Chapter 6. How Cells Harvest Chemical Energy PDF

Chapter 6 How Cells Harvest Chemical Energy 10. 10. 20 PowerPoint Lectures for...

Chapter 6 How Cells Harvest Chemical Energy 10. 10. 20 PowerPoint Lectures for 24 Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Delivered by Dr. Salamah Alwahsh Introduction ▪ In eukaryotes, cellular respiration – harvests energy from food, yields large amounts of ATP, and – Uses ATP to drive cellular work ▪ A similar process takes place in many prokaryotic organisms. Thermodynamics ▪ Thermodynamics is the study of energy transformations that occur in a collection of matter. ▪ Scientists use the word – System for the matter under study, and – Surroundings for the rest of the universe – first law of thermodynamics, energy in the universe is constant, and – second law of thermodynamics, energy conversions increase the disorder of the universe. ▪ Entropy is the measure of disorder, or randomness 5.11 Chemical reactions either release or store energy ▪ Exergonic reactions release energy. – These reactions release the energy in covalent bonds of the reactants – Burning wood releases the energy in glucose as heat and light – Cellular respiration – involves many steps, – releases energy slowly, and – uses some of the released energy to produce ATP Exergonic reaction, energy released Potential energy of molecules Reactants Amount of energy released Energy Products 5.11 Chemical reactions either release or store energy ▪ An endergonic reaction – requires an input of energy, and begins with reactant molecules that contain relatively little potential energy, but – yields products rich in potential energy 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers ▪ A metabolic pathway is a series of chemical reactions that either – builds a complex molecule, or – breaks down a complex molecule into simpler compounds ▪ Although biological molecules possess much potential energy, it is not released spontaneously. – An energy barrier must be overcome before a chemical reaction can begin – This energy is called the activation energy (EA) 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers ▪ We can think of EA – as the amount of energy needed for a reactant molecule to move “uphill” to a higher energy but an unstable state – so that the “downhill” part of the reaction can begin ▪ One way to speed up a reaction is to add heat, – which agitates atoms, so that bonds break more easily and reactions can proceed, but – could kill a cell The effect of an enzyme in lowering EA Activation energy barrier Enzyme Activation energy barrier Reactant Reactant reduced by Energy Energy enzyme Products Products Without enzyme With enzyme Enzymes can use the transfer of protons or electrons to the reactants to modify the state of the reactants 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers ▪ Enzymes – function as biological catalysts by lowering the EA needed for a reaction to begin, – increase the rate of a reaction without being consumed by the reaction, and – are usually proteins, although some RNA molecules can function as enzymes. Animation: How Enzymes Work © 2012 Pearson Education, Inc. 5.14 A specific enzyme catalyzes each cellular reaction ▪ An enzyme – is very selective in the reaction it catalyzes, and – has a shape that determines the enzyme’s specificity ▪ The specific reactant that an enzyme acts on is called the enzyme’s substrate ▪ A substrate fits into a region of the enzyme called the active site ▪ Enzymes are specific because their active site fits only specific substrate molecules.  1 Enzyme available The with empty active site catalytic Active site Substrate cycle of an (sucrose) enzyme Substrate binds 2 to enzyme with induced fit A specific Enzyme enzyme Glucose (sucrase) catalyzes each Fructose cellular H2O reaction 4 Products are released 3 Substrate is converted to products 5.14 A specific enzyme catalyzes each cellular reaction ▪ For every enzyme, there are optimal conditions under which it is most effective. ▪ Temperature affects molecular motion. – An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site. – Most human enzymes work best at 35–40ºC. ▪ The optimal pH for most enzymes is near neutrality 5.14 A specific enzyme catalyzes each cellular reaction ▪ Many enzymes require non-protein helpers called cofactors, which – bind to the active site, and – function in catalysis ▪ Some cofactors are inorganic, such as zinc, iron, or copper ▪ If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme. Enzyme inhibitors can regulate enzyme