Respiration BMZ116B Study Guide
•	To perform their many tasks, living cells require _________from outside sources.
•	Energy enters most ecosystems as _________ and leaves as _________.
•	Photosynthesis generates _________ and _________ that the mitochondria of eukaryotes use as _________ for cellular respiration.
•	Cells harvest the chemical energy stored in organic molecules and use it to regenerate _________, the molecule that drives most cellular work.
•	Respiration has three key pathways: _________, the _________, and _________.
A. The Principles of Energy Harvest
1. Cellular respiration and fermentation are catabolic, energy-yielding pathways.
•	The arrangement of atoms of organic molecules represents _________ energy.
•	Enzymes _________ the systematic degradation of organic molecules that are rich in energy to simpler waste products with _________ energy.
•	Some of the released energy is used to do _________; the rest is dissipated as _________.
•	Catabolic metabolic pathways _________ the energy stored in complex organic molecules.
•	One type of catabolic process, _________, leads to the partial degradation of sugars in the absence of oxygen.
•	A more efficient and widespread catabolic process, _________, consumes _________ as a reactant to complete the breakdown of a variety of organic molecules.
•	In eukaryotic cells, _________ are the site of most of the processes of cellular respiration.
•	Cellular respiration is similar in broad principle to the combustion of gasoline in an automobile engine after _________ is mixed with hydrocarbon fuel.
•	_________ is the fuel for respiration. The exhaust is _________ and _________.
•	The overall process is:
•	organic compounds + _________  _________ + _________ + energy (_________ + _________).
•	Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose.
•	_________ + 6O2  6CO2 + 6H2O + Energy (ATP + heat)
•	The catabolism of glucose is _________ with a  G of −686 kcal per mole of glucose.
•	Some of this energy is used to produce _________, which can perform cellular work.
2. Redox reactions release energy when electrons move closer to electronegative atoms.
•	Catabolic pathways transfer the _________ stored in food molecules, releasing energy that is used to synthesize _________.
•	Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or _________ reactions.
•	The loss of electrons is called _________.
•	The addition of electrons is called _________.
•	The formation of table salt from sodium and chloride is a redox reaction.
•	Na + Cl  Na+ + Cl−
•	Here sodium is _________ and chlorine is _________ (its charge drops from 0 to −1).
•	More generally: Xe− + Y  X + Ye−
•	X, the electron donor, is the _________ agent and _________ Y.
•	Y, the electron recipient, is the _________ agent and _________ X.
•	Redox reactions require both a _________ and _________.
•	Redox reactions also occur when the transfer of electrons is not complete but involves a change in the degree of electron _________ in _________ bonds.
•	In the combustion of methane to form water and carbon dioxide, the _________ covalent bonds of methane (C—H) and oxygen (O=O) are converted to _________ covalent bonds (C=O and O—H).
•	When methane reacts with oxygen to form carbon dioxide, electrons end up farther away from the _________atom and closer to their new covalent partners, the _________ atoms, which are very electronegative.
•	In effect, the carbon atom has partially “lost” its shared electrons. Thus, methane has been _________.
•	The two atoms of the oxygen molecule share their electrons equally. When oxygen reacts with the hydrogen from methane to form water, the electrons of the covalent bonds are drawn closer to the _________.
•	In effect, each oxygen atom has partially “gained” electrons, and so the oxygen molecule has been _________.
•	Oxygen is very _________, and is one of the most potent of all oxidizing agents.
•	Energy must be _________ to pull an electron away from an atom.
•	The more electronegative the atom, the more energy is required to take an electron _________ from it.
•	An electron _________ potential energy when it shifts from a less electronegative atom toward a more electronegative one.
•	A redox reaction that relocates electrons _________ to oxygen, such as the burning of methane, releases chemical energy that can do work.
3. The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain.
•	Cellular respiration does not _________ glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time.
•	Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific _________.
•	At key steps, _________ are stripped from the glucose.
