Understanding basic metabolic pathways in plants is essential for B.Pharm students, as these pathways underlie drug biosynthesis, herbicide action, and plant–drug interactions. This concise overview links core processes—glycolysis, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, the Calvin cycle, and photorespiration—to energy flow, ATP production, and biosynthesis of primary and secondary metabolites. Emphasis on key enzymes, regulatory steps, and metabolic control will aid comprehension of pharmacognosy and biochemical pharmacology. Familiarity with pathway intermediates, coenzymes (NADH, NADPH, FAD), and compartmentation between chloroplasts, mitochondria, and cytosol is particularly valuable. Practical understanding supports rational interpretation of MCQs and experimental data. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which enzyme catalyzes the first committed step of glycolysis in plants?
- Hexokinase
- Glucokinase
- Phosphofructokinase-1
- Pyruvate kinase
Correct Answer: Phosphofructokinase-1
Q2. In the Calvin cycle, which enzyme fixes CO2 to ribulose-1,5-bisphosphate?
- Phosphoribulokinase
- Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
- Fructose-1,6-bisphosphatase
- Sedoheptulose bisphosphatase
Correct Answer: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
Q3. Which pathway in plants generates NADPH for biosynthetic reactions and antioxidant defense?
- Glycolysis
- Oxidative pentose phosphate pathway
- TCA cycle
- Photorespiration
Correct Answer: Oxidative pentose phosphate pathway
Q4. The glyoxylate cycle is important in oil-seed germination because it:
- Produces ATP by oxidative phosphorylation
- Converts fatty acids to carbohydrates
- Fixes atmospheric nitrogen
- Generates NADPH for biosynthesis
Correct Answer: Converts fatty acids to carbohydrates
Q5. Which coenzyme is the primary electron donor for anabolic reactions in chloroplasts?
- NAD+
- NADH
- NADP+
- NADPH
Correct Answer: NADPH
Q6. In non-cyclic photophosphorylation, electrons from water are ultimately transferred to:
- FAD
- NAD+
- NADP+
- Oxygen (O2)
Correct Answer: NADP+
Q7. Which enzyme directly synthesizes ATP in chloroplasts during photophosphorylation?
- Rubisco
- ATP synthase (CF1–CF0 complex)
- Ferredoxin–NADP+ reductase
- Phosphoglycerate kinase
Correct Answer: ATP synthase (CF1–CF0 complex)
Q8. Photorespiration decreases photosynthetic efficiency because Rubisco reacts with:
- O2, producing phosphoglycolate
- CO2, producing 3-phosphoglycerate
- N2, producing nitrite
- H2O, producing glycerate
Correct Answer: O2, producing phosphoglycolate
Q9. Which pathway supplies the aromatic amino acids (phenylalanine, tyrosine, tryptophan) in plants?
- Shikimate pathway
- Methylerythritol phosphate (MEP) pathway
- Mevalonate (MVA) pathway
- Urea cycle
Correct Answer: Shikimate pathway
Q10. The main site of the TCA cycle in plant cells is the:
- Chloroplast
- Peroxisome
- Mitochondrion
- Vacuole
Correct Answer: Mitochondrion
Q11. Which enzyme converts phosphoenolpyruvate (PEP) to oxaloacetate in C4 plants during CO2 fixation?
- PEP carboxylase
- Pyruvate kinase
- PEP carboxykinase
- Malate dehydrogenase
Correct Answer: PEP carboxylase
Q12. Which product of glycolysis is the direct substrate for pyruvate dehydrogenase to enter the TCA cycle?
- Glucose-6-phosphate
- 3-Phosphoglycerate
- Pyruvate
- Lactate
Correct Answer: Pyruvate
Q13. Which inhibitor targets cytochrome c oxidase and blocks oxidative phosphorylation in plant mitochondria?
- Rotenone
- Antimycin A
- Cyanide (CN-)
- Oligomycin
Correct Answer: Cyanide (CN-)
Q14. The MEP pathway and the MVA pathway both contribute to the biosynthesis of:
- Carbohydrates
- Terpenoids/isoprenoids
- Amino acids of the shikimate family
- Fatty acids
Correct Answer: Terpenoids/isoprenoids
Q15. Which molecule is the immediate acceptor of CO2 in the Calvin cycle?
