Cellular Respiration & Fermentation (BSc1010) PDF

-©Hacisalihoglu-G >Faculty-created course materials belong to the faculty member and that permission must be given before publishing or distributing them] Chapter 9:Cellular Respiration &Fermentation Some of this energy is used to produce AT...

-©Hacisalihoglu-G >Faculty-created course materials belong to the faculty member and that permission must be given before publishing or distributing them] Chapter 9:Cellular Respiration &Fermentation Some of this energy is used to produce ATP, which can perform cellular work. Overview: Life Is Work Redox reactions release energy when electrons move closer to electronegative atoms. To perform their many tasks, living cells require energy from outside sources. Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to synthesize ATP. Energy enters most ecosystems as sunlight and leaves as heat. In contrast, the chemical elements essential for life are recycled. Reactions that result in the transfer of one or more electrons (e−) from one reactant to another are oxidation-reduction reactions, or redox reactions. Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes (including plants and algae) use as fuel for cellular respiration. o The loss of electrons from a substance is called oxidation. o The addition of electrons to another substance is called reduction. Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work. ○ Adding electrons is called reduction because negatively charged electrons added to an atom reduce the amount of positive charge of Respiration has three key pathways: glycolysis, the citric acid cycle, and that atom. oxidative phosphorylation. The formation of table salt from sodium and chloride, Na + Cl → Na+ + Cl−, Fermentation is a simpler pathway coupled to glycolysis that has deep is a redox reaction. evolutionary roots. ○ Sodium is oxidized, and chlorine is reduced (its charge drops from 0 to −1). Concept 9.1 Catabolic pathways yield energy by oxidizing organic fuels More generally: Xe− + Y → X + Ye−. Catabolic metabolic pathways release energy stored in complex organic ○ X, the electron donor, is the reducing agent and reduces Y by donating molecules. an electron to it. o Electron transfer plays a major role in these pathways. ○ Y, the electron recipient, is the oxidizing agent and oxidizes X by removing its electron. Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. Redox reactions require both a donor and an acceptor. Enzymes catalyze the systematic degradation of organic molecules that are Redox reactions also occur when the transfer of electrons is not complete but rich in energy to simpler waste products that have less energy. involves a change in the degree of electron sharing in covalent bonds. Some of the released energy is used to do work; the rest is dissipated as heat. In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C—H) and oxygen (O=O) are One type of catabolic process, fermentation, leads to the partial degradation converted to polar covalent bonds (C=O and O—H). of sugars without the use of oxygen. When methane reacts with oxygen to form carbon dioxide, electrons end up A more efficient and widespread catabolic process, aerobic respiration, farther away from the carbon atom and closer to their new covalent partners, consumes oxygen as a reactant to complete the breakdown of a variety of the oxygen atoms, which are very electronegative. organic molecules. ○ In effect, the carbon atom has partially “lost” its shared electrons. Thus, o Most eukaryotic and many prokaryotic organisms can carry out methane has been oxidized. aerobic respiration. The two atoms of the oxygen molecule (O2) share their electrons equally. o Some prokaryotes use compounds other than oxygen as reactants in a similar process called anaerobic respiration. When oxygen reacts with the hydrogen from methane to form water, the electrons of the covalent bonds are drawn closer to the oxygen. o Although cellular respiration technically includes both aerobic and anaerobic processes, the term is commonly used to refer only to the ○ In effect, each oxygen atom has partially “gained” electrons, and so the aerobic process. oxygen molecule has been reduced. ○ Oxygen is very electronegative and is one of the most potent of all Aerobic respiration is similar in broad principle to the combustion of gasoline oxidizing agents. in an automobile engine after oxygen is mixed with hydrocarbon fuel. ○ Food provides the fuel for respiration. The exhaust is carbon dioxide Energy must be added to pull an electron away from an atom. and water. The more electronegative the atom, the more energy is required to take an The overall catabolic process is: organic compounds + O2 → CO2 + H2O + electron away from it. energy (ATP + heat). An electron loses potential energy when it shifts from a less electronegative Carbohydrates, fats, and proteins can all be used as the fuel, but it is most atom toward a more electronegative one. useful to consider glucose: A redox reaction that relocates electrons closer to oxygen, such as the C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP + heat) burning of methane, releases chemical energy that can do work. The catabolism of glucose is exergonic, with G = −686 kcal per mole of glucose. Organic fuel molecules are oxidized during cellular respiration. ○ NAD+ functions as the oxidizing agent in many of the redox steps during the breakdown of glucose. Respiration, the oxidation of glucose and other molecules in food, is a redox process. The electrons carried by NADH lose very little of their potential energy in ○ In a series of reactions, glucose is oxidized and oxygen is reduced. this process. ○ The electrons lose potential energy along the way, and energy is Each NADH molecule formed during respiration represents stored energy. released. This energy is tapped to synthesize ATP as electrons “fall” down an energy gradient from NADH to oxygen. Organic molecules that contain an abundance of hydrogen are excellent fuels. How are electrons extracted from glucose and stored in NADH finally ○ The bonds of these molecules are a source of “hilltop” electrons, whose transferred to oxygen? energy may be released as the electrons “fall” down an energy Unlike the explosive release of heat energy that occurs when H2 and O2 are gradient when they are transferred to oxygen. combined (with a spark for activation energy), cellular respiration uses an ○ As hydrogen is transferred from glucose to oxygen, the energy state of electron transport chain to break the fall of electrons to O2 into several the electron changes. energy-releasing steps. ○ In respiration, the oxidation of glucose transfers electrons to a lower The electron transport chain consists of several molecules (primarily energy state, releasing energy that becomes available for ATP proteins) built into the inner membrane of mitochondria of eukaryotic cells synthesis. and the plasma membrane of aerobically respiring prokaryotes. The main energy-yielding foods, carbohydrates and fats, are reservoirs of ○ Electrons released from food are shuttled by NADH to the “top” higher- electrons associated with hydrogen. energy end of the chain. These molecules are stable because of the barrier of activation energy. ○ At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water. Without this barrier, a food molecule like glucose would combine almost instantaneously with O2. Electron transfer from NADH to oxygen is an exergonic reaction with a free-energy change of −53 kcal/mol. o If activation energy is supplied by igniting glucose, it burns in air to release 686 kcal (2,870 kJ) of heat per mole of glucose (about 180 g). Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen, the most electronegative receptor. ○ This reaction cannot happen at body temperatures. ○ Instead, enzymes within cells lower the barrier of activation energy, Each “downhill” carrier is more electronegative than, and thus capable of allowing sugar to be oxidized in a series of steps. oxidizing, its “uphill” neighbor, with oxygen at the bottom of the chain. The electrons removed from glucose by NAD+ fall down an energy gradient in the electron transport chain to a far more stable location in the The “fall” of electrons during respiration is stepwise, via NAD+ and an electron electronegative oxygen atom. transport chain. In summary, during cellular respiration, most electrons travel the following Cellular respiration does not oxidize glucose in a single step that transfers all “downhill” route: glucose → NADH → electron transport chain → oxygen. the hydrogen in the fuel to oxygen at one time. The stages of cellular respiration: a preview. ○ Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme. Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the electron transport chain and oxidative phosphorylation. At key steps, electrons are stripped from the glucose. ○ Biochemists usually reserve the term cellular respiration for stages 2 In many oxidation reactions, the electron is transferred with a proton, as a and 3. hydrogen atom. ○ Glycolysis is included here because most respiring cells deriving energy The hydrogen atoms are not transferred directly to oxygen but are passed from glucose use glycolysis to produce starting material for the citric first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide). acid cycle. ○ NAD+ is well suited as an electron carrier because it can cycle easily Glycolysis occurs in the cytosol. It begins catabolism by breaking glucose between oxidized (NAD+) and reduced (NADH) states. into two molecules of pyruvate. ○ + As an electron acceptor, NAD functions as an oxidizing agent during In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a respiration. compound called acetyl CoA, which enters the citric acid cycle. How does NAD+ trap electrons from glucose? Several steps in glycolysis and the citric acid cycle are redox reactions in ○ Dehydrogenase enzymes strip two hydrogen atoms from the substrate which dehydrogenase enzymes transfer electrons from substrates to NAD+, (glucose), thus oxidizing it. forming NADH. ○ The enzyme passes two electrons and one proton to NAD. + In the third stage of respiration, the electron transport chain accepts electrons ○ The other proton is released as H+ to the surrounding solution. from the breakdown products of the first two stages (most often via NADH). By receiving two electrons and only one proton, NAD+ has its charge neutralized when it is reduced to NADH.  In the electron transport chain, the electrons move from molecule to More than three-quarters of the original energy in glucose is still present in molecule until they combine with molecular oxygen and hydrogen ions to the two molecules of pyruvate. form water. If molecular oxygen is present in eukaryotic cells, pyruvate enters the ○ As the electrons are passed along the chain, the energy released at each mitochondrion, where enzymes of the citric acid cycle complete the step in the chain is stored in a form the mitochondrion (or prokaryotic oxidation of the organic fuel to carbon dioxide. cell) can use to make ATP. ○ In prokaryotic cells, this process occurs in the cytosol. ○ This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport After pyruvate enters the mitochondrion via active transport, it is converted chain. to a compound called acetyl coenzyme A, or acetyl CoA. In eukaryotic cells, the inner membrane of the mitochondrion is the site of This step, linking glycolysis and the citric acid cycle, is carried out by a electron transport and chemiosmosis, the processes that together constitute multienzyme complex that catalyzes three reactions: oxidative phosphorylation. 1. A carboxyl group is removed as CO2. The carbon dioxide is fully ○ In prokaryotes, these processes take place in the plasma membrane. oxidized and thus has little chemical energy. 2. The remaining two-carbon fragment is oxidized to form acetate. An Oxidative phosphorylation produces almost 90% of the ATP generated by enzyme transfers the pair of electrons to NAD+ to form NADH. respiration. 3. Acetate combines with coenzyme A to form the very reactive Some ATP is also formed directly during glycolysis and the citric acid cycle molecule acetyl CoA. by substrate-level phosphorylation, in which an enzyme transfers a phosphate group from an organic substrate molecule to ADP, forming ATP. Due to the chemical nature of the CoA group, a sulfur-containing compound derived from a B vitamin, acetyl CoA has a high potential energy. For each molecule of glucose degraded to carbon dioxide and water by ○ The reaction of acetyl CoA to yield lower-energy products is highly respiration, the cell makes up to 32 ATP, each with 7.3 kcal/mol of free exergonic. energy. Acetyl CoA now feeds its acetyl group into the citric acid cycle for further Respiration uses the small steps in the respiratory pathway to break the large oxidation. denomination of energy contained in glucose into the small change of ATP. ○ The citric acid cycle is also called the tricarboxylic acid cycle or the ○ The quantity of energy in ATP is more appropriate for the energy level Krebs cycle. The latter name honors Hans Krebs, who was largely of work required in the cell. responsible for elucidating the cycle’s pathways in the 1930s. The citric acid cycle oxidizes organic fuel derived from pyruvate. ○ Three CO2 molecules are released, including the one released during the conversion of pyruvate to acetyl CoA. Concept 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate ○ The cycle generates one ATP per turn by substrate-level phosphorylation. During glycolysis, glucose, a six-carbon sugar, is split into two three-carbon ○ Most of the chemical energy is transferred to NAD+ and a related sugars. electron carrier, the coenzyme FAD, during the redox reactions. ○ These smaller sugars are then oxidized and rearranged to form two ○ The reduced coenzymes, NADH and FADH2, transfer high-energy molecules of pyruvate, the ionized form of pyruvic acid. electrons to the electron transport chain. Each of the ten steps in glycolysis is catalyzed by a specific enzyme. The citric acid cycle has eight steps, each catalyzed by a specific enzyme. These steps can be divided into two phases. The acetyl group of acetyl CoA joins the cycle by combining with the 1. In the energy investment phase, the cell spends ATP. compound oxaloacetate, forming citrate. 