Life Processes in Plants Part 4: MCQs (Q76–Q100)

Part 4 — Respiration in Plants & Applied Questions (Q76–Q100)

76. Plant respiration is the process of:

A. Fixing CO₂ into carbohydrates only
B. Breaking down carbohydrates to release energy (ATP) ← Correct answer: B
C. Only occurring in roots
D. Producing oxygen only
Explanations:
A/D) Photosynthesis does that — incorrect.
B) Correct. Cellular respiration oxidizes sugars to produce energy.
C) Respiration occurs in all living plant cells — incorrect.

77. Aerobic respiration in plants produces:

A. Only CO₂
B. CO₂, H₂O and energy (ATP) ← Correct answer: B
C. Oxygen as main product
D. Ethanol only under all conditions
Explanations:
A/C/D) Incomplete/incorrect.
B) Correct. Aerobic respiration oxidizes glucose yielding CO₂, water, and ATP.

78. Anaerobic respiration (fermentation) in plants may produce:

A. Lactic acid only
B. Ethanol and CO₂ (in plants/yeast) ← Correct answer: B
C. Oxygen only
D. No products at all
Explanations:
A) Lactic acid is typical for animal muscles — not plants/yeasts.
B) Correct. Plant/yeast fermentation yields ethanol + CO₂ and small ATP yield.
C/D) Incorrect.

79. Which organelle is the site of aerobic respiration in plant cells?

A. Chloroplast
B. Mitochondrion ← Correct answer: B
C. Nucleus
D. Golgi apparatus
Explanations:
A) Chloroplast for photosynthesis — incorrect.
B) Correct. Mitochondria produce ATP via respiration.
C/D) Not respiration centers — incorrect.

80. Which condition favors anaerobic respiration in plant tissues?

A. Well-aerated soil
B. Waterlogged, oxygen-poor soil ← Correct answer: B
C. High light intensity only
D. Moderate temperatures always
Explanations:
A/C/D) Not promoting anaerobic pathways.
B) Correct. Lack of oxygen forces fermentation pathways.

81. The respiration rate in plants is highest in:

A. Mature, non-growing tissues
B. Growing tissues (meristems, young leaves, fruits) ← Correct answer: B
C. Dead tissues only
D. Woody heartwood only
Explanations:
A/C/D) Lower metabolic activity or dead — incorrect.
B) Correct. Growing tissues demand more energy → higher respiration.

82. Which of the following is TRUE about root respiration?

A. Roots do not respire at all
B. Roots respire only in presence of light
C. Roots respire and require oxygen from soil air ← Correct answer: C
D. Roots photosynthesize to make own O₂
Explanations:
A/B/D) Incorrect statements.
C) Correct. Roots require oxygen in soil; poor aeration limits respiration and root growth.

83. Which statement about ATP is TRUE?

A. ATP stores genetic information
B. ATP is the energy currency generated during respiration ← Correct answer: B
C. ATP produces light in plants only
D. ATP is a gas transported in xylem
Explanations:
A/C/D) Incorrect.
B) Correct. ATP supplies energy for cellular processes.

84. Which of the following can be a physiological adaptation to reduce transpiration?

A. Thin leaves with many stomata
B. Thick succulent leaves with CAM metabolism ← Correct answer: B
C. Always open stomata day and night
D. Large leaf surface area in desert plants
Explanations:
A/C/D) Increase water loss — incorrect.
B) Correct. Succulents store water and CAM opens stomata at night to conserve water.

85. Which process in plants requires oxygen?

A. Light reaction of photosynthesis only
B. Aerobic respiration (ATP production) ← Correct answer: B
C. Photorespiration only uses oxygen, not requiring it
D. Active transport of sugars in phloem that doesn’t use ATP
Explanations:
A) Light reactions do not consume oxygen; they produce O₂ — incorrect.
B) Correct. Aerobic respiration uses O₂ to oxidize substrates.
C) Photorespiration uses O₂ but is a specific process — confusing phrasing; B is broader correct.
D) Active transport uses ATP which is produced using oxygen in aerobic respiration—statement incorrect.

86. Which is the major difference between respiration and photosynthesis?

A. Respiration stores energy; photosynthesis releases energy
B. Photosynthesis stores energy in glucose; respiration releases energy from glucose ← Correct answer: B
C. Both produce oxygen only
D. Both happen only in daytime
Explanations:
A/C/D) Incorrect.
B) Correct. Photosynthesis synthesizes carbohydrates using light energy; respiration oxidizes them to release energy.

87. In respiration, which step yields the most ATP per glucose molecule?

A. Glycolysis only
B. Krebs cycle only
C. Electron transport chain (oxidative phosphorylation) ← Correct answer: C
D. Fermentation
Explanations:
A/B) Contribute ATP but less than ETC — incorrect.
C) Correct. Majority of ATP generated via proton gradient/ETC in mitochondria.
D) Fermentation yields minimal ATP.

