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Chapter Eight How Cells Harvest Energy I. Introduction – Equation for cellular respiration – Review Terms: • • • • • Autotrophs vs Heterotrophs Oxidation vs Reduction Exergonic vs Endergonic Substrate level phosphorylation vs Chemiosmotic (oxidative) phosphorylation Aerobic vs anaerobic II. Phases in Aerobic cellular respiration – Major inputs/outputs of: Glycolysis, Preparatory (transition) reaction, Krebs (Citric Acid) Cycle, ETS – Where they occur in cell, if oxygen is required to be present, if oxygen is used, # ATP’s generated III. Anaerobic cellular respiration – Types of anaerobic respiration – Yeast vs animal cells – Advantages vs Disadvantages IV. Metabolic Pool (Terms: catabolism & anabolism) • Introduction • Autotrophs - “self feeders”; organisms that produce own organic molecules – photosynthesis • Heterotrophs - “fed by others”; organisms that consume molecules produced by other organisms • Cellular Respiration - Conversion of carbohydrates and other metabolites (e.g. fats, proteins) to ATP • ATP - Provides just the right amount of energy for cellular work with no waste • Glucose - Most common molecule used to generate ATP • Conversion of glucose to ATP is about 39% efficient • What happens to the rest of the energy? – “lost” as heat [aka metabolic heat] • Glucose is a type of Carbohydrate • Why is ATP used for cell work and not glucose? – Glucose releases too much energy atone time – wasteful – ATP releases just right amount of energy Fig. 7.2 • Chemical equation for cellular respiration: Oxidation (CH2O)n + O2 CO2 + H2O + energy Reduction • Oxidation - term used to indicate that a molecule has lost hydrogen atoms – Which reactant is being oxidized during cellular respiration? (CH2O)n • Reduction - term used to indicate that a molecule has gained hydrogen atoms – Which reactant is being reduced during cellular respiration? O2 • Endergonic or Exergonic Reaction? – C6H12O6 --------> CO2 + H2O Exergonic Energy released drives the other reaction – ADP + P --------> ATP Endergonic • CONCEPT: Exergonic reactions are coupled with endergonic reactions. • During respiration, glucose is oxidized to CO2. However if electrons were given directly to O2, the reaction would be Combustible and cells would burst into flames. • So cells transfer electrons to electron carriers: 1. 2. 3. Soluble carriers that move electrons from one molecule to another Membrane-bound carriers that form a redox chain Carriers that move within membranes • Example: Enzymes catalyze these redox reactions with the help of cofactors such as nicotinamide adesosine dinucleotide (NAD+). NAD+ accepts a pair of electrons (redox) from the substrate as well as a proton. Another proton is donated to the solution. This forms NADH which is an electron carrier. This reaction is reversible. Other electron carriers include FAD+ and cytochromes. Fig. 7.3 Fig. 7.1 • Aerobic - term used to denote “with oxygen” • Anaerobic - term used to denote “without oxygen” • Does the chemical equation following reaction represent aerobic or anaerobic cellular respiration? • C6H12O6 + O2 --------> CO2 + H2O + 36 ATPs Aerobic Low energy products • Does the chemical equation following reaction represent aerobic or anaerobic cellular respiration? • C6H12O6 --------> C2H5OH + CO2 + 2 ATPs Anaerobic Still has high level of energy • CONCEPT: Oxygen MUST be present for glucose to be completely oxidized to CO2 – aerobic respiration: final e- acceptor oxygen (O2) – anaerobic respiration: final e- acceptor inorganic molecule (not O2) – fermentation: final e- acceptor is an organic molecule • In the presence of oxygen, one (1) molecule of glucose yields approximately 36-38 (net) molecules of ATP • 4 (net) ATP’s are generated via substrate-level phosphorylation • 32-34 ATP’s are generated via oxidative phosphorylation (chemiosmosis/ETS driven by atoms supplied from hydrogen) • Hydrogen atoms are supplied by Glucose • Hydrogen atoms are carried to ETS of mitochondria by coenzymes: • NAD+ gains 1 hydrogen → NADH (each produces 2-3 ATPs) • FAD gains 2 hydrogen → FADH2 (each produces 2 ATPs) • When NAD/FAD pick up hydrogen atom(s), they are Reduced • Metabolism harvests energy in stages. The larger the release of energy in any single step, the more of that energy is released as heat. • Cells make ATP by two different mechanisms. 