101. Consider the following statements about pyruvate as a metabolic branch point.
I. Glycolysis produces pyruvate before the aerobic and fermentative routes diverge.
II. Oxygen availability and organismal type influence the later fate of pyruvate.
III. Every fate of pyruvate releases carbon dioxide.
IV. Under aerobic conditions in eukaryotic cells, pyruvate can enter the mitochondrion.
ⓐ. I and III only
ⓑ. II, III and IV only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Glycolysis forms pyruvate in the cytoplasm and acts as a common initial pathway before later metabolic routes separate. In an oxygenated eukaryotic cell, pyruvate can enter the mitochondrion for aerobic oxidation. Under anaerobic conditions, its fate depends on the organism or tissue: yeast commonly forms ethanol and carbon dioxide, whereas some bacteria and oxygen-limited muscle cells form lactic acid. Carbon dioxide release is not universal among these branches. Alcoholic fermentation removes carbon dioxide from pyruvate, but lactic acid fermentation converts the three-carbon pyruvate directly into a three-carbon product without carbon loss. The valid statements identify pyruvate as the decision point connecting glycolysis with several condition-dependent pathways. Fermentation restores oxidised NAD for glycolysis; its net ATP is produced by the glycolytic substrate-level steps.
102. The following records describe three fates of glycolytic pyruvate.
| Record | Condition | Major products after pyruvate processing |
|---|
| P | Oxygen absent in yeast | Ethanol and \(\mathrm{CO_2}\) |
| Q | Oxygen limited in muscle | Lactic acid without \(\mathrm{CO_2}\) release |
| R | Oxygen available in a eukaryotic cell | Products entering complete aerobic oxidation |
The correct pathway assignment is:
ⓐ. P-alcoholic fermentation, Q-lactic acid fermentation, R-aerobic respiration
ⓑ. P-lactic acid fermentation, Q-aerobic respiration, R-alcoholic fermentation
ⓒ. P-aerobic respiration, Q-alcoholic fermentation, R-lactic acid fermentation
ⓓ. P-alcoholic fermentation, Q-aerobic respiration, R-lactic acid fermentation
Correct Answer: P-alcoholic fermentation, Q-lactic acid fermentation, R-aerobic respiration
Explanation: Yeast under oxygen-free conditions converts pyruvate into ethanol and releases carbon dioxide, identifying Record P as alcoholic fermentation. Oxygen-limited muscle reduces pyruvate directly to lactic acid, so Record Q represents lactic acid fermentation. When oxygen is available in a eukaryotic cell, pyruvate is transported into the mitochondrial matrix and prepared for complete aerobic oxidation, fitting Record R. The products provide the decisive evidence. Carbon dioxide formation distinguishes the yeast route from lactic fermentation, while mitochondrial processing and much greater energy release distinguish aerobic respiration from both fermentative routes. The table demonstrates that the same glycolytic product can enter different pathways according to cellular and environmental conditions. Changing the terminal fate of pyruvate leaves glycolysis as the common upstream ATP-producing route. The condition matters because oxygen availability and organismal enzymes determine which downstream fate the common pyruvate product follows.
103. A pathway diagram begins with pyruvate and divides into three branches. Branch P releases \(\mathrm{CO_2}\), forms a two-carbon intermediate and then produces a reduced organic end product. Branch Q retains all three carbons in its end product. Branch R enters the mitochondrial matrix. P, Q and R represent:
ⓐ. lactic fermentation, alcoholic fermentation and glycolysis
ⓑ. aerobic respiration, lactic fermentation and alcoholic fermentation
ⓒ. alcoholic fermentation, aerobic respiration and lactic fermentation
ⓓ. alcoholic fermentation, lactic fermentation and aerobic respiration
Correct Answer: alcoholic fermentation, lactic fermentation and aerobic respiration
Explanation: Branch P first removes one carbon from three-carbon pyruvate as carbon dioxide, leaving the two-carbon intermediate acetaldehyde. Acetaldehyde is then reduced to ethanol, so P is alcoholic fermentation. Branch Q preserves all three carbon atoms when pyruvate is reduced to lactic acid, identifying lactic acid fermentation. Branch R is recognised from movement into the mitochondrial matrix, where aerobic oxidation begins through conversion of pyruvate into acetyl CoA. The carbon changes and compartment together provide a complete diagnosis. Alcoholic and lactic fermentation occur outside the mitochondrial aerobic sequence, but they differ in whether carbon dioxide is released. The three branches illustrate the condition-dependent metabolic choices available after glycolysis. Using both carbon accounting and compartment location prevents the three branches from being confused by their shared pyruvate starting point.