•	In many oxidation reactions, the electron is transferred with a _________, as a hydrogen atom.
•	The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called _________.
•	How does NAD+ trap electrons from glucose?
•	Dehydrogenase enzymes strip two _________ atoms from the fuel (e.g., glucose), oxidizing it.
•	The enzyme passes two _________ and one _________ to NAD+.
•	The other _________ is released as _________ to the surrounding solution.
•	By receiving two electrons and only one proton, NAD+ has its charge _________ when it is reduced to NADH.
•	NAD+ functions as the _________ agent in many of the redox steps during the catabolism of glucose.
•	The electrons carried by NADH have lost very little of their _________ energy in this process.
•	Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize _________ as electrons “fall” from NADH to oxygen.
•	How are electrons extracted from food and stored by NADH finally transferred to oxygen?
•	Unlike the explosive release of heat energy that occurs when H2 and O2 are combined (with a spark for activation energy), cellular respiration uses an _________ _________ _________to break the fall of electrons to O2 into several steps.
•	The electron transport chain consists of several molecules (primarily proteins) built into the inner membrane of a _________.
•	Electrons released from food are shuttled by _________ to the “top” higher-energy end of the chain.
•	At the “bottom” lower-energy end, _________ captures the electrons along with H+ to form _________.
•	Electron transfer from NADH to oxygen is an _________ reaction with a free energy change of −53 kcal/mol.
•	Electrons are passed to increasingly _________ molecules in the chain until they _________ oxygen, the most electronegative receptor.
•	In summary, during cellular respiration, most electrons travel the following “downhill” route: food  NADH  electron transport chain  _________.
B. The Process of Cellular Respiration
1. These are the stages of cellular respiration: a preview.
•	Respiration occurs in three metabolic stages: _________, the _________ cycle, and the _________ transport chain and _________ phosphorylation.
•	Glycolysis occurs in the _________.
•	It begins catabolism by breaking glucose into two molecules of _________.
•	The citric acid cycle occurs in the mitochondrial _________.
•	It completes the breakdown of glucose by oxidizing a derivative of pyruvate to _________ _________.
•	Several steps in glycolysis and the citric acid cycle are _________ reactions in which dehydrogenase enzymes transfer electrons from substrates to NAD+, forming NADH.
•	NADH passes these electrons to the _________ _________ _________.
•	In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular _________ and _________ ions to form water.
•	As they are passed along the chain, the energy carried by these electrons is transformed in the mitochondrion into a form that can be used to synthesize ATP via _________ _________.
•	The _________ _________ membrane of the mitochondrion is the site of electron transport and chemiosmosis, processes that together constitute oxidative phosphorylation.
•	Oxidative phosphorylation produces almost  ____% of the ATP generated by respiration.
•	Some ATP is also formed directly during glycolysis and the citric acid cycle by _________ _________.
•	Here an enzyme transfers a phosphate group from an organic substrate to _________, forming ATP.
•	For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to _________ ATP, each with 7.3 kcal/mol of free energy.
•	Respiration uses the small steps in the respiratory pathway to break the large denomination of energy contained in glucose into the small change of ATP.
•	The quantity of energy in ATP is more appropriate for the level of work required in the cell.
2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.
•	During glycolysis, glucose, a _________ carbon-sugar, is split into two _________-carbon sugars.
•	These smaller sugars are oxidized and rearranged to form two molecules of _________, the ionized form of _________ acid.
•	Each of the ten steps in glycolysis is catalyzed by a specific _________.
•	These steps can be divided into two phases: an energy _________ phase and an energy _________ phase.
•	In the energy investment phase, the cell invests _________ to provide activation energy by phosphorylating glucose.
•	This requires ___ ATP per glucose.
•	In the energy payoff phase, ATP is produced by _________ phosphorylation and _________ is reduced to _________ by electrons released by the oxidation of glucose.
•	The net yield from glycolysis is __ ATP and __ NADH per glucose.
•	No CO2 is produced during _________.
