Chapter 12: Respiration in Plants Application Questions | biology Class 11 CBSE

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Chapter 12: Respiration in Plants Application Questions | biology Class 11 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A student noticed that dough left overnight in a warm kitchen had risen and smelt of alcohol. With reference to fermentation, explain the biological process responsible for these observations.

The dough rose because yeast cells present in it carried out alcoholic fermentation under partially anaerobic conditions, producing CO2 as a byproduct; CO2 gas became trapped in the dough, causing it to expand.
The alcoholic smell indicates that pyruvic acid produced by glycolysis in yeast was converted to ethanol by the sequential action of pyruvic acid decarboxylase and alcohol dehydrogenase.
The warm temperature (around 30–35°C) accelerated the enzymatic reactions of glycolysis and fermentation, increasing both CO2 production and ethanol formation.
This process is an example of anaerobic respiration in a unicellular eukaryote (yeast); the net energy gain is only 2 ATP per glucose, but it sustains yeast metabolism in the oxygen-limited interior of dough.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 2

Applied Concept

With reference to the concept of the amphibolic pathway, explain what would happen in a cell if only glucose oxidation occurred and no biosynthesis of fatty acids or amino acids took place.

If only catabolism occurred with no biosynthesis, the intermediates of respiration such as acetyl CoA and Krebs' cycle acids would not be withdrawn from the pathway, and the pathway would function only as a catabolic route.
Without withdrawal of acetyl CoA for fatty acid synthesis, the cell could not build cell membranes, store energy as fat, or produce steroid hormones and other lipid-based molecules essential for normal function.
Without withdrawal of Krebs' cycle intermediates (e.g., alpha-ketoglutarate, OAA) for amino acid synthesis, the cell could not synthesise proteins, enzymes, or other nitrogen-containing compounds needed for growth and repair.
This illustrates why the respiratory pathway is described as amphibolic: isolating it to purely catabolic function would make it incompatible with life, since cellular anabolism depends entirely on intermediates supplied by the same pathway.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A scientist measured the RQ of a germinating oil-rich seed and found it to be approximately 0.7. What does this indicate about the substrate being respired? How would the RQ change if the seed switched to using proteins?

An RQ of approximately 0.7 indicates that fats (lipids) are the predominant respiratory substrate; this value matches the theoretical RQ of fat oxidation as seen in the example of tripalmitin (102 CO2 / 145 O2 = 0.7).
Fats require more O2 per carbon than carbohydrates because they are highly reduced molecules with a low oxygen-to-carbon ratio, resulting in more O2 consumed relative to CO2 produced, giving RQ < 1.
If the germinating seed shifted to using proteins as the respiratory substrate, the RQ would rise to approximately 0.9, which is the value associated with protein oxidation due to the intermediate oxygen-to-carbon ratio of amino acids.
This change in RQ would provide physiological evidence that the seed had exhausted its stored fat reserves and was now catabolising storage proteins; RQ measurements are thus a useful tool for monitoring the metabolic status of organisms.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

During vigorous exercise, human muscle cells experience a shortage of oxygen and begin producing lactic acid. Explain the biochemical events responsible for this, and what happens to the lactic acid when oxygen is restored.

During vigorous exercise, oxygen delivery to muscle cells cannot match the demand; cells switch to anaerobic conditions and pyruvic acid produced by glycolysis is reduced to lactic acid by the enzyme lactate dehydrogenase.
The reducing agent for this reaction is NADH+H+, which is reoxidised to NAD+; this reoxidation is essential to allow glycolysis to continue generating ATP anaerobically for rapid muscle contraction.
Lactic acid accumulation lowers the pH of muscle tissue, causing the burning sensation and fatigue experienced during intense exercise; its buildup impairs enzyme activity if it continues.
When oxygen is restored (recovery period), lactic acid is transported to the liver where it is reconverted to pyruvate and then to glucose via gluconeogenesis, or oxidised through aerobic respiration; this is part of the oxygen debt repayment process.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

With reference to the respiratory balance sheet, a student claims that aerobic respiration of glucose always produces exactly 38 ATP molecules in a living cell. Evaluate this claim.

The claim is incorrect — 38 ATP is only a theoretical maximum calculated under idealised assumptions, not the actual yield in a living cell.
In living cells, all metabolic pathways operate simultaneously; intermediates are constantly diverted for biosynthesis (e.g., amino acids, fatty acids), reducing the substrates available for complete oxidation.
The transfer of cytoplasmic NADH from glycolysis into the mitochondria involves an energy cost (shuttle mechanism), and enzymatic rates are regulated by feedback; actual ATP yield is therefore always lower than 38.
The value of calculating 38 ATP lies in appreciating the theoretical maximum efficiency and comparing aerobic respiration with fermentation (2 ATP), but students must understand it represents a model, not the actual cellular reality.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

A biology teacher demonstrates that a sealed container of germinating seeds shows a decrease in O2 and increase in CO2 over time. Explain the process occurring and predict the RQ if the seeds are oil-rich.

Germinating seeds are metabolically active and respiring aerobically; they consume O2 to completely oxidise stored food materials (primarily fats in oil-rich seeds) and release CO2 and water as products.
The decrease in O2 and increase in CO2 in the sealed container directly reflects the aerobic respiratory process in which organic substrates undergo oxidation via glycolysis, Krebs' cycle, and ETS.
If the seeds are oil-rich (e.g., sunflower or groundnut seeds), fats are the primary respiratory substrate; fats require significantly more O2 for complete oxidation than they produce CO2.
Therefore, the RQ would be less than 1 — approximately 0.7 — since CO2 evolved would be less than O2 consumed, consistent with fat as the respiratory substrate.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

With reference to the electron transport system, explain what would happen to cellular ATP production if an inhibitor specifically blocked Complex I (NADH dehydrogenase).