2. In the energy payoff phase, this investment is repaid with interest. ○ The next seven steps decompose the citrate back to oxaloacetate. ATP is produced by substrate-level phosphorylation, and NAD+ is ○ It is the regeneration of oxaloacetate that makes this process a cycle. reduced to NADH by electrons released by the oxidation of glucose. For each acetyl group that enters the cycle, 3 NAD+ are reduced to NADH. The net yield from glycolysis is 2 ATP and 2 NADH per glucose. ○ No carbon is released as CO2 during glycolysis. In one step, electrons are transferred to FAD instead of NAD +. FAD then accepts 2 electrons and 2 protons to become FADH2. Glycolysis can occur whether or not O2 is present. In the cells of plants, bacteria, and some animal tissues, the citric acid cycle ○ If O2 is present, the chemical energy stored in pyruvate and NADH can forms an ATP molecule by substrate-level phosphorylation. be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. In many animal tissue cells, guanosine triphosphate (GTP) is formed by the same process of substrate-level phosphorylation. ○ GTP may be used to synthesize ATP or directly power work in the cell. Concept 9.3 After pyruvate is oxidized, the citric acid cycle completes the ○ The output from this step is the only ATP generated directly by the energy-yielding oxidation of organic molecules citric acid cycle.  ○ The total yield per glucose from the citric acid cycle is 6 NADHs, 2 The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, FADHs, and the equivalent of 2 ATPs. which is very electronegative. Most of the ATP produced by respiration results from oxidative ○ Each oxygen atom also picks up a pair of hydrogen ions from the phosphorylation, when the NADH and FADH2 produced by the citric acid aqueous solution to form water. cycle relay the electrons extracted from food to the electron transport chain. The electrons carried by FADH2 have lower free energy and are added at a ○ This process supplies the necessary energy for the phosphorylation of lower energy level than those carried by NADH. ADP to ATP. ○ The electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH. Concept 9.4 During oxidative phosphorylation, chemiosmosis The electron transport chain generates no ATP directly. couples electron transport to ATP synthesis Its function is to break the large free-energy drop from food to oxygen into a Only 4 of 38 ATP produced by the respiration of glucose are produced by series of smaller steps that release energy in manageable amounts. substrate-level phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. Chemiosmosis couples electron transport and energy release to ATP synthesis. NADH and FADH2 account for most of the energy extracted from glucose. A protein complex in the cristae, ATP synthase, actually makes ATP from ○ These reduced coenzymes link glycolysis and the citric acid cycle to ADP and inorganic phosphate. oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis. ATP synthase works like an ion pump running in reverse. ○ Ion pumps usually use ATP as an energy source to transport ions against The inner mitochondrial membrane couples electron transport to ATP synthesis. their gradients. The electron transport chain is a collection of molecules embedded in the Enzymes can catalyze a reaction in either direction, depending on the G for cristae, the folded inner membrane of the mitochondrion. the reaction, which is affected by the local concentrations of reactants and ○ In prokaryotes, the electron transport chain is located in the plasma products. membrane. Rather than hydrolyzing ATP to pump protons against their concentration The folding of the inner membrane to form cristae increases its surface area, gradient, under the conditions of cellular respiration, ATP synthase uses the providing space for thousands of copies of the chain in each mitochondrion. energy of an existing ion gradient to power ATP synthesis. Most components of the chain are proteins that exist in multiprotein ○ The power source for the ATP synthase is a difference in the complexes numbered I– IV. concentrations of H+ on opposite sides of the inner mitochondrial ○ Tightly bound to these proteins are prosthetic groups, nonprotein membrane. This is also a pH gradient. components essential for catalysis. This process, in which energy stored in the form of a hydrogen ion gradient Electrons drop in free energy as they pass down the electron transport chain. across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis. During electron transport along the chain, electron carriers alternate between ○ Here, osmosis refers to the flow of H+ across a membrane. reduced and oxidized states as they accept and donate electrons. ○ Each component of the chain becomes reduced when it accepts electrons From studying the structure of ATP synthase, scientists have learned how from its “uphill” neighbor, which is less electronegative. the flow of H+ through this large enzyme powers ATP generation. ○ It then returns to its oxidized form as it passes electrons to its more ATP synthase is a multisubunit complex with four main parts, each made up electronegative “downhill” neighbor. of multiple polypeptides. Electrons carried by NADH are transferred to the first molecule in the ○ Protons move one by one into binding sites on one of the parts (the electron transport chain, a flavoprotein. rotor), causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate. In the next redox reaction, the flavoprotein returns to its oxidized form as it ○ ATP synthase is the smallest molecular rotary motor known in nature. passes electrons to an iron-sulfur protein. How does the inner mitochondrial membrane or the prokaryotic plasma The iron-sulfur protein then passes the electrons to a compound called membrane generate and maintain the H+ gradient that drives ATP synthesis ubiquinone, a small hydrophobic molecule and the only member of the in the ATP synthase protein complex? electron transport chain that is not a protein. o Establishing the H+ gradient is the function of the electron transport ○ Ubiquinone is individually mobile within the membrane rather than chain. residing in a particular complex. o The chain is an energy converter that uses the exergonic flow of Most of the remaining electron carriers between ubiquinone and oxygen are electrons to pump H+ across the membrane from the mitochondrial proteins called cytochromes. matrix into the intermembrane space. ○ The prosthetic group of each cytochrome is a heme group with an iron o The H+ has a tendency to diffuse down its gradient. atom that accepts and donates electrons. 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. ▪ The mitochondrial inner membrane is impermeable to NADH, This coupling of the redox reactions of the electron transport chain to so NADH produced in glycolysis must be conveyed into the ATP synthesis is an example of chemiosmosis. mitochondrion by one of several electron shuttle systems. How does the electron transport chain pump protons? ▪ Depending on the kind of shuttle in a particular cell type, the electrons are passed either to NAD+ or to FAD in the o Certain members of the electron transport chain accept and release H + mitochondrial matrix. along with electrons. ▪ If the electrons are passed to FAD, as in brain cells, 2 ATP o At certain steps along the chain, electron transfers cause H + to be result from each NADH that was originally generated in the taken up and released into the surrounding solution. cytosol. o The electron carriers are spatially arranged in the membrane in such a ▪ If the electrons are passed to mitochondrial NAD+, as in liver way that protons are accepted from the mitochondrial matrix and cells and heart cells, the yield is about 3 ATP per NADH. deposited in the intermembrane space. 3. The proton-motive force generated by the redox reactions of The H+ gradient that results is the proton-motive force, a gradient with the respiration may drive other kinds of work, such as mitochondrial capacity to do work. uptake of pyruvate from the cytosol. o The force drives H+ back across the membrane through the specific H+ ▪ For example, the proton-motive force powers the channels provided by ATP synthases. mitochondrion’s uptake of pyruvate from the cytosol. Chemiosmosis is an energy-coupling mechanism that uses energy stored in ▪ However, if all the proton-motive force generated by the the form of an H+ gradient across a membrane to drive cellular work. electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of 28 ATP In mitochondria, the energy for proton gradient formation comes from produced by oxidative phosphorylation plus 4 ATP (net) from exergonic redox reactions, and ATP synthesis is the work performed. substrate-level phosphorylation to give a total yield of about 32 Chemiosmosis in chloroplasts also generates ATP, but light drives both the ATP (or only about 30 ATP if the less efficient shuttle were electron flow down an electron transport chain and the resulting H+ gradient functioning). formation. How efficient is respiration in generating ATP? Prokaryotes generate H+ gradients across their plasma membrane. o Complete oxidation of glucose releases 686 kcal/mol. o Prokaryotes use the proton-motive force not only to generate ATP but o Phosphorylation of ADP to form ATP requires at least 7.3 kcal/mol. also to pump nutrients and waste products across the membrane and to o Efficiency of respiration is 7.3 kcal/mol times 32 ATP/glucose rotate their flagella. divided by 686 kcal/mol glucose, which equals 0.34, or 34%. Here is an accounting of ATP production by cellular respiration. o The actual percentage is probably higher because G is lower under cellular conditions. During cellular respiration, most energy flows as follows: glucose → NADH → electron transport chain → proton-motive force → ATP. 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). Let’s consider the products generated when cellular respiration oxidizes a molecule of glucose to six molecules of CO2. Cellular respiration is remarkably efficient in energy conversion. o Four ATP molecules are produced by substrate-level phosphorylation o For example, the most efficient automobile converts only about 25% during glycolysis and the citric acid cycle. of the energy stored in gasoline to energy that moves the car. o Many more ATP molecules are generated by oxidative Under certain conditions, it may be beneficial to reduce the efficiency of phosphorylation. cellular respiration. o Each NADH from the citric acid cycle and the conversion of pyruvate A remarkable adaptation is shown by hibernating mammals, which contributes enough energy to the proton-motive force to generate a overwinter in a state of inactivity and lowered metabolism. maximum of 3 ATP. o Although their internal body temperature is lower than normal, it is There are three reasons we cannot state an exact number of ATP molecules still significantly higher than the external air temperature. generated by one molecule of glucose. o One type of tissue, called brown fat, is made up of cells packed full of 1. Phosphorylation and the redox reactions are not directly coupled to mitochondria. each other, so the ratio of the number of NADH to the number of ATP o The inner mitochondrial membrane contains a channel protein called is not a whole number. the uncoupling protein, which allows protons to flow back down their ▪ One NADH results in 10 H+ being transported across the inner concentration gradient without generating ATP. mitochondrial membrane. o Activation of these proteins in hibernating mammals results in ▪ 4 H+ must reenter the mitochondrial matrix via ATP synthase to ongoing oxidation of stored fuel stores (fats), generating heat without generate 1 ATP. ATP production. ▪ Therefore, 1 NADH generates enough proton-motive force for o In the absence of such an adaptation, the ATP level would build up to the synthesis of 2.5 ATP. a point that cellular respiration would be shut down due to regulatory 2. The ATP yield varies slightly depending on the type of shuttle used to mechanisms. transport electrons from the cytosol into the mitochondrion. Concept 9.5 Fermentation and anaerobic respiration enable cells to Human muscle cells switch from aerobic respiration to lactic acid produce ATP without the use of oxygen fermentation to generate ATP when O2 is scarce. This may occur in the early stages of strenuous exercise. Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation eventually ceases. o The waste product, lactate, was previously thought to cause muscle fatigue and pain, but recent research suggests instead that increased However, there are two general mechanisms by which certain cells can levels of potassium ions (K+) may be to blame; lactate appears to oxidize organic fuel and generate ATP without the use of oxygen: enhance muscle performance. fermentation and anaerobic respiration. o Excess lactate is gradually carried away by the blood to the liver, o An electron transport chain is present in aerobic respiration but not in where it is converted back to pyruvate by liver cells. fermentation. o The pyruvate enters the mitochondria in liver cells and completes Anaerobic respiration takes place in organisms that have an electron cellular respiration. transport chain but do not use oxygen as a final electron acceptor at the end of the chain. o Some “sulfate-reducing” marine bacteria, for instance, use the Fermentation and cellular respiration are compared. 