- 3-Phosphoglycerate
- Ribulose-1,5-bisphosphate (RuBP)
- Fructose-6-phosphate
- Glyceraldehyde-3-phosphate
Correct Answer: Ribulose-1,5-bisphosphate (RuBP)
Q16. In plant respiration, the malate-aspartate shuttle primarily functions to:
- Export ATP from mitochondria to cytosol
- Transport reducing equivalents (NADH) across membranes
- Import CO2 into chloroplasts
- Convert malate to pyruvate
Correct Answer: Transport reducing equivalents (NADH) across membranes
Q17. Which enzyme controls the rate-limiting step of the TCA cycle by catalyzing oxidative decarboxylation of isocitrate?
- Citrate synthase
- Aconitase
- Isocitrate dehydrogenase
- α-Ketoglutarate dehydrogenase
Correct Answer: Isocitrate dehydrogenase
Q18. In C3 plants, which condition increases photorespiration and reduces net photosynthesis?
- High CO2 concentration
- Low temperature
- High O2 to CO2 ratio (e.g., hot, dry conditions with closed stomata)
- Low light intensity
Correct Answer: High O2 to CO2 ratio (e.g., hot, dry conditions with closed stomata)
Q19. Which compound is the major mobile form of fixed carbon transported from leaves to sinks in most plants?
- Glucose
- Starch
- Sucrose
- Fructose
Correct Answer: Sucrose
Q20. Which enzyme catalyzes the committed step in the shikimate pathway leading to aromatic amino acid biosynthesis?
- DAHP synthase (3-deoxy-D-arabino-heptulosonate-7-phosphate synthase)
- AroA (EPSP synthase)
- Chorismate mutase
- Anthranilate synthase
Correct Answer: DAHP synthase (3-deoxy-D-arabino-heptulosonate-7-phosphate synthase)
Q21. Which organelle in plant cells houses fatty acid β-oxidation during seed germination?
- Peroxisome (glyoxysome)
- Chloroplast
- Mitochondrion
- Endoplasmic reticulum
Correct Answer: Peroxisome (glyoxysome)
Q22. ATP yield per molecule of NADH oxidized through mitochondrial electron transport is approximately:
- 1 ATP
- 2.5–3 ATP
- 4–5 ATP
- 0.5 ATP
Correct Answer: 2.5–3 ATP
Q23. Which enzyme regenerates ribulose-1,5-bisphosphate (RuBP) in the Calvin cycle?
- Ribulose-5-phosphate epimerase
- Phosphoribulokinase
- Triose phosphate isomerase
- Fructose-bisphosphate aldolase
Correct Answer: Phosphoribulokinase
Q24. In plants, nitrate assimilation begins with reduction of NO3- to NO2- by:
- Nitrite reductase
- Nitrate reductase
- Glutamine synthetase
- Glutamate synthase (GOGAT)
Correct Answer: Nitrate reductase
Q25. The primary role of ferredoxin in chloroplasts during the light reactions is to:
- Transport protons into the thylakoid lumen
- Reduce NADP+ to NADPH via ferredoxin–NADP+ reductase
- Synthesize ATP directly
- Fix CO2 in the Calvin cycle
Correct Answer: Reduce NADP+ to NADPH via ferredoxin–NADP+ reductase
Q26. Which enzyme catalyzes the conversion of acetyl-CoA and oxaloacetate to citrate in the TCA cycle?
- Citrate synthase
- Aconitase
- Malate synthase
- Isocitrate dehydrogenase
Correct Answer: Citrate synthase
Q27. The main distinction between cyclic and noncyclic electron flow in photosynthesis is that cyclic flow:
- Produces both ATP and NADPH equally
- Generates only ATP and recycles electrons to photosystem I
- Requires water splitting at photosystem II
- Reduces NADP+ directly to NADPH
Correct Answer: Generates only ATP and recycles electrons to photosystem I
Q28. Which metabolite links glycolysis and the TCA cycle and is the substrate for pyruvate dehydrogenase?
- Acetyl-CoA
- Lactate
- Pyruvate
- Oxaloacetate
Correct Answer: Pyruvate
Q29. Phenolic secondary metabolites in plants are mainly derived from which primary metabolic pathway?
- Glycolysis
- Shikimate pathway (from aromatic amino acids)
- TCA cycle directly
- MVA pathway exclusively
Correct Answer: Shikimate pathway (from aromatic amino acids)
Q30. Which regulatory mechanism commonly controls enzyme activity in plant metabolic pathways?
- Allosteric regulation by metabolites
- Permanent covalent modification only
- Random enzyme degradation exclusively
- Temperature changes as the sole regulator
Correct Answer: Allosteric regulation by metabolites
