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Chapter 8 An Introduction to Metabolism PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics • Metabolism is the totality of an organism’s chemical reactions • Metabolism is an emergent property of life that arises from interactions between molecules within the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Organization of the Chemistry of Life into Metabolic Pathways • A metabolic pathway begins with a specific molecule and ends with a product • Each step is catalyzed by a specific enzyme Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-UN1 Enzyme 1 A Reaction 1 Starting molecule Enzyme 2 B Enzyme 3 C Reaction 2 D Reaction 3 Product Forms of Energy • Energy is the capacity to cause change • Energy exists in various forms, some of which can perform work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Kinetic energy is energy associated with motion • Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules • Potential energy is energy that matter possesses because of its location or structure • Chemical energy is potential energy available for release in a chemical reaction • Energy can be converted from one form to another Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Laws of Energy Transformation • Thermodynamics is the study of energy transformations • A closed system, such as that approximated by liquid in a thermos, is isolated from its surroundings • In an open system, energy and matter can be transferred between the system and its surroundings • Organisms are open systems Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The First Law of Thermodynamics • According to the first law of thermodynamics, the energy of the universe is constant: – Energy can be transferred and transformed, but it cannot be created or destroyed • The first law is also called the principle of conservation of energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Second Law of Thermodynamics • During every energy transfer or transformation, some energy is unusable, and is often lost as heat • According to the second law of thermodynamics: – Every energy transfer or transformation increases the entropy (disorder) of the universe Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Living cells unavoidably convert organized forms of energy to heat • Spontaneous processes occur without energy input; they can happen quickly or slowly • For a process to occur without energy input, it must increase the entropy of the universe Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Biological Order and Disorder • Cells create ordered structures from less ordered materials • Organisms also replace ordered forms of matter and energy with less ordered forms • Energy flows into an ecosystem in the form of light and exits in the form of heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Catabolic pathways release energy by breaking down complex molecules into simpler compounds • Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Anabolic pathways consume energy to build complex molecules from simpler ones • The synthesis of protein from amino acids is an example of anabolism • Bioenergetics is the study of how organisms manage their energy resources Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The evolution of more complex organisms does not violate the second law of thermodynamics • Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously • Biologists want to know which reactions occur spontaneously and which require input of energy • To do so, they need to determine energy changes that occur in chemical reactions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Free-Energy Change, G • A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), change in entropy (∆S), and temperature in Kelvin (T): ∆G = ∆H – T∆S • Only processes with a negative ∆G are spontaneous • Spontaneous processes can be harnessed to perform work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Free Energy, Stability, and Equilibrium • Free energy is a measure of a system’s instability, its tendency to change to a more stable state • During a spontaneous change, free energy decreases and the stability of a system increases • Equilibrium is a state of maximum stability • A process is spontaneous and can perform work only when it is moving toward equilibrium Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-5 • More free energy (higher G) • Less stable • Greater work capacity In a spontaneous change • The free energy of the system decreases (∆G < 0) • The system becomes more stable • The released free energy can be harnessed to do work • Less free energy (lower G) • More stable • Less work capacity (a) Gravitational motion (b) Diffusion (c) Chemical reaction Fig. 8-5a • More free energy (higher G) • Less stable • Greater work capacity In a spontaneous change • The free energy of the system decreases (∆G < 0) • The system becomes more stable • The released free energy can be harnessed to do work • Less free energy (lower G) • More stable • Less work capacity Exergonic and Endergonic Reactions in Metabolism • An exergonic reaction proceeds with a net release of free energy and is spontaneous • An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-6 Reactants Free energy Amount of energy released (∆G < 0) Energy Products Progress of the reaction (a) Exergonic reaction: energy released Free energy Products Amount of energy required (∆G > 0) Energy Reactants Progress of the reaction (b) Endergonic reaction: energy required Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions • A cell does three main kinds of work: – Chemical – Transport – Mechanical • To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one • Most energy coupling in cells is mediated by ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Structure and Hydrolysis of ATP • ATP (adenosine triphosphate) is the cell’s energy shuttle • ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-8 Adenine Phosphate groups Ribose • The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis • Energy is released from ATP when the terminal phosphate bond is broken • This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-9 P P P Adenosine triphosphate (ATP) H2O Pi + Inorganic phosphate P P + Adenosine diphosphate (ADP) Energy How ATP Performs Work • The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP • In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction • Overall, the coupled reactions are exergonic Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant • The recipient molecule is now