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Cell Metabolism BIG PICTURE The sun provides the energy that powers all life Animals depend on plants to convert solar energy to chemical energy This chemical energy is in the form of organic molecules Cellular Respiration The main way that chemical energy is harvested from organic molecules and converted to ATP Series of catabolic reactions This is an aerobic process —it requires oxygen OVERALL EQUATION Carbon Glucose Oxygen Dioxide Water Energy Oxidation-Reduction Reactions Reactions transferring electrons from one molecule to another Molecules that lose electrons are said to be oxidized Molecules that gain electrons are said to be reduced Movement of electrons is usually associated with movement of hydrogen atoms Which molecule is oxidized and which is reduced during cellular respiration? Glucose Oxygen Carbon Dioxide Water Energy Glucose loses electrons (and hydrogen) Oxidation Reduction Oxygen gains electrons (and hydrogen] Enzymes Involved Dehydrogenases - enzymes that catalyze redox reactions by removing hydrogen Most require coenzymes that are able to accept and carry the electrons (electron carriers) Nicotinamide adenine dinucleotide (NAD+) – derived from niacin Flavin adenine dinucleotide (FAD) – derived from riboflavin Electron Carriers Electron Carriers NAD+ FAD Stages of cellular respiration Mechanisms of ATP Synthesis Substrate-level phosphorylation Enzymes transfer a phosphate group from a substrate to ADP Occurs during glycolysis and Krebs cycle Mechanisms of ATP Synthesis Oxidative phosphorylation The phosphorylation of ADP is powered by a series of redox reactions that transfer electrons from organic molecules to oxygen Produces the majority of the ATP molecules In electron transport system Stage 1: GLYCOLYSIS Occurs in the cytoplasm Does not require oxygen (anaerobic) Six carbon glucose molecule is broken down into 2 three carbon molecules of pyruvic acid Produces 2 net ATP and 2 NADH (electron carrier) INVESTMENT STAGE P A Y O F F S T A G E Stage 1: GLYCOLYSIS The Fate of Pyruvic Acid Depends on the availability of oxygen In aerobic conditions pyruvic acid is converted to acetyl-CoA and enters the Krebs cycle In anaerobic conditions NADH + H+ reduces pyruvic acid to form lactic acid Lactic Acid Fermentation Produces 2 ATP per glucose (less efficient) When oxygen becomes available again lactic acid is oxidized back to pyruvic acid and enters the Krebs cycle. Stage 1½ : TRANSITION Pyruvic acid enters the mitochondrial matrix through facilitated diffusion There it is converted to Acetyl-Coenzyme A to enter Krebs cycle 1 CO2 and 1 NADH is produced in this stage per pyruvate Stage 2: KREBS CYCLE Occurs in the matrix of the mitochondria Requires oxygen (aerobic) Completes the breakdown of glucose to CO2 and harvests the energy as: 2 ATP Citric acid 6 NADH 2 FADH2 4 CO2 (per 1 glucose) Two of these cycles per 1 glucose molecule Oxaloacetic acid Stage 2: KREBS CYCLE Acetate joins the 4 carbon compound oxaloacetate to form the 6 carbon compound citrate 2 decarboxylation events release 2 CO 2 Stage 2: KREBS CYCLE Four oxidation events generate 3 NADH and 1 FADH2 1 molecule of ATP is formed via substrate-level phosphorylation Note these numbers are per cycle. For each glucose molecule you have two cycles. Stage 3: Electron Transport Chain The hydrogen being delivered to the ETC by the coenzymes are split into electrons and H+ ions Electrons from NADH and FADH2 are passed down a chain of protein complexes embedded in the inner membrane of the mitochondria Stage 3: Electron Transport Chain Electrons fall to lower energy levels as they are passed down the chain (releases energy) Oxygen is the final electron acceptor The negative oxygen binds to 2 H+ to form water Stage 3: Electron Transport Chain Chemiosmosis The energy released by electrons moving down the chain is used to pump H+ from the matrix to the intermembrane space This creates a proton gradient (potential energy) Stage 3: Electron Transport Chain This gradient drives protons back in through a protein called ATPsynthase This creates kinetic energy that ATPsynthase harnesses to catalyze ADP + P ATP (oxidative-phosphorylation) Summary of ETC Metabolic pool concept: any organic molecule can be used in respiration Lipid Metabolism Lipolysis – the hydrolysis of triglycerides into glycerol and fatty acids Catalyzed by the enzyme lipase Lipid Metabolism The glycerol is converted into glyceraldehyde phosphate a glycolysis intermediate Which then enters into Krebs cycle Complete oxidation of glycerol yields 18 ATP molecules Lipid Metabolism The fatty acid chains Are broken apart into 2 carbon acetic acid fragments (Beta-oxidation) Coenzyme A is attached to the acetic acid fragments forming Acetyl CoA Enters the Krebs cycle Complete oxidation yields ~54 ATP Ketogenesis If Acetyl CoA production exceeds the capacity of the Krebs cycle to process it, the liver will convert it to ketone bodies which are released into the blood Ketones can be used as an energy source in skeletal and cardiac muscle Examples of ketones: acetoacetic acid, Bhydroxybutyric acid, acetone Ketosis Ketosis - an increase in circulating ketone bodies Occurs when lipids are the primary energy source (starvation and diabetes mellitus) May lead to ketoacidosis – decreased blood pH Depresses nervous system, may become comatose Compensatory response: Increased ventilation and large amounts of ketones excreted in urine Protein Metabolism Proteins are hydrolyzed into individual amino acids by proteases Amino acids are deaminated in the liver (amine group is removed) Amine group is removed as ammonia Combined with CO2 to form urea which is excreted by the kidneys Protein Metabolism: Deamination Also generates organic acids which can be converted to glucose or enter Krebs cycle to be oxidized for energy Protein Metabolism Glucose Synthesis Aerobic metabolism of glucose is the most efficient way for cells to make ATP It is the primary source of energy in cells and normally the ONLY source in neurons There is multiple metabolic pathways for producing glucose to ensure that there is a continuous supply for the brain Synthesis of Glucose Glycogenolysis – the breakdown of glycogen to glucose The liver and skeletal muscle contains high concentrations of glycogen Synthesis of Glucose Gluconeogenesis synthesis of glucose from non-carbohydrates Can start with glycerol, lactic acid or various amino acids Occurs in the liver and kidneys