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201. Which statement best explains why the respiratory balance sheet of aerobic respiration is considered theoretical?
ⓐ. Because glucose is never broken down in living cells
ⓑ. Because all NADH molecules always produce exactly the same ATP in every situation
ⓒ. Because intermediate products may enter other pathways and the actual ATP yield may differ from the ideal calculation
ⓓ. Because oxygen is not really used in terminal oxidation
Correct Answer: Because intermediate products may enter other pathways and the actual ATP yield may differ from the ideal calculation
Explanation: The respiratory balance sheet is called theoretical because it assumes complete and ideal oxidation of one glucose molecule with perfect ATP accounting. In actual living cells, this rarely occurs in such a neat and isolated way. Some intermediates may be withdrawn for biosynthesis, and all pyruvate may not always follow the same route. In addition, the exact ATP yield from reduced coenzymes may vary depending on cellular conditions. For these reasons, the balance sheet serves mainly as a conceptual accounting model rather than an exact measurement of every cell. Thus it is considered theoretical because actual ATP yield may differ from the ideal calculation.
202. Which statement correctly represents the classical ATP distribution from one glucose molecule in aerobic respiration?
ⓐ. 4 ATP from glycolysis, 4 ATP from link reaction, 30 ATP from Krebs cycle
ⓑ. 2 ATP from glycolysis, 2 ATP from Krebs cycle, and the remaining major ATP through electron transport
ⓒ. 10 ATP from glycolysis, 10 ATP from Krebs cycle, and 18 ATP from fermentation
ⓓ. 2 ATP from glycolysis, 2 ATP from fermentation, and 34 ATP from photosynthesis
Correct Answer: 2 ATP from glycolysis, 2 ATP from Krebs cycle, and the remaining major ATP through electron transport
Explanation: In the classical respiratory balance sheet, only a small amount of ATP is formed directly by substrate-level phosphorylation. Glycolysis contributes a net gain of 2 ATP, and the Krebs cycle contributes 2 more ATP per glucose. The much larger remaining share comes indirectly through oxidation of NADH and $FADH_2$ in the electron transport system. This is why electron transport and oxidative phosphorylation dominate total ATP production. The balance sheet therefore separates direct ATP formation from indirect ATP equivalents. Hence the correct classical distribution is 2 ATP from glycolysis, 2 ATP from the Krebs cycle, and the rest mainly through electron transport.
203. Which overall equation best represents the complete aerobic oxidation of one molecule of glucose in the classical respiratory balance sheet?
ⓐ. $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
ⓑ. $C_6H_{12}O_6 + 3O_2 \rightarrow 3CO_2 + 3H_2O + \text{energy}$
ⓒ. $C_6H_{12}O_6 + 6CO_2 \rightarrow 6O_2 + 6H_2O + \text{energy}$
ⓓ. $C_6H_{12}O_6 + 6H_2O \rightarrow 6CO_2 + 6O_2 + \text{energy}$
Correct Answer: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
Explanation: The complete aerobic oxidation of glucose means that one glucose molecule is fully broken down in the presence of oxygen. In the overall balance sheet, glucose reacts with six molecules of oxygen and yields six molecules of carbon dioxide, six molecules of water, and energy that is partly conserved in ATP. This equation summarizes the entire pathway rather than showing the details of individual stages. It captures the net material balance of aerobic respiration in a compact form. Because the carbon atoms of glucose are fully oxidized, carbon dioxide appears as the carbon product. Therefore this is the correct overall equation of aerobic respiration.
204. How many molecules of carbon dioxide are released in total during the complete aerobic respiration of one glucose molecule?
ⓐ. 2
ⓑ. 4
ⓒ. 6
ⓓ. 8
Correct Answer: 6
Explanation: One molecule of glucose contains six carbon atoms, and complete aerobic respiration ultimately removes all of them in the form of carbon dioxide. Two carbon dioxide molecules are released during the link reaction because the two pyruvate molecules each lose one carbon. The remaining four carbon dioxide molecules are released during the two turns of the Krebs cycle. Adding these together gives a total of six carbon dioxide molecules per glucose. This total fits the overall respiratory equation and shows complete oxidation of the carbon skeleton. Thus the complete aerobic respiration of one glucose yields 6 molecules of carbon dioxide.
205. Which statement correctly distributes the six carbon dioxide molecules produced from one glucose during aerobic respiration?