phosphorylated Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-10 NH2 Glu Glutamic acid NH3 + ∆G = +3.4 kcal/mol Glu Ammonia Glutamine (a) Endergonic reaction 1 ATP phosphorylates glutamic acid, making the amino acid less stable. P + Glu ATP Glu + ADP NH2 2 Ammonia displaces the phosphate group, forming glutamine. P Glu + NH3 Glu + Pi (b) Coupled with ATP hydrolysis, an exergonic reaction (c) Overall free-energy change The Regeneration of ATP • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) • The energy to phosphorylate ADP comes from catabolic reactions in the cell • The chemical potential energy temporarily stored in ATP drives most cellular work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-12 ATP + H2O Energy from catabolism (exergonic, energy-releasing processes) ADP + P i Energy for cellular work (endergonic, energy-consuming processes) Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-13 Sucrose (C12H22O11) Sucrase Glucose (C6H12O6) Fructose (C6H12O6) The Activation Energy Barrier • Every chemical reaction between molecules involves bond breaking and bond forming • The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) • Activation energy is often supplied in the form of heat from the surroundings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-14 A B C D Transition state A B C D EA Reactants A B ∆G < O C D Products Progress of the reaction How Enzymes Lower the EA Barrier • Enzymes catalyze reactions by lowering the EA barrier • Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually Animation: How Enzymes Work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-15 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Course of reaction with enzyme ∆G is unaffected by enzyme Products Progress of the reaction Substrate Specificity of Enzymes • The reactant that an enzyme acts on is called the enzyme’s substrate • The enzyme binds to its substrate, forming an enzyme-substrate complex • The active site is the region on the enzyme where the substrate binds • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-17 1 Substrates enter active site; enzyme changes shape such that its active site enfolds the substrates (induced fit). 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Substrates Enzyme-substrate complex 6 Active site is available for two new substrate molecules. Enzyme 5 Products are released. 4 Substrates are converted to products. Products 3 Active site can lower EA and speed up a reaction. Cofactors • Cofactors are nonprotein enzyme helpers • Cofactors may be inorganic (such as a metal in ionic form) or organic • An organic cofactor is called a coenzyme • Coenzymes include vitamins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by – General environmental factors, such as temperature and pH – Chemicals that specifically influence the enzyme Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Effects of Temperature and pH • Each enzyme has an optimal temperature in which it can function • Each enzyme has an optimal pH in which it can function Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-18 Rate of reaction Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria 40 60 80 Temperature (ºC) (a) Optimal temperature for two enzymes 0 20 Optimal pH for pepsin (stomach enzyme) 100 Optimal pH for trypsin Rate of reaction (intestinal enzyme) 4 5 pH (b) Optimal pH for two enzymes 0 1 2 3 6 7 8 9 10 Enzyme Inhibitors • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective • Examples of inhibitors include toxins, poisons, pesticides, and antibiotics Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-19 Substrate Active site Competitive inhibitor Enzyme Noncompetitive inhibitor (a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition Concept 8.5: Regulation of enzyme activity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated • A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Allosteric Regulation of Enzymes • Allosteric regulation may either inhibit or stimulate an enzyme’s activity • Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Allosteric Activation and Inhibition • Most allosterically regulated enzymes are made from polypeptide subunits • Each enzyme has active and inactive forms • The binding of an activator stabilizes the active form of the enzyme • The binding of an inhibitor stabilizes the inactive form of the enzyme Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-20a Allosteric enzyme with four subunits Active site (one of four) Regulatory site (one of four) Activator Active form Stabilized active form Oscillation NonInhibitor Inactive form functional active site (a) Allosteric activators and inhibitors Stabilized inactive form • Cooperativity is a form of allosteric regulation that can amplify enzyme activity • In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-20b Substrate Inactive form Stabilized active form (b) Cooperativity: another type of allosteric activation Identification of Allosteric Regulators • Allosteric regulators are attractive drug candidates for enzyme regulation • Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-21a EXPERIMENT Caspase 1 Active site Substrate SH Known active form SH Allosteric binding site Allosteric Known inactive form inhibitor SH Active form can bind substrate S–S Hypothesis: allosteric inhibitor locks enzyme in inactive form Feedback Inhibition • In feedback inhibition, the end product of a metabolic pathway shuts down the pathway • Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 8-22 Initial substrate (threonine) Active site available Isoleucine used up by cell Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Feedback inhibition Isoleucine binds to allosteric site Enzyme 2 Active site of enzyme 1 no longer binds Intermediate B threonine; pathway is Enzyme 3 switched off. Intermediate C Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine) Specific Localization of Enzymes Within the Cell • Structures within the cell help bring order to metabolic pathways • Some enzymes act as structural components of membranes • In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Chapter 9 Cellular Respiration: Harvesting Chemical Energy PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings OVERVIEW • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates O2 and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 + H2O Organic +O molecules 2 Cellular respiration in mitochondria ATP ATP powers most cellular work Heat energy Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • Fermentation is a partial degradation of sugars that occurs without O2 • Aerobic respiration