Respiration in Plants — Practice Questions with Answers
30 free MCQs on Respiration in Plants with worked answers and explanations. Glycolysis, the Krebs cycle, the electron transport chain, fermentation, and the respiratory quotient. The cellular energy-release counterpart to photosynthesis.
Below are 30 practice questions on Respiration in Plants, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Respiration in Plants notes.
Aerobic respiration unfolds in three locations: glycolysis in the cytoplasm yields a small amount of ATP directly, the Krebs cycle in the mitochondrial matrix harvests electron carriers (NADH, FADH2), and the electron transport chain in the inner mitochondrial membrane uses those carriers to generate the bulk of the ATP, with O2 needed only at this final stage.
Easy — 10 questions
Q1.
Glycolysis occurs in the:
A Mitochondrial matrix
B Cytoplasm
C Nucleus
D Chloroplast
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Answer: B. Cytoplasm
Why: Glycolysis takes place in the cytoplasm and is common to both aerobic and anaerobic respiration.
Q2.
One molecule of glucose is broken down by glycolysis into:
A 2 molecules of pyruvic acid
B 1 molecule of CO2
C 2 molecules of glucose
D Acetyl CoA directly
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Answer: A. 2 molecules of pyruvic acid
Why: Glycolysis splits one 6-carbon glucose into 2 molecules of 3-carbon pyruvic acid, with a net gain of 2 ATP and 2 NADH.
Q3.
The Krebs cycle occurs in the:
A Cytoplasm
B Mitochondrial matrix
C Inner mitochondrial membrane
D Nucleus
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Answer: B. Mitochondrial matrix
Why: The Krebs cycle (citric acid cycle) takes place in the mitochondrial matrix, using acetyl CoA derived from pyruvate.
Q4.
The Electron Transport Chain (ETC) is located in the:
A Mitochondrial matrix
B Inner mitochondrial membrane
C Outer mitochondrial membrane
D Cytoplasm
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Answer: B. Inner mitochondrial membrane
Why: The ETC is located in the inner mitochondrial membrane, where most ATP is generated via oxidative phosphorylation.
Q5.
Alcoholic fermentation by yeast converts pyruvate into:
A Lactic acid
B Ethanol and CO2
C Citric acid
D Acetyl CoA
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Answer: B. Ethanol and CO2
Why: In alcoholic fermentation, yeast converts pyruvate into ethanol and CO2 under anaerobic conditions, regenerating NAD+ for glycolysis to continue.
Q6.
The approximate net ATP yield from the complete aerobic respiration of one glucose molecule is:
A 2 ATP
B 4 ATP
C 36-38 ATP
D 100 ATP
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Answer: C. 36-38 ATP
Why: Complete aerobic respiration (glycolysis + Krebs cycle + ETC) yields approximately 36-38 ATP per glucose, far more than glycolysis alone (2 ATP).
Q7.
Respiratory Quotient (RQ) is defined as:
A O2 absorbed / CO2 released
B CO2 released / O2 absorbed
C ATP produced / O2 absorbed
D Glucose used / CO2 released
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Answer: B. CO2 released / O2 absorbed
Why: RQ = volume of CO2 released / volume of O2 absorbed. RQ=1 for carbohydrates, less than 1 for fats, greater than 1 for organic acids/proteins.
Q8.
Fermentation, an anaerobic process, yields how much net ATP per glucose molecule compared to aerobic respiration?
A Considerably more ATP than aerobic respiration gives
B Hardly any ATP is produced during fermentation
C Roughly the same amount of ATP as aerobic respiration
D Considerably less ATP than aerobic respiration gives
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Answer: D. Considerably less ATP than aerobic respiration gives
Why: Fermentation yields only 2 ATP per glucose molecule (from glycolysis), far less than the much larger ATP yield from complete aerobic respiration.
Q9.
The breakdown of glucose into pyruvate, the first step common to both aerobic and anaerobic respiration, is called:
A Krebs cycle, occurring in the mitochondrial matrix
B Glycolysis, occurring in the cytoplasm
C Oxidative phosphorylation, at the inner membrane
D Beta oxidation, breaking down fatty acids
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Answer: B. Glycolysis, occurring in the cytoplasm
Why: Glycolysis is the universal first pathway of respiration, breaking down one glucose molecule into two molecules of pyruvate in the cytoplasm.