104. An actively respiring yeast culture is shifted from an oxygenated medium to an oxygen-free medium while glucose remains abundant. The most appropriate pathway-level prediction is:
ⓐ. glycolysis stops, and pyruvate is no longer produced
ⓑ. glycolytic pyruvate enters alcoholic fermentation
ⓒ. pyruvate enters the TCA cycle at a faster rate
ⓓ. lactic acid becomes the obligatory yeast product
Correct Answer: glycolytic pyruvate enters alcoholic fermentation
Explanation: Glycolysis does not directly require oxygen, so removal of oxygen does not immediately prevent glucose from being converted into pyruvate. What changes is the later fate of pyruvate. In yeast, the oxygen-free condition favours alcoholic fermentation, in which pyruvate is converted through acetaldehyde into ethanol. This route also regenerates \(\mathrm{NAD^+}\), allowing the glycolytic oxidation step to continue. Entry into mitochondrial aerobic oxidation declines when oxygen is unavailable, since the respiratory electron-transfer system can no longer operate normally. Yeast does not characteristically switch to muscle-type lactic acid formation in this context. The prediction follows the branch-point logic: a shared upstream pathway continues, while the downstream route changes with oxygen availability.
105. Assertion: Pyruvate is described as a metabolic branch point in respiration.
Reason: Every organism converts pyruvate into the same products irrespective of oxygen availability.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is true, but Reason is false
Explanation: Pyruvate is a metabolic branch point because it can enter more than one pathway after glycolysis. Under suitable aerobic conditions it enters mitochondrial oxidation, whereas anaerobic conditions may direct it into alcoholic or lactic acid fermentation. Organismal type also matters: yeast commonly forms ethanol, while oxygen-limited muscle forms lactate. The assertion therefore describes a genuine pathway choice. The reason is false because it claims that every organism produces the same pyruvate products irrespective of oxygen availability. A branch point exists precisely because biological condition and enzymatic capacity alter the downstream fate. These different routes share glycolysis up to pyruvate but diverge in carbon loss, organic end product, ATP yield and the way oxidised coenzyme is regenerated.
106. Evaluate the following statements about glycolysis under anaerobic conditions.
I. Glycolysis does not directly use oxygen as a reactant.
II. Many anaerobic organisms depend on glycolysis for their direct ATP gain from glucose.
III. Glycolysis completely oxidises glucose into carbon dioxide and water.
IV. Fermentation can support continued glycolysis by restoring oxidised coenzyme.
ⓐ. I, II and IV only
ⓑ. I and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Glycolysis is an oxygen-independent cytoplasmic pathway that directly produces ATP through substrate-level phosphorylation. In many organisms living anaerobically, it is the only stage that supplies a net ATP gain from glucose. Glycolysis does not completely oxidise glucose; it ends with two pyruvate molecules that still contain much of the original chemical energy. Continued pathway operation also requires a supply of \(\mathrm{NAD^+}\) for oxidation of PGAL. Fermentation restores this oxidised coenzyme by transferring reducing equivalents from glycolytic NADH to an organic acceptor derived from pyruvate. The valid statements connect oxygen independence, ATP production and redox recycling without confusing glycolysis with complete aerobic oxidation. The organism and its oxygen conditions decide whether pyruvate is converted to ethanol or lactate after glycolysis.