ⓐ. 2 in glycolysis and 4 in the electron transport system
ⓑ. 2 in the link reaction and 4 in the Krebs cycle
ⓒ. 6 in glycolysis and none in the Krebs cycle
ⓓ. 4 in the link reaction and 2 in oxidative phosphorylation
Correct Answer: 2 in the link reaction and 4 in the Krebs cycle
Explanation: Glycolysis does not release carbon dioxide, so the carbon loss begins only after pyruvate is formed. In the link reaction, each pyruvate loses one carbon as carbon dioxide, giving a total of two carbon dioxide molecules per glucose. Then each turn of the Krebs cycle releases two more carbon dioxide molecules, and since the cycle turns twice per glucose, the total from the cycle becomes four. These values together account for all six carbon atoms of glucose. This distribution is important for understanding where decarboxylation occurs in respiration. Therefore the correct distribution is 2 in the link reaction and 4 in the Krebs cycle.
206. In the classical respiratory balance sheet, how many ATP molecules are attributed to oxidative phosphorylation from one glucose molecule?
ⓐ. 4
ⓑ. 30
ⓒ. 34
ⓓ. 38
Correct Answer: 34
Explanation: In the classical theoretical balance sheet, most ATP is produced indirectly through oxidative phosphorylation. The 10 NADH molecules are taken to yield 30 ATP, and the 2 $FADH_2$ molecules are taken to yield 4 ATP. When these are added together, oxidative phosphorylation contributes 34 ATP molecules. This is much greater than the 4 ATP formed directly by substrate-level phosphorylation in glycolysis and the Krebs cycle combined. The large value highlights the importance of the electron transport system in energy conservation. Hence oxidative phosphorylation is classically assigned 34 ATP per glucose.
207. A student says, “All 38 ATP of aerobic respiration are formed directly in metabolic reactions before the electron transport system begins.” Which correction is most accurate?
ⓐ. All ATP are formed directly only in glycolysis and none later
ⓑ. Only 4 ATP are formed directly, while most ATP are produced indirectly through oxidative phosphorylation
ⓒ. No ATP are formed before oxygen accepts electrons in the chain
ⓓ. All 38 ATP are formed in the Krebs cycle after acetyl-CoA enters
Correct Answer: Only 4 ATP are formed directly, while most ATP are produced indirectly through oxidative phosphorylation
Explanation: The classical respiratory balance sheet distinguishes between ATP formed directly and ATP formed indirectly. Direct ATP formation by substrate-level phosphorylation contributes only a small amount, namely 2 ATP from glycolysis and 2 ATP from the Krebs cycle, giving a total of 4 ATP. The much larger remainder comes through oxidative phosphorylation using the reduced coenzymes NADH and $FADH_2$. Therefore it is incorrect to say that all 38 ATP are formed directly before the electron transport system. The major share is produced only after reduced coenzymes enter the final stage. Thus the correct correction is that only 4 ATP are direct, while most are formed indirectly.
208. Assertion (A): The respiratory balance sheet is useful for understanding the contribution of each stage of aerobic respiration. Reason (R): It summarizes the inputs, outputs, and ATP equivalents of the major respiratory stages in one account.
ⓐ. Both A and R are true, and R is the correct explanation of A.
ⓑ. Both A and R are true, but R is not the correct explanation of A.
ⓒ. A is true, but R is false.
ⓓ. A is false, but R is true.
Correct Answer: Both A and R are true, and R is the correct explanation of A.
Explanation: The assertion is true because the respiratory balance sheet helps learners see how the different stages of respiration contribute to the final energy yield. The reason is also true, since the balance sheet brings together the major steps such as glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation in a single summarized account. It includes the reduced coenzymes formed and the ATP equivalents associated with them. This unified presentation makes stage-wise contribution much easier to understand. The reason directly explains why the balance sheet is educationally useful. Therefore both statements are true, and the reason correctly explains the assertion.
209. Which pair correctly represents the reduced coenzyme contribution in the classical balance sheet of one glucose molecule?
ⓐ. $10\ \text{NADH} \rightarrow 30\ \text{ATP}$ and $2\ \text{FADH}_2 \rightarrow 4\ \text{ATP}$
ⓑ. $10\ \text{NADH} \rightarrow 20\ \text{ATP}$ and $2\ \text{FADH}_2 \rightarrow 6\ \text{ATP}$
ⓒ. $6\ \text{NADH} \rightarrow 30\ \text{ATP}$ and $4\ \text{FADH}_2 \rightarrow 8\ \text{ATP}$
ⓓ. $2\ \text{NADH} \rightarrow 4\ \text{ATP}$ and $10\ \text{FADH}_2 \rightarrow 30\ \text{ATP}$
Correct Answer: $10\ \text{NADH} \rightarrow 30\ \text{ATP}$ and $2\ \text{FADH}_2 \rightarrow 4\ \text{ATP}$
Explanation: In the classical respiratory balance sheet, each NADH is taken to yield 3 ATP and each $FADH_2$ is taken to yield 2 ATP. Complete aerobic respiration of one glucose produces 10 NADH and 2 $FADH_2$. Therefore the ATP equivalents are calculated as $10 \times 3 = 30$ ATP from NADH and $2 \times 2 = 4$ ATP from $FADH_2$. These values account for the indirect ATP yield through oxidative phosphorylation. This is why reduced coenzymes dominate the total ATP production. Hence the correct pair is 10 NADH yielding 30 ATP and 2 $FADH_2$ yielding 4 ATP.
210. Which statement best explains why direct ATP formation contributes only a small part of the total respiratory yield?
ⓐ. Because direct ATP is formed only when oxygen is completely absent
ⓑ. Because most energy is first trapped in NADH and $FADH_2$ and later converted to ATP through the electron transport system
ⓒ. Because glycolysis and the Krebs cycle do not release any usable energy at all
ⓓ. Because ATP can be formed directly only from carbon dioxide and water
Correct Answer: Because most energy is first trapped in NADH and $FADH_2$ and later converted to ATP through the electron transport system
Explanation: The major portion of energy released during aerobic respiration is not captured immediately as ATP in the early stages. Instead, much of it is stored first in the reduced coenzymes NADH and $FADH_2$ during glycolysis, the link reaction, and the Krebs cycle. These coenzymes then transfer electrons to the electron transport system, where most ATP is synthesized through oxidative phosphorylation. This is why direct ATP formation by substrate-level phosphorylation remains comparatively small. The balance sheet clearly reflects this difference between direct and indirect ATP generation. Therefore direct ATP contributes only a small fraction because most energy is first trapped in reduced coenzymes.
211. A classmate counts only substrate-level phosphorylation and concludes that aerobic respiration yields 4 ATP from one glucose molecule. What is missing from this conclusion?
ⓐ. the ATP formed through oxidation of NADH and $FADH_2$ in the electron transport system
ⓑ. the carbon dioxide released during the link reaction and Krebs cycle
ⓒ. the water absorbed during glycolysis and fermentation
ⓓ. the chlorophyll needed to activate glucose in mitochondria
Correct Answer: the ATP formed through oxidation of NADH and $FADH_2$ in the electron transport system
Explanation: Counting only substrate-level phosphorylation includes the net 2 ATP of glycolysis and the 2 ATP of the Krebs cycle, giving 4 ATP. However, this does not account for the much larger ATP yield associated with reduced coenzymes. During respiration, NADH and $FADH_2$ carry electrons to the electron transport system, where their oxidation is linked with oxidative phosphorylation. This indirect ATP production makes up the majority of the total classical yield. Therefore the conclusion of only 4 ATP is incomplete because it ignores oxidative phosphorylation. What is missing is the ATP formed through the oxidation of NADH and $FADH_2$ in the electron transport system.
212. What is meant by calling respiration an amphibolic pathway?
ⓐ. It functions only in breaking down food molecules for energy
ⓑ. It functions only in synthesizing complex molecules in cells
ⓒ. It serves both catabolic and anabolic roles in metabolism
ⓓ. It occurs equally in chloroplasts and mitochondria only
Correct Answer: It serves both catabolic and anabolic roles in metabolism
Explanation: Respiration is called an amphibolic pathway because it is involved in both breakdown and biosynthetic functions. On one side, it is catabolic since it breaks down respiratory substrates and releases energy. On the other side, several intermediates of the pathway can be diverted and used for building important cellular compounds. This means the respiratory pathway is not limited to energy release alone. It also provides starting materials for anabolic processes in the cell. Because it participates in both directions of metabolism, it is described as amphibolic. This dual role is the key meaning of the term.
213. Which statement best explains the catabolic role of respiration?
ⓐ. It converts simple molecules into highly complex cellular structures
ⓑ. It breaks down organic substrates to release stored chemical energy
ⓒ. It stores all respiratory intermediates permanently in vacuoles
ⓓ. It uses carbon dioxide only for the formation of glucose
Correct Answer: It breaks down organic substrates to release stored chemical energy
Explanation: The catabolic role of respiration refers to the degradative side of the pathway. Organic compounds such as carbohydrates, fats, and proteins can be broken down step by step during respiration. As these molecules are degraded, the energy stored in their chemical bonds is released and captured in a usable form such as ATP. This is why respiration is considered a major energy-yielding process in living cells. The term catabolic specifically emphasizes breakdown rather than synthesis. Therefore respiration is catabolic because it breaks down substrates to release stored chemical energy.
214. Why is respiration also considered anabolic in significance?
ⓐ. Because all ATP formed in respiration is converted directly into starch
ⓑ. Because respiratory intermediates may be withdrawn for biosynthesis of other compounds
ⓒ. Because oxygen is synthesized at the end of terminal oxidation
ⓓ. Because carbon dioxide is always fixed back into pyruvate in mitochondria
Correct Answer: Because respiratory intermediates may be withdrawn for biosynthesis of other compounds
Explanation: Respiration is not only a pathway for energy release but also a source of important intermediate compounds. Several molecules formed during glycolysis and the Krebs cycle can be removed from the pathway and used in the synthesis of amino acids, fatty acids, and other cellular substances. This biosynthetic use gives respiration an anabolic significance. The pathway therefore supports building processes by supplying carbon skeletons and precursor molecules. It does not become anabolic by making oxygen or by directly converting all ATP into storage products. Hence its anabolic role comes from the use of respiratory intermediates in biosynthesis.