activity in a cell ▪ A chemical that interferes with an enzyme’s activity is called an inhibitor. ▪ Competitive inhibitors – block substrates from entering the active site, and – reduce an enzyme’s productivity ▪ Noncompetitive inhibitors – bind to the enzyme somewhere other than the active site, – change the shape of the active site, and – prevent the substrate from binding Figure 5.15A How inhibitors interfere with substrate binding Substrate Active site Enzyme Allosteric site Normal binding of substrate Competitive Noncompetitive inhibitor inhibitor Enzyme inhibition Para-amino-benzoic acid (PABA) is necessary for bacteria in producing Folic Acid Humans get their Folic acid as a dietary Vitamin B9. Sulfanilamide blocks the bacterial enzymes that convert PABA to folic acid thus killing them.  Feedback inhibition of a biosynthetic pathway Feedback inhibition Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting Product molecule Enzyme inhibitors are important in regulating cell metabolism In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition Many drugs, pesticides, and poisons are enzyme inhibitors ▪ Many beneficial drugs act as enzyme inhibitors, including – Ibuprofen, inhibiting the production of prostaglandins, – some blood pressure medicines, – some antidepressants, ‫مضادات اإلكتئاب‬ – many antibiotics, and – protease inhibitors used to fight HIV. ▪ Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare. Ibuprofen, an enzyme inhibitor  Cellular respiration: aerobic harvesting of energy ▪ In cellular respiration – glucose is broken down to carbon dioxide and water, and – the cell captures some of the released energy to make ATP ▪ Cellular respiration takes place in the mitochondria of eukaryotic cells Breathing supplies O2 for use in cellular respiration and removes CO2 – Respiration, in the breathing sense, refers to an exchange of gases – Usually an organism brings in oxygen from the environment and releases waste CO2 – Cellular respiration is the aerobic (oxygen requiring) harvesting of energy from food molecules by cells The connection O2 Breathing CO2 between breathing and cellular respiration Lungs CO2 Bloodstream O2 Muscle cells carrying out Cellular Respiration CO2 + H2O + ATP Glucose + O2 Cellular respiration banks energy in ATP molecules ▪ Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form ATP ▪ Other foods (organic molecules) can also be used as a source of energy C6H12O6 6 O2 6 CO2 6 H2O ATP Glucose Oxygen Carbon Water + Heat dioxide Cells tap energy from electrons “falling” from organic fuels to oxygen ▪ The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules ▪ An important question is how do cells extract this energy? ▪ When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen – Oxygen has a strong tendency to attract electrons – An electron loses potential energy when it “falls” to oxygen ▪ The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction, – the loss of electrons from one substance is called oxidation, – the addition of electrons to another substance is called reduction, – a molecule is oxidized when it loses one or more electrons, and – reduced when it gains one or more electrons. ▪ A cellular respiration equation is helpful to show the changes in hydrogen atom distribution. ▪ Glucose – loses its hydrogen atoms and – becomes oxidized to CO2 ▪ Oxygen – gains hydrogen atoms, and – becomes reduced to H2O Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration Loss of hydrogen atoms (becomes oxidized) C6H12O6 6 O2 6 CO2 6 H2O ATP Glucose + Heat Gain of hydrogen atoms (becomes reduced) ▪ Enzymes are necessary to oxidize glucose and other foods. ▪ NAD+ – is an important coenzyme in oxidizing glucose, – accepts electrons, and – becomes reduced to NADH Nicotinamide adenine dinucleotide is a coenzyme found in all living cells. Figure 6.5B A pair of redox reactions occurring simultaneously Becomes oxidized 2H Becomes reduced NAD+ 2H NADH H+ (carries 2 H+ 2 2 electrons) ▪ There are other electron “carrier” molecules that function like NAD+. – They form a staircase where the electrons pass from one to the next down the staircase – These electron carriers collectively are called the electron transport chain – As electrons are transported down the chain, ATP is generated  NADH NAD+ ATP 2 Controlled H+ release of energy for In cellular respiration, synthesis electrons fall down an of ATP energy staircase and finally reduce O2. 