Part 1 — Respiratory Substrates (25 MCQs)

Part 1 — Respiratory Substrates (25 MCQs)

1. Which of the following is the primary respiratory substrate in most plants under normal conditions?
   A. Starch
   B. Proteins
   C. Lipids
   D. Cellulose
   Answer: A. Starch
   Explanation:

• A (Correct): Starch (polysaccharide of glucose) is the main stored carbohydrate in plants and is readily hydrolyzed to glucose for respiration.

• B: Proteins are used only when carbohydrates are scarce; not primary under normal conditions.

• C: Lipids yield more energy per gram but are mobilized later (e.g., seeds) and require β-oxidation; not the primary everyday substrate.

• D: Cellulose is structural and not easily mobilized as a respiratory substrate.

2. During seed germination, the major respiratory substrate is often:
   A. Starch in endosperm
   B. Stored lipids in oil seeds
   C. Structural cellulose
   D. Atmospheric CO₂
   Answer: B. Stored lipids in oil seeds
   Explanation:

• A: True for many cereal seeds, but in oil seeds (e.g., castor, sunflower), lipids are major.

• B (Correct): Oil seeds use stored triacylglycerols (lipids) which are converted via β-oxidation and glyoxylate cycle for energy and gluconeogenesis.

• C: Cellulose is structural and not used for germination energy.

• D: Atmospheric CO₂ is a carbon source for photosynthesis, not a respiratory substrate.

3. Which statement about respiratory substrates is correct?
   A. All substrates are converted to acetyl-CoA before entering TCA.
   B. Only carbohydrates feed into glycolysis; lipids can’t be used.
   C. Proteins must be converted to amino acids before oxidation.
   D. Lipids enter respiration via glycolysis directly.
   Answer: C. Proteins must be converted to amino acids before oxidation.
   Explanation:

• A: Not strictly — some intermediates (like some amino acids) feed directly into TCA at different points; but many are converted to acetyl-CoA or TCA intermediates.

• B: Incorrect — lipids are broken into glycerol (can enter glycolysis) and fatty acids (β-oxidation to acetyl-CoA).

• C (Correct): Proteins are first hydrolysed to amino acids; then deamination and conversion allow entry to respiration.

• D: Lipids do not enter glycolysis directly; glycerol can, but fatty acids go via β-oxidation to acetyl-CoA.

4. Which of the following yields the highest amount of ATP per carbon atom when oxidized?
   A. Glucose
   B. Palmitic acid (C16 fatty acid)
   C. Alanine (amino acid)
   D. Pyruvate
   Answer: B. Palmitic acid (C16 fatty acid)
   Explanation:

• A: Glucose yields decent energy, but less per carbon than fatty acids.

• B (Correct): Fatty acids are more reduced (more hydrogens per carbon) so oxidation releases more ATP per carbon.

• C: Amino acids give variable yields; generally less than fatty acids per carbon due to nitrogen removal and energetic costs.

• D: Pyruvate is an oxidized 3-carbon molecule — yields less per carbon than reduced fatty acids.

5. Which enzyme catalyzes the mobilization of stored starch for respiration in plants?
   A. Lipase
   B. Amylase
   C. Protease
   D. Cellulase
   Answer: B. Amylase
   Explanation:

• A: Lipase acts on lipids, not starch.

• B (Correct): Amylase (α- and β-amylase) hydrolyzes starch to maltose/glucose for respiration.

• C: Protease digests proteins, not starch.

• D: Cellulase digests cellulose, not starch; plants generally have low cellulase activity for their own walls.

6. Which substrate requires the glyoxylate cycle to convert its breakdown products into carbohydrates?
   A. Glucose
   B. Fatty acids
   C. Sucrose
   D. Fructose
   Answer: B. Fatty acids
   Explanation:

• A: Glucose already is a carbohydrate; no glyoxylate cycle needed.

• B (Correct): Fatty acid carbons enter as acetyl-CoA, and the glyoxylate cycle in plants/seeds allows net conversion of acetyl-CoA into four-carbon compounds for gluconeogenesis.

• C & D: Sugars don’t require glyoxylate cycle for conversion to carbohydrates.

7. Which is NOT true about respiratory substrates?
   A. Proteins can be used for respiration after deamination.
   B. Fats are mobilized primarily in photosynthetically active tissues.
   C. Carbohydrates are the most readily used substrates.
   D. Different tissues may prefer different substrates.
   Answer: B. Fats are mobilized primarily in photosynthetically active tissues.
   Explanation:

• A: True.

• B (Correct — statement false): Fats are typically mobilized in storage tissues (seeds) or under starvation, not primarily in photosynthetically active (leaf) tissues.

• C: True.

• D: True.

8. Glycerol, produced from triglyceride breakdown, enters respiration at which point?
   A. Oxaloacetate
   B. Glycolysis as dihydroxyacetone phosphate (DHAP)
   C. Pyruvate decarboxylase step
   D. Directly into the TCA cycle as acetyl-CoA
   Answer: B. Glycolysis as DHAP
   Explanation:

• A: Not direct.

