Harvesting Energy: Glycolysis and Cellular Respiration

Harvesting Energy: Glycolysis and Cellular Respiration
chapter 8 Harvesting Energy: Glycolysis and Cellular Respiration 1

Chapter 8 At a Glance 8.1 How Do Cells Obtain Energy?
8.2 How Does Glycolysis Begin Breaking Down Glucose? 8.3 How Does Cellular Respiration Extract Energy from Glucose? 8.4 How Does Fermentation Allow Glycolysis to Continue When Oxygen Is Lacking? © 2017 Pearson Education, Inc.

8.1 How Do Cells Obtain Energy?
Most cellular energy is stored in the chemical bonds of storage molecules such as adenosine triphosphate (ATP) Cells are relatively efficient at capturing chemical energy during glucose breakdown when oxygen is available © 2017 Pearson Education, Inc.

Photosynthesis is the ultimate source of cellular energy Photosynthetic organisms capture the energy of sunlight and store it in the form of glucose Nearly all organisms use glycolysis and cellular respiration to break down sugar molecules to capture energy as ATP © 2017 Pearson Education, Inc.

Photosynthesis is the ultimate source of cellular energy (continued) Photosynthesis 6 CO2  6 H2O  light energy → C6H12O6  6 O2 Complete glucose breakdown C6H12O6  6 O2 → 6 CO2  6 H2O  ATP energy  heat energy © 2017 Pearson Education, Inc.

energy from sunlight chloroplast photosynthesis 6 CO2 6 H2O 6 O2
Figure 8-1 energy from sunlight chloroplast photosynthesis 6 CO2 6 H2O 6 O2 C6H12O6 Figure 8-1 The interrelationship between photosynthesis and glucose breakdown cellular respiration glycolysis ATP mitochondrion © 2017 Pearson Education, Inc.

All cells can use glucose as a source of energy All cells metabolize glucose for energy Plants convert glucose to sucrose or starch for storage 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 © 2017 Pearson Education, Inc.

All cells can use glucose as a source of energy (continued) The breakdown of glucose occurs in phases Glycolysis Cellular respiration During glycolysis and cellular respiration, energy is captured in ATP © 2017 Pearson Education, Inc.

Figure 8-2 A summary of glucose breakdown
(cytosol) 1 glucose glycolysis 2 ATP 2 lactate 2 pyruvate fermentation 2 ethanol If O2 is available If no O2 is available + 2 CO2 6 O2 cellular respiration 34 ATP Figure 8-2 A summary of glucose breakdown 6 CO2 6 H2O mitochondrion © 2017 Pearson Education, Inc.

Why must glucose be broken down to generate ATP during cellular respiration?
Glucose breakdown is the only source of ATP in the cell. For the energy released by cellular respiration, ATP is a more usable form than glucose. Without the breakdown of glucose to ATP, a cell would dehydrate and shrivel up. This breakdown gives off less heat than does any other metabolic reaction. Question: 8-2 Answer: b Diff: Moderate Text Ref: Section 8.1 Skill: Conceptual Notes: Remind students of the purpose of cellular respiration: changing a large energy source into a smaller, more universal, and usable form of energy. © 2017 Pearson Education, Inc.

Why must glucose be broken down to generate ATP during cellular respiration?
Glucose breakdown is the only source of ATP in the cell. For the energy released by cellular respiration, ATP is a more usable form than glucose. Without the breakdown of glucose to ATP, a cell would dehydrate and shrivel up. This breakdown gives off less heat than does any other metabolic reaction. Question: 8-2 Answer: b Diff: Moderate Text Ref: Section 8.2 Skill: Conceptual Notes: Remind students of the purpose of cellular respiration: changing a large energy source into a smaller, more universal, and usable form of energy. © 2017 Pearson Education, Inc.

8.2 How Does Glycolysis Begin Breaking Down Glucose?
Glycolysis has an etymological root from the Greek glyco, meaning “sweet,” and lysis, meaning to “split apart” Glycolysis begins by splitting glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon sugar) Glycolysis has energy investment and energy-harvesting stages © 2017 Pearson Education, Inc.

Energy investment stage 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 © 2017 Pearson Education, Inc.

Energy-harvesting stage The six-carbon fructose bisphosphate is split into two three-carbon molecules of glyceraldehyde-3-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 © 2017 Pearson Education, Inc.