2 1 O 2 H+ 2 2 H2O  STAGES OF CELLULAR RESPIRATION © 2012 Pearson Education, Inc. Cellular respiration occurs in three main stages – Stage 1 – Glycolysis – Stage 2 – Pyruvate oxidation and citric acid cycle – Stage 3 – Oxidative phosphorylation ▪ Hydrolysis of ATP releases energy by transferring its third phosphate from ATP to some other molecule in a process called phosphorylation. ▪ Most cellular work depends on ATP energizing molecules by phosphorylating them. Phosphorylation is the chemical addition of a phosphoryl group (PO3-) to an organic molecule (e.g., kinases, phosphotransferases) The removal of a phosphoryl group is called dephosphorylation by enzymes Hexokinase D-glucose + ATP → D-glucose-6-phosphate + ADP Figure 6.6_1 An overview of cellular respiration CYTOPLASM (cytosol) NADH Electrons NADH FADH2 carried by NADH Glycolysis Oxidative Pyruvate Citric Acid Phosphorylation Glucose Pyruvate Oxidation Cycle (electron transport and chemiosmosis) Mitochondrion ATP ATP ATP Substrate-level Substrate-level Oxidative phosphorylation phosphorylation phosphorylation Glycolysis harvests chemical energy by oxidizing glucose to pyruvate ▪ In glycolysis (in the cytosol) – a single molecule of glucose is enzymatically cut in half through a series of steps, – two molecules of pyruvate are produced, – two molecules of NAD+ are reduced to two molecules of NADH, and – a net of two molecules of ATP is produced. Glu Glu-6-P An overview of Glucose 1 glucose molecule glycolysis 2 ADP 2 NAD+ 2 P 2 NADH 2 ATP 2 H+ 2 Pyruvate Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP Enzyme Enzyme P ADP ATP P P Substrate Product Pyruvate is oxidized prior to the citric acid cycle ▪ The pyruvate formed in glycolysis is transported from the cytosol into a mitochondrion where – the citric acid cycle and – oxidative phosphorylation will occur ▪ Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which – a carboxyl group is removed and given off as CO2, – the two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH, – coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and – acetyl CoA enters the citric acid cycle The link between glycolysis and the citric acid cycle NAD+ NADH H+ 2 CoA Pyruvate 1 Acetyl coenzyme A 3 CO2 Coenzyme A The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules ▪ The citric acid cycle – is also called the Krebs cycle (after the German-British researcher Hans Krebs, who worked out much of this pathway in the 1930s), – completes the oxidation of organic molecules, and – generates many NADH and FADH2 molecules  Acetyl CoA CoA An overview of the CoA citric acid cycle Occurs in the matrix of the 2 CO2 mitochondria Citric Acid Cycle FADH2 3 NAD+ FAD 3 NADH 3 H+ ATP ADP P ▪ During the citric acid cycle – the two-carbon group of acetyl CoA molecule is added to a four-carbon compound, forming citrate, – citrate is degraded back to the four-carbon compound, – two CO2 are released, and – 1 ATP, 3 NADH, and 1 FADH2 are produced ‫هذا من جزيء واحد من‬ Acetyl CoA ▪ Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose. ▪ Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is – 4 CO2 – 2 ATP, – 6 NADH, and – 2 FADH2  Most ATP production occurs by oxidative phosphorylation ▪ Oxidative phosphorylation – involves electron transport and chemiosmosis and – requires an adequate supply of oxygen Chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient An example of this would be the generation of ATP by the movement of hydrogen ions (H+) across a membrane during cellular respiration ▪ Electrons from NADH and FADH2 travel down the electron transport chain to O2 ▪ Oxygen picks up H+ to form water ▪ Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space ▪ In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP  Oxidative phosphorylation: electron transport and chemiosmosis in a mitochondrion H+ H+ H+ H+ H+ Intermem- Protein H+ Mobile H+ brane space electron H+ H+ ATP complex carriers of electron synthase carriers III IV I Inner mito- chondrial II membrane Electron FADH2 FAD flow 1 2 H+ H2O NADH NAD+ 2 O2 Mitochondrial H+ matrix ADP P ATP H+ Electron Transport Chain Chemiosmosis Oxidative Phosphorylation Interrupting cellular respiration can have both harmful and beneficial effects ▪ Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons 1. block the electron transport chain (e.g., rotenone, cyanide, and carbon monoxide (CO)), 2. inhibit ATP synthase (e.g., the antibiotic oligomycin), or 3. make the membrane leaky to hydrogen ions (called uncouplers, e.g., dinitrophenol (DNP))  *DCMU is 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DCCD, dicyclohexylcarbodiimide; FCCP, cyanide-p- trifluoromethoxyphenylhydrazone; DNP, 