• B (Correct): Glycerol is phosphorylated and converted to DHAP, an intermediate of glycolysis.

• C: Pyruvate decarboxylase acts on pyruvate; glycerol does not directly get decarboxylated.

• D: Fatty acid chains become acetyl-CoA after β-oxidation; glycerol enters glycolysis first.

9. During prolonged starvation, which substrate becomes increasingly important for respiration in plants?
   A. Structural carbohydrates
   B. Starch only
   C. Lipids and proteins
   D. Atmospheric oxygen
   Answer: C. Lipids and proteins
   Explanation:

• A: Structural carbohydrates (cellulose) are usually preserved.

• B: Starch is used early; prolonged starvation leads to lipids/proteins catabolism.

• C (Correct): After carbohydrate depletion, lipids and then proteins are mobilized for energy.

• D: Oxygen is required for respiration but not a substrate providing carbon/energy.

10. Which respiratory substrate requires deamination before entering respiration?
    A. Glucose
    B. Fatty acids
    C. Amino acids
    D. Sucrose
    Answer: C. Amino acids
    Explanation:

• A: Glucose needs phosphorylation but not deamination.

• B: Fatty acids are processed by β-oxidation, not deamination.

• C (Correct): Amino acids must be deaminated to remove amino groups before carbon skeletons enter respiration.

• D: Sucrose is a disaccharide hydrolyzed to sugars, not deaminated.

11. Which molecule is the common entry point for many respiratory substrates into the TCA cycle?
    A. Glucose-6-phosphate
    B. Acetyl-CoA
    C. Fructose-1,6-bisphosphate
    D. Lactate
    Answer: B. Acetyl-CoA
    Explanation:

• A: G6P is a glycolytic intermediate; not the common TCA entry.

• B (Correct): Acetyl-CoA is central — carbohydrates (via pyruvate), fats (via β-oxidation) and some amino acids converge here.

• C: Glycolytic intermediate.

• D: Lactate must be converted to pyruvate and then to acetyl-CoA in aerobiosis.

12. Which substrate provides energy without releasing nitrogenous waste when used for respiration?
    A. Proteins
    B. Amino acids
    C. Lipids
    D. Nucleic acids
    Answer: C. Lipids
    Explanation:

• A & B: Proteins/amino acids yield nitrogenous wastes (ammonia/urea) upon deamination.

• C (Correct): Lipid oxidation yields CO₂ and H₂O (and acetyl-CoA) — no nitrogenous waste.

• D: Nucleic acids contain nitrogenous bases; their catabolism produces nitrogenous compounds.

13. Which of the following is FALSE concerning carbohydrate respiration?
    A. It yields CO₂ and H₂O in aerobic conditions.
    B. It can proceed anaerobically via fermentation.
    C. Carbohydrates are stored as proteins in plants.
    D. Glycolysis is common to both aerobic and anaerobic breakdown of glucose.
    Answer: C. Carbohydrates are stored as proteins in plants.
    Explanation:

• A: True.

• B: True — fermentation occurs under anaerobic conditions.

• C (Correct — false): Carbohydrates are stored as starch (polysaccharide), not proteins.

• D: True — glycolysis occurs in both conditions.

14. Which of these enzymes is involved in converting starch to glucose during respiration?
    A. Hexokinase
    B. α-Amylase
    C. Pyruvate kinase
    D. Citrate synthase
    Answer: B. α-Amylase
    Explanation:

• A: Hexokinase phosphorylates glucose in glycolysis; does not break starch.

• B (Correct): α-Amylase hydrolyzes α-1,4 glycosidic bonds in starch, releasing sugars.

• C: Late glycolytic enzyme; not involved in starch breakdown.

• D: TCA enzyme; not involved in starch hydrolysis.

15. Which statement best explains why fats yield more ATP than carbohydrates per gram?
    A. Fats have fewer carbons.
    B. Fats are more oxidized.
    C. Fats are more reduced and contain more hydrogen per carbon.
    D. Carbohydrates are not oxidized completely.
    Answer: C. Fats are more reduced and contain more hydrogen per carbon.
    Explanation:

• A: Incorrect; fats often have many carbons.

• B: Opposite — fats are less oxidized (more reduced).

• C (Correct): More hydrogens per carbon → more electrons transferred to electron carriers → more ATP.

• D: Carbohydrates can be completely oxidized; they simply have fewer hydrogens per carbon.

16. Which of these plant tissues is most likely to use lipids as the main respiratory substrate?
    A. Leaf mesophyll during daytime
    B. Oil-rich seed endosperm during germination
    C. Woody stem bark
    D. Root hair cells in moist soil
    Answer: B. Oil-rich seed endosperm during germination
    Explanation:

• A: Leaves use carbohydrates during the day (and photosynthesis) mainly.

• B (Correct): Oil seeds mobilize stored TAGs for energy during germination.

• C: Woody tissues are structural — not major lipid-using tissues.

• D: Root hairs use sugars transported from shoots.

