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CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL ENERGY 1. Catabolic pathways yield energy by Oxidizing Organic Fuels. 2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate. 3. The citric acid cycle completes the energy-yielding oxidation of organic molecules. 4. During g Oxidative Phosphorylation, y chemiosmosis couples electron transport to ATP synthesis. Life is work • cells require transfusions of energy from outside sources to perform f their h i tasks. k – In most ecosystems, energy enters as sunlight sunlight. – Light energy trapped in organic g molecules is available to both photosynthetic organisms and others that eat them. them Fig. 9.2 Catabolic pathways yield energy by Oxidizing Organic Fuels Catabolic Pathways and Production of ATP • Organic O i molecules l l store energy in i their h i arrangement off atoms. • Enzymes catalyze the degradation of organic molecules that are rich in energy to simpler waste products with less energy. • Some of the released energy is used to do work and the rest is di i dissipated d as heat. h • Cellular respiration is a catabolic process in which oxygen is usedd as a reactant t t to t complete l t the th breakdown b kd off a variety i t off organic molecules. – Most of the processes in cellular respiration occur in mitochondria. • One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence of oxygen oxygen. • Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose. C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat) • The catabolism of glucose is exergonic with : a ∆G of - 686 kcal / mole of glucose. – Some of this energy is used to produce ATP that will perform cellular work • The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (Pi). • An animal cell regenerates ATP from ADP and Pi by the catabolism of organic molecules. • The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. – This changes the shape of the receiving molecule, performing work (transport, mechanical, or chemical). – When the phosphate group leaves the molecule, the molecule returns to its alternate shape. Fig 9.2. Fig.9.2 Adenosine denosine TTri ri‐‐Phosphate ( hosphate (ATP ATP)) Adenosine P P + H2O P Triphosphate Energy Pi + P P Adenosine Di‐Phosphate(ADP) ٧ Fig. 6.8 Redox Reactions: Oxidation and Reduction • Catabolic pathways relocate the electrons stored in food molecules releasing energy that is used to synthesize ATP. molecules, ATP • Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions. – The loss of electrons is called oxidation. – The addition of electrons is called reduction. • O Oxygen yg is one of the most ppotent oxidizing g agents. g • An electron looses energy as it shifts from a less electronegative atom to a more electronegative one. • A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work. • The formation of table salt from sodium and chloride is a redox reaction. – Na + Cl Na+ + Cl- – Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to -1). • More generally: Xe- + Y X + Ye- – X, the electron donor, is the reducing agent, reduces Y and it self is oxidized. – Y, the electron recipient, is the oxidizing agent, oxidizes X and itself is reduced. • Redox reactions require both a donor and acceptor. Oxidation of Organic Fuel Molecules During Cellular Respiration • In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. • Cellular respiration: C6H12O6 + 6O2 6CO2 + 6H2O • Glucose is oxidized, oxygen is reduced, and electrons loose potential energy. • Molecules that have an abundance of hydrogen(carbohydrates and fats) are excellent fuels because their bonds are a source of “hilltop” electrons th t “fall” that “f ll” closer l to t oxygen. • However, these fuels do not spontaneously combine with O2 because they lack the activation energy energy. • Enzymes lower the barrier of activation energy, allowing these fuels to be oxidized slowly. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. • Rather Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific p enzyme. y • At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, coenzyme like NAD+ (nicotinamide adenine dinucleotide). • During the oxidation of glucose: – Dehydrogenase enzymes strip two hydrogen atoms from the glucose (2 electrones and 2 protons). – pass two t electrons l t andd one proton t to t its it coenzyme NAD+. – Release the remaining proton (H+) into the surrounding. Oxidation – H-C-OH + NAD+ dehydrogenae C=O + NADH + H+ Reduction • This changes the oxidized form, NAD+, to the reduced form NADH. • NAD + functions as the oxidizing agent in many of the redox steps during the catabolism of glucose. • The electrons carried by NADH loose very little of their potential energy in this process. • This energy is stored in NADH to synthesize ATP as electrons “fall” fall from