Plant Physiology and Biochemistry Questions and Answers

Answer: C

Delay of senescence is caused by cytokinin.

Cytokinins keep leaves green and active: they maintain protein and chlorophyll synthesis and draw nutrients to the treated tissue. In the Richmond-Lang effect (1957), a detached leaf treated with kinetin stayed green while untreated leaves yellowed. Cytokinins are used to extend the shelf life of cut flowers and vegetables.

IBA and IAA are auxins, best known for rooting and cell elongation, and gibberellins promote stem elongation and seed germination. Ethylene and abscisic acid, not these, are the hormones that hasten senescence.

Answer: D

The carrier is plastocyanin.

Plastocyanin is a small copper protein in the thylakoid lumen that carries one electron at a time, its copper switching between Cu²⁺ and Cu⁺. It takes electrons from the cytochrome b₆f complex and delivers them to P700 of photosystem I, the same role that the mobile cytochrome c plays between complexes III and IV in mitochondria. (The stem names the photosystems the other way round; electrons pass from photosystem II towards photosystem I.)

Plastoquinone is a lipid-soluble two-electron carrier, not a protein, and phycobilins and phycoerythrin are light-harvesting pigments of cyanobacteria and red algae, not electron carriers.

Answer: C

The hormone is Gibberellin.

Many long-day plants, especially rosette plants such as Hyoscyamus, flower only after bolting. Applying gibberellin makes them bolt and flower even under short days, substituting for the long photoperiod (and in some plants for the cold treatment too).

Auxin and cytokinin do not replace the photoperiodic requirement, and brassinosteroids mainly promote cell elongation and division.

Answer: A

Cytokinins (spelled Cytokinines in the paper) are Adenine derivatives.

Natural cytokinins such as zeatin and isopentenyladenine, and synthetic ones such as kinetin (6-furfurylaminopurine) and benzyladenine, are all adenine with a side chain on the N6 amino group. Kinetin was first isolated from autoclaved herring-sperm DNA, a breakdown product of its adenine.

No cytokinin is built on guanine, cytosine or thymine.

Answer: A

The hormone is Cytokinin.

Auxin from the shoot apex keeps lateral buds dormant (apical dominance). Cytokinins, made largely in the roots, promote cell division in the lateral buds and release them; applying cytokinin to a lateral bud makes it grow even with the apex intact.

Ethylene and ABA tend to inhibit bud growth, and brassinosteroids mainly promote cell elongation; none of them counteracts apical dominance in the way cytokinin does.

Answer: C

Chlorophyll-b has a formyl group in place of a methyl group.

The two chlorophylls share the same porphyrin ring with a central Mg\(^{2+}\) and the same phytol tail. They differ at one position, carbon 7 (C-7) of ring II (ring B): chlorophyll-a carries a methyl group (-CH₃), while chlorophyll-b carries a formyl (aldehyde) group (-CHO). This small change shifts chlorophyll-b's absorption peaks, letting it harvest light that chlorophyll-a absorbs poorly.

'Methyl, formyl' reverses the two. Acetyl and acetone groups are not part of this difference.

Answer: C

Artificial ripening of banana is induced by ethylene.

Banana is a climacteric fruit: ripening is triggered by a burst of ethylene production and respiration. Treating mature green bananas with ethylene gas, or with ethephon, which releases ethylene, starts the conversion of starch to sugar, softening and yellowing in ripening chambers.

Auxin promotes cell elongation and can delay some ripening processes, cytokinin delays senescence, and polyamines generally oppose ethylene action and retard ripening. None of them is used to ripen bananas.

Answer: B

The correct combination is a - 3, b - 4, c - 1, d - 2.

• Sulphur is part of the amino acid methionine (and cysteine).
• Zinc is needed to make auxin, through tryptophan, the precursor of indole-3-acetic acid; zinc deficiency causes small leaves and stunting.
• Magnesium is the central atom of chlorophyll.
• Molybdenum is a cofactor of nitrogenase (the FeMo-cofactor), and of nitrate reductase.

Each other combination breaks one of the safest pairs. Option A gives sulphur to chlorophyll, option C gives zinc to chlorophyll, and option D gives molybdenum to methionine.

Answer: A

The products are 3-phosphoglycerate and 2-phosphoglycolate.

Rubisco can add either CO₂ or O₂ to ribulose 1,5-bisphosphate. Carboxylation gives two molecules of 3-phosphoglycerate. Oxygenation splits the five-carbon sugar into one three-carbon 3-phosphoglycerate and one two-carbon 2-phosphoglycolate. The phosphoglycolate is salvaged through photorespiration in the chloroplast, peroxisome and mitochondrion, with loss of CO₂.

Two molecules of 3-phosphoglycerate is the carboxylation product. Two phosphoglycolates would need ten carbons rather than five, and glyceraldehyde 3-phosphate is made later in the Calvin cycle, not by rubisco.

Answer: D

The false statement is Phenylalanine is an intermediate in the biosynthesis of most plant alkaloids.

Alkaloids are built from several different amino acids: lysine, ornithine, tyrosine and tryptophan are the main precursors (for example nicotine from ornithine, morphine from tyrosine, the indole alkaloids from tryptophan). Phenylalanine is the starting point of the phenylpropanoid pathway that leads to lignin, flavonoids and other phenolics, not of most alkaloids.

The other statements are true. Alkaloids are by definition nitrogen-containing compounds. All terpenes are built from five-carbon isoprene units (isopentenyl diphosphate). Flavonoids have a C6-C3-C6 skeleton of 15 carbons: two aromatic rings joined by a three-carbon bridge.