Q10.
In lactic acid fermentation, pyruvate is reduced directly to lactic acid using:
A ATP generated by the electron transport chain
B NADH produced earlier during glycolysis
C CO2 released from the Krebs cycle reactions
D Oxygen drawn in from the surrounding air
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Answer: B. NADH produced earlier during glycolysis
Why: In lactic acid fermentation, pyruvate accepts electrons from NADH (generated during glycolysis) and is reduced to lactic acid, regenerating NAD+ for glycolysis to continue.
Medium — 10 questions
Q11.
Pyruvate is converted to Acetyl CoA before entering the Krebs cycle, releasing:
A O2
B CO2
C ATP only
D Glucose
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Answer: B. CO2
Why: Pyruvate oxidative decarboxylation: pyruvate (3C) loses one carbon as CO2 and is converted to acetyl CoA (2C), which then enters the Krebs cycle by combining with oxaloacetate.
Q12.
Lactic acid fermentation occurs in muscle cells when:
A Mitochondrial electron transport chains are saturated with surplus oxygen
B Oxygen supply is insufficient (anaerobic conditions/oxygen debt)
C Glycogen stores are fully depleted after prolonged starvation
D Cytoplasmic ATP concentration exceeds the cell's regulatory threshold
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Answer: B. Oxygen supply is insufficient (anaerobic conditions/oxygen debt)
Why: Under oxygen debt (vigorous exercise), muscle cells convert pyruvate to lactic acid to regenerate NAD+ for glycolysis to continue, without needing O2.
Q13.
A germinating seed respiring purely on stored fat would show an RQ:
A Equal to 1
B Less than 1
C Greater than 1
D Exactly 0
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Answer: B. Less than 1
Why: Fats are more reduced (less oxidised) than carbohydrates, so more O2 is needed relative to CO2 released during their oxidation, giving RQ < 1.
Q14.
Each NADH produced during respiration contributes approximately how many ATP via the electron transport chain?
A 1 ATP
B 2.5-3 ATP
C 10 ATP
D 0 ATP
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Answer: B. 2.5-3 ATP
Why: Each NADH yields roughly 2.5-3 ATP via the ETC (chemiosmotic oxidative phosphorylation), while each FADH2 yields roughly 1.5-2 ATP, since FADH2 enters the chain at a later, lower-energy point.
Q15.
Glycolysis, the first stage of respiration, takes place in which part of the cell?
A Cytoplasm
B Nucleus
C Mitochondrial matrix
D Inner mitochondrial membrane
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Answer: A. Cytoplasm
Why: Glycolysis occurs in the cytoplasm (cytosol), breaking down glucose into two molecules of pyruvate before further oxidation in the mitochondria.
Q16.
The net ATP yield from glycolysis (glucose to pyruvate) is:
A 4 ATP
B 2 ATP
C 6 ATP
D 8 ATP
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Answer: B. 2 ATP
Why: Glycolysis uses 2 ATP for activation steps and produces 4 ATP, giving a net gain of 2 ATP per glucose molecule, along with 2 NADH and 2 pyruvate.
Q17.
In anaerobic conditions, yeast cells convert pyruvate into:
A Acetyl CoA and water
B Glucose and oxygen
C Ethanol and carbon dioxide
D Lactic acid only
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Answer: C. Ethanol and carbon dioxide
Why: Yeast carries out alcoholic fermentation, converting pyruvate into ethanol and CO2 in the absence of oxygen, regenerating NAD+ needed to sustain glycolysis.
Q18.
Which step of the Krebs cycle directly combines a 2-carbon compound with a 4-carbon compound to begin the cycle?
A Citrate combining with NADH to form isocitrate
B Acetyl CoA combining with oxaloacetate to form citrate
C Pyruvate combining with CO2 to form oxaloacetate
D Succinyl CoA combining with fumarate to form malate
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Answer: B. Acetyl CoA combining with oxaloacetate to form citrate
Why: The Krebs cycle begins when the 2-carbon acetyl CoA combines with the 4-carbon oxaloacetate to form the 6-carbon citrate, catalyzed by citrate synthase.
Q19.
