How Cells Harvest Chemical Energy 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. © 2012 Pearson Education, Inc. 2

Cellular Respiration: Aerobic Harvesting of Energy Figure 6.0_1 Chapter 6: Big Ideas Cellular Respiration: Aerobic Harvesting of Energy Stages of Cellular Respiration Figure 6.0_1 Chapter 6: Big Ideas Fermentation: Anaerobic Harvesting of Energy Connections Between Metabolic Pathways 3

CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY © 2012 Pearson Education, Inc. 4

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. 5

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. 6

Photosynthesis in chloroplasts Cellular respiration in mitochondria Figure 6.1_1 Sunlight energy ECOSYSTEM Photosynthesis in chloroplasts CO2 Glucose H2O O2 Figure 6.1_1 The connection between photosynthesis and cellular respiration Cellular respiration in mitochondria (for cellular work) ATP Heat energy 7

6.2 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. © 2012 Pearson Education, Inc. 8

Muscle cells carrying out Figure 6.2 Breathing O2 CO2 Lungs Figure 6.2 The connection between breathing and cellular respiration CO2 Bloodstream O2 Muscle cells carrying out Cellular Respiration Glucose  O2 CO2  H2O  ATP 9

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. 10

Glucose Oxygen Carbon dioxide Water  Heat C6H12O6 6 O2 6 CO2 6 H2O Figure 6.3 C6H12O6 6 O2 6 CO2 6 H2O ATP Figure 6.3 Summary equation for cellular respiration Glucose Oxygen Carbon dioxide Water  Heat 11

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. About 75% of these calories are used to maintain a healthy body. The remaining 25% is used to power physical activities. A kilocalorie (kcal) is the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC, the same as a food Calorie, and used to measure the nutritional values indicated on food labels. © 2012 Pearson Education, Inc. 12

kcal consumed per hour by a 67.5-kg (150-lb) person* Figure 6.4 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 Figure 6.4 Energy consumed by various activities Dancing (slow) 204 Driving a car 61 Sitting (writing) 28 *Not including kcal needed for body maintenance 13

6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen 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? 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. © 2012 Pearson Education, Inc. 14

6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen 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. 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 ATP. © 2012 Pearson Education, Inc. 15

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. 16

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. 17

Loss of hydrogen atoms (becomes oxidized) Figure 6.5A Loss of hydrogen atoms (becomes oxidized) C6H12O6 6 O2 6 CO2 6 H2O ATP Figure 6.5A Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration Glucose  Heat Gain of hydrogen atoms (becomes reduced) 18

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. 19

Becomes oxidized 2H Becomes reduced NAD 2H NADH H Figure 6.5B Becomes oxidized 2H Figure 6.5B A pair of redox reactions occuring simultaneously Becomes reduced NAD 2H NADH H (carries 2 electrons) 2 H 2 20

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. 21

Controlled release of energy for synthesis of ATP H Figure 6.5C NADH NAD ATP 2 Controlled release of energy for synthesis of ATP H Figure 6.5C In cellular respiration, electrons fall down an energy staircase and finally reduce O2. Electron transport chain 2 2 1 2 H O2 H2O 22

STAGES OF CELLULAR RESPIRATION © 2012 Pearson Education, Inc. 23

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. 24

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 three-carbon compound called pyruvate. 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. 25

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. 26

Electrons carried by NADH NADH FADH2 Figure 6.6 CYTOPLASM NADH Electrons carried by NADH NADH FADH2 Glycolysis Figure 6.6 An overview of cellular respiration Oxidative Phosphorylation (electron transport and chemiosmosis) Pyruvate Oxidation Glucose Pyruvate Citric Acid Cycle Mitochondrion ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation ATP ATP 27

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. 28

Glucose 2 ADP 2 NAD 2 P 2 NADH ATP 2 2 H 2 Pyruvate Figure 6.7A Figure 6.7A An overview of glycolysis 2 ATP 2 H 2 Pyruvate 29

