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Essentials of Biology Sylvia S. Mader Chapter 7 Lecture Outline Prepared by: Dr. Stephen Ebbs Southern Illinois University Carbondale Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7.1 Cellular Respiration • The ATP molecules that provide energy to eukaryotic cells are produced during cellular respiration. • During cellular respiration, the mitochondria take in O2 and release CO2. • Cellular respiration is the reason that animals breathe. 7.1 Cellular Respiration (cont.) • Oxidation, the removal of hydrogen atoms from a molecule, is a central reaction in cellular respiration. oxidation C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy reduction 7.1 Cellular Respiration (cont.) • The breakdown of glucose during cellular respiration releases energy. • The slow oxidation of glucose in the mitochondria allows the energy to be removed slowly and stored as ATP. 7.1 Cellular Respiration (cont.) Phases of Complete Glucose Breakdown • Cellular respiration involves a metabolic pathway of enzymes assisted by coenzymes. • The two coenzymes involved in cellular respiration, NAD+ and FAD+, receive the hydrogen atoms removed from glucose. Phases of Complete Glucose Breakdown (cont.) • The complete oxidation of glucose involves four phases. – Glycolysis, the splitting of glucose into two 3-carbon molecules – The preparatory reaction, which divides each 3carbon molecules into a 2-carbon molecule and CO2 – The citric acid cycle, which produces CO2, NADH, FADH2, and ATP – The electron transport chain, which assists in the production of the largest amount of ATP Phases of Complete Glucose Breakdown (cont.) 7.2 Outside the Mitochondria: Glycolysis • Glycolysis takes place in the cytoplasm of the cell. • During glycolysis, glucose (a 6-carbon molecule) is broken down to two pyruvate (3-carbon) molecules. • Glycolysis is divided into two stages. – Energy-Investment Steps – Energy-Harvesting Steps 7.2 Outside the Mitochondria: Glycolysis (cont.) Energy-Investment Steps • Some molecules must be energized before they can be broke down. • To facilitate glucose breakdown during glycolysis, 2 ATP molecules energize glucose by donating their phosphate groups. Energy-Harvesting Steps • During the energy-harvesting steps, substrates are oxidized and the hydrogen atoms removed are used to form NADH. • This oxidation also produces substrates with high-energy phosphate groups, which can be used to synthesize ATP. • The transfer of a phosphate group from a molecule to form ATP is called substrate-level ATP synthesis. Energy-Harvesting Steps (cont.) • Glycolysis produces a total of four ATP. • Since two ATP were used to initiate glycolysis, the net ATP production from glycolysis is two ATP. • The metabolic fate of pyruvate, the product of glycolysis, depends upon the presence of oxygen. Energy-Harvesting Steps (cont.) 7.3 Inside the Mitochondria • The remaining stages of cellular respiration occur in the mitochondria. • These steps require the presence of oxygen. • The structural features of the mitochondria contribute to cellular respiration. 7.3 Inside the Mitochondria (cont.) Preparatory Reaction • The preparatory (prep) reaction of glycolysis, which occurs twice for each glucose molecule, produces the substrate that enters the subsequent citric acid cycle. • Several events occur in the preparatory reaction. – Pyruvate is oxidized and releases a molecule of CO2 and a 2-carbon acetyl group. – NAD+ accepts a hydrogen atom, producing NADH. – The acetyl group is attached to coenzyme A (CoA) to form acetyl-CoA. The Citric Acid Cycle • The citric acid cycle occurs in the matrix of the mitochondria. • Several events occur during the citric acid cycle. – The acetyl group is oxidized to CO2. – Both NAD+ and FAD+ accept hydrogen atoms, forming NADH and FADH respectively. – Substrate-level ATP synthesis occurs, forming ATP. • The citric acid cycle turns twice for each glucose molecule. The Citric Acid Cycle (cont.) The Electron Transport Chain • The electron transport chain is located in the cristae of the mitochondria. • The members of the electron transport chain accept electrons from the hydrogen atoms accepted by NADH and FADH2. • As the electrons are passed down the electron transport chain, energy is released and captured for ATP production. The Electron Transport Chain (cont.) • At the end of the electron transport chain, the electrons are donated to oxygen atoms to form water. • The number of ATP molecules formed depends upon the electron donor. – The electrons from NADH provide energy for the synthesis of three ATP molecules. – The electrons from FADH2 provide energy for the synthesis of two ATP molecules. The Electron Transport Chain (cont.) The Cristae of a Mitochondrion • The members of the electron transport chain are imbedded in the cristae of the mitochondria in a specific pattern. • As the members of the electron transport chain accept electrons from NADH and FADH2, the H+ are pumped into the intermembrane space. The Cristae of a Mitochondrion (cont.) • This pumping creates an H+ reservoir in the intermembrane space. • This reservoir can be released through an ATP synthase complex to synthesize ATP. The Cristae of a Mitochondrion (cont.) Energy Yield from Glucose Metabolism • The complete breakdown of glucose yields 36 ATP molecules. – Glycolysis provides 2 net ATP. – The NADH produced by the prep reaction and the citric acid cycle yield 30 ATP. – The electron transport chain uses FADH2 to produce 4 ATP. Energy Yield from Glucose Metabolism (cont.) Alternative Metabolic Pathways • Cells can breakdown other molecules, such as lipids and proteins, to yield ATP. • Lipids can be broken down to produce more ATP than glucose. – The glycerol from lipids enters cellular respiration at glycolysis. – The fatty acids from lipids can be metabolized into acetyl groups, which enter the citric acid cycle. Alternative Metabolic Pathways (cont.) • The hydrocarbon backbone of amino acids can enter cellular respiration at several points and can be broken down to produce energy. • The small molecules produced during cellular respiration can be used to synthesize larger molecules. Alternative Metabolic Pathways (cont.) 7.4 Fermentation • Fermentation is the anaerobic breakdown of glucose, forming 2 ATP and a toxic by-product. • In animal cells during fermentation, pyruvate from glycolysis is reduced to lactate, reforming NAD+. • Although fermentation produces only 2 ATP molecules per glucose, it is essential as a quick source of ATP energy for cells. 7.4 Fermentation (cont.) • When fermentation occurs in muscles during vigorous exercise, the lactate builds up, as does an oxygen deficit. • The increase in lactate changes the pH, creating the “burn” associated with exercise. 7.4 Fermentation (cont.) Microorganisms and Fermentation • Bacterial fermentation produces either lactate or alcohol + CO2. • Yeast are well known microorganisms that produce alcohol and CO2 during fermentation. – CO2 production is what causes bread to rise. – Ethanol production is critical for the making of beer and wine.