•	Glycolysis can occur whether _________ is present or not.
3. The citric acid cycle completes the energy-yielding oxidation of organic molecules.
•	More than three-quarters of the original energy in glucose is still present in the two molecules of _________.
•	If oxygen is present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle complete the oxidation of the organic fuel to _________ _________.
•	After pyruvate enters the mitochondrion via active transport, it is converted to a compound called _________ _________ A or acetyl _________.
•	This step is accomplished by a multienzyme complex that catalyzes three reactions:
1.	A carboxyl group is removed as _________.
2.	The remaining two-carbon fragment is _________ to form acetate. An enzyme transfers the pair of electrons to NAD+ to form NADH.
3.	Acetate combines with coenzyme A to form the very reactive molecule _________ _________.
•	Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further _________.
•	The citric acid cycle is also called the Krebs cycle in honor of Hans Krebs, who was largely responsible for elucidating its pathways in the 1930s.
•	The citric acid cycle oxidizes organic fuel derived from _________.
•	The citric acid cycle has eight steps, each catalyzed by a specific _________.
•	The acetyl group of acetyl CoA joins the cycle by combining with the compound _________, forming citrate.
•	The next seven steps decompose the _________ back to oxaloacetate. It is the regeneration of _________ that makes this process a cycle.
•	_________ CO2 molecules are released, including the one released during the conversion of pyruvate to acetyl CoA.
•	The cycle generates one ATP per turn by _________ phosphorylation.
•	A _________ molecule is formed by substrate-level phosphorylation.
•	The GTP is then used to synthesize an _________, the only _________ generated directly by the citric acid cycle.
•	Most of the chemical energy is transferred to _________ and _________ during the redox reactions.
•	The reduced coenzymes NADH and FADH2 then transfer high-energy _________ to the electron transport chain.
•	Each cycle produces _________ ATP by substrate-level phosphorylation, _________ NADH, and _________ FADH2 per acetyl CoA.
4. The inner mitochondrial membrane couples electron transport to ATP synthesis.
•	Only 4 of 38 ATP ultimately produced by respiration of glucose are produced by substrate-level phosphorylation.
•	Two are produced during _________, and 2 are produced during the _________ _________ cycle.
•	NADH and FADH2 account for the vast majority of the energy extracted from the food.
•	These reduced coenzymes link glycolysis and the citric acid cycle to _________ phosphorylation, which uses energy released by the electron transport chain to power _________ synthesis.
•	The electron transport chain is a collection of molecules embedded in the _________, the folded inner membrane of the mitochondrion.
•	The folding of the cristae increases its _________ area, providing space for thousands of copies of the chain in each mitochondrion.
•	Most components of the chain are proteins bound to _________ groups, nonprotein components essential for catalysis.
•	Electrons _________ in free energy as they pass down the electron transport chain.
•	During electron transport along the chain, electron carriers alternate between reduced and _________ states as they _________ and donate electrons.
•	Each component of the chain becomes _________ when it accepts electrons from its “uphill” neighbor, which is less electronegative.
•	It then returns to its _________ form as it passes electrons to its more electronegative “downhill” neighbor.
•	Electrons carried by _________ are transferred to the first molecule in the electron transport chain, a flavoprotein.
•	The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier.
•	The prosthetic group of each cytochrome is a heme group with an _________ atom that accepts and donates electrons.
•	The last cytochrome of the chain, cyt a3, passes its electrons to _________, which is very electronegative.
•	Each oxygen atom also picks up a pair of _________ ions from the aqueous solution to form water.
•	For every two electron carriers (four electrons), one O2 molecule is reduced to two molecules of _________.
•	The electrons carried by _________ have lower free energy and are added at a lower energy level than those carried by NADH.
•	The electron transport chain provides about _________ less energy for ATP synthesis when the electron donor is FADH2 rather than NADH.
•	The electron transport chain generates no _________ 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.
•	How does the mitochondrion couple electron transport and energy release to ATP synthesis?