consumes organic molecules and O2 and yields ATP • Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Redox Reactions: Oxidation and Reduction • The transfer of electrons during chemical reactions releases energy stored in organic molecules • This released energy is ultimately used to synthesize ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Principle of Redox • Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions • In oxidation, a substance loses electrons, or is oxidized • In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-UN1 becomes oxidized (loses electron) becomes reduced (gains electron) Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced: Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-UN3 becomes oxidized becomes reduced Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • In cellular respiration, glucose and other organic molecules are broken down in a series of steps • Electrons from organic compounds are usually first transferred to NAD+, a coenzyme • As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • NADH passes the electrons to the electron transport chain • Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction • O2 pulls electrons down the chain in an energyyielding tumble • The energy yielded is used to regenerate ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-5 H2 + 1/2 O2 2H (from food via NADH) Controlled release of + – 2H + 2e energy for synthesis of ATP 1/ 2 O2 Explosive release of heat and light energy 1/ (a) Uncontrolled reaction (b) Cellular respiration 2 O2 The Stages of Cellular Respiration: A Preview • Cellular respiration has three stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) – The citric acid cycle or Krebs cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-6-1 Electrons carried via NADH Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation Fig. 9-6-2 Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Mitochondrion Cytosol ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Fig. 9-6-3 Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion Cytosol ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions BioFlix: Cellular Respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration • A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-7 Enzyme Enzyme ADP P Substrate + Product ATP Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate • Glycolysis occurs in the cytoplasm and has two major phases: – Energy investment phase – Energy payoff phase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used 4 ATP formed Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • In the presence of O2, pyruvate enters the mitochondrion • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-10 CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 Pyruvate Transport protein 3 CO2 Coenzyme A Acetyl CoA • The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix (inside the inner membrane) • The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn • Two CO2 molecules are also produced per turn Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The citric acid cycle has eight steps, each catalyzed by a specific enzyme • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-11 Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle FADH2 2 CO2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Pathway of Electron Transport • The electron transport chain is in the cristae (inner membrane) of the mitochondrion • Most of the chain’s components are proteins, which exist in multiprotein complexes • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H 2O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-13 NADH 50 2 e– NAD+ FADH2 2 e– 40 FMN FAD Multiprotein complexes FAD Fe•S Fe•S Q Cyt b 30 Fe•S Cyt c1 I V Cyt c Cyt a Cyt a3 20 10 2 e– (from NADH or FADH2) 0 2 H+ + 1/2 O2 H2O • Electrons are transferred from NADH or FADH2 to the electron transport chain • Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 • The electron transport chain generates no ATP • The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps that release energy in manageable amounts Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Chemiosmosis: The Energy-Coupling Mechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through channels in ATP synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-14 INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Catalytic knob ADP + P i ATP MITOCHONDRIAL MATRIX • The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis • The H+ gradient is referred to as a protonmotive force, emphasizing its capacity to do work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-16 H+ H+ H+ H+ Protein complex of electron carriers Cyt c V Q ATP synthase FADH2 NADH 2 H+ + 1/2O2 H2O FAD NAD+ ADP + P i (carrying electrons from food) ATP H+ 1 Electron transport chain Oxidative phosphorylation 2 Chemiosmosis An Accounting of ATP Production by Cellular Respiration • During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways • Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Versatility of Catabolism • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) • Fatty acids are broken down by beta oxidation and yield acetyl CoA • An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-20 Proteins Carbohydrates Amino acids Sugars Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Fats Glycerol Fatty acids Biosynthesis (Anabolic Pathways) • The body uses small molecules to build other substances • These small molecules may come directly from food, from glycolysis, or from the citric acid cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Regulation of Cellular Respiration via Feedback Mechanisms • Feedback inhibition is the most common mechanism for control • If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down • Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen • Most cellular respiration requires O2 to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate • Fermentation uses phosphorylation instead of an electron transport chain to generate ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Types of Fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 • Alcohol fermentation by yeast is used in brewing, winemaking, and baking Animation: Fermentation Overview Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-18a 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 Pyruvate 2 NAD+ 2 Ethanol (a) Alcohol fermentation 2 NADH + 2 H+ 2 CO2 2 Acetaldehyde • In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as an end product, with no release of CO2 • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-18b 