Energy-harvesting stage (continued) As each G3P is converted to pyruvate, two high-energy electrons and a hydrogen ion are added to an “empty” electron-carrier nicotinamide adenine dinucleotide (NAD) to make the high-energy electron-carrier molecule NADH Because two G3P molecules are produced per glucose molecule, two NADH carrier molecules are formed as well © 2017 Pearson Education, Inc.

Summary of glycolysis During the energy investment stage, phosphate groups and energy from each of the two ATP are added to glucose to produce fructose bisphosphate Fructose bisphosphate is broken down into two G3P molecules During the energy-harvesting stage, the two G3P molecules are converted into two pyruvate molecules, resulting in four ATP and two NADH molecules A net of two ATP molecules and two NADH (high-energy electron carriers) is formed © 2017 Pearson Education, Inc.

Animation: Glycolysis and Fermentation
© 2017 Pearson Education, Inc.

Figure 8-3 The essentials of glycolysis
2 ATP 2 ADP 4 ADP 4 ATP 1 glucose 1 fructose bisphosphate 2 G3P 2 pyruvate 2 NAD+ 2 NADH Energy investment stage Energy harvesting stage Figure 8-3 The essentials of glycolysis © 2017 Pearson Education, Inc.

Glycolysis is distinct from cellular respiration because ___________.
glycolysis does not produce ATP glycolysis utilizes oxygen glycolysis does not occur in the mitochondria the products of glycolysis do not enter the mitochondria Question: 8-3 Answer: c Diff: Moderate Text Ref: Section 8.1 Skill: Factual Notes: Many students have the perception that all parts of glucose breakdown occur in mitochondria. This question will be helpful in talking about the difference between anaerobic respiration and aerobic respiration and the way in which glycolysis is a necessary part of both of them. © 2017 Pearson Education, Inc.

8.3 How Does Cellular Respiration Extract Energy From Glucose?
Cellular respiration breaks down the two pyruvate molecules into six carbon dioxide molecules and six water molecules The chemical energy from the two pyruvate molecules aids in the production of 32 ATP © 2017 Pearson Education, Inc.

Cellular respiration (continued) Cellular respiration occurs in mitochondria (powerhouses of the cell), organelles specialized for the aerobic breakdown of pyruvate A mitochondrion has two membranes 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 © 2017 Pearson Education, Inc.

Figure 8-4 The mitochondrion
Outer membrane: Separates the mitochondrion from the cytosol and confines the intermembrane space. Intermembrane space: Hydrogen ions are transported here, allowing chemiosmosis to occur. Inner membrane: The electron transport chain and ATP synthase are embedded here. Matrix: Acetyl CoA is produced and the Krebs cycle occurs here. Figure 8-4 The mitochondrion (a) Mitochondrial structures and their functions matrix inner membrane outer membrane © 2017 Pearson Education, Inc. (b) TEM of a mitochondrion

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle Pyruvate is synthesized in the cytosol Before cellular respiration can occur, pyruvate is actively transported from the cytosol matrix to the mitochondrial matrix The mitochondrial matrix is where cellular respiration begins © 2017 Pearson Education, Inc.

(in mitochondrial matrix)
Figure 8-5 (in mitochondrial matrix) formation of acetyl CoA 3 NADH 3 NAD+ FAD FADH2 CO2 coenzyme A coenzyme A Figure 8-5 Reactions in the mitochondrial matrix: acetyl CoA formation and the Krebs cycle Krebs cycle − CoA CO2 pyruvate acetyl CoA NAD+ NADH ADP ATP © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (continued) 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 © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (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 © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (continued) The Krebs cycle Hans Krebs won the Nobel Prize in 1953 for his discovery of the Krebs cycle The Krebs cycle is also known as the citric acid cycle because citrate is produced first in the cycle © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (continued) The Krebs cycle (continued) The Krebs cycle begins by combining acetyl CoA with a four-carbon molecule to form six-carbon 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 four-carbon molecule for use in future cycles © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (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 © 2017 Pearson Education, Inc.

Cellular respiration stage 1: CoA is formed and travels through the Krebs cycle (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 © 2017 Pearson Education, Inc.

Why is the Krebs cycle appropriately named a cycle?
The molecules that enter this chain of reactions are regenerated at the end. All the molecules involved in this chain of reactions are round. The reactions typically proceed both backward and forward. The energy carriers NADH and FADH2 are broken down and regenerated during this chain of reactions. Question: 8-8 Answer: a Diff: Moderate Text Ref: Section 8.3 Skill: Conceptual Notes: Point out that the molecules that enter the Krebs cycle are regenerated at the completion of a cycle. © 2017 Pearson Education, Inc.