2- electronegative sulfate ion (SO4 ) at the end of their respiratory chain. Fermentation, anaerobic respiration, and aerobic respiration are three o Operation of the chain builds up a proton-motive force used to alternative cellular pathways for producing ATP from sugars. produce ATP, but H2S (hydrogen sulfide) is produced as a by-product o All three use glycolysis to oxidize sugars to pyruvate with a net rather than H2O (water). production of 2 ATP by substrate-level phosphorylation. Fermentation provides a mechanism by which some cells can oxidize o In all three, NAD+ is the oxidizing agent that accepts electrons from organic fuel and generate ATP without the use of oxygen or any electron food during glycolysis. transport chain (that is, without cellular respiration). A key difference is the mechanisms for oxidizing NADH to NAD+, which is o Glycolysis oxidizes glucose to two pyruvate molecules, with NAD + as required to sustain glycolysis. the oxidizing agent. o In fermentation, the final electron acceptor is an organic molecule o Glycolysis is exergonic and produces 2 ATP (net) by substrate-level such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol phosphorylation. fermentation). If oxygen is present, additional ATP can be generated when NADH delivers o In cellular respiration, electrons carried by NADH are transferred to its electrons to the electron transport chain. an electron transport chain and move stepwise down a series of redox o However, glycolysis generates 2 ATP whether oxygen is present reactions to a final electron acceptor. (aerobic) or not (anaerobic). In aerobic respiration, the final electron acceptor is oxygen; in anaerobic Fermentation allows generation of ATP from glucose by substrate-level respiration, the final acceptor is another molecule that is less electronegative phosphorylation. than oxygen. o Glycolysis continues as long as there is a supply of NAD+ to accept Transfer of electrons from NADH to the electron transport chain not only electrons during the oxidation step. If the NAD+ pool is exhausted, regenerates the NAD+ required for glycolysis but also pays an ATP bonus glycolysis shuts down. when the stepwise electron transport from this NADH to oxygen drives oxidative phosphorylation. Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD+. More ATP is produced by the oxidation of pyruvate in the mitochondrion, which is unique to respiration. o Without an electron transport chain, the energy still stored in pyruvate Fermentation pathways recycle NAD+ by transferring electrons from NADH to is unavailable to most cells. pyruvate or derivatives of pyruvate. o Thus, cellular respiration harvests much more energy from each sugar In alcohol fermentation, pyruvate is converted to ethanol in two steps. molecule than fermentation can. o Pyruvate is converted to a two-carbon compound, acetaldehyde, by Aerobic respiration yields up to 16 times as much ATP per glucose molecule the removal of CO2. as does fermentation—up to 32 molecules of ATP for respiration, compared o Acetaldehyde is reduced by NADH to ethanol. This process with 2 molecules of ATP produced by substrate-level phosphorylation in regenerates the supply of NAD+ needed for the continuation of fermentation. glycolysis. o Alcohol fermentation by yeast is used in brewing, baking, and winemaking. Organisms vary in the pathways available to them to break down sugars. During lactic acid fermentation, pyruvate is reduced directly by NADH to Obligate anaerobes carry out only fermentation or anaerobic respiration form lactate (the ionized form of lactic acid) without the release of CO 2. and cannot survive in the presence of oxygen. o Lactic acid fermentation by some fungi and bacteria is used to make A few cell types, such as the cells of the vertebrate brain, can carry out only cheese and yogurt. aerobic oxidation of pyruvate, not fermentation.  Yeast and many bacteria are facultative anaerobes that can survive using Catabolism can also harvest energy stored in fats obtained from food or from either fermentation or respiration. storage cells in the body. o At a cellular level, human muscle cells can behave as facultative After fats are digested to glycerol and fatty acids, glycerol can be converted anaerobes. to glyceraldehyde-3-phosphate, an intermediate of glycolysis. For facultative anaerobes, pyruvate is a fork in the metabolic road that leads The rich energy of fatty acids is accessed as fatty acids are split into two- to two alternative catabolic routes. carbon fragments via beta oxidation. o Under aerobic conditions, pyruvate is converted to acetyl CoA and o These molecules enter the citric acid cycle as acetyl CoA. oxidation continues in the citric acid cycle via aerobic respiration. NADH and FADH2 are also generated during beta oxidation; they can enter o Under anaerobic conditions, lactic acid fermentation occurs and the electron transport chain, leading to further ATP production. pyruvate serves as an electron acceptor to recycle NAD+. A gram of fat oxidized by respiration generates twice as much ATP as a To make the same amount of ATP, a facultative anaerobe must consume gram of carbohydrate. sugar at a much faster rate when fermenting than when respiring. The metabolic pathways of respiration also play a role in anabolic pathways of the cell. The role of glycolysis in both fermentation and respiration has an evolutionary basis. In addition to calories, food must provide the carbon skeletons that cells Ancient prokaryotes likely used glycolysis to make ATP long before oxygen require to make their own molecules. was present in Earth’s atmosphere. Some organic monomers obtained from digestion can be used directly. The oldest bacterial fossils are more than 3.5 billion years old, appearing long before appreciable quantities of O2 accumulated in the atmosphere Intermediaries in glycolysis and the citric acid cycle can be diverted to about 2.7 billion years ago. anabolic pathways as precursors from which the cell can synthesize the o Cyanobacteria produced this O2 as a by-product of photosynthesis. molecules it requires. o For example, a human cell can synthesize about half the 20 different The first prokaryotes may have generated ATP exclusively from glycolysis, amino acids by modifying compounds from the citric acid cycle. The which does not require oxygen. rest are “essential amino acids” that must be obtained in the diet. The fact that glycolysis is a ubiquitous metabolic pathway and occurs in the o Glucose can be synthesized from pyruvate; fatty acids can be cytosol without requiring any of the membrane-enclosed organelles suggests synthesized from acetyl CoA. that this pathway evolved very early in the history of life on Earth. Anabolic, or biosynthetic, pathways do not generate ATP but instead consume it. Concept 9.6 Glycolysis and the citric acid cycle connect to many other Glycolysis and the citric acid cycle function as metabolic interchanges that metabolic pathways enable cells to convert one kind of molecule to another as needed. Glycolysis and the citric acid cycle are major intersections of various o For example, excess carbohydrates and proteins can be converted to catabolic and anabolic (biosynthetic) pathways. fats through intermediaries of glycolysis and the citric acid cycle. o If