107. Anaerobic yeast cells receive glucose and initially produce both ATP and ethanol. A drug then blocks the first committed reactions of glycolysis. Pyruvate, ethanol and ATP production all decline sharply. The strongest inference is:
ⓐ. ethanol formation is the main ATP-producing step of fermentation
ⓑ. oxygen must be a direct reactant in glycolysis
ⓒ. yeast produces pyruvate independently of glucose breakdown
ⓓ. fermentation uses glycolytic pyruvate; ATP arises in glycolysis
Correct Answer: fermentation uses glycolytic pyruvate; ATP arises in glycolysis
Explanation: Blocking glycolysis removes the pathway that supplies pyruvate to alcoholic fermentation. Ethanol production declines as its carbon precursor becomes unavailable. Direct ATP generation also falls since the net fermentative ATP yield is produced during glycolysis through substrate-level phosphorylation, not during conversion of pyruvate into ethanol. The experiment links two observations to one upstream block: reduced pyruvate explains reduced ethanol, while loss of glycolytic phosphate-transfer reactions explains reduced ATP. Oxygen is absent throughout the setup, yet glycolysis operated before the drug was added, so oxygen cannot be a required glycolytic reactant. The results identify glycolysis as both the carbon-supplying core and the ATP-yielding stage of anaerobic glucose metabolism in yeast.
108. The most accurate relationship between glycolysis and fermentation is:
ⓐ. Fermentation precedes glycolysis and supplies glucose to it
ⓑ. Glycolysis precedes anaerobic processing of pyruvate
ⓒ. Both processes occur only in the mitochondrial matrix
ⓓ. Fermentation completely oxidises glucose after glycolysis
Correct Answer: Glycolysis precedes anaerobic processing of pyruvate
Explanation: Glycolysis converts glucose into pyruvate in the cytoplasm and forms a small net ATP gain together with reduced coenzyme. Fermentation follows glycolysis when suitable cells operate without adequate oxygen. It acts on pyruvate or a pyruvate-derived intermediate and restores \(\mathrm{NAD^+}\), permitting the common glycolytic pathway to continue. Fermentation does not supply glucose to glycolysis and does not precede it. Neither process requires the mitochondrial matrix, and fermentative end products remain incompletely oxidised. The relationship is hierarchical: glycolysis supplies both the carbon product and reduced coenzyme context, while fermentation maintains redox continuity under anaerobic conditions. This classification separates an ATP-yielding upstream pathway from the later reactions that recycle the oxidised coenzyme.
109. Fermentation is best described as:
ⓐ. incomplete anaerobic oxidation of respiratory substrate
ⓑ. complete aerobic oxidation to carbon dioxide and water
ⓒ. mitochondrial electron transfer coupled to ATP synthase
ⓓ. photosynthetic reduction of carbon dioxide into carbohydrate
Correct Answer: incomplete anaerobic oxidation of respiratory substrate
Explanation: Fermentation occurs without oxygen and leaves substantial chemical energy in organic end products such as ethanol or lactic acid. It is incomplete oxidation since the carbon substrate is not fully converted into carbon dioxide and water. The small net ATP gain associated with fermentation comes from glycolysis, while later fermentative reactions primarily regenerate \(\mathrm{NAD^+}\). Complete aerobic oxidation requires mitochondrial reactions and an electron-transport system in eukaryotic cells. Fermentation lacks this large oxidative-phosphorylation contribution. The definition must include both the anaerobic condition and the incomplete nature of substrate oxidation. Merely describing fermentation as glucose breakdown would be insufficient, since glycolysis also breaks down glucose and can feed either an aerobic or an anaerobic route.