During aerobic respiration, the role of oxygen is to:
A Activate the enzyme hexokinase to begin glucose breakdown under most conditions studied
B Provide carbon atoms that are incorporated into ATP in standard reference material
C Act as the final electron acceptor at the end of the electron transport chain
D Directly combine with glucose in the cytoplasm during glycolysis as widely reported
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Answer: C. Act as the final electron acceptor at the end of the electron transport chain
Why: In aerobic respiration, oxygen serves as the final electron acceptor of the electron transport chain, combining with electrons and protons to form water, allowing the chain to continue functioning.
Q20.
A respiratory substrate that gives a respiratory quotient (RQ) of exactly 1 is most likely:
A A protein rich in nitrogen
B A fat such as tripalmitin
C A carbohydrate such as glucose
D An organic acid such as oxalic acid
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Answer: C. A carbohydrate such as glucose
Why: Carbohydrate respiration consumes O2 and releases CO2 in equal volumes, giving an RQ of 1, since carbohydrates already contain oxygen in the same ratio needed for complete oxidation.
Hard — 10 questions
Q21.
Respiration is described as an 'amphibolic pathway' because:
A It mainly channels every carbon atom from glucose toward ATP generation, with little diversion permitted regardless of the cell's current biosynthetic demand for amino acid or lipid precursors
B Its intermediates (pyruvate, acetyl CoA, oxaloacetate, etc.) serve as starting points for biosynthesis of fatty acids and amino acids, not just energy release
C It is physically split between the cytoplasm for glycolysis and the mitochondrial matrix for the Krebs cycle, a spatial separation that defines compartmentalization rather than its amphibolic character
D It can be run largely in reverse to regenerate glucose directly from carbon dioxide using the same enzymes and without any net energy input required to drive the reaction
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Answer: B. Its intermediates (pyruvate, acetyl CoA, oxaloacetate, etc.) serve as starting points for biosynthesis of fatty acids and amino acids, not just energy release
Why: Amphibolic = both catabolic and anabolic. Glycolysis/Krebs cycle intermediates are diverted into biosynthetic pathways for fatty acids, amino acids, and other biomolecules, not solely used for ATP production.
Q22.
In the electron transport chain, the final electron acceptor is:
A NAD+, regenerated for further use in glycolysis
B FAD, embedded in Complex II of the inner membrane
C Oxygen, reduced to water
D Pyruvate, shuttled into the mitochondrial matrix
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Answer: C. Oxygen, reduced to water
Why: O2 is the terminal electron acceptor of the ETC, reduced to water at Complex IV (cytochrome c oxidase). Without O2 as an acceptor, the chain backs up and aerobic respiration halts.
Q23.
The chemiosmotic mechanism of ATP synthesis in mitochondria is functionally analogous to:
A The semi-conservative unwinding and copying of the double helix by DNA polymerase
B The proton gradient mechanism in chloroplast thylakoids during photosynthesis
C The stepwise cytoplasmic breakdown of glucose into pyruvate
D The carrier-mediated movement of glucose against its concentration gradient
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Answer: B. The proton gradient mechanism in chloroplast thylakoids during photosynthesis
Why: Both mitochondrial oxidative phosphorylation and chloroplast photophosphorylation use a proton gradient across a membrane to drive ATP synthase — the same fundamental chemiosmotic principle, just in different organelles and directions.
Q24.
Substrate-level phosphorylation, in contrast to oxidative phosphorylation, refers to ATP synthesis that occurs:
A Generally during the light reactions of photosynthesis in chloroplasts according to standard texts in general clinical practice
B Directly during a reaction step of glycolysis or the Krebs cycle, without involving the electron transport chain
C Mainly within the mitochondrial inner membrane through ATP synthase activity under most conditions studied
D Mostly after oxygen acts as the final electron acceptor at the chain's end in most observed cases under typical physiological conditions
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Answer: B. Directly during a reaction step of glycolysis or the Krebs cycle, without involving the electron transport chain
Why: Substrate-level phosphorylation generates ATP directly from a high-energy intermediate during specific glycolysis and Krebs cycle reactions, distinct from the chemiosmotic ATP synthesis of oxidative phosphorylation.
Q25.
