Plant Physiology and Biochemistry Questions and Answers

Answer: B

Free oxygen stays low because of binding of oxygen to leghemoglobin.

Nitrogenase is irreversibly inactivated by oxygen, yet the bacteroids in the nodule need oxygen for respiration to supply the ATP that nitrogen fixation demands. Leghemoglobin, a plant haemoglobin made in large amounts in infected nodule cells (giving them their pink colour), binds oxygen tightly and delivers it to the respiring bacteroids while keeping the free oxygen concentration very low.

Rhizobia are aerobic, not anaerobic, so that option is wrong. Neither the reductase nor the nitrogenase component of the nitrogenase complex removes oxygen; both are themselves damaged by it.

Answer: A

The plant is Mesembryanthemum crystallinum, the common ice plant.

The ice plant is the classic facultative CAM species. Well watered, it fixes carbon by the ordinary C₃ pathway; under salt or drought stress it induces PEP carboxylase and the other CAM enzymes and switches to opening its stomata at night, which saves a great deal of water.

Cynodon dactylon (Bermuda grass) and Eleusine coracana (finger millet, spelt Eleucine in the paper) are C₄ grasses, and Hordeum vulgare (barley) is a C₃ plant; none of them switches to CAM.

Answer: B

The true statements are Q, S.

Q is true: gibberellin synthesis begins in the plastid, where geranylgeranyl diphosphate is cyclised to ent-kaurene; the later steps take place in the endoplasmic reticulum and cytosol. S is true: wounding and herbivore attack trigger jasmonic acid signalling, which switches on genes for proteinase inhibitors.

P is false: the release of cellulase and polygalacturonase that dissolves the cell walls in abscission zones is promoted by ethylene, not abscisic acid, despite the latter's name. R is false: zeatin is an isoprenoid (adenine-type) cytokinin; the aromatic cytokinins are a separate group such as benzyladenine. So the pairs with P or R are wrong.

Answer: D

The correct statements are Q, R.

Q is correct: cell suspension cultures of Coleus blumei accumulate rosmarinic acid in high yield and are a well-known example of producing this antioxidant in culture. R is correct: hairy root cultures of Tagetes patula (French marigold), produced by Agrobacterium rhizogenes, synthesise nematicidal thiophenes.

P is false: the tropane alkaloids of Atropa belladonna, such as hyoscyamine, derive from ornithine (or arginine) through putrescine, not from tyrosine. S is false: cyanidin is made via the phenylpropanoid and flavonoid pathway from phenylalanine, through p-coumaroyl CoA and chalcone, not from cinnamaldehyde. So pairs with P or S are excluded.

Answer: A

The true statements are P, R.

P is true. Pyruvate dehydrogenase releases one CO₂ and makes one NADH; one turn of the citric acid cycle releases two more CO₂ and makes three NADH (plus one FADH₂ and one ATP or GTP). That is three CO₂ and four NADH per pyruvate. R is true: glucose 6-phosphate dehydrogenase oxidises glucose 6-phosphate (to 6-phosphogluconolactone, hydrolysed to 6-phosphogluconate), the committed first step of the oxidative pentose phosphate pathway.

Q is false: glycolysis starts from glucose (or, in plants, sucrose and starch), and fructose 6-phosphate is an intermediate. S is false: the alternative oxidase takes electrons from ubiquinol straight to oxygen, bypassing the proton-pumping complexes III and IV, which is why it releases energy as heat.

Answer: A

The bacteria gather in the blue and red regions, as in option (A).

This is Engelmann's experiment. The algae photosynthesise and release O₂ fastest where they absorb light best, and chlorophyll absorbs most strongly in the blue and red parts of the spectrum. Aerotactic bacteria swim toward high oxygen, so they pile up under the blue and red lights and stay sparse under green and yellow light, which chlorophyll largely transmits or reflects.

Option (B) puts a peak at green, the region chlorophyll absorbs least. Options (C) and (D) place peaks at green or yellow and miss the red peak, which would require photosynthesis to be driven mainly by poorly absorbed wavelengths.

Answer: D

The correct order is P-V, Q-I, R-II, S-IV, T-III.

• Myrcene is a C₁₀ acyclic monoterpene, made of two isoprene units (V).
• β-Farnesene is a C₁₅ sesquiterpene, three isoprene units, known as an aphid alarm pheromone (I).
• Amygdalin, from bitter almond and apricot kernels, is a cyanogenic glycoside that releases HCN on hydrolysis (II).
• Nicotine, from tobacco, is a pyridine alkaloid (IV).
• Luteolin is a flavone (III).

Option A swaps the two terpenes, and options B and C misplace amygdalin and nicotine as well.

Answer: A

The correct combination is P-III-a, Q-IV-d, R-I-b, S-II-c.

• PEP carboxylase fixes HCO₃^- onto phosphoenolpyruvate to give oxaloacetate (III), in the mesophyll cytosol, the first step of the C₄ cycle (a).
• Rubisco carboxylates ribulose 1,5-bisphosphate to two molecules of 3-phosphoglycerate (IV), in the chloroplast stroma, in the C₃ (Calvin) cycle (d).
• Glycolate oxidase oxidises glycolate to glyoxylate with O₂, producing H₂O₂ (I), in the peroxisome, as part of photorespiration, the C₂ cycle (b).
• Pyruvate dehydrogenase converts pyruvate, NAD⁺ and CoA to acetyl-CoA, CO₂ and NADH (II), in the mitochondrial matrix, linking glycolysis to aerobic respiration (c).

Options B, C and D attach the wrong reaction to PEP carboxylase, Rubisco or pyruvate dehydrogenase.

Answer: C

The first labelled metabolite is 3-phosphoglycerate.

In the carboxylation step of the Calvin cycle, rubisco adds \(CO_{2}\) to ribulose 1,5-bisphosphate, and the unstable six-carbon intermediate splits at once into two molecules of 3-phosphoglycerate, one of which carries the new carbon. Calvin's short-exposure experiments with algae found the label in 3-phosphoglycerate within seconds.

1,3-bisphosphoglycerate and glyceraldehyde 3-phosphate are formed from 3-phosphoglycerate afterwards, in the reduction steps. Ribulose 1,5-bisphosphate is labelled only later, when the cycle regenerates it.

Answer: A

The correct combination is S-iv.

Nitrate reductase reduces nitrate to nitrite using NAD(P)H, the first step of nitrate assimilation in plants:

\[ \begin{aligned} &\mathrm{NO_3^- + NAD(P)H + H^+} \\ &\rightarrow \mathrm{NO_2^- + NAD(P)^+ + H_2O} \end{aligned} \]

The other pairs are wrong. Phenylalanine to cinnamic acid is catalysed by phenylalanine ammonia-lyase; cinnamate-4-hydroxylase converts cinnamate to p-coumarate. Glyceraldehyde 3-phosphate to dihydroxyacetone phosphate is triose phosphate isomerase, not glycerate kinase. Glycolate oxidation to glyoxylate and H₂O₂ is glycolate oxidase in photorespiration, not PEP carboxylase. Only one pair is correct, the whole key of this multiple-select question.