1. Substrate-level phosphorylation (SLP) – ATP formed by transferring a phosphate group directly to ADP – Occurs in the cytoplasm – all cells can perform this type of phosphorylation – Glycolysis 2. Oxidative phosphorylation (OP) – ATP synthesized by the enzyme ATP Synthase by using energy from a proton gradient – Requires oxygen – Occurs in the mitochondria – Krebs Cycle (citric acid cycle) and Electron Transport Chain (ETS) Phases in Aerobic cellular respiration Inputs/Outputs Where Occurs 02 required / 02used Net ATP Glycolysis glucose → pyruvate Cytoplasm No / ---- 2 (SLP) Preparatory Stage pyruvate → acetyl Mitochondria Yes/ No ------ CoA / Matrix oxaloacetate → citrate Mitochondria Yes / No 2 (SLP) Yes / Yes 32 (OP) Krebs Cycle / Matrix ETS Hydrogen → H20 Mitochondria/ Cristae Fig. 7.5 • IMPORTANT: Aerobic cellular respiration includes glycolysis (an anaerobic process). – See Aerobic cellular respiration flow chart. • Interesting Facts about ATP production – In men, have enough to sustain life for one minute – Some poisons inhibit ATP synthase – Some tissues have more mitochondria • Chicken: dark - legs (more myoglobin) vs. light – breasts (less myoglobin) – Rigor mortis due to lack of ATP (which is required for muscle relaxation. • Glycolysis A. Glucose priming – 3 reactions that prime glucose to be cleaved and requires energy (2 ATP) B. Cleavage and rearrangement – split glucose into G3P C. Oxidation – 2e- and 1 proton are transferred from G3P to NAD+ forming NADH D. ATP generation – produces a pyruvate and 2 ATP molecules from each G3P ( 2 pyruvate and 4 ATP made) • 4 ATP formed but 2 ATP used which results in 2 ATP gained. Glycolysis • Preparatory stage – Occurs if oxygen is present – Location – mitochondria – Pyruvate + NAD+ + CoA → acetyl – CoA + NADH + CO2 + H+ • NADH used later • Acetyl group enters the next step – Krebs (citric acid) cycle • Krebs Cycle - Has Three segments 1. Acetyl-CoA + oxaloacetate → 6-carbon citrate molecule 2. Citrate rearrangement and decarboxylation • reduces citrate to a 5-carbon intermediate and then to a 4-carbon succinate • produces 2 NADH and 1 ATP 3. Regeneration of oxaloacetate • Succinate undergoes a series of reactions to return back to oxaloacetate so it can reenter the cycle • Producing one NADH and one FADH2 • NOTE: Through a series of 9 reactions the Krebs cycle extracts electrons and synthesizes 1 ATP each cycle. The Krebs cycle turns twice with each glucose molecule. Krebs Cycle • Electron Transport Chain and Chemiosmosis – ETC produces a proton gradient • Gradient is formed as electrons move through electron carriers located in the mitochondrial membrane. • Each electron attracts a proton and transfers them into the intermembrane space of the mitochondria. – Chemiosmosis utilizes the electrochemical gradient to produce ATP • The mitochondrial matrix is negative compared to the intermembrane space • The positive protons are attracted to the negative matrix and want to diffuse from high concentration to low concentration • The protons can only pass through a protein channel created by ATP Synthase • As protons pass through ATP synthase the protein rotates taking kinetic energy and storing it as potential energy in newly formed ATP. http://vcell.ndsu.nodak.edu/animations/etc/ movie-flash.htm http://vcell.ndsu.nodak.edu/animations/atp gradient/movie-flash.htm Fig. 7.16 • Anaerobic cellular respiration = FERMENTATION (pg. 140) • Pyruvate is the pivotal metabolite in cellular respiration. • In man, the major driving force affecting the fate of pyruvate is the presence or absence of oxygen!!! • If oxygen is present, pyruvate enters into the preparatory stage and aerobic cellular respiration occurs. • If oxygen is not present: • fermentation occurs • oxygen is NOT used as a final electron acceptor • water is not formed FERMENTATION Glycolysis Glucose net 2 ATP pyruvate PRODUCTS Yeast Cells CO2 + C2H5OH Carbon dioxide + ethanol Alcoholic Fermentation Animal Cells C2H5 COOH (lactic acid) pH causes muscle fatigue In liver cells some lactic acid is converted back to pyruvate and reprocesses it via aerobic respiration (O2 recovery) NOTE: Fermentation includes glycolysis. The NADH created during glycolysis must be recycled to continue respiration. The NADH reduces the organic compound present (ethanol or lactic acid) in order to regenerate NAD+. Advantages Disadvantages Rapid burst of energy short lived Economic Benefits: (pg. 141) Low ATP yield Yeast: making of beers, wines and 2% efficiency bread. Anaerobic bacteria: making of cheese, yogurt, ect. And Isopropanol, acetic acid, ect. Products are toxic • Some organisms use inorganic molecules to regenerate NAD+ from NADH – Methanogens • CO2 reduced to CH4 • Methanobacterium – Sulfur Bacteria • SO4 reduced to H2S • Archaeon - Thermoproteus tenax Metabolic Pool Anabolism Metabolism = + Catabolism Synthetic Degradative Building up Breaking down Catabolic reactions release (produce) energy which may be used to drive Anabolic reactions • Other types of molecules (metabolites) may be used for the generation of ATP such as Proteins & Fats. • Proteins remove amino groups – – – – First the protein is broken into individual amino acids Amno group is then removed in a process called deamination Carbon chains that remain enter glycolysis and the Krebs cycle Non-essential (11) vs. Essential (9) amino acids • 11 can be synthesized (non-essential) • 9 can not be synthesized – supplied by diet (essential due to lack of enzymes) • Fats – Broken into fatty acids and glycerol – Produce acetyl group that then enters the Krebs cycle Fig. 7.20