•	The answer is a mechanism called _________.
•	A protein complex, _________ _________, in the cristae actually makes ATP from ADP and Pi.
•	ATP synthase uses the energy of an existing _________ gradient to power ATP synthesis.
•	The proton gradient develops between the _________ space and the matrix.
•	The proton gradient is produced by the movement of _________ along the electron transport chain.
•	The chain is an energy converter that uses the exergonic flow of electrons to pump _________ from the matrix into the intermembrane space.
•	The protons pass back to the _________ through a channel in ATP synthase, using the exergonic flow of H+ to drive the _________ of ADP.
•	Thus, the energy stored in a H+ gradient across a membrane couples the redox reactions of the _________ _________ _________ to ATP synthesis.
•	From studying the structure of ATP synthase, scientists have learned how the flow of _________ through this large enzyme powers ATP generation.
•	ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides:
1.	A _________ in the inner mitochondrial membrane.
2.	A _________ that protrudes into the mitochondrial matrix.
3.	An internal _________ extending from the rotor into the knob.
4.	A _________, anchored next to the rotor, which holds the knob stationary.
•	Protons flow down a narrow space between the stator and rotor, causing the rotor and its attached rod to _________.
•	The spinning rod causes _________ changes in the stationary knob, activating three catalytic sites in the knob where ADP and inorganic phosphate combine to make _________.
•	How does the inner mitochondrial membrane generate and maintain the H+ gradient that drives ATP synthesis in the ATP synthase protein complex?
•	Creating the H+ gradient is the function of the _________ _________ _________.
•	The ETC is an energy converter that uses the exergonic flow of _________ to pump _________ across the membrane from the mitochondrial _________ to the _________ space.
•	The H+ has a tendency to _________ down its gradient.
•	The ATP synthase molecules are the only place that H+ can diffuse back to the _________.
•	The exergonic flow of H+ is used by the enzyme to generate _________.
•	This coupling of the _________ reactions of the electron transport chain to ATP synthesis is called chemiosmosis.
•	How does the electron transport chain pump protons?
•	Certain members of the electron transport chain accept and release _________ along with electrons.
•	At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution.
•	The electron carriers are spatially arranged in the membrane in such a way that protons are accepted from the mitochondrial _________ and deposited in the _________ space.
•	The H+ gradient that results is the _________ force.
•	The gradient has the capacity to do _________.
•	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.
•	Chemiosmosis in chloroplasts also generates ATP, but light drives the electron flow down an electron transport chain and H+ gradient formation.
•	Prokaryotes generate H+ gradients across their _________ _________.
•	They can use this proton-motive force not only to generate ATP, but also to pump nutrients and waste products across the membrane and to rotate their _________.
5. Here is an accounting of ATP production by cellular respiration.
•	During cellular respiration, most energy flows from _________  _________  _________ _________ _________  _________ force  ATP.
•	Let’s consider the products generated when cellular respiration oxidizes a molecule of glucose to six CO2 molecules.
•	_________ ATP molecules are produced by substrate-level phosphorylation during glycolysis and the citric acid cycle.
•	Many more ATP molecules are generated by _________ phosphorylation.
•	Each NADH from the citric acid cycle and the conversion of pyruvate contributes enough energy to the proton-motive force to generate a maximum of _________ ATP.
•	The NADH from glycolysis may also yield _________ ATP.
•	Each FADH2 from the citric acid cycle can be used to generate about _________ ATP.
•	Why is our accounting so inexact?
•	There are three reasons that we cannot state an exact number of ATP molecules generated by one molecule of glucose.
1.	_________ and the _________ reactions are not directly coupled to each other, so the ratio of number of NADH to number of ATP is not a whole number.
•	One NADH results in _________ H+ being transported across the inner mitochondrial membrane.
•	Between 3 and 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate _________ ATP.
•	Therefore, 1 NADH generates enough proton-motive force for synthesis of 2.5 to 3.3 ATP.
