Lesson 5 Harvesting Energy: Glycolysis and Cellular Respiration Introduction to Life Processes - SCI 102 1
How Cells Obtain Energy Cells require a constant flow of energy Most cellular energy is stored in adenosine triphosphate (ATP) Photosynthesis is the ultimate source of cellular energy Photosynthetic organisms capture and store the energy of sunlight in sugar and other organic molecules The chemical equation for glucose formation during photosynthesis is essentially the reverse of the equation of glucose breakdown by glycolysis and cellular respiration Introduction to Life Processes - SCI 102 2
How Cells Obtain Energy Glucose is a key energy-storage molecule All cells metabolize glucose for energy Glucose breakdown occurs in stages Glycolysis in the cytoplasm begins the process If oxygen is present, cellular respiration occurs If oxygen is absent, fermentation occurs Introduction to Life Processes - SCI 102 3
Glycolysis Glycolysis breaks down glucose to pyruvate, releasing chemical energy Glycolysis has two stages: energy investment and energy harvesting 1) The energy investment steps of glycolysis are energy requiring Glucose is converted to fructose bisphosphate, a 6-C glucose with two phosphate groups Fructose bisphosphate is unstable and high in energy Glucose activation costs two ATP This is an endergonic reaction 2) The energy harvesting steps yield ATP and NADH Fructose bisphosphate splits into two 3-C molecules of G3P Each G3P molecule undergoes a series of steps to be converted to pyruvate Energy-harvesting steps produce two NADH and four ATP Glycolysis produces a net two ATP and two NADH (high-energy electron carriers) for each molecule of glucose converted to two pyruvate Introduction to Life Processes - SCI 102 4
Glycolysis Glycolysis does not require oxygen to occur If a cell (ex: bacteria) shifts from an environment with oxygen to one without, it will need to increase its rate of glycolysis in order to have energy In an environment with oxygen, the bacteria can perform cellular respiration which produces much more energy than glycolysis Metabolic poison can interfere with glycolysis when the poison has a structure which is very similar to glucose but is unable to be metabolized Introduction to Life Processes - SCI 102 5
Cellular Respiration In most organisms, if oxygen is present, cellular respiration occurs Cellular respiration in eukaryotic cells occurs in mitochondria in three stages A mitochondrion has two membranes that produce two compartments: the matrix and the intermembrane space Introduction to Life Processes - SCI 102 6
Cellular Respiration Stage 1of cellular respiration: pyruvate is broken down First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO 2 In the mitochondrial matrix, pyruvate reacts with a molecule of coenzyme A to produce acetyl-coa and one CO 2 and one NADH Each acetyl-coa combines with a 4-C molecule to produce 6-C citrate, releasing coenzyme A Citrate goes through a series of rearrangements in a cycle of reactions called the Krebs cycle The end products of the Krebs cycle per molecule of pyruvate are two CO 2, one ATP, one FADH 2, and three NADH; the 4-C molecule is regenerated Introduction to Life Processes - SCI 102 7
Cellular Respiration Stage 2 of cellular respiration: high-energy electrons travel through the electron transport chain From glycolysis and the mitochondrial matrix reactions, the cell has accumulated 4 ATP, 10 NADH, and 2 FADH 2 The electron carriers NADH and FADH 2 release their electrons to the electron transport chains located in the inner mitochondrial membrane Energy released by these electrons is used to pump hydrogen ions from the matrix to the intermembrane space to produce ATP by chemiosmosis At the end of the electron transport chain (ETC), the energydepleted electrons are transferred to oxygen, forming water Introduction to Life Processes - SCI 102 8
Cellular Respiration Stage 3 of cellular respiration: chemiosmosis generates ATP During chemiosmosis, the flow of hydrogen ions provides enough energy to produce 32 to 34 ATP The ATP diffuses out of the mitochondria to the cytoplasm through the outer membrane, which is permeable to ATP Introduction to Life Processes - SCI 102 9
Cellular Respiration A summary of glucose breakdown in eukaryotic cells Glycolysis occurs in the cytoplasmic fluid This process produces two pyruvate molecules, two ATP molecules, and two NADH molecules Cellular respiration breaks down the two pyruvates during the Krebs cycle This process produces NADH and FADH 2 and a small amount of ATP Electrons from NADH and FADH 2 are donated to the electron transport chain, producing 32 or 34 ATP through chemiosmosis Cellular respiration can extract energy from a variety of molecules Cellular respiration can extract energy from sugars, fats, and amino acids Cyanide poisoning Occurs because cyanide inhibits an enzyme in the electron transport pathway This becomes deadly because ATP can no longer be produced by chemiosmosis Introduction to Life Processes - SCI 102 10
Fermentation Fermentation allows NAD + to be recycled when oxygen is absent Under aerobic conditions, most organisms use cellular respiration, regenerating NAD + from the ETC Under anaerobic conditions, cellular respiration does not occur, so NAD + must be regenerated another way to allow glycolysis to occur Introduction to Life Processes - SCI 102 11
Fermentation Some cells ferment pyruvate to form lactate Muscle cells undergo lactate fermentation during vigorous exercise when not enough oxygen is available As soon as oxygen is available, lactate will be converted back to pyruvate in the liver, and cellular respiration will resume Introduction to Life Processes - SCI 102 12
Fermentation Some cells ferment pyruvate to form alcohol and carbon dioxide Many microorganisms, including yeast, convert pyruvate to ethanol and carbon dioxide Alcoholic fermentation can be used to produce alcoholic beverages and bread Introduction to Life Processes - SCI 102 13
Fermentation Pyruvate in the cytosol is converted into lactate or ethanol and carbon dioxide Lactic acid fermentation produces lactic acid from pyruvate Alcoholic fermentation produces alcohol and CO 2 from pyruvate Does not produce ATP Fermentation is needed to convert the NADH produced during glycolysis back to NAD +, which needs to be continuously available for glycolysis to happen Introduction to Life Processes - SCI 102 14
Fermentation Lactate fermentation When muscles are deprived of oxygen, they do not stop working immediately During vigorous activity, muscles become sufficiently low on oxygen and perform glycolysis to produce two ATP molecules per glucose This provides a brief burst of speed The muscle cells ferment the resulting pyruvate molecules to lactate, using electrons from NADH and hydrogen ions Introduction to Life Processes - SCI 102 15
Fermentation Lactate fermentation Example: Joe bicycles up a hill during the neighborhood biking race. As pedals up the hill, he feels the burn in his legs. His muscles are shifting away from cellular respiration due to the lack of oxygen and shifting towards lactic acid fermentation to produce energy in the leg muscles. Example: Bacteria in the mouth feed off of sugars that we eat. As they ferment the sugar, they produce lactic acid which causes cavities in the teeth. Introduction to Life Processes - SCI 102 16
Fermentation Alcohol fermentation Pyruvate is converted into ethanol (an alcohol) and carbon dioxide This releases NAD +, which is then able to accept more highenergy electrons during glycolysis Many microorganisms use alcoholic fermentation when they are in anaerobic conditions Example: yeast Introduction to Life Processes - SCI 102 17
Fermentation Fermentation of yeast: When yeast ferments, it produces carbon dioxide gas which causes bread dough to rise (the carbon dioxide gas takes up space and pushes the dough to expand) If a single yeast cell undergoes alcohol fermentation and uses 50 molecules of glucose, it will only generate 100 molecules of ATP (for every molecule of glucose, 2 ATPs are produces) This is much less energy than in cellular respiration Introduction to Life Processes - SCI 102 18