The complete oxidation of one glucose molecule via aerobic respiration yields a much higher net ATP than fermentation mainly because:
A Aerobic respiration fully oxidizes glucose to CO2 and water, extracting far more energy via the electron transport chain
B Aerobic respiration skips glycolysis largely, conserving more energy in typical laboratory settings under usual circumstances
C Fermentation occurs mainly in the mitochondrial matrix, limiting enzyme access under normal conditions as generally observed
D Fermentation produces additional CO2 that aerobic respiration does not release as frequently documented in most reference accounts
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Answer: A. Aerobic respiration fully oxidizes glucose to CO2 and water, extracting far more energy via the electron transport chain
Why: Aerobic respiration completely oxidizes glucose to CO2 and H2O, with most ATP generated through the electron transport chain, whereas fermentation only partially oxidizes glucose, yielding far less ATP.
Q26.
Cyanide inhibits aerobic respiration mainly by:
A Blocking cytochrome oxidase, the terminal enzyme of the electron transport chain
B Competing with glucose for the active site of hexokinase in glycolysis according to most studies
C Inhibiting ATP synthase directly at the F0 proton channel as widely reported
D Preventing pyruvate from entering the mitochondrial matrix in the majority of documented cases
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Answer: A. Blocking cytochrome oxidase, the terminal enzyme of the electron transport chain
Why: Cyanide binds and inhibits cytochrome c oxidase (Complex IV), the terminal enzyme of the electron transport chain that transfers electrons to oxygen, halting ATP production via oxidative phosphorylation.
Q27.
In the Krebs cycle, the step converting succinate to fumarate is notable because it is the only step where:
A ATP is generated directly through substrate-level phosphorylation in most observed cases under typical physiological conditions
B The enzyme involved sits in the inner mitochondrial membrane and feeds electrons to the transport chain
C Acetyl CoA condenses with oxaloacetate to begin the cycle according to standard texts in general clinical practice
D Carbon dioxide is released as a byproduct of the reaction in standard reference material under most conditions studied
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Answer: B. The enzyme involved sits in the inner mitochondrial membrane and feeds electrons to the transport chain
Why: Succinate dehydrogenase, unlike other Krebs cycle enzymes, is embedded in the inner mitochondrial membrane and directly passes electrons to FAD, linking the cycle to the electron transport chain as Complex II.
Q28.
A respiratory quotient (RQ) greater than 1 is typically observed when a plant tissue is respiring substrates that are:
A Proteins mainly, since amino acid oxidation usually yields RQ above 1 in typical laboratory settings
B Fats, since their high hydrogen content raises the RQ above 1 under normal conditions as generally observed
C Pure carbohydrates, since glucose oxidation usually gives an RQ above 1 as frequently documented in most reference accounts
D Organic acids, which are more oxidized than carbohydrates and release relatively more CO2 per O2 consumed
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Answer: D. Organic acids, which are more oxidized than carbohydrates and release relatively more CO2 per O2 consumed
Why: Organic acids are already partially oxidized, so their respiration releases relatively more CO2 per O2 consumed, giving an RQ greater than 1; pure carbohydrate respiration gives RQ = 1, and fat/protein respiration gives RQ less than 1.
Q29.
Mitochondria are described as semi-autonomous organelles mainly because they:
A Replicate mainly when directed by chloroplast DNA in plant cells according to most studies
B Contain their own DNA and ribosomes, enabling synthesis of some of their own proteins
C Can move independently between cells without any external signal under usual circumstances
D Function largely independently of any nuclear-encoded proteins in the majority of documented cases
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Answer: B. Contain their own DNA and ribosomes, enabling synthesis of some of their own proteins
Why: Mitochondria possess their own circular DNA and ribosomes, allowing them to synthesize some of their own proteins, though they still depend on the nucleus for many other proteins, hence 'semi-autonomous'.
Q30.
During oxidative decarboxylation of pyruvate, the pyruvate dehydrogenase complex requires several cofactors. Which of the following is essential for this multi-step reaction?
A Coenzyme A, NAD+, and thiamine pyrophosphate acting together
B Water alone, since the reaction is a straightforward hydrolysis
C ATP alone, since the reaction is a simple phosphorylation step
D Oxygen alone, since this is where O2 is first consumed in respiration
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Answer: A. Coenzyme A, NAD+, and thiamine pyrophosphate acting together
Why: The pyruvate dehydrogenase complex uses multiple cofactors, including thiamine pyrophosphate, coenzyme A, and NAD+, to convert pyruvate into acetyl CoA, releasing CO2 in the process.