•	We round off and say that 1 NADH generates _________ ATP.
2.	The ATP yield varies slightly depending on the type of shuttle used to transport electrons from the _________ into the mitochondrion.
•	The mitochondrial inner membrane is impermeable to NADH, so the two electrons of the NADH produced in _________ must be conveyed into the mitochondrion by one of several electron shuttle systems.
•	In some shuttle systems, the electrons are passed to NAD+, which generates _________ ATP. In others, the electrons are passed to FAD, which generates only _________ ATP.
3.	The proton-motive force generated by the redox reactions of respiration may drive other kinds of work, such as mitochondrial uptake of pyruvate from the _________.
•	If all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of _________ ATP by oxidative phosphorylation plus _________ ATP (net) from substrate-level phosphorylation to give a total yield of _________–_________ ATP (depending on the efficiency of the shuttle).
•	How efficient is respiration in generating ATP?
•	Complete _________ of glucose releases 686 kcal/mol.
•	_________ of ADP to form ATP requires at least 7.3 kcal/mol.
•	Efficiency of respiration is 7.3 kcal/mol times 38 ATP/glucose divided by 686 kcal/mol glucose, which equals _________ or _________%.
•	Approximately 60% of the energy from glucose is lost as _________.
•	Some of that heat is used to maintain our high body temperature (37°C).
•	Cellular respiration is remarkably efficient in energy conversion.
C. Related Metabolic Processes
1. Fermentation enables some cells to produce ATP without the help of oxygen.
•	Without electronegative _________ to pull electrons down the transport chain, oxidative phosphorylation ceases.
•	However, _________ provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen.
•	In glycolysis, glucose is oxidized to two _________ molecules with NAD+ as the oxidizing agent.
•	Glycolysis is ____gonic and produces 2 ATP (net).
•	If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron transport chain.
•	Glycolysis generates _________ ATP whether oxygen is present (aerobic) or not (anaerobic).
•	Anaerobic catabolism of sugars can occur by fermentation.
•	Fermentation can generate ATP from glucose by substrate-level phosphorylation as long as there is a supply of _________ to accept electrons.
•	If the NAD+ pool is exhausted, _________ shuts down.
•	Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling _________.
•	Under anaerobic conditions, various fermentation pathways generate ATP by _________ and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate.
•	In _________ fermentation, pyruvate is converted to ethanol in two steps.
•	First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of _________.
•	Second, acetaldehyde is reduced by NADH to _________.
•	Alcohol fermentation by yeast is used in brewing and winemaking.
•	During _________ acid fermentation, pyruvate is reduced directly by NADH to form _________ (the ionized form of lactic acid) without release of _________.
•	Lactic acid fermentation by some fungi and bacteria is used to make cheese and _________.
•	Human muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when _________ is scarce.
•	The waste product, lactate, may cause _________ fatigue, but ultimately it is converted back to pyruvate in the _________.
•	Fermentation and cellular respiration are anaerobic and aerobic alternatives, respectively, for producing _________ from sugars.
•	Both use glycolysis to oxidize sugars to pyruvate with a net production of _________ ATP by _________ phosphorylation.
•	Both use _________ as an oxidizing agent to accept electrons from food during glycolysis.
•	The two processes differ in their mechanism for oxidizing NADH to NAD+.
•	In fermentation, the electrons of NADH are passed to an _________ molecule to regenerate NAD+.
•	In respiration, the electrons of NADH are ultimately passed to _________, generating ATP by oxidative phosphorylation.
•	More ATP is generated from the oxidation of pyruvate in the citric acid cycle.
•	Without oxygen, the energy still stored in pyruvate is unavailable to the cell.
•	Under aerobic respiration, a molecule of glucose yields _________ ATP, but the same molecule of glucose yields only _________ ATP under anaerobic respiration.
•	Yeast and many bacteria are _________ anaerobes that can survive using either fermentation or respiration.
•	At a cellular level, human muscle cells can behave as facultative anaerobes.