110. Carbon flow from pyruvate is compared in two anaerobic pathways.
| Pathway | Starting carbon compound | Organic end product | \(\mathrm{CO_2}\) release |
|---|
| P | \(\mathrm{3C}\) pyruvate | \(\mathrm{2C}\) ethanol | Present |
| Q | \(\mathrm{3C}\) pyruvate | \(\mathrm{3C}\) lactic acid | Absent |
The most accurate inference is:
ⓐ. Both pathways retain all pyruvate carbon in the organic product
ⓑ. Pathway Q removes one carbon before forming lactic acid
ⓒ. Pathway P includes decarboxylation, whereas Pathway Q does not
ⓓ. Neither pathway changes the carbon skeleton of pyruvate
Correct Answer: Pathway P includes decarboxylation, whereas Pathway Q does not
Explanation: Pathway P begins with three-carbon pyruvate but ends with two-carbon ethanol and releases carbon dioxide. The missing carbon is removed during decarboxylation of pyruvate to acetaldehyde. Pathway Q converts three-carbon pyruvate directly into three-carbon lactic acid, so no carbon atom is lost and no carbon dioxide is produced. The table allows the reaction type to be inferred from carbon accounting rather than from pathway names alone. Both pathways are fermentative and regenerate \(\mathrm{NAD^+}\), yet their carbon transformations differ. Alcoholic fermentation contains a carbon-removal step, while lactic acid fermentation is primarily a reduction of the existing three-carbon skeleton. Aerobic and fermentative branches diverge after glycolysis, so the initial net glycolytic ATP is shared. Redox recycling, not an extra ATP payoff from pyruvate reduction, is the central role of fermentation. The comparison is valid because fermentation restores \(\mathrm{NAD^+}\) through an organic electron acceptor so glycolytic PGAL oxidation can continue.
111. Arrange the events of alcoholic fermentation in their correct order.
P. Pyruvate decarboxylase acts on pyruvate.
Q. Acetaldehyde and \(\mathrm{CO_2}\) are formed.
R. Alcohol dehydrogenase transfers reducing equivalents to acetaldehyde.
S. Ethanol forms while \(\mathrm{NAD^+}\) is regenerated.
ⓐ. \(P\rightarrow R\rightarrow Q\rightarrow S\)
ⓑ. \(P\rightarrow Q\rightarrow R\rightarrow S\)
ⓒ. \(Q\rightarrow P\rightarrow S\rightarrow R\)
ⓓ. \(R\rightarrow S\rightarrow P\rightarrow Q\)
Correct Answer: \(P\rightarrow Q\rightarrow R\rightarrow S\)
Explanation: Alcoholic fermentation begins when pyruvate decarboxylase removes carbon dioxide from three-carbon pyruvate, forming the two-carbon compound acetaldehyde. Acetaldehyde must therefore appear before the reduction step can occur. Alcohol dehydrogenase then transfers reducing equivalents from \(\mathrm{NADH+H^+}\) to acetaldehyde, producing ethanol and restoring \(\mathrm{NAD^+}\). Product formation fixes the order: carbon dioxide release belongs to the first fermentative reaction, whereas ethanol formation and coenzyme regeneration belong to the second. The regenerated \(\mathrm{NAD^+}\) returns to glycolysis and permits continued PGAL oxidation under anaerobic conditions. Reversing Q and R would require alcohol dehydrogenase to act before its acetaldehyde substrate had been formed. The condition matters because organismal enzymes and oxygen availability determine whether pyruvate forms ethanol, lactate or enters aerobic oxidation. Mechanistically, coenzyme regeneration, rather than extensive carbon oxidation, is the immediate metabolic function of the fermentative branch.
112. Pyruvate decarboxylase is completely inhibited in anaerobic yeast while glycolysis initially continues. The expected immediate pattern is:
ⓐ. decreased pyruvate with increased acetaldehyde and ethanol
ⓑ. increased acetaldehyde with unchanged carbon-dioxide release
ⓒ. normal ethanol formation with no change in \(\mathrm{NAD^+}\) regeneration
ⓓ. more pyruvate with less acetaldehyde, ethanol and \(\mathrm{CO_2}\)
Correct Answer: more pyruvate with less acetaldehyde, ethanol and \(\mathrm{CO_2}\)
Explanation: Pyruvate decarboxylase consumes pyruvate and produces acetaldehyde together with carbon dioxide. When the enzyme is blocked, glycolysis can initially continue supplying pyruvate, so pyruvate accumulates upstream of the inhibition. Acetaldehyde formation falls, removing the substrate needed by alcohol dehydrogenase. Ethanol production declines, and carbon-dioxide release from the decarboxylation step also decreases. Reduced ethanol formation eventually limits regeneration of \(\mathrm{NAD^+}\), which can slow glycolysis after the available oxidised coenzyme is depleted. The predicted pattern follows both carbon flow and redox consequences. A block in the first fermentative reaction affects not only its immediate products but also the downstream reaction that depends on acetaldehyde. The branch decision acts on pyruvate processing rather than replacing the preceding cytoplasmic pathway.