Cellular respiration stage 2: High-energy electrons traverse the electron transport chain, and chemiosmosis generates ATP 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 Depleted carriers are available for recharging by glycolysis and the Krebs cycle © 2017 Pearson Education, Inc.

Cellular respiration stage 2: High-energy electrons traverse the electron transport chain, and chemiosmosis generates ATP (continued) The electron transport chain releases energy in steps 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+ © 2017 Pearson Education, Inc.

Cellular respiration stage 2: High-energy electrons traverse the electron transport chain, and chemiosmosis generates ATP (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 © 2017 Pearson Education, Inc.

Cellular respiration stage 2: High-energy electrons traverse the electron transport chain, and chemiosmosis generates ATP (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 © 2017 Pearson Education, Inc.

Figure 8-6 The electron transport chain and chemiosmosis
Energy from high-energy electrons powers active transport of H+ through the inner membrane as they travel through the ETC. H+ H+ A high H+ concentration is created in the intermembrane space. H+ (intermembrane space) H+ H+ H+ H+ H+ H+ H+ H+ H+ ATP synthase inner membrane H+ H+ H+ H+ Figure 8-6 The electron transport chain and chemiosmosis 2 e- 2 e- H+ H+ H+ H+ FADH2 NADH NAD FAD ADP ATP + H+ 1 /2 O2 + 2 H e- H2O Pi (matrix) The high-energy electron carriers FADH and NADH2 donate electrons to the ETC. The flow of H+ down its concentration gradient powers ATP synthesis. O2 is required to accept energy-depleted electrons. © 2017 Pearson Education, Inc.

Chemiosmosis captures energy in ATP 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 © 2017 Pearson Education, Inc.

Chemiosmosis captures energy in ATP (continued) The energy present in this nonuniform H+ distribution across the inner membrane is released when hydrogen ions flow down their concentration gradient The hydrogen ions flow across the membrane through the ATP synthase channels, and their movement generates ATP from ADP and phosphate © 2017 Pearson Education, Inc.

Chemiosmosis captures energy in ATP (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 Without this continuous recycling, life would cease People produce, use, and then regenerate the equivalent of their body weight of ATP daily © 2017 Pearson Education, Inc.

How is ATP synthesized in the electron transport chain?
The energy given off by the high-energy electrons is coupled to the synthesis of NADH and FADH2. In a reaction coupled to the breakdown of NADH and FADH2, high-energy electrons synthesize ATP in the inner membrane. In chemiosmosis, as H+ flow down their concentration gradient across the inner membrane, the energy released is coupled to ATP synthesis. In chemiosmosis, as H+ flow against their concentration gradient across the inner membrane, the energy released is coupled to ATP synthesis. Question: 8-9 Answer: c Diff: Moderate Text Ref: Section 8.3 Skill: Conceptual Also relates to: Chapters 5 and 6 Notes: Students should know how chemiosmosis works to produce ATP. Students should apply what they learned about concentration gradients (Chapter 5) to understand why the hydrogen ions move across the membrane. © 2017 Pearson Education, Inc.

Summing up cellular respiration In the mitochondrial matrix, each pyruvate molecule is converted into acetyl CoA, producing one NADH per pyruvate molecule and releasing one CO2 As each acetyl CoA passes through the Krebs cycle, its energy is captured in one ATP, three NADH, and one FADH2. The carbons of acetyl CoA are released in two CO2 molecules © 2017 Pearson Education, Inc.

Summing up cellular respiration (continued) During cellular respiration, the two pyruvate molecules enter the mitochondrion and are completely broken down, yielding two ATP and ten high-energy electron carriers: eight NADH and two FADH2. The carbon atoms from the pyruvates are released in six molecules of CO2 High-energy electrons release energy that is harnessed to pump H into the intermembrane space as they pass through the ETC © 2017 Pearson Education, Inc.

Summing up cellular respiration (continued) The NADH and FADH2 molecules donate their energetic electrons to the ETC embedded in the inner mitochondrial membrane 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 ATP Energy-depleted electrons exiting the ETC are picked up by H+ released from NADH and FADH2 and combine with oxygen to form water © 2017 Pearson Education, Inc.

BioFlix Animation: Summary of Cellular Respiration

Figure 8-7 (cytosol) 1 glucose 2 NADH glycolysis 2 ATP 2 pyruvate mitochondrion (matrix) CoA 2 NADH 2 CO2 2 acetyl CoA 6 NADH Krebs cycle 2 ATP 2 FADH2 Figure 8-7 A summary of the ATP harvest from glycolysis and cellular respiration 4 CO2 O2 H2O electron transport chain and chemiosmosis 32 ATP total from complete glucose breakdown: 36 ATP © 2017 Pearson Education, Inc.

