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

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?

8.1 How Do Cells Obtain Energy? Most cellular energy is stored in the chemical bonds of energy-carrier molecules such as 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

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 Nearly all organisms use glycolysis and cellular respiration to break down sugar molecules to capture energy as ATP

8.1 How Do Cells Obtain Energy? 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

energy from sunlight photosynthesis C6H12O6 6 CO2 6 H2O 6 O2 cellular Figure 8-1 Photosynthesis provides the energy released during glycolysis and cellular respiration energy from sunlight photosynthesis C6H12O6 6 CO2 6 H2O 6 O2 cellular respiration glycolysis ATP 6

8.1 How Do Cells Obtain Energy? Glucose is a key energy-storage molecule 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

8.1 How Do Cells Obtain Energy? Glucose is a key energy-storage molecule (continued) The breakdown of glucose occurs in phases Glycolysis Fermentation Cellular respiration During glycolysis and cellular respiration, energy is captured in ATP

Figure 8-2 A summary of glucose breakdown (cytosol) 1 glucose glycolysis ATP 2 lactate 2 pyruvate fermentation 2 ethanol If O2 is available If no O2 is available  2 CO2 6 O2 cellular respiration ATP 6 CO2 6 H2O mitochondrion 9

8.2 What Happens During Glycolysis? 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

8.2 What Happens During Glycolysis? 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

8.2 What Happens During Glycolysis? Energy harvesting The 6-carbon fructose bisphosphate is split into two 3-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

8.2 What Happens During Glycolysis? Energy harvesting (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

8.2 What Happens During Glycolysis? 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) are formed

Animation: Glycolysis

The essentials of glycolysis Fig. 8-3

8.3 What Happens During Cellular Respiration? 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 Cellular respiration occurs in mitochondria (powerhouses of the cell), organelles specialized for the aerobic breakdown of pyruvate 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

Figure 8-4 A mitochondrion matrix inner membrane intermembrane space outer membrane 18

8.3 What Happens During Cellular Respiration? Cellular respiration occurs in three stages Pyruvate breakdown Transfer of electrons along the electron transport chain Generation of ATP by chemiosmosis

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down Pyruvate is synthesized in the cytoplasmic matrix Before cellular respiration can occur, the pyruvate is actively transported from the cytoplasmic matrix to the mitochondrial matrix The mitochondrial matrix is where cellular respiration begins

Figure 8-5 Reactions in the mitochondrial matrix Formation of acetyl CoA 3 NADH 3 NAD FAD FADH2 CO2 coenzyme A coenzyme A Krebs cycle acetyl CoA pyruvate NAD NADH ADP ATP 21

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (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

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (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

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (continued) The Krebs cycle Discovered by Hans Krebs 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

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (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

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (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

8.3 What Happens During Cellular Respiration? During the first stage of cellular respiration, pyruvate is broken down (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

8.3 What Happens During Cellular Respiration? During 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 Depleted carriers are available for recharging by glycolysis and the Krebs cycle

8.3 What Happens During Cellular Respiration? During 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

8.3 What Happens During Cellular Respiration? During 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

8.3 What Happens During Cellular Respiration? During the second stage of cellular respiration, high-energy 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

Figure 8-6 The electron transport chain (matrix) ADP ATP  P NADH FADH2 NAD FAD ATP synthase (inner membrane) ETC (intermembrane space) 32

8.3 What Happens During Cellular Respiration? During the third stage of cellular respiration, chemiosmosis generates 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

8.3 What Happens During Cellular Respiration? During the third stage of cellular respiration, chemiosmosis generates 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 H ions flow across the membrane through the ATP synthase channels, and their movement generates ATP from ADP and phosphate

8.3 What Happens During Cellular Respiration? During the third stage of cellular respiration, chemiosmosis generates 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

8.3 What Happens During Cellular Respiration? 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

8.3 What Happens During Cellular Respiration? 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

8.3 What Happens During Cellular Respiration? 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

BioFlix Animation: Summary of Cellular Respiration

Figure 8-7 The energy sources and ATP harvest from glycolysis and cellular respiration (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 4 CO2 O2 H2O electron transport chain 32 ATP Total: 36 ATP 40

8.3 What Happens During Cellular Respiration? Cellular respiration can extract energy from a variety of molecules 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

8.3 What Happens During Cellular Respiration? Cellular respiration can extract energy from a variety of molecules (continued) 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

8.4 What Happens During Fermentation? Glycolysis is used by virtually every organism on Earth 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 Stomach and intestines of animals Deep in soil Bogs and marshes

8.4 What Happens During Fermentation? Fermentation allows NAD to be recycled when oxygen is absent 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

8.4 What Happens During Fermentation? Fermentation allows NAD to be recycled when oxygen is absent (continued) Under anaerobic conditions, with no oxygen to allow the ETC to function, the cell must regenerate the NAD for glycolysis using fermentation Under aerobic (with oxygen) conditions, NADH donates its high-energy 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

8.4 What Happens During Fermentation? Fermentation allows NAD to be recycled when oxygen is absent (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

8.4 What Happens During Fermentation? Some cells ferment pyruvate to form lactate Lactic acid 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

Figure 8-8 Glycolysis followed by lactic acid fermentation 2 NAD 2 NADH 2 NADH 2 NAD (glycolysis) (fermentation) 1 glucose 2 pyruvate 2 lactate 2 ADP 2 ATP 48

Figure 8-9a Fermentation in action 49

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 Generates alcohol and CO2 from pyruvate 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

Animation: Fermentation

Figure 8-10 Glycolysis followed by alcoholic fermentation 2 NAD 2 NADH 2 NADH 2 NAD (glycolysis) (fermentation) 1 glucose 2 pyruvate 2 ethanol 2 CO2 2 ADP 2 ATP 52

Figure 8-9b Fermentation in action 53