113. Four glucose molecules complete glycolysis and all resulting pyruvate enters alcoholic fermentation. How many acetaldehyde molecules and carbon-dioxide molecules are formed during the decarboxylation step?
ⓐ. \(\mathrm{8}\) acetaldehyde and \(\mathrm{8CO_2}\)
ⓑ. \(\mathrm{4}\) acetaldehyde and \(\mathrm{8CO_2}\)
ⓒ. \(\mathrm{8}\) acetaldehyde and \(\mathrm{4CO_2}\)
ⓓ. \(\mathrm{4}\) acetaldehyde and \(\mathrm{4CO_2}\)
Correct Answer: \(\mathrm{8}\) acetaldehyde and \(\mathrm{8CO_2}\)
Explanation: Each glucose molecule produces two pyruvate molecules during glycolysis. Four glucose molecules supply \(4\times2=8\) pyruvate molecules. In alcoholic fermentation, every pyruvate undergoes one decarboxylation event, producing one acetaldehyde molecule and one carbon-dioxide molecule. Applying the per-pyruvate relation gives \(8\times1=8\) acetaldehyde molecules and \(8\times1=8\) carbon-dioxide molecules. Carbon accounting confirms the result: the eight three-carbon pyruvates contain \(\mathrm{24C}\); the eight two-carbon acetaldehydes contain \(\mathrm{16C}\), while eight carbon-dioxide molecules contain the remaining \(\mathrm{8C}\). Ethanol formation occurs later and does not release an additional carbon atom. Alcoholic fermentation requires decarboxylation to acetaldehyde before its reduction can regenerate the oxidised coenzyme. Lactate formation preserves all three pyruvate carbons, whereas alcoholic fermentation removes one carbon as carbon dioxide. Continued anaerobic glycolysis depends on coupling reduction of the organic acceptor with oxidation of NADH. Both oxygen-rich and oxygen-poor routes begin with the same conversion of glucose to pyruvate.
114. Anaerobic yeast is treated with an inhibitor of alcohol dehydrogenase. Acetaldehyde and \(\mathrm{NADH+H^+}\) accumulate, while ethanol and \(\mathrm{NAD^+}\) formation decline. The observations indicate that alcohol dehydrogenase normally:
ⓐ. converts glucose directly into pyruvate and ATP
ⓑ. removes carbon dioxide from pyruvate
ⓒ. reduces acetaldehyde using \(\mathrm{NADH+H^+}\)
ⓓ. transfers electrons from oxygen to ethanol
Correct Answer: reduces acetaldehyde using \(\mathrm{NADH+H^+}\)
Explanation: The accumulated compounds identify the substrates of the blocked reaction, while the declining compounds identify its products. Alcohol dehydrogenase normally uses acetaldehyde as an organic electron acceptor and transfers reducing equivalents from \(\mathrm{NADH+H^+}\) to it. Acetaldehyde is reduced to ethanol, and the reduced coenzyme is oxidised back to \(\mathrm{NAD^+}\). Inhibition prevents both changes, producing the observed rise in acetaldehyde and NADH together with reduced ethanol and oxidised coenzyme. Carbon-dioxide removal occurs one step earlier through pyruvate decarboxylase. The experiment distinguishes the decarboxylation reaction from the redox reaction and identifies the specific role of alcohol dehydrogenase through paired metabolite changes. Depletion of oxidised NAD links failure of redox recycling to a later fall in glycolytic flux.
115. Assertion: Formation of ethanol during alcoholic fermentation is associated with regeneration of \(\mathrm{NAD^+}\).
