Chapter 9 How Cells Harvest Energy Cellular Respiration Life is work Living cells require a constant input of free energy from outside sources for growth, reproduction and maintenance of organization. The energy stored in the bonds of organic molecules (C-H) is the ultimate result of the transfer of free energy from the sun. Overview All of the reactions involved in the breakdown and harvesting of free energy from organic molecules are oxidation reactions. These reactions are not simple transfers of electrons, they are also dehydrogenations. Enzyme facilitated REDOX Intermediate molecules are used throughout the stages of cell respiration to prevent direct transfer of electrons to O2 directly BOOM!! This is accomplished via; 1. Soluble carriers 2. Membrane bound carriers 3. Within membrane carriers Generation of ATP In cells, there are 2 methods that are used to make ATP: 1. Substrate-level phosphorylation 1. Oxidative Phosphorylation Substrate level phosphorylation ATP is formed when a phosphate is transferred to an ADP from a phosphate bearing intermediate molecule. 3 Main Stages of Respiration Cellular Respiration occurs in 3 main stages: Glycolysis: exergonic / occurs in cytoplasm. Krebs cycle: exergonic / occurs in mitochondria. Electron Transport: endergonic / occurs in the mitochondria. Glycolysis splitting of sugar Approximately 10 steps involved Occurs in virtually all living cells. Considered ancient metabolism Occurs in the cytosol Requires the expenditure of 2 ATP molecules to initiate the reaction. Pyruvic acid is toxic to cells and must be detoxed. Glycolysis Results 1. 4 ATP 2. 2 NADH Total net gain = 2 ATP 2 NADH 2 NADH * Organisms such as yeast and bacteria can satisfy their energy needs with glycolysis alone. Citric Acid Cycle (TCA) This step occurs within the mitochondrial matrix only if molecular oxygen is present. This path is also called the Krebs cycle after Hans Krebs in the 1930s. This cycle oxidizes pyruvate releasing much of its stored energy. Results of Krebs At this point the original glucose molecule has been completely dismanteled providing the following: 1. 4 ATP NADH 3. 2 FADH CO2 * This total accounts for glycolysis and Krebs. Electron Transport and Chemiosmosis The spatial arrangement of membrane proteins allows the mitochondrion to use a gradient of H+ ions to drive ATP synthesis. Grand Finale Total 3 ATP x 2 NADH glycolysis 3 ATP x 2 NADH pyruvic acid to acetyl Co 3 ATP x 6 NADH Krebs 2 ATP x 2 FADH2 Krebs __________________________ 34 ATP 2 ATP glycolysis 2ATP Krebs __________________________ Total 36 38 ATP Fermentation/Anaerobic Respiration Some organisms can thrive in an environment without oxygen as long as: continuous supply of glucose continuous supply of NAD+ Types of Fermentation Alcoholic Fermentation: Yeast and bacteria convert glucose to ethanol and CO2. Lactic Acid Fermentation: Bacteria in dairy industry and muscle cells convert glucose into lactic acid. In humans this is transported to the liver to be converted back into pyruvic acid. Glucose 2 Pyruvic acid 2 ethanol Glucose 2 pyruvic acid 2 lactic acid Poisons Affect ETC Rotenone : Binds to an electron carrier in the 1 st protein complex. Prevents electrons from passing to next carrier. Cyanide and carbon monoxide: Bind with a carrier in the 3 rd complex blocking flow of electrons to oxygen. No H+ gradient formed. Oligomycin: Antibiotic that blocks the passage of H+ ions through the ATP synthase. Dinitrophenol (uncouplers): Make the membrane leaky to H+ preventing a concentration build up. Organic Molecules as Food Polysaccharides are converted to glucose directly. Proteins are hydrolyzed to amino acids. The needed parts are converted into pyruvic acid, acetyl CoA or an organic of the Krebs cycle. Fats are digested. The glycerol is converted into G3P for glycolysis and the fatty acids are changed into acetyl CoA.