Cellular Respiration and Fermentation Lecture Note

Chapter 9 → Cellular Respiration and Fermentation Lecture Notes Overview: Life Is Work • Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work. Concept 9.1 Catabolic pathways yield energy by oxidizing organic fuels • Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. • Enzymes catalyze the systematic degradation of organic molecules that are rich in energy. • Some of the released energy is used to do work; the rest is dissipated as heat. • Fermentation, leads to the partial degradation of sugars without the use of oxygen (anaerobic.) • A more efficient catabolic process, aerobic respiration, consumes oxygen as a reactant. o Although cellular respiration technically includes both aerobic and anaerobic processes, the term is commonly used to refer only to the aerobic process. • Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP + heat) • The catabolism of glucose is exergonic, with G = −686 kcal per mole of glucose. Redox reactions release energy when electrons move closer to electronegative atoms. • Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to synthesize ATP. • • • • Reactions that result in the transfer of one or more electrons (e−) from one reactant to another are oxidation- reduction reactions, or redox reactions. o The loss of electrons from a substance is called oxidation. o The addition of electrons to another substance is called reduction. Xe− + Y → X + Ye−. o X, the electron donor, is the reducing agent and reduces Y by donating an electron to it. Y, the electron recipient, is the oxidizing agent and oxidizes X by removing its electron. o Oxygen is very electronegative and is one of the most potent of all oxidizing agents. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one. A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work. Organic fuel molecules are oxidized during cellular respiration. • Organic molecules that contain an abundance of hydrogen are excellent fuels. o The bonds of these molecules are a source of “hilltop” electrons, whose energy may be released as the electrons “fall” down an energy gradient when they are transferred to oxygen. The “fall” of electrons during respiration is stepwise, via NAD+ and an 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. o Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme which lowers the activation energy barrier. o In many reactions, electrons are stripped from the glucose and the electron is transferred with a proton, as a hydrogen atom. 9-1 • • • • • The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide). o NAD+ is well suited as an electron carrier because it can cycle easily between oxidized (NAD+) and reduced (NADH) states. Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize ATP as electrons “fall” down an energy gradient from NADH to oxygen. Cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several energy-releasing steps. The electron transport chain consists of several molecules (primarily proteins) built into the inner membrane of mitochondria of eukaryotic cells. o Electrons released from food are shuttled by NADH to the “top” higher-energy end of the chain. o At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water. In summary, during cellular respiration, most electrons travel the following “downhill” route: glucose → NADH → electron transport chain → oxygen. The stages of cellular respiration: a preview • Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain and oxidative phosphorylation. • Glycolysis occurs in the cytosol. It begins catabolism by breaking glucose into two molecules of pyruvate. • In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which enters the citric acid cycle (Krebs cycle.) • In the third stage of respiration, the electron transport chain accepts electrons from the breakdown products of the first two stages (via NADH and FADH2). o As the electrons are passed along the chain, the energy released at each step in the chain is stored in a form the mitochondrion can use to make ATP. o This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain. • Some ATP is also formed directly during glycolysis and the citric acid cycle by substrate-level phosphorylation, in which an enzyme transfers a phosphate group from an organic substrate molecule to ADP, forming ATP. Concept 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • During glycolysis, glucose, a six-carbon sugar, is split into two three-carbon sugars and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid. • Each of the ten steps in glycolysis is catalyzed by a specific enzyme. These steps can be divided into two phases. 1. In the energy investment phase, the cell spends ATP. 2. In the energy payoff phase, this investment is repaid with interest. ATP is produced by substratelevel phosphorylation, and NAD+ is reduced to NADH by electrons released by the oxidation of glucose. • The net yield from glycolysis is 2 ATP and 2 NADH per glucose. • Glycolysis can occur whether or not O2 is present. 9-2 Concept 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules • After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A, or acetyl CoA. • This process (the intermediate step), linking glycolysis and the citric acid cycle catalyzes three reactions: 1. A carboxyl group is removed as CO2. 2. The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers the pair of electrons to NAD+ to form NADH. 3. Acetate combines with coenzyme A to form acetyl CoA. • The citric acid cycle oxidizes organic fuel derived from pyruvate. o Two CO2 molecules are released. o The cycle generates one ATP per turn by substrate-level phosphorylation. o During the redox reactions, chemical energy is transferred to NAD+ and FAD forming NADH and FADH2 (electron carriers!) which bring the electrons to the ETC • • • • 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 the compound oxaloacetate, forming citrate. o The next seven steps decompose the citrate back to oxaloacetate. For each acetyl group that enters the cycle, 3 NAD+ are reduced to NADH. NOTE: this does not include the NADH formed in the intermediate step In one step, electrons are transferred to FAD instead of NAD+. FAD then accepts 2 electrons and 2 protons to become FADH2. The citric acid (Krebs) cycle forms an ATP molecule by substrate-level phosphorylation. MAIN FUNCTION OF KREBS → to make electron carriers! • OVERALL SUMMARY: • • • • 1 Glucose → 2 turns of the Krebs (one for each acetyl CoA) Each turn makes/releases: ▪ 1 ATP ▪ 3 NADH ▪ 1 FADH2 ▪ 2 CO2 NOTE: Totals listed above do not include the intermediate step where pyruvate is converted to acetate!! Concept 9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • NADH and FADH2 account for most of the energy extracted from glucose. The inner mitochondrial membrane couples electron transport to ATP synthesis. • The electron transport