2,4-dinitrophenol Valinomycin is an antibiotic and ionophore that triggers rapid loss of mitochondrial membrane potential, It is a K+/H+ antiport How some poisons affect the electron transport chain and chemiosmosis Rotenone Cyanide, Oligomycin H+ H+ carbon monoxide H+ H+ ATP synthase H+ H+ H+ DNP FADH2 FAD 1 NADH NAD+ O 2 H+ 2 2 H+ H2O ADP P ATP Interrupting cellular respiration can have both harmful and beneficial effects ▪ Brown fat is – a special type of tissue associated with the generation of heat and – more abundant in hibernating mammals and newborn infants ‫ بيات شتوي‬/‫سبات‬ ▪ In brown fat, – the cells are packed full of mitochondria, – the inner mitochondrial membrane contains an uncoupling protein, which allows H+ to flow back down its concentration gradient without generating ATP, and – ongoing oxidation of stored fats generates additional heat An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration Cytosol Electron shuttles Mitochondrion across membrane 2 NADH 2 NADH or 2 FADH2 2 NADH 6 NADH 2 FADH2 Glycolysis Pyruvate Oxidative 2 Oxidation Phosphorylation Glucose Citric Acid Pyruvate 2 Acetyl (electron transport Cycle CoA and chemiosmosis) Maximum per glucose: +2 +2 + about ATP ATP 28 ATP About 32 ATP by substrate-level by substrate-level by oxidative phosphorylation phosphorylation phosphorylation Fermentation: anaerobic harvesting of energy Fermentation enables cells to produce ATP without O2 ▪ Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation – takes advantage of glycolysis, – produces two ATP molecules per glucose, and – reduces NAD+ to NADH ▪ The trick of fermentation is to provide an anaerobic path for recycling NADH back to NAD+ ▪ Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which – NADH is oxidized to NAD+, and – pyruvate is reduced to lactate ▪ Lactate is carried by the blood to the liver, where it is converted back to pyruvate and oxidized in the mitochondria of liver cells ▪ The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt  Glucose Lactic acid fermentation: 2 ADP NAD+ is regenerated as 2 NAD+ Glycolysis 2 P pyruvate is reduced to lactate. 2 ATP 2 NADH Most organisms will use some form of fermentation to 2 Pyruvate accomplish the 2 NADH regeneration of NAD+, ensuring the continuation of 2 NAD+ glycolysis 2 Lactate ▪ The baking and winemaking industries have used alcohol fermentation for thousands of years. ▪ In this process yeasts (single-celled fungi) – oxidize NADH back to NAD+ and – convert pyruvate to CO2 and ethanol.  Glucose Alcohol fermentation: NAD+ is regenerated as 2 NAD+ pyruvate is broken down to CO2 2 ADP Glycolysis 2 P and ethanol 2 ATP 2 NADH 2 Pyruvate 2 NADH 2 CO2 2 NAD+ 2 Ethanol ▪ Which metabolic pathway is common to both fermentation and cellular respiration of a glucose molecule? …………………. ▪ Muscle cells, when an individual is exercising heavily and when the muscle becomes oxygen deprived, convert pyruvate to lactate. What happens to the lactate in skeletal muscle cells? ▪ A) It is converted to NAD+. ▪ B) It produces CO2 and water. ▪ C) It is taken to the liver and converted back to pyruvate… ▪ D) It reduces FADH2 to FAD+. ▪ E) It is converted to alcohol.  The area of the inner membrane is about five times as large as the outer membrane, this ratio is variable Inner membrane is studded with pin head particles called Oxysomes or elementary particles or F0-F1 particles or subunits of Fernandez Moran (104 to 106 in number). Each F0 and F1 particle consists of three parts - Basal piece, Stalk and Head. ATP synthesis occur in head region of oxysome because here ATPase enzyme is present. Oxysomes (F0F1 particle) refers to small round structures present within the folds of the cristae of the inner mitochondrial membrane Is a tennis racket- shaped particle PDH Complex: Covalent Regulation Allosteric regulation Figure 9.10 MITOCHONDRION CYTOSOL CO2 Coenzyme A 1 3 2 NAD+ NADH + H+ Acetyl CoA Pyruvate Transport protein Figure 9.13 NADH 50 2 e− NAD+ FADH2 2 e− FAD Multiprotein Free energy (G) relative to O2 (kcal/mol) 40 FMN I complexes Fe S II Fe S Q III Cyt b Fe S 30 Cyt c1 IV Cyt c Cyt a Cyt a3 20 2 e− 10 (originally from NADH or FADH2) 0 2 H+ + 1/2 O2 H2O Figure 9.14 INTERMEMBRANE SPACE H+ Stator Rotor F0 Internal rod F1 Catalytic knob ADP + Pi ATP MITOCHONDRIAL MATRIX Figure 9.15 H+ H+ H + Protein complex H+ Cyt c of electron carriers IV Q III I ATP II synth- 2 H+ + 1/2O2 H2O ase FADH2 FAD NADH NAD+ ADP + P i ATP (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation