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Study Guide 31. An organism’s metabolism transforms matter and energy, subject to the laws of

thermodynamicsa. The totality of an organism’s chemical reactions is called metabolism.

Metabolism is an emergent property of life that arises from interactions between molecules within the orderly environment of the cell.

b. Organization of the Chemistry of Life into Metabolici. A metabolic pathway begins with a specific molecule, which is then

altered in a series of defined steps, resulting in a certain product. Each step of the pathway is catalyzed by a specific enzyme.

c. Metabolism as a whole manages the material and energy resources of cell. Some metabolic pathway release energy by breaking down complex molecules to simpler compounds.

i. These degradative processes are called catabolic pathways, or breakdown pathways. A major pathway of catabolism is cellular respiration, in which the sugar glucose and other organic fuels are broken down in the presence of oxygen to carbon dioxide and water. (Pathways can have more than one starting molecule and/or product). Energy that was stored in the organic molecules becomes available to do the work of the cell, such as ciliary beating or membrane transport.

ii. Anabolic pathways, in contrast, consume energy to build complicated molecules from simpler ones; they are sometimes called biosynthetic pathways. An example of anabolism is the synthesis of a protein from amino acids.

iii. Catabolic and anabolic pathways are the “downhill” and “uphill” avenues of the metabolic map. Energy released from the downhill reactions of catabolic pathways can be stored and then used to drive the uphill reactions of anabolic pathways.

2. Energy is the capacity to cause change.a. Energy can be associated with the relative motion of objects is called kinetic

energy.b. Heat or thermal energy is the kinetic energy associated with the random

movement of atoms of molecules.c. Energy that is not kinetic is called potential energy; that matter possesses because

of its location or structure. d. Chemical energy is a term used by biologists to refer to the potential energy

available for release in a chemical reaction.3. The Laws of Energy Transformation

a. The study of energy transformations that occur in a collection of matter is called thermodynamics.

i. First law of thermodynamics – the energy of the universe is constant, energy can be transferred and transformed, but it cannot be created or destroyed.

ii. Second law of thermodynamics – every transfer or transfer or transformation increases the entropy of the universe.

1. Entropy is the measure of disorder.

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4. Free energy is the portion of a system’s energy that can perform work when the temperature and pressure are uniform throughout the system, as in a living cell.

a. Delta G = Delta H – T(Delta S)b. Delta G = Final – Initialc. Thus, Delta G can be negative only when the process involves a loss of free

energy during the change from initial state to final state. Because it has less free energy, the system is in its final state is less likely to change and is therefore more stable than it was previously.

d. More free energy (higher G), Less stable, Greater work capacitye. Less free energy (lower G), More stable, Less work capacityf. Enzymes do nothing to the delta G

5. Exergonic reactions – proceeds with a net release of free energy. Because the chemical mixture loses free energy. Spontaneous reaction.

6. Endergonic reaction – is one that absorbs free energy from its surroundings. Nonspontaneous.

7. ATP Powers cellular work by coupling exergonic reactions to endergonic reactionsa. A cell does three main kinds of work: chemical, transport, and mechanical

i. Chemical work – the pushing of endergonic reactions, which would not occur spontaneously, such as the synthesis of polymers from monomers.

ii. Transport work – the pumping of substances across membranes against the direction of spontaneous movement.

iii. Mechanical work – such as the beating of cilia, the contraction of muscle cells, and the movement of chromosomes during cellular reproduction.

b. A key feature in the way cells manage their energy resources to do this work is energy coupling, the use of an exergonic process to drive an endergonic one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy that power cellular work.

8. The Structure and Hydrolysis of ATPa. ATP (adenosine triphosphate) contains the sugar ribose, with the nitrogenous base

adenine and a chain of three phosphate group bonded to it. In addition to its role in energy coupling, ATP is also one of the nucleoside triphosphates used to make RNA.

b. ATP can be incorporated by a nucleotide.c. The bond between the phosphate groups of ATP can be broken by hydrolysis.

When the terminal phosphate bond is broken a molecule of inorganic phosphate (HOPO32-, abbreviated P1) leaves the ATP, which become adenosine diphosphate, ADP. The reaction is exergonic and releases 7.3 kcal of energy per mol.

i. ATP + H2O –> ADP + P1ii. Delta G = -7.3 kcal/mol

d. This is the free energy change measured under standard conditions. In the cell, conditions do not conform to standard conditions, primarily because reactant and product concentration differ from 1 M. For example, when ATP hydrolysis occurs under cellular conditions, the actual delta G is about -13 kcal/mol, 78% greater than the energy released by ATP hydrolysis under standard conditions.

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e. Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as the high-energy phosphate bonds, but the term is misleading. The phosphate bonds of ATP are not usually strong bonds, as “high-energy” may imply; rather, the reactants (ATP and water) themselves have high energy relative to the energy of the products (ADP and P). The release of energy during hydrolysis of ATP comes from the chemical change to a state of lower free energy, not from the phosphate bond themselves.

f. ATP is useful to the cell because the energy it releases on losing a phosphate group is somewhat greater than the energy most other molecules could deliver.

9. How ATP Performs Worka. When ATP is hydrolyzed in a test tube, the release of free energy merely heats the

surrounding water. In an organism, this same generation of heat can sometimes be beneficial. Cell’s proteins harness the energy released during ATP hydrolysis in several ways to perform the three types of cellular work.

b. With the specific enzymes, the cell is able to use the energy released by ATP hydrolysis directly to drive chemical reactions that, by themselves, are endergonic. If the delta G of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that, overall, the coupled reactions are exergonic. This usually involves the transfer of a phosphate group from ATP to some other molecule, such as the reactant. The recipient of the phosphate group is then said to be phosphorylated.

c. Transport and mechanical work in the cell are also nearly always powered by the hydrolysis of ATP. In these cases, ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind another molecule. Sometime this occurs via a phosphorylated intermediate.

i. In most instances of mechanical work involving motor proteins “walking” along cytoskeletal elements, a cycle occurs in which ATP is first bound non-covalently to the motor protein.

ii. If the delta G of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that the overall reaction is exergonic. This usually involves the transfer of a phosphate group from ATP to some other molecule such as the reactant. The recipient of the phosphate group is then said to be phosphorylated.

iii. Next, ATP is hydrolyzed, releasing ADP and P; another ATP molecule can then bind.

iv. At each stage, the motor protein changes its shape and ability to bind to the cytoskeleton, resulting in movement of the protein along the cytoskeletal track.

d. The Regeneration of ATPi. An organism at work uses ATP continuously, but ATP is a renewable

resource that can be regenerated by the addition of phosphate to ADP. The free energy required to phosphorylate ADP comes from exergonic breakdown reactions (catabolism) in the cell.

1. This shuttling of inorganic phosphate and energy is called the ATP cycle, and it couples the cell’s energy-yielding (exergonic) processes to the energy-consuming (endergonic) ones.

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2. The ATP cycle moves at an astonishing pace.3. If ATP could not be regenerated by the phosphorylation of ADP,

humans would use up nearly their body weight in ATP each day.4. Because both directions of a reversible process cannot go downhill,

the regeneration of ATP from ADP and P is necessarily endergonic.

5. Because ATP formation from ADP and P is not spontaneous, free energy must be spent to make it occur. Catabolic (exergonic) pathways, especially cellular respiration, provide the energy for the endergonic process of making ATP. Plants also use light energy to produce ATP.

10. Enzymes speed up metabolic reactions by lowering energy barriersa. An enzyme is a macromolecule that acts as a catalyst, a chemical agent that

speeds up a reaction without being consumed by the reaction. b. The initial investment of energy for starting a reaction – the energy required to

contort the reactant molecules so the bonds can break is known as the free energy of activation, or activation energy. We can think of activation energy as the amount of energy needed to push the reactants over an energy barrier, or hill, so that the “down-hill” part of the reaction can begin.

c. The lower the Ea, the fast the reactiond. An enzyme catalyzes a reaction by lowering the Ea barrier, enabling the reactant

molecules to absorb enough energy to reach the transition state even at moderate temperatures.

i. An enzyme cannot change the delta G for a reaction; it cannot make an endergonic reaction exergonic. Enzymes can only hasten reactions that would occur eventually anyway, but this function makes it possible for the cell to have a dynamic metabolism, routing chemicals smoothly through the cell’s metabolic pathways.

11. Substrate Specificity of Enzymesa. The reactant an enzyme acts on is referred to as the enzyme’s substrate. The

enzyme binds to its substrate forming an enzyme-substrate complex. While enzyme and substrate are joined, the catalytic action of the enzyme converts the substrate to the product.

b. The region called the active site is the restricted region of the enzyme where the substrate can attach.

12. One catabolic process, fermentation, is a partial degradation of sugars that occurs without the use of oxygen. However, the most prevalent and efficient catabolic pathway is aerobic respiration, in which oxygen is consumed as a reactant along with the organic fuel.

a. Technically, the term cellular respiration includes both aerobic and anaerobic processes. But cellular respiration is often used to refer to the aerobic process.

13. Redox Reactionsi. Four kinds of chemical reactions you should know:

1. Oxidation2. Reduction3. Hydrolysis

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4. Oxidationii. Redox are the two reactions that occur simultaneously because the highly

reactive products.b. The Principle of Redox

i. In many chemical reactions, there is a transfer of one or more electrons from one reactant to another. These are redox.

1. Oxidation is the loss of electrons2. Reduction is the addition of electrons

ii. The electron donor is the reducing agentiii. The electron acceptor is the oxidizing agent.

14. Stepwise Energy Harvest via NAD+ and the Electron Transport Chaina. If energy is released from a fuel all at once, it cannot be harnessed efficiently for

constructive work.b. Cellular respiration does not oxidize glucose in a single explosive step. Rather,

glucose and other organic fuels are broken down in a series of steps, each one catalyzed by an enzyme.

i. At key steps, electrons are stripped from the glucose. As is often the case in oxidation reactions, each electron travels with a proton-thus, as hydrogen atom.

ii. The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme called NAD+ (nicotinamide adenine dinucleotide, a derivative of the vitamin niacin). As an electron acceptor, NAD+ functions as an oxidizing agent during respiration.

iii. Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate(glucose, in this example), thereby oxidizing it.

iv. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+. The other proton is released as a hydrogen ion (H+) into the surrounding solution.

v. By receiving 2 negatively charged electrons but only 1 positively charged proton, NAD+ has its charge neutralized when its reduced to NADH. The name NADH shows the hydrogen has been received in the reaction. NAD+ is the most versatile electron acceptor in cellular respiration and functions in several of the redox steps during the breakdown of glucose.

vi. Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down an energy gradient from NADH to oxygen.

vii. Respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy releasing steps. An electron transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of mitochondria of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes.

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1. Electrons removed from the glucose are shuttled by NADH to the top, higher energy end of the chain.

2. At the bottom, lower-energy end, O2 captures these electrons along with hydrogen nuclei, forming water.

viii. In summary, during cellular respiration, most electrons travel the following “downhill” routine: glucose NADH electron transport chain oxygen.

ix. Three important biochemical molecules involved in aerobic respiration: ATP, FAD, and NAD.

15. The Stages of Cellular Respiration: A Previewi. Glycolysis

ii. The Citric Acid Cycleiii. Oxidative Phosporylation: electron transport and chemiosmosis (color-

coded violet)b. Cellular respiration is sometimes defined as including only the citric acid cycle

and oxidative phosphorylation. We include glycolysis, however, because most respiring cells deriving energy from glucose use this process to produce starting materials for citric acid cycle.

i. The first two stages of cellular respiration, glycolysis and the citric acid cycle, are the catabolic pathways that break down glucose and other organic fuels.

ii. Glycolysis, which occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of compounds called pyruvate.

iii. The citric acid cycle, which takes place within the mitochondrial matrix of eukaryotic cells or simply in the cytosol of prokaryotes, completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide. Thus, the carbon dioxide produced by respiration represents fragment of oxidizing organic molecules.

iv. In the third stage of respiration, the electron transport chain accepts electrons from the breakdown products of the first two stages (most often via NADH) and passes these electrons from one molecule to another.

v. At the end of the chain, the electrons form one molecule to another. At the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions, forming water.

vi. The energy released at each step of the chain is stored in a form the mitochondrion can use to take ATP.

vii. This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain.

viii. In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, the process that constitute oxidative phosphorylation.

ix. In prokaryotes, these processes take place in the plasma membrane. Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration. A smaller amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle by a mechanism called substrate-level phosphorylation.

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1. This mode of ATP synthesis occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation.

2. During glycolysis, each glucose molecule is broken down into two molecules of the compound pyruvate.

a. In eukaryotic cells, the pyruvate enters the mitochondrion, where the citric acid cycle oxidizes it to carbon dioxide.

b. NADH and a similar electron carrier, a coenzyme called FADH2, transfers electrons derived from glucose to electron transport chains, which are built into the inner mitochondrial membrane.

c. (In prokaryotes, the electron transport chains are located in the plasma membrane.)

d. During oxidative phosphorylation, electron transport chains convert the chemical energy to form used for ATP synthesis in the process called chemiosmosis.

c. Substrate-level phosphorylation gives 4 ATP while oxidative phosphorylation gives 34 to 36 ATP.

16. Glycolysis harvests chemical energy by oxidizing glucose to pyruvatea. The word glycolysis means “sugar splitting” and that is exactly what happens.b. Glucose, a six-carbon sugar, is split into two three-carbon sugar. These smaller

sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.)

c. Glycolysis can be divided into two phases: energy investment and energy payoff. During the energy investment phase, the cell actually spends ATP. It spends 2 molecules of ATP. The investment is repaid with interest during the energy pay-off phase, when ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electron released from the oxidation of glucose.

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d. The net energy yield from glycolysis, per glucose molecule, is 2 ATP plus 2 NADH.

e. In the end, all of the carbon originally present in glucose is accounted for in the two molecules of pyruvate; no CO2 is released during glycolysis. Glycolysis occurs whether or not O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation.

f. Stepsi. Glucose enters the cell and is phosphorylated by the enzyme hexokinase,

which transfers a phosphate group from ATP to the sugar. Forming glucose-6-phosphate.

ii. Glucose-6-phosphate is converted to its isomer fructose-6-phoshate.iii. Phospfructokinase transfers a phosphate group from ATP to the sugar,

investing another molecule of ATP in glycolysis. With phosphate groups on its opposite ends, the sugar will now split in half.

iv. It breaks into two different 3 carbon sugars: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate which are isomers.

v. Isomerase catalyzes the reversible conversion between the two three-carbon sugars. The reaction never reaches equilibrium because the next enzyme only uses glyceraldehyde-3-phosphate as its substrate. This pulls the equilibrium in the direction of glyceraldehyde-3-phosphate, which is removed as fast as it forms. The isomerase turns the other to glyceraldehyde-3-phosphate.

vi. Triose phosphate dehydrogenase catalyzes two sequential reactions while it holds G3P in its active site. First, the sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH. This reaction is very exergonic, and the enzyme uses the released energy to attach a phosphate group to the oxidized substrate, making a product of very high potential energy. The source of phosphates is the pool of inorganic phosphate ions that are always present in the cytosol.

vii. Glycolysis produces some ATP by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to ADP in an exergonic reaction. For each glucose molecule that began glycolysis, step 7 produces 2 ATP, since every product after the sugar splitting step (4) is doubled. Recall that 2 ATP were invested to get sugar ready for splitting; this ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar. The carbonyl group that characterizes a sugar has been oxidized to a carbonyl group, the hallmark of an organic acid. The sugar was oxidized in step 6, and now the energy made available by that oxidation has been used to make ATP.

viii. Phosphogylceromutase relocates the remaining phosphate groups, preparing the substrate for the next reaction.

ix. Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP). The electrons of the substrate are rearranged in such a way that the resulting phosphorylated compound has a very high potential energy, allowing step 10 to occur.

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x. The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second instance of substrate level phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced. Overall, glycolysis has used 2 ATP in the energy investment phase and produced 4 ATP in the energy payoff phase, for a net gain of 2 ATP. Glycolysis has repaid. Addition energy was stored by step 6 in NADH, which can be used to make ATP by oxidative phosphorylation if oxygen is present. Glucose has been broken down and oxidized to two molecules of pyruvate, the end product of the glycolytic pathway. If oxygen is present, the chemical energy in pyruvate can be extracted by the citric acid cycle. If oxygen is not present, fermentation may occur.

17. The Citric acid cycle completes the energy-yielding oxidation of organic moleculesa. Glycolysis releases less than a quarter of the chemical energy store in glucose;

most of the energy remains stockpile in the molecules of pyruvate.b. If molecular oxygen is present, the pyruvate enters a mitochondrion, where the

enzymes of the citric acid cycle complete the oxidation of glucose.c. Upon entering the mitochondrion via active transport, pyruvate is first converted

to a compound called acetyl coenzyme A, or acetyl CoA. This step, the junction between glycolysis and the citric acid cycle, is accomplished by a multi-enzyme complex that catalyzes three reactions:

i. Pyruvate’s carboxyl group, which is already fully oxidized and thus has little chemical energy is removed and given off as a molecule of CO2.

ii. The remaining two-carbon fragment is oxidized, forming a compound named acetate. An enzyme transfers the extracted electrons to NAD+, storing in the form of NADH.

iii. Coenzyme A (CoA), a sulfur containing compound derived from a B vitamin, is attached to the acetate by an unstable bond that makes the acetyl group very reactive.

d. The citric acid cycle is also called the tri-carboxylic acid cycle or the Krebs cycle.i. Acetyl-CoA combined with a 4 carbon compound named oxaloacetate to

form citric acid.e. The cycle functions as a metabolic furnace that oxidizes organic fuel derived from

pyruvate. f. The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most

of the chemical energy is transferred to NAD+ and a related electron carrier, the coenzyme FAD (flavin adenine dinucleotide), during the redox reactions. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high energy electrons to the electron transport chain.

g. For each acetyl group entering the cycle, 3 NAD+ are reduced to NADH (steps 3, 4, and 8). In step 6, electrons are transferred not to NAD+, but FAD, which accepts 2 electrons and 2 protons to become FADH2.

h. Steps 3 and 4, CO2 is released along when pyruvate enters the mitochondria.i. In many animal tissue cells, step 5 produces a guanosine triphosphate (GTP)

molecule by substrate-level phosphorylation. GTP is a molecule similar to ATP in its structure and cellular function.

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j. This GTP may be used to make an ATP molecule or directly power work in the cell. In the cells of plants, bacteria, and some animal tissues, step 5 froms an ATP molecule directly by substrate-level phosphorylation. The output from step 5 represents the only ATP generated by the citric acid cycle.

k. In the Krebs Cycle, 4 moles of NADH, 1 mole of FADH2, 1 mole of ATP, and 1 mole of CO2 was produced per pyruvate. So in total, 8 NAD molecules are produced.

18. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

a. The Pathway of Electron Transporti. The electron transport is a collection of molecules embedded in the inner

membrane of the mitochondrion in eukaryotic cells. 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. Most components of the chain are proteins, which exist in multi-protein complexes numbered I through IV. Tightly bound to these proteins are prosthetic groups, non-protein components essential for the catalytic functions of certain enzymes.

ii. Electrons removed from glucose by NAD+, during glycolysis and the citric acid cycle, are transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononuceleotide (FMN).

iii. In the next redox reaction, the flavinprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein, one of a family of proteins with both iron and sulfur tightly bound.

iv. The iron-sulfur protein then passes the electrons to a compound called ubiquinone.

v. This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is individually mobile within the membrane rather than residing in a particular complex.

vi. Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes. Their prosthetic group, called a heme group, has an iron atom that accepts and donates electrons. (It is similar to the heme group in hemoglobin, the protein of red blood cells, except that the iron in hemoglobin carries oxygen, not electrons).

vii. The electron transport chain has several types of cytochromes, each a different protein with a slightly different electron-carrying heme group.

viii. The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution, forming.

ix. Another source of electrons for the transport chain is FADH2, the other reduced product of the citric acid cycle.

1. FADH2 adds its electrons to the electron transport chain at complex II, at a lower energy level than NADH does.

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2. Although NADH and FADH2 each donate an equivalent number of electrons (2) for oxygen reduction, the electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH.

x. The electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts. How does the mitochondrion couple this electron transport and energy release to ATP synthesis? Chemiosmosis.

xi. NADH occurs first so it pumps 3 protons while FADH2 occurs in the second chain so it powers 2 protons.

19. Chemiosmosis: The Energy-Coupling Mechanisma. Populating the inner membrane of the mitochondrion or the prokaryotic plasma

membrane are many copies of a protein complex called ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate. ATP synthase works like an ion pump running in reverse.

b. Ion pumps usually use ATP as an energy source to transport ions against their gradients.

c. ATP synthase uses the energy of an existing ion gradient to power ATP synthesis. d. The power source for the ATP synthase is a difference in the concentration of H+

on opposite sides of the inner mitochondrial membrane. e. This process, in which energy stored in the form of hydrogen ion gradient across a

membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis.

f. From studying the structure of ATP synthase, scientists have learned how the flow of H+ through this large enzyme powers ATP generation. ATP synthase is a multi-subunit complex with four main parts, each made up of multiple polypeptides. Protons move one by one into binding sites on one of the parts, causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate. The proton thus behaves somewhat like a rushing stream that turns a waterwheel.

g. At certain steps along the chain, electron transfers cause H+ to be taken up and released surrounding solution. In eukaryotic cells, the electron carriers are spatially arranged in the membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the intermembrane space. The H+ gradient that results is referred to as a proton-motive force, emphasizing the capacity of the gradient to perform work. The force drives H+ back across the membrane through the H+ channels provided by ATP synthases.

i. In general terms, chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.

20. An Accounting of ATP Production by Cellular Respirationa. During respiration, most energy flows in this sequence: glucose NADH

electron transport chain proton-motive force ATP.b. When cellular respiration oxidizes a molecule of glucose to six molecule of

carbon dioxide. The three main departments of this metabolic enterprise are

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glycolysis, the citric acid cycle, and the electron transport chain, which drives oxidative phosphorylation.

c. In aerobic respiration, the final acceptor for electrons from NADH is oxygen.

21. Fermentation and anaerobic respirationa. Two general mechanisms by which certain cells can oxidize organic fuel and

generate ATP without the use of oxygen: anaerobic respiration and fermentation.b. The distinction between these two is based on whether an electron transport chain

is present.c. Fermentation is a way of harvesting chemical energy without using either oxygen

or any electron transport chain – in other words without cellular respiration.d. Oxidation is the loss of electrons to an electron acceptor, so it does not need to

involve oxygen. Glycolysis oxides glucose to two molecules of pyruvate. The oxidizing agent of glycolysis is NAD+. Overall, the reaction is exergonic and 2 ATP is produced by substrate-level-phosphorylation. ADP is unavailable.

e. If oxygen is present, then addition ATP is made by oxidative phosphorylation.f. Amino acids can be fed into aerobic respiration.g. Carbohydrates feed into glycolysis in aerobic respiration

22. Types of Fermentationa. Alcohol fermentation – pyruvate is converted to ethanol in two steps.

i. The first step releases carbon dioxide form the pyruvate, which is converted to the two-carbon compound acetaldehyde.

ii. In the second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD+ needed for the continuation of glycolysis.

iii. Irreversible due to the release of CO2.b. Lactic acid fermentation – pyruvate is reduced directly by NADH to form lactate

as an end product, with no release of CO2. Lactic acid fermentation by certain fungi and bacteria is used in the diary industry to make cheese and yogurt.

c. Lactic acid can be converted into pyruvate when oxygen is present.d. Human muscle cells make ATP by lactic acid fermentation when oxygen is

scarce.23. The inactive state of a glycolytic enzyme would be stabilized by a inhibitor molecule.24. The active state of a glycolytic enzyme would be stabilized by an activator molecule.