If Complex I (NADH dehydrogenase) were blocked, NADH produced in the mitochondrial matrix (from the Krebs' cycle and pyruvate oxidation) could not donate its electrons to the ETS, stalling oxidation of NADH.
Without electron flow through Complex I, no proton pumping would occur at this site, reducing the proton gradient across the inner membrane and severely impairing ATP synthesis by Complex V.
NAD+ would not be regenerated from NADH, so the Krebs' cycle would quickly halt due to unavailability of the oxidised coenzyme needed as an electron acceptor at multiple cycle steps.
FADH2-linked electrons (via Complex II) could still reach ubiquinone and proceed through Complexes III and IV to O2, allowing some residual ATP production, but overall ATP yield would be dramatically reduced, threatening cell survival.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

Explain why brewers can produce wine (with ~13% alcohol) naturally by fermentation, but must use distillation to produce spirits like whisky. Relate this to the biology of yeast.

Yeast cells produce ethanol as a byproduct of alcoholic fermentation; as ethanol concentration in the fermentation medium rises, it becomes increasingly toxic to the yeast cells themselves.
When ethanol concentration reaches approximately 13%, yeast cells are poisoned and die; beyond this concentration, the yeast can no longer carry out fermentation, so alcohol content naturally plateaus at ~13%.
This biological ceiling on yeast-mediated fermentation means that all naturally fermented beverages (wine, beer) have alcohol content at or below 13%; the yeast essentially kills itself before higher concentrations are reached.
To produce spirits with alcohol content above 13% (e.g., whisky at 40%, vodka at 40%), distillation is used — a physical process that exploits the lower boiling point of ethanol to concentrate it above what fermentation alone can achieve.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 9

Applied Concept

A patient is given a drug that inhibits the F0 component of ATP synthase in mitochondria. Predict the consequences for cellular respiration and the patient's health.

F0 forms the proton channel through which H+ ions flow from the intermembrane space back into the mitochondrial matrix; inhibiting it would block proton flow and therefore halt ATP synthesis by Complex V entirely.
With ATP synthase blocked, the proton gradient across the inner membrane would build up but not be dissipated; this could eventually inhibit the ETS itself (back pressure), stalling electron transfer and O2 reduction.
Cells throughout the body would be severely deprived of ATP, as oxidative phosphorylation — responsible for ~89% of aerobic ATP production — would cease; cells would be unable to maintain basic functions like ion gradients, movement, and biosynthesis.
The patient would experience rapid cellular failure, particularly in high-energy tissues like the brain (neurotransmission), heart (constant muscle contraction), and kidneys; without immediate intervention, the outcome would be life-threatening.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 10

Applied Concept

A plant tissue sample is kept in a nitrogen atmosphere (no O2). Using your knowledge of glycolysis and fermentation, describe the energy metabolism that would occur and the products formed.

Without O2, the plant tissue cannot carry out aerobic respiration; instead, glycolysis continues in the cytoplasm, converting glucose to pyruvic acid with a net gain of 2 ATP and 2 NADH+H+ per glucose.
Pyruvic acid then undergoes anaerobic fermentation; in plant tissues, alcoholic fermentation predominates — pyruvic acid decarboxylase converts pyruvate to acetaldehyde + CO2, then alcohol dehydrogenase reduces it to ethanol using NADH+H+.
The reoxidation of NADH to NAD+ in fermentation is critical; it allows glycolysis to continue, sustaining ATP production in the absence of oxygen, albeit at very low efficiency (2 ATP per glucose vs 38 in aerobic conditions).
Over time, accumulation of ethanol would become toxic to the tissue; the energy deficit (only 2 ATP vs 38) would also impair all energy-demanding cellular processes, eventually leading to cell death if anaerobic conditions persist.

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Chapter 12: Respiration in Plants | Application Questions

A student noticed that dough left overnight in a warm kitchen had risen and smelt of alcohol. With reference to fermentation, explain the biological process responsible for these observations.

With reference to the concept of the amphibolic pathway, explain what would happen in a cell if only glucose oxidation occurred and no biosynthesis of fatty acids or amino acids took place.

A scientist measured the RQ of a germinating oil-rich seed and found it to be approximately 0.7. What does this indicate about the substrate being respired? How would the RQ change if the seed switched to using proteins?

During vigorous exercise, human muscle cells experience a shortage of oxygen and begin producing lactic acid. Explain the biochemical events responsible for this, and what happens to the lactic acid when oxygen is restored.

With reference to the respiratory balance sheet, a student claims that aerobic respiration of glucose always produces exactly 38 ATP molecules in a living cell. Evaluate this claim.

A biology teacher demonstrates that a sealed container of germinating seeds shows a decrease in O2 and increase in CO2 over time. Explain the process occurring and predict the RQ if the seeds are oil-rich.

With reference to the electron transport system, explain what would happen to cellular ATP production if an inhibitor specifically blocked Complex I (NADH dehydrogenase).

Explain why brewers can produce wine (with ~13% alcohol) naturally by fermentation, but must use distillation to produce spirits like whisky. Relate this to the biology of yeast.

A patient is given a drug that inhibits the F0 component of ATP synthase in mitochondria. Predict the consequences for cellular respiration and the patient's health.

A plant tissue sample is kept in a nitrogen atmosphere (no O2). Using your knowledge of glycolysis and fermentation, describe the energy metabolism that would occur and the products formed.

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