Chapter 6 How Cells Harvest Chemical Energy

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1 Introduction Chapter 6 How Cells Harvest Chemical Energy oweroint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Lecture by Edward J. Zalisko In eukaryotes, cellular respiration harvests energy from food, yields large amounts of AT, and Uses AT to drive cellular work. A similar process takes place in many prokaryotic organisms. Figure 6.0_1 Chapter 6: Big Ideas Figure 6.0_ Cellular Respiration: Aerobic Harvesting of Energy Stages of Cellular Respiration Fermentation: Anaerobic Harvesting of Energy Connections Between Metabolic athways CELLULAR RESIRATION: AEROBIC HARVESTING OF ENERGY 6.1 hotosynthesis 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.

2 6.1 hotosynthesis 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 AT. Cellular respiration takes place in the mitochondria of eukaryotic cells. Figure 6.1 Sunlight energy ECOSYSTEM CO hotosynthesis in chloroplasts Cellular respiration in mitochondria Glucose H O O (for cellular work) AT Heat energy 6. Breathing supplies O for use in cellular respiration and removes CO 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 CO. Cellular respiration is the aerobic (oxygen requiring) harvesting of energy from food molecules by cells. Figure 6. Breathing O CO Lungs CO Bloodstream O Muscle cells carrying out Cellular Respiration Glucose O CO H O AT 6.3 Cellular respiration banks energy in AT molecules Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form AT. Figure 6.3 Cellular respiration C 6 H 1 O 6 O 6 CO 6 H O AT 6 produces up to 3 AT 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. Glucose Oxygen Carbon Water dioxide Heat

3 6.4 CONNECTION: The human body uses energy from AT for all its activities The average adult human needs about,00 kcal of energy per day. About 75% of these calories are used to maintain a healthy body. The remaining 5% is used to power physical activities. 6.4 CONNECTION: The human body uses energy from AT for all its activities A kilocalorie (kcal) is the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1 o C, the same as a food Calorie, and used to measure the nutritional values indicated on food labels. Figure 6.4 Activity Running (8 9 mph) Dancing (fast) Bicycling (10 mph) Swimming ( mph) Walking (4 mph) Walking (3 mph) Dancing (slow) Driving a car Sitting (writing) *Not including kcal needed for body maintenance kcal consumed per hour by a 67.5-kg (150-lb) person* Cells tap energy from electrons falling 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? 6.5 Cells tap energy from electrons falling When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen. Oxygen has a strong tendency to attract electrons. An electron loses potential energy when it falls to oxygen. 6.5 Cells tap energy from electrons falling Energy can be released from glucose by simply burning it. The energy is dissipated as heat and light and is not available to living organisms.

4 6.5 Cells tap energy from electrons falling On the other hand, cellular respiration is the controlled breakdown of organic molecules. Energy is gradually released in small amounts, captured by a biological system, and stored in AT. 6.5 Cells tap energy from electrons falling 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. 6.5 Cells tap energy from electrons falling A cellular respiration equation is helpful to show the changes in hydrogen atom distribution. Glucose loses its hydrogen atoms and becomes oxidized to CO. Oxygen gains hydrogen atoms and becomes reduced to H O. Figure 6.5A C 6 H 1 O 6 6 O 6 CO 6 H O AT Glucose Loss of hydrogen atoms (becomes oxidized) Gain of hydrogen atoms (becomes reduced) Heat 6.5 Cells tap energy from electrons falling Figure 6.5B Enzymes are necessary to oxidize glucose and other foods. Becomes oxidized H NAD + is an important enzyme in oxidizing glucose, accepts electrons, and becomes reduced to NADH. NAD Becomes reduced H NADH (carries electrons)

5 6.5 Cells tap energy from electrons falling Figure 6.5C NADH 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, AT is generated. NAD Electron transport chain AT Controlled release of energy for synthesis of AT 1 O H O STAGES OF CELLULAR RESIRATION 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 yruvate oxidation and citric acid cycle Stage 3 Oxidative 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. 6.6 Overview: Cellular respiration occurs in three main stages Stage : The citric acid cycle takes place in mitochondria, oxidizes pyruvate to a two-carbon compound, and supplies the third stage with electrons.

