pyruvate Oxidation Pyruvate Glucose NADH In glycolysis, FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) Citric Acid Cycle – 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 Mitochondrion – a net of two molecules of ATP is produced. ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation © 2012 Pearson Education, Inc. Figure 6.7A 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glucose 2 ADP 2 NAD+ 2 P ATP is formed in glycolysis by substrate-level phosphorylation during which – an enzyme transfers a phosphate group from a substrate molecule to ADP and 2 NADH 2 ATP 2 H+ – ATP is formed. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates. 2 Pyruvate © 2012 Pearson Education, Inc. 6 Figure 6.7B 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Enzyme P The steps of glycolysis can be grouped into two main phases. Enzyme – In steps 1–4, the energy investment phase, ADP – energy is consumed as two ATP molecules are used to energize a glucose molecule, ATP P P Substrate Product – which is then split into two small sugars that are now primed to release energy. – In steps 5–9, the energy payoff, – two NADH molecules are produced for each initial glucose molecule and – ATP molecules are generated. © 2012 Pearson Education, Inc. Figure 6.7Ca_s1 ENERGY INVESTMENT PHASE Glucose Steps 1 – 3 A fuel ATP molecule is energized, using ATP. ADP Step 1 Figure 6.7Ca_s2 Step 1 Glucose 6-phosphate P 2 P Glucose 6-phosphate P Fructose 6-phosphate P Fructose 1,6-bisphosphate 2 P Fructose 6-phosphate ATP ATP 3 3 ADP ADP P Fructose 1,6-bisphosphate P Step 4 A six-carbon intermediate splits into two three-carbon intermediates. P 4 P P Figure 6.7Cb_s1 Step 5 A redox reaction generates NADH. ENERGY INVESTMENT PHASE Glucose Steps 1 – 3 A fuel ATP molecule is energized, using ATP. ADP NAD+ NAD+ 5 P NADH H+ P ENERGY PAYOFF PHASE P P P 5 P NADH H+ P P Figure 6.7Cb_s2 Step 5 A redox reaction generates NADH. NAD+ 5 P NADH H+ 5 P NADH H+ P P ADP Steps 6 – 9 ATP and pyruvate are produced. ENERGY PAYOFF PHASE P P NAD+ 1,3-Bisphosphoglycerate Glyceraldehyde 3-phosphate (G3P) P 1,3-Bisphosphoglycerate P 3-Phosphoglycerate ADP 6 6 ATP ATP P 7 7 P P 8 H 2O 8 H 2O P 2-Phosphoglycerate P ADP Phosphoenolpyruvate (PEP) ADP 9 9 ATP P ATP Pyruvate 7 6.8 Pyruvate is oxidized prior to the citric acid cycle 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 Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. – the citric acid cycle and Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which – oxidative phosphorylation will occur. – 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. © 2012 Pearson Education, Inc. Figure 6.8 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules NAD+ NADH The citric acid cycle H+ 2 CoA Pyruvate Acetyl coenzyme A 1 CO2 3 – 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. Coenzyme A © 2012 Pearson Education, Inc. Figure 6.9A 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules Acetyl CoA CoA CoA During the citric acid cycle 2 CO2 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, 3 NAD+ FADH2 3 NADH FAD – two CO2 are released, and – 1 ATP, 3 NADH, and 1 FADH2 are produced. 3 H+ ATP ADP P © 2012 Pearson Education, Inc. 8 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules Figure 6.9B_s1 Acetyl CoA CoA CoA 2 carbons enter cycle Oxaloacetate 1 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 Citric Acid Cycle – 2 ATP, – 6 NADH, and – 2 FADH2. Step 1 Acetyl CoA stokes the furnace. © 2012 Pearson Education, Inc. Figure 6.9B_s2 Acetyl CoA Figure 6.9B_s3 CoA Acetyl CoA CoA CoA 2 carbons enter cycle Oxaloacetate CoA 2 carbons enter cycle Oxaloacetate 1 1 Citrate Citrate NADH NAD+ NADH 2 Citric Acid Cycle H+ NAD+ 5 NAD+ Alpha-ketoglutarate CO2 leaves cycle NADH 2 Citric Acid Cycle CO2 leaves cycle 3 H+ CO2 leaves cycle Malate FADH2 H+ Alpha-ketoglutarate 4 3 FAD NAD+ CO2 leaves cycle NAD+ Succinate ADP Step 1 Acetyl CoA stokes the furnace. P NADH ADP H+ Steps 2 – 3 ATP NADH, ATP, and CO2 are generated during redox reactions. Step 1 Acetyl CoA stokes the furnace. P Steps 2 – 3 ATP NADH, ATP, and CO2 are generated during redox reactions. NADH H+ Steps 4 – 5 Further redox reactions generate FADH2 and more NADH. 6.10 Most ATP production occurs by oxidative phosphorylation 6.10 Most ATP production occurs by oxidative phosphorylation Oxidative phosphorylation – involves electron transport and chemiosmosis and Electrons from NADH and FADH2 travel down the electron transport chain to O2. – requires an adequate supply of oxygen. 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. © 2012 Pearson Education, Inc. 9 Figure 6.10 Figure 6.10_1 H+ H+ Intermembrane space H+ H+ H+ Mobile electron carriers Protein complex of electron carriers H+ H+ III H+ ATP synthase Mitochondrial matrix NAD+ H+ H+ H+ H+ III H+ ATP synthase IV I II II FADH2 Electron flow NADH H+ H+ H+ Mobile electron carriers Protein complex of electron carriers IV I Inner mitochondrial membrane H+ H+ FAD 2 H+ 1 2 O2 FADH2 Electron flow NADH H 2O H+ ADP P FAD 2 H+ NAD+ 1 O 2 2 H 2O H+ ATP ADP H+ Electron Transport Chain P ATP H+ Chemiosmosis Chemiosmosis Electron Transport Chain Oxidative Phosphorylation Oxidative Phosphorylation 6.11 CONNECTION: Interrupting cellular respiration Figure 6.11 can have both harmful and beneficial effects Rotenone Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons Cyanide, carbon monoxide H+ H+ H+ ATP synthase H+ H+ H+ 1. block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide), DNP 2. inhibit ATP synthase (for example, the antibiotic oligomycin), or FADH2 NAD+ NADH 3. make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol). Oligomycin H+ H+ FAD 1 O 2 2 2 H+ H 2O ADP P ATP © 2012 Pearson Education, Inc. 6.11 CONNECTION: Interrupting cellular respiration 6.11 CONNECTION: Interrupting cellular respiration Brown fat is In brown fat, can have both harmful and beneficial effects – a special type of tissue associated with the generation of heat and – more abundant in hibernating mammals and newborn infants. can have both harmful and beneficial effects – the cells are packed full of mitochondria, – the inner mitochondrial membrane contains an uncoupling protein, which allows H+ to flow back down its concentration gradient without generating ATP, and – ongoing oxidation of stored fats generates additional heat. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 10 6.12 Review: Each molecule of glucose yields many molecules of ATP 6.12 Review: Each molecule of glucose yields many molecules of ATP Recall that the energy payoff of cellular respiration involves The total yield is about 32 ATP molecules per glucose molecule. 1. glycolysis, 2. alteration of pyruvate, This is about 34% of the potential energy of a glucose molecule. 3. the citric acid cycle, and In addition, water and CO2 are produced. 4. oxidative phosphorylation. © 2012 Pearson Education, Inc. © 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 FERMENTATION: ANAEROBIC HARVESTING OF ENERGY Oxidative Phosphorylation (electron transport and chemiosmosis) Maximum per glucose: +2 +2 ATP ATP by substrate-level phosphorylation by substrate-level phosphorylation + about 28 ATP About 32 ATP by oxidative phosphorylation © 2012 Pearson Education, Inc. 6.13 Fermentation enables cells to produce ATP without oxygen 6.13 Fermentation enables cells to produce ATP without oxygen Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which – takes advantage of glycolysis, – NADH is oxidized to NAD+ and – produces two ATP molecules per glucose, and – pyruvate is reduced to lactate. – reduces NAD+ to NADH. The trick of fermentation is to provide an anaerobic path for recycling NADH back to NAD+. Animation: Fermentation Overview © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 11 Figure 6.13A Glucose 2 ADP 2 P Lactate is carried by the blood to the liver, where it is converted back to pyruvate and oxidized in the mitochondria of liver cells. 2 NAD+ Glycolysis 6.13 Fermentation enables cells to produce ATP without oxygen 2 ATP 2 NADH The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt. 2 Pyruvate Other types of microbial fermentation turn 2 NADH – soybeans into soy sauce and – cabbage into sauerkraut. 2 NAD+ 2 Lactate © 2012 Pearson Education, Inc. Figure 6.13B Glucose 2 ADP 2 P The baking and winemaking industries have used alcohol fermentation for thousands of years. 2 ATP 2 NAD+ Glycolysis 6.13 Fermentation enables cells to produce ATP without oxygen 2 NADH In this process yeasts (single-celled fungi) – oxidize NADH back to NAD+ and – convert pyruvate to CO2 and ethanol. 2 Pyruvate 2 NADH 2 CO2 2 NAD+ 2 Ethanol © 2012 Pearson Education, Inc. 6.13 Fermentation enables cells to produce ATP without oxygen 6.14 EVOLUTION CONNECTION: Glycolysis evolved early in the history of life on Earth Obligate anaerobes Glycolysis is the universal energy-harvesting process of life. – are poisoned by oxygen, requiring anaerobic conditions, and – live in stagnant ponds and deep soils. Facultative anaerobes The role of glycolysis in fermentation and respiration dates back to – life long before oxygen was present, – include yeasts and many bacteria, and – when only prokaryotes inhabited the Earth, – can make ATP by fermentation or oxidative phosphorylation. – about 3.5 billion years ago. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 12 6.14 EVOLUTION CONNECTION: 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 CONNECTIONS BETWEEN METABOLIC PATHWAYS – location within the cell, using pathways that do not involve any membrane-bounded organelles. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 6.15 Cells use many kinds of organic molecules as fuel for cellular respiration 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 Fats make excellent cellular fuel because they – carbohydrates, – 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. – fats, and – proteins. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 6.15 6.16 Food molecules provide raw materials for biosynthesis Food, such as peanuts Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules. Carbohydrates Sugars Fats Proteins Glycerol Fatty acids Amino acids Amino groups Glucose G3P Pyruvate Glycolysis Pyruvate Oxidation Acetyl CoA Citric Acid Cycle Oxidative Phosphorylation ATP © 2012 Pearson Education, Inc. 13 Figure 6.16 ATP needed to drive biosynthesis Citric Acid Cycle 6.16 Food molecules provide raw materials for biosynthesis ATP Pyruvate Oxidation Acetyl CoA Glucose Synthesis Pyruvate G3P Glucose Amino groups Amino acids Proteins Fatty acids Glycerol Fats Sugars Metabolic pathways are often regulated by feedback inhibition in which an accumulation of product suppresses the process that produces the product. Carbohydrates Cells, tissues, organisms © 2012 Pearson Education, Inc. 14