we eat more food than we need, we store fat even if our diet is fat- free. A variety of organic molecules can be used to make ATP. Metabolism is remarkably versatile and adaptable. Glycolysis can accept a wide range of carbohydrates for catabolism. o Polysaccharides like starch or glycogen can be hydrolyzed to glucose Feedback mechanisms control cellular respiration. monomers that enter glycolysis and the citric acid cycle. Basic principles of supply and demand regulate the metabolic economy. o The digestion of disaccharides, including sucrose, provides glucose o If a cell has an excess of a certain amino acid, it typically uses and other monosaccharides as fuel for respiration. feedback inhibition to prevent the diversion of intermediary molecules The other two major fuels, proteins and fats, can also enter the respiratory from the citric acid cycle to the synthesis pathway of that amino acid. pathways used by carbohydrates. o The end product of the anabolic pathway inhibits the enzyme catalyzing an early step of the pathway, preventing diversion of key Proteins must first be digested to individual amino acids. metabolic intermediates from more urgent uses. o Many of the amino acids are used by the organism to build new proteins. The rate of catabolism is also regulated: if ATP levels drop, catabolism speeds up to produce more ATP. Amino acids that will be catabolized must have their amino groups removed o When there is plenty of ATP to meet demand, respiration slows down, via deamination. sparing valuable organic molecules for other functions. o The nitrogenous waste is excreted as ammonia, urea, or another waste product. Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway. o The carbon skeletons are modified by enzymes to intermediates of glycolysis and the citric acid cycle.  One strategic point occurs in the third step of glycolysis, catalyzed by ○ Plants are photoautotrophs, using light as a source of energy to phosphofructokinase, an enzyme which functions as the pacemaker of synthesize organic compounds. respiration. ○ Photosynthesis also occurs in algae, some other protists, and some o Phosphofructokinase catalyzes the earliest step that irreversibly prokaryotes. commits the substrate to glycolysis. Heterotrophs live on organic compounds produced by other organisms. o By controlling the rate of this step, the cell can speed up or slow down Heterotrophs are the consumers of the biosphere. the entire catabolic process. Phosphofructokinase is thus considered the pacemaker of respiration. ○ The most obvious type of heterotrophs feeds on other organisms. Animals feed this way. Phosphofructokinase is an allosteric enzyme with receptor sites for specific ○ Other heterotrophs decompose and feed on dead organisms or on inhibitors and activators. organic litter, like feces and fallen leaves. These are decomposers. o It is inhibited by ATP and stimulated by AMP (derived from ADP). ○ Most fungi and many prokaryotes get their nourishment this way. o When ATP levels are high, inhibition of this enzyme slows glycolysis. ○ Almost all heterotrophs are completely dependent on photoautotrophs o As ATP levels drop and ADP and AMP levels rise, the enzyme for food and for oxygen, a by-product of photosynthesis. becomes active again and glycolysis speeds up. The Earth’s supply of fossil fuels was formed from remains of organisms Citrate, the first product of the citric acid cycle, is also an inhibitor of that died hundreds of millions of years ago. phosphofructokinase. ○ Fossil fuels thus represent stores of the sun’s energy from the distant o This synchronizes the rate of glycolysis and the citric acid cycle. past. If intermediaries from the citric acid cycle are diverted to other uses (for ○ Because fossil fuels are not renewable, researchers are exploring example, amino acid synthesis), glycolysis speeds up to replace these methods of capitalizing on photosynthesis for alternative fuels. molecules. Metabolic balance is augmented by the control of other enzymes at other key locations in glycolysis and the citric acid cycle. Concept 10.1 Photosynthesis converts light energy to chem energy of food Cells are thrifty, expedient, and responsive in their metabolism. Photosynthetic enzymes and other molecules are grouped together in a Cellular respiration and metabolic pathways play a role of central biological membrane, allowing the necessary series of chemical reactions to importance in organisms. be carried out efficiently. o Cellular respiration also functions in the broad context of energy flow and chemical cycling in ecosystems. The process of photosynthesis likely originated in a group of bacteria with infolded regions of the plasma membrane containing clusters of such The energy that keeps us alive is released, not produced, by cellular molecules. respiration. ○ In existing photosynthetic bacteria, infolded photosynthetic membranes o These processes tap energy that was stored in food by photosynthesis. function similarly to the internal membranes of the chloroplast. --------------------------------------------------- ○ The endosymbiont theory suggests that original chloroplast was a photosynthetic prokaryote that lived inside a eukaryotic cell. Chapter 10: Photosynthesis Chloroplasts are the sites of photosynthesis in plants. All green parts of a plant have chloroplasts, but leaves are the major sites of Overview: The Process That Feeds the Biosphere photosynthesis for most plants. Life on Earth is solar powered. ○ There are about half a million chloroplasts per square millimeter of leaf surface. The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars Chloroplasts are found mainly in cells of mesophyll, the tissue in the interior and other organic molecules. of the leaf. Photosynthesis nourishes almost all the living world directly or indirectly. O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf. Organisms obtain organic compounds by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Veins deliver water from the roots and carry off sugar from mesophyll cells to roots and other nonphotosynthetic areas of the plant. Autotrophs produce organic molecules from CO2 and other inorganic raw materials obtained from the environment. A typical mesophyll cell has 30–40 chloroplasts, each measuring about 2–4 Autotrophs are the ultimate sources of organic compounds for all µm by 4–7 µm. heterotrophic organisms. Each chloroplast has two membranes around a dense fluid called the Autotrophs are the producers of the biosphere. stroma. ○ Almost all plants are autotrophs; the only nutrients they require are Suspended within the stroma is an internal membrane system of sacs, the water and minerals from the soil and carbon dioxide from the air. thylakoids.  ○ The interior of the thylakoids forms another compartment, the thylakoid ○ Plants: CO2 + 2H2O → [CH2O] + H2O + O2 space. ○ General: CO2 + 2H2X → [CH2O] + H2O + X2 ○ Thylakoids may be stacked in columns called grana. Twenty years later, scientists confirmed van Niel’s hypothesis. Chlorophyll, the green pigment in the chloroplasts, is located in the ○ Researchers used 18O, a heavy isotope, as a tracer to follow the fate of thylakoid membranes. oxygen atoms during photosynthesis. Chlorophyll plays an important role in the absorption of light energy during ○ When they labeled either C18O2 or H218O, they found that the 18O label photosynthesis. appeared in the oxygen produced in photosynthesis only when water was the source of the tracer. The photosynthetic membranes of prokaryotes arise from infolded regions of the plasma membranes, also called thylakoid membranes. Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the atmosphere. Powered by light, photosynthesis produces org compounds and O 2 from CO2 and H2O. Photosynthesis is a redox reaction. The equation describing the process of photosynthesis is 6CO2 + 12H2O + light energy → C6H12O6 + 6O2+ 6H2O Both photosynthesis and aerobic respiration involve redox reactions. C6H12O6 is glucose, although the direct product of photosynthesis is actually During cellular respiration, energy is released from sugar when electrons a three-carbon sugar that can be used to make glucose. associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. Water appears on both sides of the equation because 12 molecules of water are consumed and 6 molecules are newly formed during photosynthesis. The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that We can simplify the equation by showing only the net consumption of energy to synthesize ATP. water: Photosynthesis reverses the direction of electron flow. 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 Water is split and electrons are transferred with H+ from water to CO2, Written this way, the overall chemical change during photosynthesis is the reducing it to sugar. reverse of cellular respiration. Because the electrons increase in potential energy as they move from water Both of these metabolic processes occur in plant cells. However, to sugar, the process requires energy. The energy boost is provided by light. chloroplasts do not synthesize sugars by simply reversing the steps of respiration. A preview of the two stages of photosynthesis. In its simplest possible form, CO2 + H2O + light energy → [CH2O] + O2, Photosynthesis is two processes, each with multiple steps: light reactions and where [CH2O] represents the general formula for a carbohydrate. the Calvin cycle. Evidence that chloroplasts split water molecules enabled researchers to track atoms ○ The light reactions (photo) convert solar energy to chemical energy. through photosynthesis. ○ The Calvin cycle (synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar. One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2. In the light reactions, water is split, providing a source of electrons and protons (H+ ions) and giving off O2 as a by-product. Before the 1930s, the prevailing hypothesis was that photosynthesis split carbon dioxide and then added water to the carbon: Light absorbed by chlorophyll drives the transfer of electrons and hydrogen Step 1: CO2 → C + O2 ions from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH. Step 2: C + H2O → CH2O The light reactions also generate ATP using chemiosmosis, in a process Stanford University’s van Niel challenged this hypothesis. In the bacteria called photophosphorylation. that he was studying, hydrogen sulfide (H2S), rather than water, is used in photosynthesis. Thus, light energy is initially converted to chemical energy in the form of ○ These bacteria produce yellow globules of sulfur as a waste, rather than two compounds: NADPH, a source of electrons as reducing power that can oxygen. be passed along to an electron acceptor, and ATP, the energy currency of cells. He proposed this chemical equation for photosynthesis in sulfur bacteria: The light reactions produce no sugar; that happens in the second stage of CO2 + 2H2S → [CH2O] + H2O + 2S photosynthesis, the Calvin cycle. He generalized this idea and applied it to plants, proposing this reaction for The Calvin cycle is named for Melvin Calvin, who, with his colleagues, their photosynthesis: worked out many of its steps in the 1940s. CO2 + 2H2O → [CH2O] + H2O + O2 The cycle begins with the incorporation of CO2 into organic molecules, a Thus, van Niel hypothesized that plants split water as a source of electrons process known as carbon fixation. from hydrogen atoms, releasing oxygen as a by-product. The fixed carbon is reduced with electrons provided by NADPH. ○ Sulfur bacteria: CO2 + 2H2S → [CH2O] + H2O + 2S  ATP from the light reactions also powers parts of the Calvin cycle. ○ A spectrophotometer beams narrow wavelengths of light through a solution containing the pigment and then measures the fraction of Thus, it is the Calvin cycle that makes sugar, but only with the help of ATP light transmitted at each wavelength. and NADPH from the light reactions. ○ An absorption spectrum plots a pigment’s light absorption versus The metabolic steps of the Calvin cycle are sometimes referred to as light- wavelength. independent reactions because none of the steps requires light directly. The light reactions can perform work with wavelengths of light that are ○ Nevertheless, the Calvin cycle in most plants occurs during daylight absorbed. because that is when the light reactions can provide the NADPH and ATP the Calvin cycle requires. Several pigments in the thylakoid differ in their absorption spectra. In essence, the chloroplast uses light energy to make sugar by ○ Chlorophyll a, which participates directly in the light reactions, absorbs coordinating the two stages of photosynthesis. best in the red and violet-blue wavelengths and absorbs least in the green. Whereas the light reactions occur at the thylakoids, the Calvin cycle occurs ○ Accessory pigments include chlorophyll b and a group of molecules called in the stroma. carotenoids. ○ In the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to An overall action spectrum for photosynthesis profiles the relative the stroma, where they play crucial roles in the Calvin cycle. effectiveness of different wavelengths of radiation in driving the process. ○ An action spectrum measures changes in some measure of photosynthetic activity (for example, O2 release) as the wavelength is Concept 10.2 The light reactions convert solar energy to the chemical varied. energy of ATP and NADPH The action spectrum of photosynthesis was first demonstrated in 1883 in a Light is a form of electromagnetic energy or radiation. clever experiment performed by Thomas Engelmann. Like other forms of electromagnetic energy, light travels in rhythmic waves. ○ Different segments of a filamentous alga were exposed to different wavelengths of light. The distance between crests of electromagnetic waves is called the ○ Areas receiving wavelengths favorable to photosynthesis produced wavelength. excess O2. ○ Wavelengths of electromagnetic radiation range from shorter than a ○ Engelmann used the abundance of aerobic bacteria that clustered along nanometer (gamma rays) to longer