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. 30

Enzyme Enzyme P ADP ATP P P Substrate Product Figure 6.7B Figure 6.7B Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP ATP P P Substrate Product 31

6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate The steps of glycolysis can be grouped into 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 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. 32

ENERGY INVESTMENT PHASE Figure 6.7Ca_s2 Glucose ENERGY INVESTMENT PHASE Steps – A fuel molecule is energized, using ATP. 1 3 ATP Step 1 ADP P Glucose 6-phosphate 2 P Fructose 6-phosphate Figure 6.7Ca_s2 Details of glycolysis: energy investment phase (step 2) ATP 3 ADP Step A six-carbon intermediate splits into two three-carbon intermediates. 4 Fructose 1,6-bisphosphate P P 4 Glyceraldehyde 3-phosphate (G3P) P P 33

Step A redox reaction generates NADH. 5 5 5 Figure 6.7Cb_s2 P P ENERGY PAYOFF PHASE Step A redox reaction generates NADH. 5 NAD 5 NAD 5 NADH P P NADH H H P P P P 1,3-Bisphospho- glycerate ADP ADP 6 6 Steps – ATP and pyruvate are produced. 6 9 ATP ATP P P 3-Phospho- glycerate 7 7 Figure 6.7Cb_s2 Details of glycolysis: energy payoff phase (step 2) P P 2-Phospho- glycerate 8 8 H2O H2O P P Phosphoenol- pyruvate (PEP) ADP ADP 9 9 ATP ATP Pyruvate 34

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. 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. 35

NAD NADH H CoA Pyruvate Acetyl coenzyme A CO2 Coenzyme A 2 1 3 Figure 6.8 NAD NADH H 2 CoA Figure 6.8 The link between glycolysis and the citric acid cycle Pyruvate 1 Acetyl coenzyme A 3 CO2 Coenzyme A 36

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. 37

Acetyl CoA Citric Acid Cycle CoA CoA 2 CO2 3 NAD FADH2 FAD 3 NADH Figure 6.9A Acetyl CoA CoA CoA 2 CO2 Citric Acid Cycle Figure 6.9A An overview of the citric acid cycle 3 NAD FADH2 FAD 3 NADH 3 H ATP ADP P 38

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 four-carbon 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. 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. 39

Step Acetyl CoA stokes the furnace. 1 Figure 6.9B_s3 Acetyl CoA CoA CoA 2 carbons enter cycle Oxaloacetate 1 Citrate NADH H NAD 5 NAD NADH H 2 Citric Acid Cycle Figure 6.9B_s3 A closer look at the citric acid cycle (step 3) Malate CO2 leaves cycle FADH2 Alpha-ketoglutarate 4 3 FAD CO2 leaves cycle NAD Succinate ADP P NADH H Step Acetyl CoA stokes the furnace. 1 Steps – NADH, ATP, and CO2 are generated during redox reactions. 2 3 Steps – Further redox reactions generate FADH2 and more NADH. 4 5 ATP 40

6.10 Most ATP production occurs by oxidative phosphorylation involves electron transport and chemiosmosis and requires an adequate supply of oxygen. 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. 41

Figure 6.10 H H H H H Intermem- brane space H Mobile electron carriers H Protein complex of electron carriers H H ATP synthase III IV I Inner mito- chondrial membrane II Figure 6.10 Oxidative phosphorylation: electron transport and chemiosmosis in a mitochondrion Electron flow FADH2 FAD 1 2 2 H NADH NAD O2 H2O Mito- chondrial matrix H ADP P ATP H Electron Transport Chain Chemiosmosis Oxidative Phosphorylation 42

6.11 CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide), inhibit ATP synthase (for example, the antibiotic oligomycin), or make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol). © 2012 Pearson Education, Inc. 43