17. Which of the following is an immediate product of starch degradation that feeds into glycolysis?
    A. Glycogen
    B. Maltose/glucose
    C. Amino acids
    D. Fatty acids
    Answer: B. Maltose/glucose
    Explanation:

• A: Glycogen is animal storage polysaccharide.

• B (Correct): Starch yields maltose and glucose, which enter glycolysis (after phosphorylation).

• C/D: Not products of starch degradation.

18. Which plant organelle contains the enzymes for β-oxidation of fatty acids?
    A. Mitochondrion only
    B. Glyoxysome (peroxisome) and mitochondrion
    C. Chloroplast
    D. Vacuole
    Answer: B. Glyoxysome (peroxisome) and mitochondrion
    Explanation:

• A: Mitochondria perform some β-oxidation, but in plants glyoxysomes/peroxisomes are key especially in seeds.

• B (Correct): Plant peroxisomes (glyoxysomes) carry out β-oxidation and glyoxylate cycle; mitochondria further oxidize acetyl-CoA.

• C: Chloroplasts are for photosynthesis; not β-oxidation.

• D: Vacuole is storage, not site of β-oxidation.

19. Which is the main reason plants use sugars rather than lipids for quick energy needs?
    A. Sugars are energy-rich per gram.
    B. Sugars are soluble and mobilized faster.
    C. Sugars yield more ATP per carbon.
    D. Lipids cannot be respired in plants.
    Answer: B. Sugars are soluble and mobilized faster.
    Explanation:

• A: Lipids are more energy-dense per gram.

• B (Correct): Solubility and ease of enzymatic breakdown make sugars convenient for immediate energy.

• C: Incorrect; per carbon, lipids yield more.

• D: False; lipids are respired.

20. During high aerobic activity in plants (e.g., active root growth), the preferred substrate is usually:
    A. Proteins
    B. Lipids
    C. Soluble sugars (sucrose/glucose)
    D. Cellulose
    Answer: C. Soluble sugars (sucrose/glucose)
    Explanation:

• A: Proteins are reserved for structural/enzymatic roles; used only under stress.

• B: Lipids are slower to mobilize.

• C (Correct): Soluble sugars are quickly transported and metabolized to meet immediate high-energy demand.

• D: Cellulose is structural.

21. Which process converts storage fat into substrates for gluconeogenesis in germinating seeds?
    A. Glycolysis
    B. Glyoxylate cycle
    C. Calvin cycle
    D. Fermentation
    Answer: B. Glyoxylate cycle
    Explanation:

• A: Glycolysis breaks glucose, not convert fatty acids into carbohydrate intermediates.

• B (Correct): Glyoxylate cycle bypasses CO₂-releasing steps of TCA, enabling net synthesis of four-carbon compounds from acetyl-CoA for gluconeogenesis.

• C: Calvin cycle is photosynthetic CO₂ fixation.

• D: Fermentation occurs in anaerobic conditions; not for converting fats to sugars.

22. Which of the following gives the fastest increase in ATP production per unit time when available?
    A. Fatty acid oxidation
    B. Glycolysis of glucose
    C. Amino acid deamination
    D. Cellulose breakdown
    Answer: B. Glycolysis of glucose
    Explanation:

• A: Fatty acid oxidation yields more ATP per molecule but is slower to mobilize/oxidize.

• B (Correct): Glycolysis is rapid and provides quick ATP (and intermediates).

• C: Deamination is slower and often under stress/starvation.

• D: Cellulose is not readily hydrolyzed in situ.

23. Which substrate produces NADH during its catabolism?
    A. Glucose
    B. Lipids (fatty acids)
    C. Amino acids
    D. All of the above
    Answer: D. All of the above
    Explanation:

• A (True): Glycolysis and pyruvate dehydrogenase produce NADH.

• B (True): β-oxidation produces NADH (and FADH₂).

• C (True): Deamination and oxidation of carbon skeletons produce NADH.

• D (Correct): All produce reduced electron carriers (NADH) during breakdown.

24. Which of the following is NOT a reason plants store carbohydrates rather than free glucose?
    A. Osmotic considerations
    B. Easy mobilization by amylase
    C. Starch is compact and insoluble
    D. Starch forms strong cell walls
    Answer: D. Starch forms strong cell walls
    Explanation:

• A (True): Free glucose would increase osmotic pressure; stored polymer reduces osmotic effect.

• B (True): Amylase can mobilize starch when needed.

• C (True): Starch is insoluble and compact.

• D (Correct — not a reason): Cell walls are mainly cellulose, not starch.

25. Which plant hormone stimulates mobilization of reserves (like starch) in germinating seeds?
    A. Auxin
    B. Gibberellin
    C. Abscisic acid (ABA)
    D. Ethylene
    Answer: B. Gibberellin
    Explanation:

• A: Auxin influences growth but not primary starch mobilization in seeds.

• B (Correct): Gibberellins stimulate synthesis of hydrolytic enzymes (e.g., α-amylase) in aleurone layer leading to starch breakdown.

• C: ABA promotes dormancy and inhibits germination.

• D: Ethylene regulates fruit ripening and some stress responses, not primary starch mobilization in germination.

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