•	For facultative anaerobes, _________ is a fork in the metabolic road that leads to two alternative routes.
•	Under aerobic conditions, pyruvate is converted to _________ _________ and oxidation continues in the citric acid cycle.
•	Under anaerobic conditions, pyruvate serves as an _________ acceptor to recycle NAD+.
•	The oldest bacterial fossils are more than 3.5 _________ years old, appearing long before appreciable quantities of O2 accumulated in the atmosphere.
•	Therefore, the first prokaryotes may have generated ATP exclusively from _________.
•	The fact that glycolysis is a ubiquitous metabolic pathway and occurs in the _________ without membrane-enclosed organelles suggests that glycolysis evolved early in the history of life.
2. Glycolysis and the citric acid cycle connect to many other metabolic pathways.
•	Glycolysis can accept a wide range of carbohydrates for catabolism.
•	Polysaccharides like starch or glycogen can be hydrolyzed to glucose monomers that enter glycolysis.
•	Other hexose sugars, such as galactose and fructose, can also be modified to undergo glycolysis.
•	The other two major fuels, proteins and fats, can also enter the respiratory pathways used by carbohydrates.
•	Proteins must first be digested to individual _________ _________.
•	Amino acids that will be catabolized must have their _________ groups removed via deamination.
•	The nitrogenous waste is excreted as ammonia, urea, or another waste product.
•	The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the citric acid cycle, depending on their structure.
•	Catabolism can also harvest energy stored in fats.
•	Fats must be digested to glycerol and fatty acids.
•	Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis.
•	The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via beta oxidation.
•	These molecules enter the citric acid cycle as _________ _________.
•	A gram of fat oxides by respiration generates _________ as much ATP as a gram of carbohydrate.
•	The metabolic pathways of respiration also play a role in anabolic pathways of the cell.
•	Intermediaries in glycolysis and the citric acid cycle can be diverted to anabolic pathways.
•	For example, a human cell can synthesize about half the _________ different amino acids by modifying compounds from the citric acid cycle.
•	Glucose can be synthesized from _________; fatty acids can be synthesized from _________ _________.
•	Glycolysis and the citric acid cycle function as metabolic interchanges that enable cells to convert one kind of molecule to another as needed.
•	For example, excess carbohydrates and proteins can be converted to fats through intermediaries of glycolysis and the citric acid cycle.
•	Metabolism is remarkably versatile and adaptable.
3. Feedback mechanisms control cellular respiration.
•	Basic principles of supply and demand regulate the metabolic economy.
•	If a cell has an excess of a certain amino acid, it typically uses _________ _________ to prevent the diversion of intermediary molecules from the citric acid cycle to the synthesis pathway of that amino acid.
•	The rate of catabolism is also regulated, typically by the level of ATP in the cell.
•	If ATP levels drop, catabolism speeds up to produce more ATP.
•	Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway.
•	One strategic point occurs in the third step of glycolysis, catalyzed by phosphofructokinase.
•	_________ regulation of phosphofructokinase sets the pace of respiration.
•	This enzyme catalyzes the earliest step that irreversibly commits the substrate to glycolysis.
•	Phosphofructokinase is an allosteric enzyme with receptor sites for specific inhibitors and activators.
•	It is inhibited by _________ and stimulated by _________ (derived from ADP).
•	When ATP levels are high, inhibition of this enzyme slows _________.
•	As ATP levels drop and ADP and AMP levels rise, the enzyme becomes active again and _________ speeds up.
•	Citrate, the first product of the citric acid cycle, is also an _________ of phosphofructokinase.
•	This synchronizes the rate of glycolysis and the citric acid cycle.
•	If intermediaries from the citric acid cycle are diverted to other uses (e.g., amino acid synthesis), _________ speeds up to replace these molecules.
•	Metabolic balance is augmented by the control of other enzymes at other key locations in glycolysis and the citric acid cycle.
•	Cells are thrifty, expedient, and responsive in their metabolism.