Cellular respiration can extract energy from a variety of foods Glucose often enters the human body as starch or table sugar, but energy can come from the consumption of fats and proteins in the diet Intermediate molecules of cellular respiration can be formed by other metabolic pathways Molecules enter at appropriate stages and then are broken down to produce ATP Amino acids of protein serve as energy sources © 2017 Pearson Education, Inc.

Cellular respiration can extract energy from a variety of molecules Fats are excellent sources of energy Serve as major energy-storage molecule in animals Fatty acids combine with CoA then are broken down to produce acetyl CoA molecules, which enter the first stage of the Krebs cycle A limited intake of fats will allow this process to happen Overindulgence of fats will cause the body to store excess fat © 2017 Pearson Education, Inc.

Figure 8-8 proteins carbohydrates fats amino acids sugar (glucose) glycerol fatty acids glycolysis pyruvate acetyl CoA Figure 8-8 Proteins, carbohydrates, and fats are broken down and release ATP Krebs cycle electron carriers electron transport chain ATP © 2017 Pearson Education, Inc.

8.4 How Does Fermentation Allow Glycolysis to Continue When Oxygen Is Lacking?
Glycolysis is used by virtually every organism on Earth Under aerobic conditions, when oxygen is available, cellular respiration usually follows Earlier forms of life appeared under the anaerobic (no oxygen) conditions existing before photosynthesis Some microbes lack enzymes for cellular respiration and rely solely on fermentation Various microorganisms still thrive in places where oxygen is limited or absent © 2017 Pearson Education, Inc.

Fermentation produces either lactate or alcohol and carbon dioxide 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 © 2017 Pearson Education, Inc.

Fermentation produces either lactate or alcohol and carbon dioxide (continued) Under anaerobic conditions, with no oxygen to allow the ETC to function, the cell must regenerate the NAD+ for glycolysis using fermentation © 2017 Pearson Education, Inc.

Fermentation produces either lactate or alcohol and carbon dioxide (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 Organisms use one of two types of fermentation to regenerate NAD+ Lactic acid fermentation Alcohol fermentation © 2017 Pearson Education, Inc.

Some cells ferment pyruvate to form lactate Lactate fermentation produces lactic acid from pyruvate 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 © 2017 Pearson Education, Inc.

Some cells engage in alcoholic fermentation During alcoholic fermentation, pyruvate is converted into ethanol and carbon dioxide This process converts NADH into NAD+, which is then available for glycolysis © 2017 Pearson Education, Inc.

e n e g r a e t i r o n 2 NAD+ 2 NADH 2 NADH 2 NAD+ (glycolysis)
Figure 8-9 e n e g r a e t i r o n 2 NAD+ 2 NADH 2 NADH 2 NAD+ (glycolysis) (fermentation) Figure 8-9 Glycolysis followed by lactic acid fermentation 1 glucose 2 pyruvate 2 lactate 2 ADP 2 ATP © 2017 Pearson Education, Inc.

Figure 8-10 Lactic acid fermentation in action

Animation: Glycolysis and Fermentation

n e e g e r a t i r o n 2 NAD+ 2 NADH 2 NADH 2 NAD+ (glycolysis)
Figure 8-11 n e e g e r a t i r o n 2 NAD+ 2 NADH 2 NADH 2 NAD+ (glycolysis) (fermentation) 1 glucose 2 pyruvate 2 ethanol 2 CO2 Figure Glycolysis followed by alcoholic fermentation 2 ADP 2 ATP © 2017 Pearson Education, Inc.

How can glycolysis continue producing energy when oxygen is not present?
Fermentation regenerates the NAD+ needed for glycolysis by allowing pyruvate to accept electrons and H+ from NADH. Fermentation regenerates the ATP needed for glycolysis by allowing pyruvate to accept phosphates from ADP formation. Fermentation produces oxygen, allowing the organism to make more energy in cellular respiration. Fermentation keeps oxygen, which is poisonous, from entering the cell. Question: 8-7 Answer: a Diff: Moderate Text Ref: Section 8.2 Skill: Conceptual Notes: Students are often confused about how fermentation is not a part of glycolysis under anaerobic conditions. It is important to show how fermentation will keep the organism alive when oxygen is not present. © 2017 Pearson Education, Inc.

Figure 8-12 Some products of fermentation

Harvesting Energy: Glycolysis and Cellular Respiration

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