NADH to oxygen. oxygen •Unlike the explosive release of heat energy that would occur when H2 and O2 combine, cellular respiration p uses an electron transport chain to break the fall of electrons to O2 into several steps. p Fig. 9.5 • The electron transport chain, consisting of several molecules (primarily proteins), proteins) is built into the inner membrane of a mitochondrion. • NADH shuttles h l electrons l ffrom ffood d to the h ““top”” off the h chain. • At the “bottom”, oxygen captures the electrons and H+ to form water. • The free energy change from “top” to “bottom” is -53 kcal/mole of NADH. • Electrons are passed by increasingly electronegative molecules in the chain until they are caught by oxygen, oxygen the most electronegative. During cellular respiration, most electrons travel this downhill pathway: Food NADH electron transport chain oxygen Stages of Cellular respiration The three Stages of Cellular Respiration are : 1. Glycolysis is the process that harvest chemical energy by oxidizing glucose to pyruvate . 2.The Citric acid cycle is the process that completes the energy-yielding oxidation of organic molecules. 3. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis . The Stages of Cellular Respiration: a Preview • Respiration occurs in three metabolic stages: 1. Glycolysis 2. The Citric Acid Cycle 3. Oxidative phosphorylation: electron transport p and chemiosmosis Fi 9.6 Fig. 96 1.Glycolysis: – occurs in the cytosol cytosol. – It doesn’t require O2. – It begins b i catabolism t b li b by b breaking ki glucose l iinto t ttwo molecules of pyruvate. 2 Citric Acid cycle: 2. – occurs in the mitochondrial matrix. – It is a catabolic pathway that degrades pyruvate to carbon dioxide. • Several steps in glycolysis and the citric acid cycle transfer electrons from substrates to NAD+, forming NADH. • NADH passes these electrons to the electron transport chain. chain 3. Electron transport chain: − the electrons move from molecule to molecule until theyy combine with oxygen yg and hydrogen y g ions to form water. − the energy released at each step of the chain is stored in the mitochondrion in a form that can be used to make ATP. ATP • The mode of ATP synthesis is called oxidative phosphorylation: • – Oxidative phosphorylation produces almost 90% of the ATP generated by respiration. Some ATP is also generated in glycolysis and the citric acid cycle by substrate-level phosphorylation. – Here an enzyme y transfers a phosphate group from an organic g molecule (the substrate) to ADP, forming ATP. • Ultimately 38 ATP are produced per a molecule of glucose that is degraded to carbon dioxide and water by respiration. Fig. 9.7 1. Glycolysis harvests chemical energy by oxidizing glucose l to t pyruvate t • During glycolysis, glucose, a six-carbon sugar, is split into two, three-carbon sugars. • These smaller Th ll sugars are oxidized idi d and d rearranged d tto fform two molecules of pyruvate. • Each E h off the th ten t steps t i glycolysis in l l i iis catalyzed t l db by a specific enzyme. • These steps Th t can be b divided di id d iinto t two t phases: h 1. energy investment phase 2. energy payoff phase. • In the energy investment phase, ATP provides activation energy by phosphorylating glucose. – This requires 2 ATP per glucose. • In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH. • 2 ATP ((net)) and 2 NADH are produced per glucose. Fig. 9.8 Fig. 9.9a Energy Investment Phase (steps 1-5) Fig. 9.9b Energy-Payoff Phase (Steps 6-10) • The net yield from glycolysis is 2 ATP and 2 NADH per glucose. l • No CO2 is pproduced during g gglycolysis. y y • Glycolysis occurs whether O2 is present or not. – If O2 is present, pyruvate moves to the citric acid cycle y and the energy gy stored in NADH can be converted to ATP by the electron transport system and oxidative phosphorylation. p p y 2. The Citric acid cycle (Krebs cycle) • • If oxygen is present present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide. As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix: – A carboxyl group is removed as CO2. – A pair of electrons is transferred from the remaining two-carbon fragment to NAD+ to form NADH. – The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA. Fig. 9.10 – This cycle begins when acetate from acetyl CoA combines bi with ith oxaloacetate l t t to t form f citrate. it t – The oxaloacetate is recycled y and the acetate is broken down to CO2. –E Eachh cycle l produces d one ATP by b substrate-level bt t l l phosphorylation, three NADH, and one FADH2 ( th electron (another l t carrier) i ) per acetyl t l CoA. C A • The conversion of pyruvate t andd th the Citirc Citi Acid Cycle produces l large quantities titi off electron carriers: 6 NADH 2 FADH2 per glucose molecule Fig. 9.12 3.Oxidative phosphorylation: ( chemiosmosis couples