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K. Muma Bio 6 Cell Metabolism Study Objectives: 1. Define oxidation and reduction. 2. Describe the mechanisms of ATP synthesis: substrate level phosphorylation vs. oxidative phosphorylation 3. Write the overall general equation for cellular respiration. 4. Describe the role of dehydrogenases and coenzymes NAD and FAD in cellular respiration. 5. Distinguish between anaerobic (lactic acid fermentation) and aerobic respiration in terms of when they occur and the total number of ATP produced per glucose molecule. 6. Describe the basic stages of aerobic respiration: glycolysis, transitional stage, Krebs cycle and Electron Transport System. For each stage include: What goes into each stage and what is produced in each stage? Where does each stage occur in the cell? Is it anaerobic or aerobic? 7. Define lipolysis and explain how glycerol and fatty acids are used for cellular respiration. 8. Define beta-oxidation, ketogenesis, ketosis, and ketoacidosis 9. Describe protein metabolism. Explain the role of proteases, deamination, and organic acids in utilizing proteins for cellular respiration. 10. Define glycogenolysis and gluconeogenesis. 1 Cell Metabolism Outline I. The big picture: a. The sun provides the energy that powers all life b. Animals depend on plants to convert solar energy to chemical energy c. This chemical energy is in the form of organic molecules which animals then eat to convert into ATP through cellular respiration II. Cellular Respiration a. Overview i. Cell respiration is the main way that chemical energy is harvested from organic molecules and converted to ATP ii. It involves a series of catabolic reactions iii. This is an aerobic process —it requires oxygen iv. OVERALL EQUATION b. Oxidation-Reduction Reactions (review these types of reactions on page 103 of your textbook). i. Reactions transferring electrons from one molecule to another ii. Molecules that lose electrons are said to be oxidized iii. Molecules that gain electrons are said to be reduced iv. Movement of electrons is usually associated with movement of hydrogen atoms v. Look at the overall equation above: which molecule is oxidized and which is reduced during cellular respiration? c. Enzymes Involved i. Dehydrogenases - enzymes that catalyze redox reactions by removing hydrogen ii. Most require coenzymes that are able to accept and carry the electrons (electron carriers) 1. Nicotinamide adenine dinucleotide (NAD+) – derived from niacin 2. Flavin adenine dinucleotide (FAD) – derived from riboflavin 2 d. Three overall stages of cellular respiration i. Glycolysis ii. Krebs Cycle (a.k.a. Citric Acid Cycle) iii. Electron Transport Chain e. Mechanisms of ATP Synthesis i. Substrate-level phosphorylation 1. Enzymes transfer a phosphate group from a substrate to ADP 2. Occurs during glycolysis and Krebs cycle ii. Oxidative phosphorylation 1. The phosphorylation of ADP is powered by a series of redox reactions that transfer electrons from organic molecules to oxygen 2. Produces the majority of the ATP molecules 3. In electron transport system III. Stage 1: GLYCOLYSIS a. Overview i. Occurs in the cytoplasm ii. Does not require oxygen (anaerobic) iii. Six carbon glucose molecule is broken down into 2 three carbon molecules of pyruvic acid iv. Produces 2 net ATP and 2 NADH (electron carrier) *The image above is a simplified version of glycolysis showing what goes in and what is produced. Each little gray ball represents a carbon. Also see figure 2-11 in your textbook. 3 b. The Fate of Pyruvic Acid (see figure 2-16) i. Depends on the availability of oxygen ii. In aerobic conditions pyruvic acid is converted to acetyl-CoA and enters the Krebs cycle iii. In anaerobic conditions NADH + H+ reduces pyruvic acid to form lactic acid c. Lactic Acid Fermentation i. Produces 2 ATP per glucose (less efficient) ii. When oxygen becomes available again lactic acid is oxidized back to pyruvic acid and enters the Krebs cycle IV. Stage 1½ : TRANSITION a. Pyruvic acid enters the mitochondrial matrix through facilitated diffusion b. There it is converted to Acetyl-Coenzyme A to enter Krebs cycle c. 1 CO2 and 1 NADH is produced in this stage per pyruvate V. Stage 2: KREBS CYCLE a. Overview i. Occurs in the matrix of the mitochondria ii. Requires oxygen (aerobic) iii. Completes the breakdown of glucose to CO2 and harvests the energy as: 1. 