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Transcript
Introduction to Metabolism What is energy? • the ability to do work – carry out life functions Energy is used to: • Break down larger compounds (catabolism) + • Build complex substances (anabolism) ______________ metabolism Types of Energy Plants use solar (light ) energy Glucose bonds store chemical potential energy Moving objects possess kinetic energy Living organisms release thermal (heat) energy Laws of Thermodynamics How energy flows between organisms is governed by the Laws of Thermodynamics 1st law of Thermodynamics – energy cannot be created or destroyed. It can only be converted from one form to another. Example: photosynthesis: plants absorb solar energy chemical potential energy (in bonds of a sugar molecule) When energy transformations occur, some energy is lost as heat and is not available to do work (not free energy). When energy is lost, we say the entropy (disorder) is increasing 2nd law of thermodynamics – all energy transformations increase the entropy of the universe Introduction to Metabolism continued Most of our available energy is stored as chemical potential energy in the covalent bonds of the food we eat. Main source of energy: Glucose – C6H12O6 C6H12O6 + O2 CO2 + H2O + Energy energy When the bonds in a glucose molecule are broken, and atoms are rearranged to form products - energy is released. Some of this energy becomes available to do work (called Gibbs free energy). Some energy is lost as heat. The change is free energy represented as ΔG. Oxidation of glucose: ΔG = - 2870 kJ/mol Why a negative value? ΔG = G final – G initial The negative sign shows that the products have less chemical potential energy (i.e stronger bonds, more stable) than the reactants. This means: The energy released when the products form is greater than the energy absorbed when the reactants’ bonds broke. Some of this difference in energy is now available or “free” to do work – move, produce light, sound. Some is lost as heat. Reactions that release energy are called Exergonic. Catabolic reactions are exergonic. Compare to photosynthesis: Energy + CO2 + H2O C6H12O6 + O2 ΔG = 2870 kJ/mol The products have more chemical potential energy energy. There has to be an investment of energy to make this reaction possible. The energy came from the sun. Reactions that require energy to be absorbed before they proceed are called Endergonic reactions. Anabolic reactions are endergonic. Balanced equation: C6H12O6 + 6O2 6CO2 + 6H2O + 36 ATP The controlled stepwise oxidation of sugar that occurs in the cell preserves useful energy, unlike the simple burning of the same fuel molecule. In the cell, enzymes catalyze oxidation via a series of small steps in which free energy is transferred in conveniently sized packets to carrier molecules — most often ATP and NADH. At each step, an enzyme controls the reaction by reducing the activation energy barrier that has to be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (A) and (B). Thousands of chemical reactions just occurred in your body. Some required energy (anabolic). From where did this energy come? ATP – adenosine triphosphate ATP is a universal molecule of energy transfer – like a cell’s currency. Any energy made available by some cellular process (ex: cell respiration) is first transferred to ATP. If that energy is needed later, it is released by ATP ATP –Structure & Synthesis A Inorganic phosphate P P Adenine O P ribose - the high energy phosphate bonds (~) are very unstable & can be easily hydrolyzed (by adding H2O) Energy + ADP + Pi ATP + H2O ATP + H2O ADP + Pi + Energy ∆ G = - 31 kJ/mol exergonic (energy released) – available to do work (free energy) ∆ G = 31 kJ/mol endergonic (energy taken in) The process by which ATP is synthesized (from ADP) and broken down (to ADP) is the basis of cell metabolism. ADP and ATP are shuttled throughout your cells to: a) Provide energy for endergonic reactions like building macromolecules, contracting muscles b) Store the energy released when exergonic reactions occur (ex: glucose broken down) Note: When ADP binds a Pi, ATP is made. This reaction is catalyzed by an enzyme. ATP made in this manner is called substrate-level phosphorylation. The addition of a Pi group to any molecule is termed phosphorylation. The Release of Energy in the Cell Recall: the explosive burning (oxidation) of glucose How do you convert the great deal of free energy in glucose into the small more easily managed energy molecules called ATP? Need the help of: • Enzymes • Coenzymes – not proteins, much smaller, assist enzymes, act as electron carriers – involved in redox reactions. Include: NAD+ – nicotinamide adenine dinucleotide NADP - nicotinamide adenine dinucleotide phosphate FAD – flavin adenine dinucleotide The transfer of glucose energy to ATP can be accomplished by: i) aerobic respiration a) glycolysis b) kreb’s cycle c) electron transport system ii) Anaerobic respiration- a) ethanol fermentation b) lactic acid fermentation Both aerobic and anaerobic respiration begin with the same set of reactions called glycolysis – “sugar splitting” - occurs in the cytoplasm - No oxygen is required - involves 9 enzyme mediated reactions Glycolysis – occurs in cytoplasm glucose ATP outside cell membrane ADP Glucose – 6 – phosphate Hexokinase – breaks down ATP and 1 Pi attaches to glucose (phosphorylation of glucose) & ADP is released Phosphoglucoisomerase – takes molecule and rearranges it into a fructose Fructose – 6 – phosphate Phosphofructokinase (PFK) – takes Pi from ATP and attaches it; ADP is release ATP ADP - it is the rate limiting step aka the flux generating step Fructose – 1, 6 - bisphosphate Aldolase – splits molecule in 2 PGAL (phosphoglyceraldehyde) -aka glyceraldehyde 3-phosphate (G3P) PGAL (phosphoglyceraldehyde) NAD+ Pi NADH +H+ Same as right side 1, 3 - bisphosphoglycerate ADP ATP 3 - phosphoglycerate Phosphate dehydrogenase – Pi (present in cytoplasm) attaches to PGAL , 2H+ & 2 e- are taken away (from PGAL) and given to NAD+ to make NADH + H+ Phosphoglycerokinase – 1 Pi group removed and added to ADP to form ATP (substrate level phosphorylation) Phosphoglyceromutase – changes location of Pi group onto 2nd C so it is more balanced 2 - phosphoglycerate H2O Enolase – takes out H2O and makes molecule symmetrical PEP (phosphophenol pyruvic acid) Same as right side PEP (phosphophenol pyruvic acid) ADP ATP Pyruvate kinase – P group removed and joined with ADP to make ATP (substrate level phosphorylation) pyruvate w/ O2 – aerobic respiration in Kreb’s Cycle 2 ATP net gain 2 NADH + 2 H+ w/out O2 – fermentation Net Glycolytic Equation C6H12O6 + 2 ATP + 2NAD+ 2 PYRUVATE + 4 ATP + 2 NADH + 2 H2O there is a net gain of 2 ATP from glycolysis recall: glucose = – 2870 kJ/mol (total possible released) glycolysis = 2 x ( – 31 kJ/mol) (1 ATP = – 31 kJ/mol) = – 62 kJ/mol efficiency = – 62 kJ/mol x 100% – 2870 kJ/mol = 2% there is only 2% of the possible energy released by glycolysis glycolysis is extremely inefficient @ harvesting energy - very small cells, like yeast, bacteria can live like this but our cells cannot ...if O2 is present, the pyruvate enters the inner Matrix of the mitochondria for the Krebs cycle & ETS Structure of Mitochondrion (pl: mitochondria) Preparatory stage (aka Transition Stage) Multiply all products by 2. Why? Decarboxylation – CO2 removed from pyruvate Acetyl – CoA Net products: 6 NADH 2 ATP 2 FADH2 4 CO2 CoA x 2 everything oxaloacetate NADH + H+ NAD+ malate Coenzyme A (aka Citric Acid Cycle) NAD+ NADH + H+ succinate CO2 - ketoglutarate fumarate FAD H2 O H2 O isocitrate Krebs Cycle H2 O FADH2 citrate ATP Pi + ADP GTP (guanosine Pi + GDP triphosphate) NAD+ NADH + H+ Coenzyme A CO2 Succinyl – CoA Inner mitochondrial membrane Kreb cycle Proton cyt b Proton pump e- e - Q FMN NADH pump H+ H+ O2 O O 2eO2e + 2H+ H2O ATP to make ADP + Pi (+) H+ FAD NAD+ -) ETS A T P a s e cyt a cyt a3 Proton pump INNER MATRIX H+ cyt c1 FADH2 (6 NADH, 2 FADH2, 2 ATP) ( H+ H+ Space b/w inner & outer m/b H+ H+ H+ H+ + H H+ H+ H+ + + + H + H H H H+ H+ H+ So, how many ATP are made? For every H+ ion that is pumped out, 1 re-enters and the energy it releases (as it moves down the electrochemical gradient) is used to make 1 ATP So, 34 H+ pumped out, 34 re-enter = 34 ATP made by ETC 34 ATP + 2 ATP + 2 ATP = 38 ATP ETS glycolysis Kreb’s Efficiency of aerobic respiration : 38 x (31kJ/mol) ÷ 2870 kJ/mol = 41% Note: One small problem- the 2 NADHs produced in glycolysis (in the cytoplasm) must be brought into the mitochondrion at a cost of some energy, usually estimated to be 1 ATP per NADH. So, final ATP count is 36. How many O2 molecules are required as the final electron acceptors (for each glucose molecule)? 10 NADH pass on 2e- each 20 e2 FADH2 pass on 2e- each 4 eEach oxygen atom has room for 2 electrons in outer shell 2 6 O2 = 2 oxygen atoms, each accept 2e- = 4e- 24e- ÷ 4e-(per O2 molecule) = 6O2 Recall: Oxygen atom accepts 2e-, 6O2 + 24e- + 24 H+ = 12 H2O molecules 6 H2O molecules get used up in previous reactions, net gain of 6H2O Alleluia, alleluia, alleluia………. Overall equation for cell respiration (aerobic respiration), the process by which the energy stored in glucose is released and stored in ATP is: C6H12O6 + O2 6CO2 + 6H2O + 36 ATP Why is this chemical equation somewhat deceiving? Cell Respiration song http://www.youtube.com/w atch?v=3aZrkdzrd04 Fermentation Recall: NAD functions in the cell as an energy transport compound. The cell has a limited supply of this compound. In glycolysis, 2 molecules of NAD+ are reduced to NADH + H+. Under aerobic conditions, the NADH transfer their H and 2e- to the ETC (where O2 is the final electron acceptor). But, without O2, how would NADH unload the electrons it picked up? If NADH doesn’t get oxidized the cell’s supply of NAD+ would run out. The result …… glycolysis would stop (no NAD+ as a reactant) and the cell would die from lack of ATP. All single celled organisms (like bacteria) that can currently live in areas without oxygen (anaerobic) would cease to exist. So...how to keep glycolysis going? NADH must find another acceptor for H & its electron(s). That acceptor is…… ….Pyruvate. The process that enables a cell to continue synthesizing ATP by the breakdown of glucose under anaerobic conditions is FERMENTATION. 2 types: 1. Lactic Acid (lactate) fermentation 2. Ethanol Fermentation (aka alcoholic fermentation) Lactic Acid (lactate) Fermentation • • • • • • Occurs in fungi (cheese making), bacteria (in yogurt) and muscles depleted of O2 pyruvate becomes the acceptor of H atoms and e- from NADH. NAD+ is shuttled back to the glycolytic pathways so ATP can continue to be made (rate ↑). Pyruvate becomes lactate Enzyme LDH – lactate dehydrogenase mediates this process. During strenuous exercise, muscles cells have greater demand for ATP, not enough O2. Lactic acid produced – causes muscle soreness O2 debt “paid back” by deep breathing Lactic acid removed to the liver and converted to glucose Ethanol Fermentation • • • • Involve yeast – single celled fungi Occurs in bread making (CO2 forms bubbles in dough and alcohol evaporates during baking) beer, wine, champagne (CO2 does not escape) CO2 is removed from pyruvate to become acetaldehyde, then acetaldehyde accepts H to become ethanol (aka ethyl alcohol, grain alcohol) Final products= ethanol + CO2 Other Fuel Sources