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate 2 Lactate (b) Lactic acid fermentation Fermentation and Aerobic Respiration Compared • Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate • The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration • Cellular respiration produces 36 (net) ATP per glucose molecule; fermentation produces 2 (net) ATP per glucose molecule Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 • Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration • In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-19 Glucose CYTOSOL Glycolysis Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle The Evolutionary Significance of Glycolysis • Glycolysis occurs in nearly all organisms • Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Chapter 10 Photosynthesis PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: The Process That Feeds the Biosphere • Photosynthesis is the process that converts solar energy into chemical energy • Directly or indirectly, photosynthesis nourishes almost the entire living world Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Autotrophs sustain themselves without eating anything derived from other organisms • Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 10.1: Photosynthesis converts light energy to the chemical energy of food • Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria • The structural organization of these cells allows for the chemical reactions of photosynthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants • Leaves are the major locations of photosynthesis • Their green color is from chlorophyll, the green pigment within chloroplasts • Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast • CO2 enters and O2 exits the leaf through microscopic pores called stomata Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf • A typical mesophyll cell has 30–40 chloroplasts • The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana • Chloroplasts also contain stroma, a dense fluid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-3b Chloroplast Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) • The light reactions (in the thylakoids): – Split H2O – Release O2 – Reduce NADP+ to NADPH – Generate ATP from ADP by photophosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH • Chloroplasts are solar-powered chemical factories • Their thylakoids transform light energy into the chemical energy of ATP and NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes • A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes • The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a • Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-12 Photosystem STROMA Light-harvesting Reaction-center complex complexes Primary electron acceptor Thylakoid membrane Photon e– Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) • There are two types of photosystems in the thylakoid membrane • Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm • The reaction-center chlorophyll a of PS II is called P680 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Photosystem I (PS I) is best at absorbing a wavelength of 700 nm • The reaction-center chlorophyll a of PS I is called P700 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Linear Electron Flow • During the light reactions, there are two possible routes for electron flow: cyclic and linear • Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 • An excited electron from P680 is transferred to the primary electron acceptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-1 Primary acceptor e– 2 P680 1 Light Pigment molecules Photosystem II (PS II) • P680+ (P680 that is missing an electron) is a very strong oxidizing agent • H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680 • O2 is released as a by-product of this reaction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-2 Primary acceptor 2 H+ + 1/ O 2 2 H2O e– 2 3 e– e– P680 1 Light Pigment molecules Photosystem II (PS II) • Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I • Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane • Diffusion of H+ (protons) across the membrane drives ATP synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-3 4 Primary acceptor 1/ 2 H+ 2 + O2 H2O e– 2 Pq Cytochrome complex 3 Pc e– e– 5 P680 1 Light ATP Pigment molecules Photosystem II (PS II) • In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor • P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-4 4 Primary acceptor 1/ 2 H+ 2 + O2 H2O e– 2 Primary acceptor e– Pq Cytochrome complex 3 Pc e– e– P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) • Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) • The electrons are then transferred to NADP+ and reduce it to NADPH • The electrons of NADPH are available for the reactions of the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-5 4 Primary acceptor 2 H+ + 1/ O 2 2 H2O e– 2 Primary acceptor e– Pq Cytochrome complex 7 Fd e– e– 8 NADP+ reductase 3 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) NADP+ + H+ Photosystem I (PS I) A Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy • Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP • Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix • In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Stroma Matrix Key ADP + P i [H+] Higher Lower [H+] H+ ATP • ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place • In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-17 STROMA (low H+ concentration) Cytochrome Photosystem I complex Light Photosystem II 4 H+ Light Fd NADP+ reductase NADP+ + H+ NADPH Pq H2O THYLAKOID SPACE (high H+ concentration) e– 1 e– 1/ Pc 2 2 3 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+ Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle • The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) • For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2 • The Calvin cycle has three phases: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO2 acceptor (RuBP) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-18-3 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 6 P 3-Phosphoglycerate 3P P Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP 3 Calvin Cycle 6 P P 1,3-Bisphosphoglycerate ATP 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 Pi P 5 G3P 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Glucose and other organic compounds Phase 2: Reduction Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain RuBP ATP NADPH 3-Phosphoglycerate Calvin Cycle G3P Starch (storage) Chloroplast O2 Sucrose (export)