6 6.6 Overview: Cellular respiration occurs in three main stages Figure 6.6 Stage 3: Oxidative involves electrons carried by NADH and FADH, shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane, involves chemiosmosis, and generates AT through oxidative associated with chemiosmosis. CYTOLASM NADH Electrons carried by NADH Glycolysis yruvate Glucose yruvate Oxidation Mitochondrion NADH FADH Oxidative Citric Acid hosphorylation Cycle (electron transport and chemiosmosis) AT Substrate-level AT Substrate-level AT Oxidative 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Figure 6.7A Glucose 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 AT is produced. AD AT NAD NADH yruvate 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Figure 6.7B AT is formed in glycolysis by substrate-level during which an enzyme transfers a phosphate group from a substrate molecule to AD and Enzyme AD Enzyme AT is formed. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates. Substrate roduct AT

7 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate The steps of glycolysis can be grouped into two main phases. Figure 6.7Ca_s Steps 1 3 A fuel AT molecule is energized, using AT. AD Step 1 Glucose ENERGY INVESTMENT HASE Glucose 6-phosphate In steps 1 4, the energy investment phase, energy is consumed as two AT molecules are used to energize a glucose molecule, AT Fructose 6-phosphate 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 Step 4 A six-carbon intermediate splits into two three-carbon intermediates. AD 3 4 Fructose 1,6-bisphosphate AT molecules are generated. Glyceraldehyde 3-phosphate (G3) Figure 6.7Cb_s Step 5 NAD 5 NAD 5 A redox reaction NADH generates NADH. NADH Steps 6 9 AT and pyruvate are produced. AT AD AD 6 6 AT 7 7 ENERGY AYOFF HASE 1,3-Bisphosphoglycerate 3-hosphoglycerate 6.8 yruvate 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 will occur. H O 8 8 H O -hosphoglycerate AD AD 9 9 hosphoenolpyruvate (E) AT AT yruvate 6.8 yruvate is oxidized prior to the citric acid cycle Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. yruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which a carboxyl group is removed and given off as CO, 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. Figure 6.8 yruvate NAD NADH CoA 1 Acetyl coenzyme A 3 CO Coenzyme A

8 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH molecules Figure 6.9A Acetyl CoA CoA CoA 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), Citric Acid Cycle CO completes the oxidation of organic molecules, and generates many NADH and FADH molecules. FADH FAD 3 NAD 3 NADH 3 AT AD 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 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 CO are released, and 1 AT, 3 NADH, and 1 FADH are produced. 6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 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 AT, 6 NADH, and FADH. Figure 6.9B_s3 Acetyl CoA Oxaloacetate CoA CoA carbons enter cycle Most AT production occurs by oxidative Oxidative NADH NAD 5 Citric Acid Cycle Citrate NAD NADH involves electron transport and chemiosmosis and requires an adequate supply of oxygen. Malate CO leaves cycle FADH FAD 4 3 Alpha-ketoglutarate CO leaves cycle Succinate AD NAD NADH Step 1 Steps 3 AT Steps 4 5 Acetyl CoA stokes NADH, AT, and CO Further redox reactions generate the furnace. are generated during redox reactions. FADH and more NADH.

9 6.10 Most AT production occurs by oxidative Figure 6.10 Electrons from NADH and FADH travel down the electron transport chain to O. 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 AT synthase complexes, driving the synthesis of AT. Intermembrane space Inner mitochondrial membrane Mitochondrial matrix rotein complex of electron carriers I Mobile electron carriers FADH FAD Electron flow NADH NAD H II III Electron Transport Chain 1 O IV Oxidative hosphorylation H O AT synthase AD AT Chemiosmosis 6.11 CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects Figure 6.11 Three categories of cellular poisons obstruct the process of oxidative. These poisons Rotenone Cyanide, carbon monoxide Oligomycin AT synthase 1. block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide), DN. inhibit AT synthase (for example, the antibiotic oligomycin), or 3. make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol). FADH NADH NAD FAD 1 O H O AD AT 6.11 CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects Brown fat is a special type of tissue associated with the generation of heat and more abundant in hibernating mammals and newborn infants CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects In brown fat, 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 AT, and ongoing oxidation of stored fats generates additional heat.

