Q.45 Identify the INCORRECT statement with relation to plant secondary metabolites.
(A) Atropine is a member of indole alkaloids
(B) Limonene is a cyclic terpene found in citrus plants
(C) Green tea is rich in polyphenols
(D) Cyanidin contributes to the red color in rose petals
Option (A) is the incorrect statement. Atropine belongs to tropane alkaloids, not indole alkaloids. The other options accurately describe plant secondary metabolites.
Option Analysis
(A) Atropine is a member of indole alkaloids
Atropine is a tropane alkaloid derived from plants like Atropa belladonna, featuring a bicyclic tropane ring system from ornithine and acetate precursors. Indole alkaloids contain an indole moiety from tryptophan, with examples like reserpine or vinblastine, but atropine lacks this structure. This classification mismatch makes the statement incorrect.
(B) Limonene is a cyclic terpene found in citrus plants
Limonene is a cyclic monoterpene (C10) abundant in citrus peels, such as oranges, responsible for their aroma. Produced via the mevalonate pathway, it exemplifies terpenoids as plant secondary metabolites for defense and attraction.
(C) Green tea is rich in polyphenols
Green tea contains high levels of polyphenols, mainly catechins like EGCG, contributing to its antioxidant properties. These phenolic compounds, up to 5% by dry weight, arise from the shikimate pathway.
(D) Cyanidin contributes to the red color in rose petals
Cyanidin, an anthocyanin, produces red to magenta hues in rose petals as cyanidin-3,5-diglucoside. Its accumulation, influenced by genetics and environment, determines petal coloration intensity.
Plant secondary metabolites play key roles in plant defense, pigmentation, and human health, including alkaloids, terpenes, polyphenols, and anthocyanins. This guide analyzes a CSIR NET-style question: “Identify the INCORRECT statement with relation to plant secondary metabolites,” focusing on atropine, limonene, green tea polyphenols, and cyanidin in rose petals.
Atropine: Tropane, Not Indole Alkaloid
Alkaloids form from amino acids like ornithine or tryptophan. Indole alkaloids derive from tryptophan, featuring an indole ring (e.g., reserpine). Atropine, however, is a tropane alkaloid with a distinct bicyclic structure from Atropa belladonna, used medically as an anticholinergic. Statement (A) errs by classifying it as indole.
Limonene in Citrus: Classic Cyclic Terpene
Terpenes, the largest secondary metabolite class, arise from isoprene units via mevalonate or MEP pathways. Limonene, a monoterpene, gives citrus fruits their scent and acts as a solvent or biofuel precursor. Statement (B) holds true.
Polyphenols Abound in Green Tea
Phenolics, including polyphenols, provide antioxidants via the shikimate pathway. Green tea excels with catechins like EGCG, aiding health benefits over black tea. Statement (C) is accurate.
Cyanidin’s Role in Rose Red Pigmentation
Anthocyanins, flavonoid pigments, create red colors through vacuolar accumulation. Cyanidin derivatives dominate many rose varieties’ red petals. Statement (D) is correct.