electron transport to ATP synthesis) • The majority of the ATP comes from the energy in the electrons carried by NADH and d (FADH2). ) • This energy is used in the electron transport system to power ATP synthesis. synthesis • Thousands of copies of the electron transport chain (ETC) are found in the extensive surface of the cristae,, the inner membrane of the mitochondrion. – Most components of the chain are proteins that can alternate between reduced and oxidized states as they accept and donate electrons. • Electrons drop in free energy as they pass down the electron transport chain. • Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein. – The h electrons l continue i along the chain which includes several cytochrome proteins and one lipid carrier. • The electrons carried by FADH2 have lower free energy and are added to a later point in the chain. Fig. 9.13 • Cytochrome = Type of protein molecule that contains a heme h which hi h acts t as an electron l t carrier i in i the th electron transport chains of mitochondria . • Electrons from NADH or FADH2 ultimately pass to oxygen. – For every two electron carriers (four electrons), one O2 molecule is reduced to two molecules of water. • The electron transport chain generates no ATP directly. • The movement of electrons along the electron transport chain does contribute to ATP synthesis. ATP Production by Cellular Respiration • During respiration, most energy flows from: glucose NADH electron transport chain proton-motive t ti force f ATP ATP. • Considering the fate of carbon, one six-carbon glucose molecule is oxidized to six CO2 molecules. • Each NADH from the Citric Acid cycle and the conversion of pyruvate generate a maximum of 3 ATP . • The NADH from gglycolysis y y mayy also yield y 3 ATP. • Each FADH2 from the Citric Acid cycle generate about 2ATP. Process ATP Produced Directly y by y Substrate-Level Phosphorylation Glycolysis Net 2 ATP Oxidation of Pyruvate Citric Acid Cycle 2 ATP Reduced Coenzyme y ATP Produced by Oxidative Phosphorylation Total 2 NADH 6 ATP 8 2 NADH 6 ATP 6 6 NADH 2 FADH2 18 ATP 4 ATP 24 38 Fig. 9.16 ATP-synthase,an ATPsynthase,an enzyme in the cristae of the mitochondria that makes ATP from ADP and Pi. ATP--synthase uses the energy of an existing proton gradient to power ATP synthesis. ATP This proton gradient develops between the intermembrane space and the matrix. proton--motive force force.. This difference in the concentration of H+ is the proton The p power source for the ATP synthase y is a difference in the concentration of H+ on the opposite sides of the inner mitochondrial membrane. The process ,in in which energy stored in the form of a hydrogen ion gradient (difference in concentration on the two sides) across a membrane i used is d to t drive d i cellular ll l work k such h as ATP ,is i called ll d CHEMIOSMOSIS Fig. 9.14 Energy of NADH and FADH2 give a maximum yield of ATP p production by oxidative production by y oxidative phosphorylation phosphorylation. p p y . Fig. 9.15 ٣٧ Fermentation: Some cells produce ATP without the use of oxygen • Oxidation refers to the loss of electrons to any electron acceptor, acceptor not just to oxygen. – In gglycolysis, y y , gglucose is oxidized to two py pyruvate molecules with NAD+ as the oxidizing agent, not O2. – Some energy from this oxidation produces 2 ATP (net). – If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron transport chain. • Glycolysis Gl l i generates t 2 ATP whether h th oxygen iis presentt (aerobic) ( bi ) or not (anaerobic). • Anaerobic catabolism of sugars can occur by fermentation. • Fermentation can generate ATP from glucose by substrate-level substrate level phosphorylation as long as there is a supply of NAD+ to accept electrons. – If the NAD+ pool is exhausted, glycolysis shuts down. – Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD+. • Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. • In alcohol fermentation, pyruvate is converted to ethanol in two steps. steps – First, pyruvate is converted to a two-carbon compound, acetaldehyde by the removal of CO2. – Second, acetaldehyde is reduced by NADH to ethanol. – Alcohol fermentation by yeast is used in brewing and winemaking. Fig. 9.17a • During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). – Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. – Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce. • The waste product, product lactate, may cause muscle fatigue, but it is converted back to pyruvate in the liver. Fig. 9.17b Some organisms ( Some organisms (facultative anaerobes facultative anaerobes , , including yeast and many bacteria, can survive using either fermentation or respiration. At a cellular level, human muscle cells can b h behave as facultative anaerobes, but f l i b b nerve cells cannot. Fig. 9.18