88. Plant leaves turning yellow (chlorosis) often indicates deficiency of:

A. Water only
B. Nitrogen or magnesium (affect chlorophyll) ← Correct answer: B
C. Oxygen only
D. Carbon dioxide only
Explanations:
A/C/D) Not primary causes of chlorosis.
B) Correct. Lack of N or Mg reduces chlorophyll causing yellowing.

89. Which statement is TRUE about transpiration stream?

A. Moves organic molecules only
B. Drags dissolved mineral ions upward with water in xylem ← Correct answer: B
C. Is confined to phloem vessels
D. Occurs from roots to soil only
Explanations:
A/C/D) Incorrect.
B) Correct. Bulk flow in xylem carries water and dissolved minerals upward.

90. Which is an effect of excessive watering (waterlogged soil) on roots?

A. Enhanced root respiration due to more O₂
B. Reduced root respiration due to O₂ deficiency; roots may rot ← Correct answer: B
C. Increased photosynthesis in roots
D. Roots become leaves
Explanations:
A) Waterlogging reduces soil aeration, not increase O₂ — incorrect.
B) Correct. Lack of oxygen impairs aerobic respiration causing root damage.
C/D) False statements.

91. Which enzyme catalyzes the fixation of CO₂ in the Calvin cycle?

A. ATP synthase
B. RuBisCO ← Correct answer: B
C. Hexokinase only
D. Amylase
Explanations:
A/C/D) Not CO₂-fixing enzymes — incorrect.
B) Correct. RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes CO₂ fixation.

92. Which adaptation reduces transpiration in xerophytes?

A. Large thin leaves
B. Sunken stomata and thick cuticle ← Correct answer: B
C. Stomata on upper leaf surface only
D. No cuticle at all
Explanations:
A/C/D) Increase water loss — incorrect.
B) Correct. Xerophytic adaptations limit water loss.

93. Which of the following pathways produces ethanol in plants (and microbes) under anaerobic conditions?

A. Aerobic respiration path in mitochondria
B. Fermentation (glycolysis followed by alcoholic fermentation) ← Correct answer: B
C. Photosynthetic electron transport chain
D. Calvin cycle only
Explanations:
A/C/D) Incorrect.
B) Correct. Anaerobic glycolysis in yeast produces ethanol + CO₂ with small ATP yield.

94. Source-sink relationship in phloem translocation means:

A. Sugars only move from roots to leaves
B. Sugars move from sources (leaves) to sinks (growing parts or storage organs) ← Correct answer: B
C. Water moves from sink to source only
D. Nutrients flow randomly
Explanations:
A/C/D) False.
B) Correct. Direction depends on where sugars are produced and used/stored.

95. Which of the following is a respiratory substrate in plants?

A. Cellulose only
B. Glucose (and other carbohydrates, lipids, proteins) ← Correct answer: B
C. Oxygen only
D. Water only
Explanations:
A/C/D) Incorrect.
B) Correct. Glucose and other macromolecules are oxidized to generate ATP.

96. If stomata remain closed for a long period, which immediate effect will occur?

A. Increase in photosynthesis due to CO₂ buildup
B. Decrease in photosynthesis due to reduced CO₂ uptake ← Correct answer: B
C. Increase in transpiration
D. No change in gas exchange
Explanations:
A/C/D) Wrong.
B) Correct. Closed stomata conserve water but limit CO₂ entry, reducing photosynthesis.

97. Which molecule temporarily stores energy released during respiration for cellular use?

A. DNA
B. ATP ← Correct answer: B
C. Starch only
D. Oxygen only
Explanations:
A/C/D) Incorrect.
B) Correct. ATP is the main immediate energy currency.

98. Which statement correctly compares xylem and phloem?

A. Xylem transports sugars; phloem transports water — incorrect
B. Xylem transports water/minerals upward; phloem translocates sugars in various directions ← Correct answer: B
C. Both are composed of living sieve tubes only — incorrect
D. Phloem vessels are dead xylem vessels — incorrect
Explanations:
A/C/D) False statements.
B) Correct. Standard transport functions of vascular tissues.

99. Which of these experiments demonstrates that light is necessary for photosynthesis?

A. Place a plant in full light and observe leaf growth only
B. Keep a potted plant in the dark for a few days and observe starch absence in leaves (iodine test) ← Correct answer: B
C. Water plant daily and expect photosynthesis to stop
D. Plant in shade grows faster always
Explanations:
A/C/D) Not conclusive.
B) Correct. Iodine test after dark treatment shows lack of starch synthesis proving light necessity.

100. Which of the following best summarizes the relationship between photosynthesis and respiration in plants?

A. Photosynthesis and respiration are independent and unrelated
B. Photosynthesis stores energy in sugars during daylight; respiration releases energy from sugars day and night ← Correct answer: B
C. Respiration produces glucose; photosynthesis consumes glucose only
D. Both processes produce oxygen only
Explanations:
A/C/D) Incorrect summaries.
B) Correct. Photosynthesis synthesizes carbohydrates using light; respiration oxidizes them to supply ATP continuously. (more…)