Transcript File - Pomp
Chapter 9: Cellular Respiration AP Biology Introduction • Chemical E exists in the arrangement of atoms in organic compounds • Cellular respiration is a catabolic pathway Introduction • To replenish ATP in cell, food molecules are constantly broken down • Enzyme mediated reaction Cellular Respiration • Oxidation of food molecules, releasing energy using an inorganic substance as the final electron acceptor. • Usually oxygen is the final electron acceptor (aerobic) Respiration W/O O2 Anaerobic respiration: uses nitrate or sulfate as final electron acceptor Fermentation: the anaerobic breakdown of food molecules in which the final e- acceptor is an organic molecule Oxidation and Reduction • E is gained by the transfer of e’s • The relocation of e-’s releases the stored E and the E ultimately makes ATP Redox Reactions • Oxidation: the loss of e-’s from a substance • Reduction: the gain of e-’s by a substance • E must be added to pull e-’s from an atom Redox Reactions • A redox reaction that relocates an e- from a less electronegative atom to a more electronegative atom loses potential E Cellular Respiration C6H12O6 + 6O2 6H2O + 6CO2 + E Hydrogen • In most redox reactions, it is not only the e- that is transferred but in most biological reactions the whole hydrogen atom is transferred Carbohydrates and Fats • Contain high levels of hydrogen and their electrons • There is a barrier that keeps sugar from combining immediately with O2 • This barrier is reduced inside the body with the help of enzymes Glycolysis • Main function: produce 3C pyruvate molecule from glucose • Takes place in cytoplasm (cytosol) of the cell Steps of Glycolysis 1. Invest 2 molecules of ATP to donate two phosphate groups to ends of glucose molecule creating instability 2. 6C molecule split into two 3C molecules with phosphate group attached(Pi comes from ATP) 3. 3C molecule receives an additional Pi group from cytosol, not from ATP Steps of Glycolysis 4. Each 3C molecule gives up H atom to form two NADH molecules 5.Two 3C molecules lose all phosphate groups to ADP forming 4 ATP molecules Results of Glycolysis • Two 3C pyruvate molecules • 2 NADH molecules • Net of 2 ATP's (4 created - 2 invested in the beginning) Mitochondria Structure • Outer membrane very permeable • Inner membrane: selectively permeable, pyruvate molecules can diffuse in • Matrix: inside inner membrane, protein-rich solution. Many enzymes of Citric Acid cycle dissolved in matrix fluid, rest are attached to inner membrane surface • ETS molecules also located on inner membrane surface • Inner surface of mitochondria similar to the plasma membrane of bacteria Respiration Series of redox reactions using 02 as the final e- acceptor that breaks down organic molecules and releases their energy that was stored as covalent bonds • Occurs inside inner membrane of mitochondria in eukaryotes and the plasma membrane of prokaryotes Preparation of Acetyl Coenzyme A • 2 pyruvate molecules formed by glycolysis enter mitochondrial matrix • Steps –Pyruvate loses one C and two O atoms as a CO2 molecule –Remaining 2C acetyl group is attached to CoA molecule forming acetyl CoA –Reduction of NAD+ molecule to NADH Citric Acid Cycle • Also known as Krebs Cycle, named after the founder Sir Hans Krebs who worked out the cycle in 1937 Cyclic Nature of Citric Acid Cycle • CoA transfers its 2C acetyl group to a 4C molecule of oxaloacetate forming a 6C citrate molecule • Throughout the cycle, the 6C citrate gives up two C atoms in the form of CO2 • Remaining 4C molecule is converted into oxaloacetate, ready to accept another acetyl group from CoA Important Features • H atoms are removed during the cycle and picked up by NAD+ and FAD. H are used in oxidative phosphorylation to power formation of most ATP • 1 ATP molecule is formed by substrate level phosphorylation during each turn of cycle (net per glucose = 2 ATP) • After 2 cycles, all 6 C molecules of original glucose are given off as CO2. 