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Chapter 8 Harvesting Energy: Glycolysis and Cellular Respiration Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 8 At a Glance 8.1 How Do Cells Obtain Energy? 8.2 What Happens During Glycolysis? 8.3 What Happens During Cellular Respiration? 8.4 What Happens During Fermentation? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? Most cellular energy is stored in the chemical bonds of energy-carrier molecules like adenosine triphosphate (ATP) Cells break down glucose in two stages: glycolysis, which liberates a small quantity of ATP, followed by cellular respiration, which produces far more ATP Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? Photosynthesis is the ultimate source of cellular energy – Photosynthetic organisms capture the energy of sunlight and store it in the form of glucose – The overall equation for photosynthesis is 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Overview: Photosynthesis and Respiration Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration energy from sunlight photosynthesis 6 CO2 6 H2O 6 O2 cellular respiration C6H12O6 glycolysis ATP Fig. 8-1 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? Glucose is a key energy-storage molecule – All cells metabolize glucose for energy – In humans, energy is stored as long chains of glucose, called glycogen, or as fat – These storage molecules are converted to glucose to produce ATP for energy harvesting Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? An overview of glucose breakdown – The first stage of glucose breakdown is glycolysis – Glycolysis begins by splitting glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon sugar) – Two ATP molecules are produced in glycolysis – Glycolysis proceeds in the same way under aerobic (with oxygen) or anaerobic (without oxygen) conditions – Glycolysis occurs in the cytoplasm Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? An overview of glucose breakdown (continued) – The second stage of glucose breakdown is cellular respiration and occurs when oxygen is available – In this stage, two pyruvate molecules produced by glycolysis are broken down into six carbon dioxide molecules and six water molecules – For every two pyruvate molecules, an additional 34 or 36 ATP molecules are generated – Cellular respiration occurs in mitochondria, organelles specialized for the aerobic breakdown of pyruvate Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? An overview of glucose breakdown (continued) – If oxygen is not available, the second stage of glucose breakdown is fermentation –Fermentation does not produce any ATP –In fermentation, pyruvate remains in the cytoplasm and is converted into lactate or ethanol + CO2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Fate of Pyruvate Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.1 How Do Cells Obtain Energy? An overview of glucose breakdown (continued) – The overall equation for the complete breakdown of glucose is C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + heat Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Summary of Glucose Breakdown (cytoplasmic fluid) glucose glycolysis 2 ATP lactate fermentation 2 pyruvate If O2 is available If no O2 is available ethanol + CO2 6 O2 cellular respiration 6 CO2 34 or ATP 36 6 H2O mitochondrion Biology: Life on Earth, 9e Fig. 8-2 Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Glycolysis has two parts, each with several steps – Glucose activation – Energy harvesting Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Glucose activation – A glucose molecule is activated when it receives two phosphates from two ATPs, becoming fructose bisphosphate –Two ATPs are converted into two low-energy adenosine diphosphate (ADP) molecules Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Glucose activation (continued) – Although the formation of fructose bisphosphate costs the cell two ATP molecules, this initial investment of energy is necessary to produce greater energy returns later – The glucose molecule is relatively stable; the added phosphates make fructose bisphosphate highly reactive Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Energy harvesting – The six-carbon fructose bisphosphate is split into two, three-carbon molecules of glyceraldehyde3-phosphate (G3P) – In a series of reactions, each of the two G3P molecules is converted into a pyruvate, generating two ATPs per conversion, for a total of four ATPs – Because two ATPs were used to activate the glucose molecule, there is a net gain of two ATPs per glucose molecule Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Energy harvesting (continued) – As each G3P is converted to pyruvate, two highenergy electrons and a hydrogen ion are added to an “empty” electron-carrier nicotinamide adenine dinucleotide (NAD+) to make the highenergy electron-carrier molecule NADH – Because two G3P molecules are produced per glucose molecule, two NADH carrier molecules are formed as well Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.2 What Happens During Glycolysis? Summary of glycolysis – Each molecule of glucose is broken down to two molecules of pyruvate – A net of two ATP molecules and two NADH (high-energy electron carriers) are formed Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Glycolysis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Essentials of Glycolysis 2 ATP C C C C C C glucose 4 ADP 2 ADP C C C C C C P fructose P bisphosphate 1 Glucose activation 4 ATP 2 C C C G3P P 2 C C C pyruvate 2 NAD+ 2 NADH 2 Energy harvest Fig. 8-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? Cellular respiration in eukaryotic cells occurs in mitochondria in three stages – First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 – Keep in mind that each glucose molecule produces two pyruvate molecules – Second, high-energy electrons travel through the electron transport chain – Third, ATP