215. Which term correctly describes a pathway that can participate in both degradation and synthesis?
ⓐ. Exergonic
ⓑ. Cyclic
ⓒ. Diffusive
ⓓ. Amphibolic
Correct Answer: Amphibolic
Explanation: A pathway that serves both degradative and synthetic roles is called amphibolic. The word distinguishes such a pathway from one that is only catabolic or only anabolic. In respiration, this term is appropriate because substrates are broken down for energy, while intermediates are also used as starting materials for biosynthetic reactions. This dual function makes the pathway metabolically versatile. The term therefore refers to role and function rather than simply to shape or energy release. Thus a pathway involved in both degradation and synthesis is called amphibolic.
216. When fats are used as respiratory substrates, they are first broken down mainly into:
ⓐ. glucose and amino acids
ⓑ. glycerol and fatty acids
ⓒ. pyruvate and oxygen
ⓓ. carbon dioxide and water
Correct Answer: glycerol and fatty acids
Explanation: Fats do not usually enter the respiratory pathway directly in their original stored form. They are first hydrolyzed into glycerol and fatty acids. These two components can then enter metabolism through different routes. Glycerol may be converted into an intermediate that joins the glycolytic pathway, while fatty acids are broken down further into smaller units that enter respiration. This is the fundamental first step in the utilization of fats as respiratory substrates. Therefore fats are initially broken down into glycerol and fatty acids.
217. The glycerol obtained from fat breakdown enters respiration mainly after conversion into:
ⓐ. glyceraldehyde-3-phosphate
ⓑ. acetyl-CoA
ⓒ. oxaloacetic acid
ⓓ. succinic acid
Correct Answer: glyceraldehyde-3-phosphate
Explanation: Glycerol released from fats does not remain outside the respiratory pathway. It is converted into a three-carbon intermediate that can enter glycolysis. In standard textbook treatment, this entry point is at the level of glyceraldehyde-3-phosphate, also called PGAL. This allows the carbon from glycerol to be processed through the glycolytic pathway and contribute to energy production. The step shows how fat metabolism can connect with carbohydrate respiratory intermediates. Thus glycerol joins respiration mainly after conversion into glyceraldehyde-3-phosphate.
218. Fatty acids formed from fat breakdown generally enter the respiratory pathway after conversion into:
ⓐ. pyruvate
ⓑ. ethanol
ⓒ. acetyl-CoA
ⓓ. fructose-1,6-bisphosphate
Correct Answer: acetyl-CoA
Explanation: Fatty acids are broken down through reactions that generate two-carbon units. These units are converted into acetyl-CoA, which is the same molecule that enters the Krebs cycle during aerobic respiration. This provides a direct metabolic connection between fat breakdown and the central respiratory pathway. Because acetyl-CoA is a common entry point into the Krebs cycle, fatty acids can serve as effective respiratory substrates. The conversion does not normally pass first through pyruvate or ethanol. Therefore fatty acids enter respiration mainly as acetyl-CoA.
219. Before proteins can be used in respiration, they are first broken down into:
ⓐ. starch molecules
ⓑ. amino acids
ⓒ. fatty acids
ⓓ. nucleotides
Correct Answer: amino acids
Explanation: Proteins are complex organic molecules and cannot enter the respiratory pathway in their intact form. They are first digested or degraded into their smaller building units, which are amino acids. These amino acids can then undergo further modification before their carbon skeletons enter respiratory metabolism. This initial breakdown is essential because respiration works with smaller metabolically usable compounds. The process therefore begins with conversion of proteins into amino acids. Hence amino acids are the first products formed before proteins can be used in respiration.
220. What happens to amino acids before their carbon skeletons enter the respiratory pathway?
ⓐ. They undergo deamination
ⓑ. They are converted directly into oxygen
ⓒ. They are stored unchanged in stomata
ⓓ. They are split only into water molecules
Correct Answer: They undergo deamination
Explanation: Amino acids contain both a carbon skeleton and an amino group, but the amino group is not part of the normal respiratory carbon pathway. Before the remaining carbon framework can enter respiration, the amino group is removed in a process called deamination. This leaves behind an organic acid or related carbon compound that can be fed into respiratory metabolism. Deamination is therefore a necessary preparatory step in the use of proteins for respiration. It links protein catabolism with central metabolic pathways. Thus amino acids undergo deamination before entering respiration through their carbon skeletons.