Chapter 6 How Cells Harvest Chemical Energy Power

Chapter 6 How Cells Harvest Chemical Energy Power. Point Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN © 2015 Pearson Education, Inc. Lecture by Edward J. Zalisko

CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY © 2015 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 • sugar and oxygen are produced. © 2015 Pearson Education, Inc.

6. 1 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 CO 2 + H 2 O Photosynthesis in chloroplasts Organic Cellular respiration in mitochondria ATP Heat energy © 2015 Pearson Education, Inc. molecules + O 2 ATP powers most cellular work

6. 2 Breathing supplies O 2 for use in cellular respiration and removes CO 2 • 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 2. • Cellular respiration is the aerobic (oxygen-requiring) harvesting of energy from food molecules by cells. © 2015 Pearson Education, Inc.

Figure 6. 2 -0 O 2 Breathing CO 2 Lungs O 2 Transported in bloodstream CO 2 Muscle cells carrying out Cellular Respiration Glucose + O 2 ➞ CO 2 + H 2 O + ATP © 2015 Pearson Education, Inc.

6. 3 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.

6. 3 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.

Figure 6. 3 C 6 H 12 O 6 6 O 2 6 CO 2 Glucose Oxygen Carbon dioxide © 2015 Pearson Education, Inc. 6 H 2 O Water ATP Heat

6. 4 CONNECTION: 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.

6. 4 CONNECTION: The human body uses energy from ATP 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 C, • the same as a food Calorie, and • used to measure the nutritional values indicated on food labels. © 2015 Pearson Education, Inc.

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 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.

Figure 6. 4 -1 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.

6. 5 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. © 2015 Pearson Education, Inc.

6. 5 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. © 2015 Pearson Education, Inc.

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.

6. 5 Cells capture energy from electrons “falling” from organic fuels to oxygen • 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. © 2015 Pearson Education, Inc.

6. 5 Cells capture 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 • a molecule is reduced when it gains one or more electrons. © 2015 Pearson Education, Inc.

6. 5 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 CO 2. • Oxygen gains hydrogen atoms and becomes reduced to H 2 O. © 2015 Pearson Education, Inc.

Figure 6. 5 a Loss of hydrogen atoms (becomes oxidized) C 6 H 12 O 6 + 6 O 2 (Glucose) 6 CO 2 + 6 H 2 O + ATP + Heat Gain of hydrogen atoms (becomes reduced) © 2015 Pearson Education, Inc.

6. 5 Cells capture energy from electrons “falling” from organic fuels to oxygen • An important player in the process of oxidizing glucose is a coenzyme called NAD+, which • accepts electrons and • becomes reduced to NADH. © 2015 Pearson Education, Inc.

Figure 6. 5 b Becomes oxidized NAD+ + 2 H Becomes reduced 2 H+ + 2 © 2015 Pearson Education, Inc. +2 H NADH (carries) 2 electrons) H+

6. 5 Cells capture energy from electrons “falling” from organic fuels to oxygen • 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 O 2), which • accepts two electrons, • picks up two H+, and • becomes reduced to water. © 2015 Pearson Education, Inc.

Figure 6. 5 c NAD+ NADH 2 Energy released and available for making ATP 2 1 − 2 O 2 2 H+ © 2015 Pearson Education, Inc. H 2 O

STAGES OF CELLULAR RESPIRATION © 2015 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 the citric acid cycle • Stage 3: Oxidative phosphorylation © 2015 Pearson Education, Inc.

6. 6 Overview: 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. © 2015 Pearson Education, Inc.

6. 6 Overview: 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. © 2015 Pearson Education, Inc.

6. 6 Overview: Cellular respiration occurs in three main stages • Stage 3: Oxidative phosphorylation • NADH and a related electron carrier, FADH 2, 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 FADH 2 to oxygen to phosphorylate ADP. © 2015 Pearson Education, Inc.

6. 6 Overview: Cellular respiration occurs in three main stages • 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. © 2015 Pearson Education, Inc.

Figure 6. 6 -1 Electrons carried by Glycolysis Glucose Pyruvate CYTOSOL ATP © 2015 Pearson Education, Inc. Pyruvate Oxidation Citric Acid Cycle NADH FADH 2 Oxidative Phosphorylation (Electron transport and chemiosmosis) MITOCHONDRION Substrate-level phosphorylation Substrate-level ATP 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 • there is a net gain of two molecules of ATP. © 2015 Pearson Education, Inc.

Figure 6. 7 a Glucose 2 ADP 2 NAD+ +2 P 2 NADH 2 ATP 2 Pyruvate © 2015 Pearson Education, Inc. +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 known as intermediates. © 2015 Pearson Education, Inc.

Figure 6. 7 b Enzyme P Enzyme ADP ATP P Substrate © 2015 Pearson Education, Inc. P Product

6. 7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • 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. © 2015 Pearson Education, Inc.

Figure 6. 7 c-1 -2 Glucose ATP Steps 1 – 3 Glucose is energized, using ATP. Step ENERGY INVESTMENT PHASE 1 ADP P Glucose 6 -phosphate P Fructose 1, 6 -bisphosphate 2 ATP 3 Step 4 A six-carbon intermediate splits into two three-carbon intermediates. ADP P 4 P © 2015 Pearson Education, Inc. P Glyceraldehyde 3 -phosphate (G 3 P)

Figure 6. 7 c-2 -2 P Step 5 A redox reaction generates NADH. P NAD+ 5 P NADH + H+ P 5 P NADH ENERGY PAYOFF PHASE + H+ P ADP Steps 6 – 9 ATP and pyruvate are produced. Glyceraldehyde 3 -phosphate (G 3 P) P P 1, 3 -Bisphoglycerate P 3 -Phosphoglycerate ADP 6 6 ATP P 7 7 P P 2 -Phosphoglycerate 8 H 2 O P P ADP Phosphoenolpyruvate (PEP) ADP 9 9 ATP 8 H 2 O 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.

6. 8 Pyruvate is oxidized in preparation for the citric acid cycle • Pyruvate does not enter the citric acid cycle but undergoes some chemical grooming in which • a carboxyl group is removed and given off as CO 2, • 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 Co. A. • Then two molecules of acetyl Co. A enter the citric acid cycle. © 2015 Pearson Education, Inc.

Figure 6. 8 NAD+ NADH + H+ 2 Co. A Pyruvate Acetyl coenzyme A 1 CO 2 © 2015 Pearson Education, Inc. 3 Coenzyme A

6. 9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 2 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 1930 s), • completes the oxidation of organic molecules, and • generates many NADH and FADH 2 molecules. © 2015 Pearson Education, Inc.

Figure 6. 9 a Acetyl Co. A Citric Acid Cycle 2 CO 2 3 NAD+ FADH 2 3 NADH FAD + 3 H+ ATP © 2015 Pearson Education, Inc. ADP + P

6. 9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 2 molecules • During the citric acid cycle • the two-carbon group of acetyl Co. A is joined to a four-carbon compound, forming citrate, • citrate is degraded back to the four-carbon compound, • two CO 2 are released, and • one ATP, three NADH, and one FADH 2 are produced. © 2015 Pearson Education, Inc.

6. 9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 2 molecules • Remember that the citric acid cycle processes two molecules of acetyl Co. A 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 FADH 2. © 2015 Pearson Education, Inc.

6. 9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH 2 molecules • Thus, after glycolysis and the citric acid cycle, the cell has gained • 4 ATP, • 10 NADH, and • 2 FADH 2. • To harvest the energy banked in NADH and FADH 2, these molecules must shuttle their highenergy electrons to an electron transport chain. © 2015 Pearson Education, Inc.

Figure 6. 9 b-3 Co. A Acetyl Co. A 2 carbons enter cycle 1 Oxaloacetate Citrate NADH + H+ NAD+ 6 NAD+ NADH + H+ 2 Malate Citric Acid Cycle CO 2 H 2 O leaves cycle 5 Alpha-ketoglutarate Fumarate FADH 2 CO 2 4 3 FAD leaves cycle NAD+ Succinate ADP + P NADH + H+ ATP Step 1 Acetyl Co. A stokes the furnace. © 2015 Pearson Education, Inc. Steps 2 – 3 NADH, ATP, and CO 2 are generated during redox reactions. Steps 4 – 6 Further redox reactions generate FADH 2 and more NADH.

6. 10 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. © 2015 Pearson Education, Inc.

6. 10 Most ATP production occurs by oxidative phosphorylation • Electrons from NADH and FADH 2 travel down the electron transport chain to O 2, 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.

6. 10 Most ATP production occurs by oxidative phosphorylation • In chemiosmosis, the H+ diffuses back across the inner membrane, through ATP synthase complexes, driving the synthesis of ATP. © 2015 Pearson Education, Inc.

Figure 6. 10 a OUTER MITOCHONDRIAL MEMBRANE Intermembrane space H+ Protein complex of electron carriers H+ Mobile electron H+ carriers III H+ H+ Inner mitochondrial membrane H+ H+ H+ Cyt c Q I H+ ATP synthase IV II Electron flow FADH 2 NADH Mitochondrial matrix FAD 1 − O 2 + 2 H+ 2 NAD+ H+ H 2 O Electron Transport Chain Oxidative Phosphorylation © 2015 Pearson Education, Inc. ADP + P ATP H+ Chemiosmosis

6. 12 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.

6. 12 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 FADH 2 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.

Figure 6. 12 CYTOSOL MITOCHONDRION 2 NADH Glycolysis 2 Pyruvate Glucose 6 NADH + 2 FADH 2 2 NADH Pyruvate Oxidation 2 Acetyl Co. A Oxidative Phosphorylation (electron transport and chemiosmosis) Citric Acid Cycle O 2 +2 ATP by substrate-level phosphorylation © 2015 Pearson Education, Inc. CO 2 Maximum per glucose: H 2 O +2 ATP by substrate-level phosphorylation + about 28 ATP About 32 ATP by oxidative phosphorylation

FERMENTATION: ANAEROBIC HARVESTING OF ENERGY © 2015 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 • 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.

6. 13 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. © 2015 Pearson Education, Inc.

Figure 6. 13 a 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 NADH 2 NAD+ 2 Lactate © 2015 Pearson Education, Inc.

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. © 2015 Pearson Education, Inc.

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, yeast (single-celled fungi) • oxidize NADH back to NAD+ and • convert pyruvate to CO 2 and ethanol. © 2015 Pearson Education, Inc.

Figure 6. 13 b 2 ADP +2 P 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 NADH 2 CO 2 2 NAD+ 2 Ethanol © 2015 Pearson Education, Inc.

6. 13 Fermentation enables cells to produce ATP without oxygen • 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. © 2015 Pearson Education, Inc.

CONNECTIONS BETWEEN METABOLIC PATHWAYS © 2015 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. © 2015 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 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.

Figure 6. 15 -1 Food Carbohydrates Sugars Fats Proteins Glycerol Fatty acids Amino groups Glucose G 3 P Pyruvate Glycolysis Acetyl Co. A ATP © 2015 Pearson Education, Inc. Citric Acid Cycle Oxidative Phosphorylation

6. 16 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. © 2015 Pearson Education, Inc.

Figure 6. 16 -1 ATP needed to drive biosynthesis Citric Acid Cycle ATP Acetyl Co. A Glucose Synthesis Pyruvate G 3 P Glucose Amino groups Amino acids Proteins Fatty Glycerol acids Fats Cells, tissues, organisms © 2015 Pearson Education, Inc. Sugars Carbohydrates

6. 16 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. © 2015 Pearson Education, Inc.