is generated by chemiosmosis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – In eukaryotic cells, cellular respiration occurs within mitochondria, organelles with two membranes that produce two compartments –The inner membrane encloses a central compartment containing the fluid matrix –The outer membrane forms the outer surface of the organelle, and an intermembrane space lies between the two membranes Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Mitochondrion matrix inner membrane intermembrane space outer membrane Biology: Life on Earth, 9e Fig. 8-4 Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – Glucose is first broken down into pyruvate, through glycolysis, in the cell cytoplasm – Pyruvate is next transported into the mitochondrion matrix (in eukaryotes), where further breakdown occurs in two stages: –The formation of acetyl coenzyme A (acetyl CoA) –The Krebs cycle Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – The formation of acetyl CoA –To generate acetyl CoA, pyruvate is split, forming an acetyl group and releasing CO2 –The acetyl group reacts with CoA, forming acetyl CoA –During this reaction, two high-energy electrons and a hydrogen ion are transferred to NAD+, forming NADH Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – The Krebs cycle –The Krebs cycle begins by combining acetyl CoA with a four-carbon molecule to form sixcarbon citrate, and coenzyme A is released –As the Krebs cycle proceeds, enzymes in the matrix break down the acetyl group, releasing two CO2 molecules and regenerating the fourcarbon molecule for use in future cycles Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – The Krebs cycle (continued) – Chemical energy released by breaking down each acetyl group is captured in energy-carrier molecules – Each acetyl group produces one ATP, three NADH, and one FADH2 – Flavin adenine dinucleotide (FAD), a high-energy electron carrier similar to NAD, picks up two electrons and two H+, forming FADH2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Acetyl CoA and the Krebs Cycle Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Reactions in the Mitochondrial Matrix 1 Formation of acetyl CoA coenzyme A 3 NADH 3 NAD+ FADH2 C CO2 coenzyme A C C – CoA acetyl CoA C C C pyruvate NAD+ FAD 2 Krebs cycle NADH ADP ATP Biology: Life on Earth, 9e 2 C CO2 Fig. 8-5 Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued) – During the mitochondrial reactions, CO2 is generated as a waste product – CO2 diffuses out of cells and into the blood, which carries it to the lungs, where it is exhaled Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain – During glycolysis and the mitochondrial matrix reactions, the cell captures many high-energy electrons in carrier molecules: 10 NADH and 2 FADH2 for every glucose molecule that was broken down – These carriers each release two electrons into an electron transport chain (ETC), many copies of which are embedded in the inner mitochondrial membrane Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain (continued) – These high-energy electrons jump from molecule to molecule in the ETC, losing small amounts of energy at each step – This resembles the process that occurs in the thylakoid membrane of chloroplasts during photosynthesis – Much of this energy is harnessed to pump H+ from the matrix across the inner membrane and into the intermembrane space, producing a concentration gradient of H+ Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain (continued) – The buildup of H+ in the intermembrane space is used to generate ATP during chemiosmosis – At the end of the ETC, the energy-depleted electrons are transferred to oxygen, which acts as an electron acceptor – Energy-depleted electrons, oxygen, and hydrogen ions combine to form water – One water molecule is produced for every two electrons that traverse the ETC Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? In the second stage of cellular respiration, highenergy electrons travel through the electron transport chain (continued) – Without oxygen, electrons would be unable to move through the ETC, and H+ would not be pumped across the inner membrane – The H+ gradient would dissipate, and ATP synthesis by chemiosmosis would stop – ATP generation continues only when there is a steady supply of oxygen Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Electron Transport Chain (matrix) ADP P H+ 3 1 NADH 2 e– 2 e– FADH2 NAD+ 1 2 O2 2 H+ 2 FAD e– ATP 4 H2O ATP synthase (inner membrane) ETC H+ 2 H+ H+ H+ (intermembrane space) Biology: Life on Earth, 9e H+ H+ Fig. 8-6 Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? The third stage of cellular respiration generates ATP by chemiosmosis – Chemiosmosis is the process by which energy is first used to generate a gradient of H+, and then captured in the bonds of ATP as H+ flows down its gradient – As the ETC pumps H+ across the inner membrane, it produces a high concentration of H+ in the intermembrane space and a low concentration in the matrix Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? The third stage of cellular respiration generates ATP by chemiosmosis (continued) – The energy present in this non-uniform H+ distribution across the inner membrane is released when hydrogen ions flow down their concentration gradient – The H+ ions flow across the membrane through the ATP synthase channels, and their movement generates ATP from ADP and phosphate Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? The third stage of cellular respiration generates ATP by chemiosmosis (continued) – The flow of H+ through the synthase channel provides the energy to synthesize 32 or 34 molecules of ATP for each molecule of glucose – The newly formed ATP leaves the mitochondrion and enters the cytoplasm, where it provides the energy needed by the cell Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: The Electron Transport Chain Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? A