1C is lost from each pyruvate to make acetyl groups and 2C’s from Krebs Cycle Oxidative Phosphorylation • Some E has been made by substrate level phosphorylation of ATP during glycolysis and the Citric Acid Cycle • Most E is now in the form of H atoms carried by NADH and FADH2 • Oxidative Phosphorylation couples the oxidation of H to ATP synthesis by using an electrochemical gradient as an E intermediate Electron Transport Chain • Complexes mostly made of proteins that are embedded on the mitochondrial inner membrane • Multiprotein complexes numbered IIV • Prosthetic groups: non-protein components essential to certain enzymes • ETC makes no ATP directly Actions of the ETC • Electron carriers of the ETC are reduced and oxidized as they accept and donate e-‘s • 1st carrier of chain has lowest e- affinity and last carrier has the highest Steps • NADH passes e- to 1st complex of the ETC called a flavoprotein • FADH2 passes e- to 2nd complex of ETC (1/3 less E than NADH) • Most electron carriers after Complex I are called cytochromes • When the carriers of the 4 complexes are reduced they attract H+ from the matrix and transfer them into the intermembrane space Chemiosmosis • Use of a H+ gradient across a membrane used to drive cellular work Animation • http://www.science.smith.edu/d epartments/Biology/Bio231/etc. html ETS • Consists of a series of carrier molecules in the inner mitochondrial membrane. Some carry H atoms, some only carry e-'s. • FMN and CoQ carry 2 H atoms each • Cytochromes: proteins containing heme carry only e-'s • Groups of carrier molecules have heigthening levels of eaffinity ending at O which has the highest e- affinity of all • Oxygen dissolves into the mitochondria and becomes sandwiched b/w a heme ion and copper ion in the last cytochrome of the ETS H+ Gradient & Phosphorylation The H+ gradient is a store of elctrochemical potential E Chemical: concentration difference of H+ • Electrical: difference of charges on either side of a membrane Protonmotive force: H+ moving down the [gradient] Chemiosmotic ATP synthesis: coupling of H+ flowing down [gradient] and phosphorylation of ADP • H+ [gradient] powers ATP synthesis indirectly by freeing ATP sythetase's active site to make more ATP Oxidation • H+ potential E is stockpiled inside the inner membrane of mitochondria via redox reactions • E-'s from H atoms are passed to ETS and uses the energy to pump H+ through membrane against its [gradient] • At end, e-'s are passed to O2 which picks up some H+ to form H2O Phosphorylation • Potential E of [gradient] used to make ATP • H+ recrosses membrane via channel proteins connected to ATP synthetase Energy Yield of Glucose Glycolysis: 2 ATP Citric Acid Cycle: 2 ATP • Equals: 4 ATP produced by substrate level phosphorylation Pair of e-'s from NADH 3 ATP Pair of e-'s from FADH2 2ATP ETS: 34 ATP created by oxidative phosphorylation • Maximum total: 38ATP Fermentation If a cell runs out of O2, all the ecarriers are stuck in reduced form, halting system • Pyruvate produced by glycolysis acts as alternative acceptor of H from NADH, keeping glycolysis going to allow small ATP production Alcoholic Fermentation • Yeasts break down sugar into pyruvate. • Each pyruvate is dismantled into a molecule of CO2 and a 2C compound acetaldehyde • Acetaldehyde is reduced by accepting 2H's from NADH and H+ forming 2C alcohol ethanol (ethyl alcohol) Lactic Acid Fermentation • Occurs during strenuous exercise • Pyruvate from glycolysis is reduced by accepting hydrogens from NADH and H+ • Pyruvate converted into 3C compound, lactate There are three metabolic pathways that provide the energy for all human action. These “metabolic engines” are known as the phosphagen pathway, the glycolytic pathway, and the oxidative pathway.The first, the phosphagen, dominates the highest-powered activities, those that last less than about ten seconds. The second pathway, the glycolytic, dominates moderate-powered activities, those that lastup to several minutes. The third pathway, the oxidative, dominates lowpowered activities, those that last in excess of several minutes.