Reason: Carbon dioxide is released when pyruvate is converted into acetaldehyde.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, but Reason does not explain Assertion
Explanation: Ethanol forms when acetaldehyde accepts reducing equivalents from \(\mathrm{NADH+H^+}\). This reaction converts the reduced coenzyme back into \(\mathrm{NAD^+}\), so the assertion is true. Carbon dioxide is indeed released in the preceding reaction, when pyruvate decarboxylase converts pyruvate into acetaldehyde. The reason is therefore also true, but decarboxylation does not regenerate the oxidised coenzyme. That redox function belongs to the alcohol-dehydrogenase reaction. The two reactions are consecutive and linked through acetaldehyde, yet they have different biochemical roles: one lowers the carbon number, while the other changes the reduction state of both the organic intermediate and the coenzyme. Thus the reason does not explain the assertion. Carbon-dioxide release creates acetaldehyde, but regeneration of \(\mathrm{NAD^+}\) occurs only when that acetaldehyde is subsequently reduced to ethanol.
116. A two-step pathway is described. Reaction P converts \(\mathrm{3C}\) pyruvate into a \(\mathrm{2C}\) compound with release of \(\mathrm{CO_2}\). Reaction Q converts the \(\mathrm{2C}\) compound into ethanol while \(\mathrm{NADH+H^+}\) becomes \(\mathrm{NAD^+}\). P and Q are catalysed by:
ⓐ. alcohol dehydrogenase and pyruvate dehydrogenase
ⓑ. lactate dehydrogenase and pyruvate decarboxylase
ⓒ. hexokinase and alcohol dehydrogenase
ⓓ. pyruvate decarboxylase and alcohol dehydrogenase
Correct Answer: pyruvate decarboxylase and alcohol dehydrogenase
Explanation: Reaction P removes one carbon from pyruvate and releases it as carbon dioxide, leaving the two-carbon intermediate acetaldehyde. Pyruvate decarboxylase catalyses this decarboxylation. Reaction Q reduces acetaldehyde to ethanol and simultaneously oxidises \(\mathrm{NADH+H^+}\) to \(\mathrm{NAD^+}\). Alcohol dehydrogenase catalyses that redox step. The carbon-number change identifies the first enzyme, while the paired organic reduction and coenzyme oxidation identify the second. Neither reaction belongs to the ATP-investment part of glycolysis or to aerobic conversion of pyruvate into acetyl CoA. The described sequence is the complete pyruvate-processing part of alcoholic fermentation.
117. Consider the following statements about lactic acid fermentation.
I. Pyruvate is reduced directly to lactic acid.
II. Carbon dioxide is not released during this conversion.
III. The pathway occurs only in bacteria.
IV. \(\mathrm{NADH+H^+}\) is oxidised to regenerate \(\mathrm{NAD^+}\).
ⓐ. I and III only
ⓑ. II, III and IV only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: In lactic acid fermentation, three-carbon pyruvate accepts reducing equivalents from \(\mathrm{NADH+H^+}\) and becomes three-carbon lactic acid. Since the carbon number remains unchanged, carbon dioxide is not released. The accompanying oxidation of NADH regenerates \(\mathrm{NAD^+}\), which can return to the glycolytic PGAL-oxidation step. The pathway is not confined to bacteria. It also occurs in animal muscle cells when oxygen availability becomes inadequate during strenuous activity. The valid statements describe the carbon transformation and redox purpose of the process, while the excluded statement incorrectly restricts its biological occurrence. The route supports continued glycolysis without adding a separate ATP-producing stage. The fermentative branch sustains glycolytic ATP production by reoxidising NADH. This redox link is what allows repeated anaerobic turnover.