than a kilometer (radio waves). the alga at different segments as a measure of O2 production. The entire range of electromagnetic radiation is the electromagnetic ○ His results are a striking match to the modern action spectrum. spectrum. The action spectrum of photosynthesis does not match exactly the absorption The most important segment of the electromagnetic spectrum for life is a spectrum of any one photosynthetic pigment, including chlorophyll a. narrow band between 380 and 750 nm, the band of visible light detected as colors by the human eye. Only chlorophyll a participates directly in the light reactions, but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a. Although light travels as a wave, many of its properties are those of a ○ Chlorophyll b, with a slightly different structure than chlorophyll a, has discrete particle, a photon. a slightly different absorption spectrum and funnels the energy from Photons are not tangible objects, but do have fixed quantities of energy. these wavelengths to chlorophyll a. ○ The amount of energy packaged in a photon is inversely related to its ○ Carotenoids can funnel the energy from other wavelengths to wavelength: Photons with shorter wavelengths pack more energy. chlorophyll a and also participate in photoprotection against excessive light. Although the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to ○ These compounds absorb and dissipate excessive light energy that pass in significant quantities. would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules that could damage the cell. ○ Visible light is the radiation that drives photosynthesis. When chlorophyll and other pigments absorb light, an electron is boosted to an excited Photosynthetic pigments are light receptors. state. When light meets matter, it may be reflected, transmitted, or absorbed. When a molecule absorbs a photon, one of the molecule’s electrons is Substances that absorb visible light are known as pigments. elevated to an orbital with more potential energy. ○ Different pigments absorb photons of different wavelengths, and the The electron moves from its ground state to an excited state. wavelengths that are absorbed disappear. The only photons that a molecule can absorb are those whose energy ○ A leaf looks green because chlorophyll, the dominant pigment, absorbs matches exactly the energy difference between the ground state and the red and violet-blue light while transmitting and reflecting green light. excited state of this electron. A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.  Because this energy difference varies among atoms and molecules, a Photosystem II has a reaction-center chlorophyll a known as P680, with an particular compound absorbs only photons corresponding to specific absorption peak at 680 nm. wavelengths. Photosystem I has a reaction-center chlorophyll a known as P700, with an ○ This is the reason each pigment has a unique absorption spectrum. absorption peak at 700 nm. Excited electrons are unstable. Generally, they drop to their ground state in a ○ These two pigments, P680 and P700, are nearly identical chlorophyll a billionth of a second, releasing heat energy. molecules. In isolation, some pigments emit light after absorbing photons, in a process Their association with different proteins in the thylakoid membrane affects called fluorescence. the electron distribution in the chlorophyll molecules and accounts for the ○ If a solution of chlorophyll isolated from chloroplasts is illuminated, it slight differences in light-absorbing properties. fluoresces in the red-orange part of the spectrum and gives off heat. These two photosystems work together in using light energy to generate Chlorophyll excited by absorption of light energy produces very different ATP and NADPH. results in an intact chloroplast than it does in isolation. In the thylakoid membrane, chlorophyll is organized along with proteins and During the light reactions, there are two possible routes for electron flow: linear and smaller organic molecules into photosystems. cyclic. A photosystem is composed of a reaction-center complex surrounded by Linear electron flow drives the synthesis of ATP and NADPH by several light-harvesting complexes. energizing the two photosystems embedded in the thylakoid membranes of ○ The reaction-center complex is an organized association of proteins chloroplasts. holding a special pair of chlorophyll a molecules. During the light reactions, electrons flow through the photosystems and Each light-harvesting complex consists of pigment molecules (which may other molecular components built into the thylakoid membrane. include chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. o Photosystem II absorbs a photon of light. One of the electrons of P680 ○ The number and variety of pigment molecules enable a photosystem to is excited to a higher energy state. harvest light over a larger surface area and a larger portion of the o This electron is captured by the primary electron acceptor, leaving spectrum than could any single pigment molecule. P680 oxidized (P680+). Together, the light-harvesting complexes act as an antenna for the reaction- o An enzyme extracts electrons from water and supplies them to the center complex. oxidized P680+ pair. This reaction splits water into two hydrogen ions and an oxygen atom that combines with another oxygen atom to form When a pigment molecule absorbs a photon, the energy is transferred from O2. The H+ are released into the thylakoid lumen. pigment molecule to pigment molecule until it is funneled into the reaction- center complex. o Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain. The electron transport At the reaction center is a primary electron acceptor, which accepts an chain between PS II and PS I is made up of the electron carrier excited electron from the reaction center chlorophyll a. plastoquinone (Pq), a cytochrome complex, and a protein called The solar-powered transfer of an electron from a special chlorophyll a plastocyanin (Pc). molecule to the primary electron acceptor is the first step of the light o As these electrons “fall” to a lower energy level, their energy is reactions. harnessed to produce ATP. As electrons pass through the cytochrome complex, H+ are pumped into the thylakoid lumen, contributing to the As soon as the chlorophyll electron is excited to a higher energy level, the proton gradient that is subsequently used in chemiosmosis. primary electron acceptor captures it in a redox reaction. o Meanwhile, light energy has excited an electron of PS I’s P700 Isolated chlorophyll fluoresces because there is no electron acceptor, so reaction center. The photoexcited electron was captured by PS I’s electrons of photoexcited chlorophyll drop right back to the ground state. primary electron acceptor, creating an electron “hole” in P700 (to ○ In a chloroplast, the potential energy represented by the excited electron produce P700+). This hole is filled by an electron that reaches the is not dissipated as light and heat.