10 6.1 Review: Each molecule of glucose yields many molecules of AT Recall that the energy payoff of cellular respiration involves 1. glycolysis,. alteration of pyruvate, 3. the citric acid cycle, and 4. oxidative. 6.1 Review: Each molecule of glucose yields many molecules of AT The total yield is about 3 AT molecules per glucose molecule. This is about 34% of the potential energy of a glucose molecule. In addition, water and CO are produced. Figure 6.1 CYTOLASM Electron shuttles across membrane NADH NADH or FADH NADH 6 NADH FADH Mitochondrion FERMENTATION: ANAEROBIC HARVESTING OF ENERGY Glycolysis Glucose yruvate yruvate Oxidation Acetyl CoA Citric Acid Cycle Oxidative hosphorylation (electron transport and chemiosmosis) Maximum per glucose: AT by substrate-level AT by substrate-level about 8 AT by oxidative About 3 AT 6.13 Fermentation enables cells to produce AT without oxygen Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation takes advantage of glycolysis, produces two AT molecules per glucose, and reduces NAD + to NADH Fermentation enables cells to produce AT 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. The trick of fermentation is to provide an anaerobic path for recycling NADH back to NAD +. Animation: Fermentation Overview

11 6.13 Fermentation enables cells to produce AT 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. Figure 6.13A AD AT Glucose Glycolysis NAD NADH yruvate NADH NAD Lactate 6.13 Fermentation enables cells to produce AT 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 CO and ethanol. Figure 6.13B AD AT Glucose Glycolysis NAD NADH yruvate NADH CO NAD Ethanol 6.13 Fermentation enables cells to produce AT without oxygen Obligate anaerobes are poisoned by oxygen, requiring anaerobic conditions, and live in stagnant ponds and deep soils. Facultative anaerobes include yeasts and many bacteria, and can make AT by fermentation or oxidative. Figure 6.13C_1

12 Figure 6.13C_ 6.14 EVOLUTION CONNECTION: 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 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 location within the cell, using pathways that do not involve any membrane-bounded organelles. CONNECTIONS BETWEEN METABOLIC ATHWAYS 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, AT 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 energyrich electrons and yield more than twice as much AT per gram than a gram of carbohydrate or protein.

13 Figure 6.15 Food, such as peanuts 6.16 Food molecules provide raw materials for biosynthesis Carbohydrates Fats roteins Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules. Sugars Glycerol Fatty acids Amino acids Amino groups Glucose G3 yruvate Glycolysis yruvate Oxidation Acetyl CoA Citric Acid Cycle Oxidative hosphorylation AT Figure 6.16 AT needed to drive biosynthesis AT 6.16 Food molecules provide raw materials for biosynthesis Amino groups Citric Acid Cycle yruvate Oxidation Acetyl CoA Glucose Synthesis yruvate G3 Glucose Amino acids Fatty acids Glycerol Sugars Metabolic pathways are often regulated by feedback inhibition in which an accumulation of product suppresses the process that produces the product. roteins Fats Carbohydrates Cells, tissues, organisms You should now be able to You should now be able to 1. Compare the processes and locations of cellular respiration and photosynthesis.. Explain how breathing and cellular respiration are related. 3. rovide the overall chemical equation for cellular respiration. 4. Explain how the human body uses its daily supply of AT. 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.

14 You should now be able to 9. Explain how rotenone, cyanide, carbon monoxide, oligomycin, and uncouplers interrupt critical events in cellular respiration. 10. Compare the reactants, products, and energy yield of alcohol and lactic acid fermentation. 11. Distinguish between strict anaerobes and facultative anaerobes. 1. Explain how carbohydrates, fats, and proteins are used as fuel for cellular respiration. Figure 6.UN0 energy for cellular work chemiosmosis Cellular respiration generates has three stages oxidizes produce some produces many (a) (b) (c) by a process called uses diffuse through AT synthase gradient (d) uses to pull electrons down uses (e) pumps to create glucose and organic fuels (f) (g) to