118. Two systems are maintained under limited oxygen.
| System | Observation |
|---|
| P. Bacterial culture | Pyruvate falls, lactic acid rises and no fermentative \(\mathrm{CO_2}\) is detected |
| Q. Contracting muscle | Lactic acid rises while the \(\mathrm{NAD^+}\) supply is temporarily maintained |
The common pathway operating in P and Q is:
ⓐ. reduction of pyruvate through lactic acid fermentation
ⓑ. decarboxylation of pyruvate through alcoholic fermentation
ⓒ. complete mitochondrial oxidation of acetyl CoA
ⓓ. direct conversion of lactate into ethanol
Correct Answer: reduction of pyruvate through lactic acid fermentation
Explanation: Both systems show evidence of pyruvate reduction to lactic acid. In the bacterial culture, lactate accumulation without fermentative carbon-dioxide release fits retention of all three pyruvate carbons in the product. In muscle, lactate formation under restricted oxygen is linked with maintenance of \(\mathrm{NAD^+}\), indicating oxidation of glycolytic NADH during pyruvate reduction. The shared evidence identifies lactic acid fermentation even though the organisms differ. Alcoholic fermentation would include acetaldehyde formation and carbon-dioxide release. Complete mitochondrial oxidation would require a functioning aerobic pathway rather than the oxygen-limited conditions supplied. The comparison demonstrates that one fermentative mechanism can occur in both some prokaryotes and animal tissues. The downstream route changes carbon and redox products, not the identity of the initial glucose-splitting pathway. In terms of carbon flow, coenzyme regeneration, rather than extensive carbon oxidation, is the immediate metabolic function of the fermentative branch.
119. During intense muscular activity, oxygen supply becomes inadequate and lactate dehydrogenase is experimentally inhibited. The most likely immediate metabolic effect is:
ⓐ. increased conversion of pyruvate into lactate with rapid \(\mathrm{NAD^+}\) formation
ⓑ. complete oxidation of pyruvate in the cytoplasm
ⓒ. increased carbon-dioxide release from lactic acid fermentation
ⓓ. reduced \(\mathrm{NAD^+}\) regeneration followed by slowing of glycolysis
Correct Answer: reduced \(\mathrm{NAD^+}\) regeneration followed by slowing of glycolysis
Explanation: Lactate dehydrogenase normally transfers reducing equivalents from \(\mathrm{NADH+H^+}\) to pyruvate, forming lactate and restoring \(\mathrm{NAD^+}\). When the enzyme is inhibited, pyruvate cannot serve efficiently as the organic electron acceptor. NADH accumulates, the available oxidised coenzyme declines and the PGAL-oxidation step of glycolysis becomes limited. ATP production through glycolysis then slows, which is especially important when oxygen shortage restricts the larger aerobic supply. Lactic fermentation does not release carbon dioxide, so the enzyme block cannot increase carbon-dioxide production. The predicted sequence is therefore enzyme inhibition, impaired \(\mathrm{NAD^+}\) regeneration and loss of glycolytic continuity, not merely reduced accumulation of an end product. This is consistent because the end product identifies the pathway, while restoration of oxidised coenzyme explains its continuity. The mechanism depends on the fact that carbon-dioxide release distinguishes alcoholic from lactic fermentation but not their shared redox purpose.
120. A sprinter’s muscle cells consume ATP rapidly. During a period when oxygen delivery cannot match demand, lactic acid production rises. The immediate metabolic advantage of this response is:
ⓐ. complete extraction of all energy remaining in pyruvate
ⓑ. regeneration of \(\mathrm{NAD^+}\) for continued glycolysis
ⓒ. production of carbon dioxide for use by mitochondria
ⓓ. formation of a large oxidative-phosphorylation ATP yield
Correct Answer: regeneration of \(\mathrm{NAD^+}\) for continued glycolysis
Explanation: Glycolysis requires \(\mathrm{NAD^+}\) during oxidation of PGAL. Under well-oxygenated conditions, reduced coenzymes can ultimately transfer electrons through aerobic pathways. When oxygen delivery becomes inadequate, that route cannot maintain redox recycling at the required rate. Muscle cells then reduce pyruvate to lactic acid, using \(\mathrm{NADH+H^+}\) and restoring \(\mathrm{NAD^+}\). This permits glycolysis to continue supplying a small but rapid ATP yield through substrate-level phosphorylation. Lactic acid formation does not completely oxidise pyruvate and does not add a major ATP-producing stage. Its immediate value is preservation of the oxidised coenzyme pool needed for short-term glycolytic energy production during oxygen limitation.