Cyanide, carbon monoxide Figure 6.11 Rotenone Cyanide, carbon monoxide Oligomycin H H H ATP synthase H H H H DNP Figure 6.11 How some poisons affect the electron transport chain and chemiosmosis FADH2 FAD 2 1 O2 2 H NADH NAD H H2O ADP P ATP 44

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. 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 ATP, and ongoing oxidation of stored fats generates additional heat. © 2012 Pearson Education, Inc. 45

6.12 Review: Each molecule of glucose yields many molecules of ATP Recall that the energy payoff of cellular respiration involves glycolysis, alteration of pyruvate, the citric acid cycle, and oxidative phosphorylation. 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. 46

   CYTOPLASM Electron shuttles across membrane Mitochondrion 2 NADH Figure 6.12 CYTOPLASM Electron shuttles across membrane Mitochondrion 2 NADH 2 or NADH 2 FADH2 6 2 2 NADH NADH FADH2 Pyruvate Oxidation 2 Acetyl CoA Glycolysis Oxidative Phosphorylation (electron transport and chemiosmosis) 2 Pyruvate Citric Acid Cycle Glucose Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration Maximum per glucose: ATP  2 ATP  2 about  28 ATP About ATP 32 by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation 47

FERMENTATION: ANAEROBIC HARVESTING OF ENERGY © 2012 Pearson Education, Inc. 48

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. 49

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. 50

6.13 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. © 2012 Pearson Education, Inc. 51

Glucose 2 ADP 2 NAD 2 P Glycolysis 2 ATP 2 NADH 2 Pyruvate 2 NADH Figure 6.13A Glucose 2 ADP 2 NAD 2 P Glycolysis 2 ATP 2 NADH Figure 6.13A Lactic acid fermentation: NAD+ is regenerated as pyruvate is reduced to lactate. 2 Pyruvate 2 NADH 2 NAD 2 Lactate 52

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. 53

Glucose 2 ADP 2 NAD Glycolysis 2 P 2 ATP 2 NADH 2 Pyruvate 2 NADH Figure 6.13B Glucose 2 ADP 2 NAD 2 P Glycolysis 2 ATP 2 NADH Figure 6.13B Alchohol fermentation: NAD is regenerated as pyruvate is broken down to CO2 and ethanol. 2 Pyruvate 2 NADH 2 CO2 2 NAD 2 Ethanol 54

6.13 Fermentation enables cells to produce ATP 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 ATP by fermentation or oxidative phosphorylation. © 2012 Pearson Education, Inc. 55

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. © 2012 Pearson Education, Inc. 56

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 location within the cell, using pathways that do not involve any membrane-bounded organelles. © 2012 Pearson Education, Inc. 57

CONNECTIONS BETWEEN METABOLIC PATHWAYS © 2012 Pearson Education, Inc. 58

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. 59

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 energy-rich electrons and yield more than twice as much ATP per gram than a gram of carbohydrate or protein. © 2012 Pearson Education, Inc. 60

Pyruvate Oxidation Acetyl CoA Oxidative Phosphorylation Figure 6.15 Food, such as peanuts Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Figure 6.15 Pathways that break down various food molecules Amino groups Citric Acid Cycle Pyruvate Oxidation Acetyl CoA Oxidative Phosphorylation Glucose G3P Pyruvate Glycolysis ATP 61

6.16 Food molecules provide raw materials for biosynthesis Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules. Metabolic pathways are often regulated by feedback inhibition in which an accumulation of product suppresses the process that produces the product. © 2012 Pearson Education, Inc. 62

Pyruvate Oxidation Acetyl CoA Figure 6.16 ATP needed to drive biosynthesis ATP Citric Acid Cycle Pyruvate Oxidation Acetyl CoA Glucose Synthesis Pyruvate G3P Glucose Amino groups Amino acids Fatty acids Glycerol Sugars Figure 6.16 Biosynthesis of large organic molecules from intermediates of cellular respiration Proteins Fats Carbohydrates Cells, tissues, organisms 63