2 ATP 2. 6 NADH 3. 2 FADH2 (Numbers based on per one glucose molecule) b. Events (see figure 2-12): i. Acetate joins the four carbon compound oxaloacetate to form the 6 carbon compound citrate ii. 2 decarboxylation events release 2 CO 2 iii. Four oxidation events generate 3 NADH and 1 FADH2 iv. 1 molecule of ATP is formed via substrate-level phosphorylation 4 VI. Stage 3: Electron Transport Chain a. The hydrogen being delivered to the ETC by the coenzymes are split into electrons and H+ ions b. Electrons from NADH and FADH2 are passed down a chain of protein complexes embedded in the inner membrane of the mitochondria c. Electrons fall to lower energy levels as they are passed down the chain (releases energy) d. Oxygen is the final electron acceptor e. The negative oxygen binds to 2 H+ to form water f. Chemiosmosis (see figure 2-13) i. The energy released by electrons moving down the chain is used to pump H+ from the matrix to the intermembrane space ii. This creates a proton gradient (potential energy) iii. This gradient drives protons back in through a protein called ATPsynthase iv. This creates kinetic energy that ATPsynthase harnesses to catalyze ADP + P ATP (oxidative-phosphorylation) 5 VII. Metabolic pool concept: any organic molecule can be used in respiration a. Lipid Metabolism i. Lipolysis – the hydrolysis of triglycerides into glycerol and fatty acids 1. Catalyzed by the enzyme lipase ii. The glycerol 1. is converted into glyceraldehyde phosphate a glycolysis intermediate 2. then enters into Krebs cycle 3. complete oxidation of glycerol yields 18 ATP molecules iii. The fatty acid chains 1. Are broken apart into 2 carbon acetic acid fragments (Betaoxidation) 2. Coenzyme A is attached to the acetic acid fragments forming Acetyl CoA 3. Enters the Krebs cycle 4. Complete oxidation yields ~54 ATP 6 iv. Ketogenesis 1. 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 2. Ketones can be used as an energy source in skeletal and cardiac muscle a. Examples of ketones: acetoacetic acid, Bhydroxybutyric acid, acetone 3. Ketosis - an increase in circulating ketone bodies a. Occurs when lipids are the primary energy source (starvation and diabetes mellitus) b. May lead to ketoacidosis – decreased blood pH c. Depresses nervous system, may become comatose d. Compensatory response to ketosis: increased ventilation and large amounts of ketones excreted in urine b. Protein Metabolism i. Proteins are hydrolyzed into individual amino acids by proteases ii. Amino acids are deaminated in the liver (amine group is removed) iii. Amine group is removed as ammonia iv. Combined with CO2 to form urea which is excreted by the kidneys v. Generates organic acids which can be converted to glucose or enter Krebs cycle to be oxidized for energy c. Glucose Synthesis i. Aerobic metabolism of glucose is the most efficient way for cells to make ATP ii. It is the primary source of energy in cells and normally the ONLY source neurons prefer iii. There is multiple metabolic pathways for producing glucose to ensure that there is a continuous supply for the brain iv. Synthesis of Glucose 1. Glycogenolysis – the breakdown of glycogen to glucose a. The liver and skeletal muscle contains high concentrations of glycogen 2. Gluconeogenesis - synthesis of glucose from noncarbohydrates a. Can start with glycerol, lactic acid or various amino acids b. Occurs in the liver and kidneys 7 Post-lecture Practice Cell Respiration Worksheet 1. Overall equation for cellular respiration: + What you eat. + What you inhale. What you exhale. + What your cells use for energy! 2. Fill out the table below to organize the main ideas of cellular metabolism. (Assume per glucose molecule for number of electron carriers and ATP): Aerobic or Anaerobic? Location in cell Starting Molecule Ending Molecules # of Electron Carriers made Number of ATP made Glycolysis Transition stage Krebs Cycle (citric acid cycle) Electron Transport Chain Fermentation 8