chain is a collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion. • The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion. • Electrons drop in free energy as they pass down the electron transport chain. • During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons. 9-3 • • • • • o Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which is less electronegative. o It then returns to its oxidized form as it passes electrons to its more electronegative “downhill” neighbor. Electrons carried by NADH and FADH2 are transferred to carriers of the electron transport chain. These carriers include: flavoprotein, iron sulfur protein, ubiquinone (a lipid), and cytochromes. The last cytochrome of the chain passes its electrons to oxygen, which is very electronegative. o Each oxygen also picks up a pair of hydrogen ions to form water. (OXYGEN = FINAL ELECTRON ACCEPTOR!!) The electrons carried by FADH2 have lower free energy and are added at a lower energy level than those carried by NADH. The electron transport chain generates no ATP directly. Its function is to break the large free-energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. Chemiosmosis couples electron transport and energy release to ATP synthesis. • A protein complex in the cristae, ATP synthase, actually makes ATP from ADP and inorganic phosphate. o The power source for the ATP synthase is a difference in the concentrations of H+ on opposite sides of the inner mitochondrial membrane. This is also a pH gradient. • This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis. o Establishing the H+ gradient is the function of the electron transport chain. • ATP synthase molecules are the only place where H+ can diffuse back to the matrix. o The exergonic flow of H+ is used by the enzyme to generate ATP. This coupling of the redox reactions of the electron transport chain to ATP synthesis is an example of chemiosmosis. o The electron carriers are spatially arranged in the membrane in such a way that protons are accepted from the mitochondrial matrix and deposited in the intermembrane space. • So, as electrons flow down the ETC, H+ are pumped FROM THE MATRIX into the INTERMEMBRANE SPACE, and the H+ diffuse BACK INTO the matrix via the ATP SYNTHASE • The H+ gradient that results is the proton-motive force, a gradient with the capacity to do work. • Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work. • In mitochondria, the energy for proton gradient formation comes from exergonic redox reactions, and ATP synthesis is the work performed. • How efficient is respiration in generating ATP? o Efficiency of respiration is 7.3 kcal/mol times 32 ATP/glucose divided by 686 kcal/mol glucose, which equals 0.34, or 34%. (compared to 25% for an automobile converting gasoline to energy.) • The rest of the stored energy is lost as heat, although some of this heat is used to maintain our high body temperature (37°C). Concept 9.5 Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen • Fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen or any electron transport chain (that is, without cellular respiration). o Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic). • Fermentation allows generation of ATP from glucose by substrate-level phosphorylation. 9-4 • o Glycolysis continues as long as there is a supply of NAD+ to accept electrons during the oxidation step. If the NAD+ pool is exhausted, glycolysis shuts down. NOTE: the term fermentation generally includes the glycolysis process which produces 2 ATP. Fermentation pathways recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. • In alcohol fermentation, pyruvate is converted to ethanol in two steps. o Alcohol fermentation by yeast is used in brewing, baking, and winemaking. • During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (the ionized form of lactic acid) without the release of CO2. o Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. • Human muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce, as in strenuous exercise. o The waste product, lactate, was previously thought to cause muscle fatigue and pain, but recent research suggests instead that it may be increased levels of potassium ions (K+). Organisms vary in the pathways available to them to break down sugars. • Obligate aerobes require oxygen and cannot carry out fermentation. • Obligate anaerobes carry out only fermentation or anaerobic respiration and cannot survive in the presence of oxygen. • Facultative anaerobes can survive using either fermentation or respiration. o At a cellular level, human muscle cells can behave as facultative anaerobes. The role of glycolysis in both fermentation and respiration has an evolutionary basis. • Ancient prokaryotes likely used glycolysis to make ATP long before oxygen was present in Earth’s atmosphere. • The evidence suggests that this pathway evolved very early in the history of life on Earth. Concept 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways • Glycolysis and the citric acid cycle are major intersections of various catabolic and anabolic (biosynthetic) pathways. A variety of organic molecules can be used to make ATP. • Glycolysis can accept a wide range of carbohydrates for catabolism. • Proteins must first be digested to individual amino acids. o Many of the amino acids are used by the organism to build new proteins. • Amino acids that will be catabolized must have their amino groups removed via deamination. o The nitrogenous waste is excreted as ammonia, urea, or another waste product. o The carbon skeletons are modified by enzymes to intermediates of glycolysis and the citric acid cycle. • After fats are digested to glycerol and fatty acids, glycerol can be converted to an intermediate of glycolysis. • The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via beta oxidation. The metabolic pathways of respiration also play a role in anabolic pathways of the cell. • In addition to calories, food must provide the carbon skeletons that cells require to make their own molecules. 9-5 Feedback mechanisms control cellular respiration. • The rate of catabolism is also regulated: if ATP levels drop, catabolism speeds up to produce more ATP and when there is plenty of ATP to meet demand, respiration slows down. o Phosphofructokinase (PFK) catalyzes the earliest step that irreversibly commits the substrate to glycolysis. o By controlling the rate of this step, the cell can speed up or slow down the entire catabolic process. Phosphofructokinase is thus considered the pacemaker of respiration. • Phosphofructokinase is an allosteric enzyme with receptor sites for specific inhibitors and activators. o It is inhibited by ATP and stimulated by AMP (derived from ADP). • Citrate, the first product of the citric acid cycle, is also an inhibitor of phosphofructokinase. o This synchronizes the rate of glycolysis and the citric acid cycle. 9-6