summary of glucose breakdown in eukaryotic cells – Glycolysis occurs in the cytoplasm, with one glucose molecule producing two three-carbon pyruvate molecules and releasing a small fraction of the energy stored in glucose – Some of the energy is used to generate two ATP molecules, and some is captured in two NADH molecules, which will feed their electrons into the ETC during cellular respiration, generating more ATP Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? A summary of glucose breakdown in eukaryotic cells (continued) – During cellular respiration, the two pyruvate molecules enter the mitochondrion – First, each reacts with coenzyme A (CoA), a process that captures high-energy electrons in two NADH, produces two molecules of acetyl CoA, and liberates two molecules of CO2 – The acetyl CoA enters the Krebs cycle Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? A summary of glucose breakdown in eukaryotic cells (continued) – The Krebs cycle releases four molecules of CO2, produces two ATP, and captures high-energy electrons in six NADH and two FADH2 – These electrons are passed to the ETC, where their energy is used during chemiosmosis to generate a gradient of H+, yielding a net of 32 or 34 ATP Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? A summary of glucose breakdown in eukaryotic cells (continued) – Energy-depleted electrons exiting the ETC are picked up by H+ released from NADH and FADH2, and combine with oxygen to form water Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.3 What Happens During Cellular Respiration? A summary of glucose breakdown in eukaryotic cells (continued) – The total energy captured from the breakdown of a single glucose molecule from glycolysis and cellular respiration is 36 or 38 ATP – The reason two different numbers exist for ATP synthesis is that some cell types have to expend two ATPs to transport the two NADH molecules created during glycolysis into the mitochondrion; others are more efficient and don’t require two – The ATP enters the cytoplasm for cellular metabolic activities Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Overview of Glucose Metabolism Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Summary of Cellular Respiration Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Energy Sources and ATP Harvest from Glycolysis and Cellular Respiration glucose (cytoplasmic fluid) glycolysis 2 NADH 2 ATP 2 pyruvate mitochondrion CoA 2 NADH 2 CO2 2 acetyl CoA Krebs cycle 6 NADH 2 ATP 2 FADH2 4 CO2 O2 H2O electron transport chain Biology: Life on Earth, 9e 32 or ATP 34 total: 36 or 38 ATP Fig. 8-7 Copyright © 2011 Pearson Education Inc. 8.4 What Happens During Fermentation? Why is fermentation necessary? – For glycolysis to continue, the NAD+ used to generate NADH must constantly be regenerated – Under aerobic conditions, NADH donates its highenergy electrons and hydrogen produced in glycolysis to ATP-generating reactions in the mitochondria, ultimately being donated to oxygen during the creation of water and making NAD+ available to recycle during glycolysis – Under anaerobic conditions, with no oxygen to allow the ETC to function, the cell must regenerate the NAD+ for glycolysis using fermentation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.4 What Happens During Fermentation? Why is fermentation necessary? (continued) – Fermentation does not produce more ATP, but is necessary to regenerate NAD+, which must be available for glycolysis to continue – If the supply of NAD+ were to be exhausted, glycolysis would stop, energy production would cease, and the organism would rapidly die Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.4 What Happens During Fermentation? Why is fermentation necessary? (continued) – Organisms use one of two types of fermentation to regenerate NAD+ – Lactic acid fermentation produces lactic acid from pyruvate – Alcohol fermentation generates alcohol and CO2 from pyruvate – Because fermentation does not break down glucose completely and does not use the energy of NADH to produce more ATP, organisms that rely on fermentation must consume far more sugar to generate the same amount of ATP than do those relying on cellular respiration Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.4 What Happens During Fermentation? Some cells ferment pyruvate to form lactate – Muscles that are working hard enough to use up all the available oxygen ferment pyruvate to lactate – To regenerate NAD+, muscle cells ferment pyruvate to lactate, using electrons from NADH and hydrogen ions – A variety of microorganisms use lactic acid fermentation, including the bacteria that convert milk into yogurt, sour cream, and cheese Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Glycolysis Followed by Lactic Acid Fermentation 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 C C C 2 C C C C C C C C C (glycolysis) (fermentation) glucose pyruvate lactate 2 ADP 2 ATP Fig. 8-8 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 8.4 What Happens During Fermentation? Some cells ferment pyruvate to form alcohol and carbon dioxide – Many microorganisms, such as yeast, engage in alcohol fermentation under anaerobic conditions – As in lactic acid fermentation, the NAD+ must be regenerated to allow glycolysis to continue – During alcohol fermentation, H+ and electrons from NADH are used to convert pyruvate into ethanol and CO2; this releases NAD+, which can accept more high-energy electrons during glycolysis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Fermentation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Glycolysis Followed by Alcohol Fermentation 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 C C C C C C C C C 2 C C +2 C (glycolysis) (fermentation) glucose pyruvate ethanol CO2 2 ADP 2 ATP Fig. 8-9 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Fermentation in Action Fig. 8-10 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc.