Examples of Secondary Metabolites (Alkaloids, Terpenoids, Toxins)
Biology · Biomolecules · NEET
Secondary metabolites are special plant products with no direct role in growth. NCERT Table 9.3 groups them by class: alkaloids (morphine, codeine, nicotine), terpenoids (monoterpenes, diterpenes), toxins (abrin, ricin), lectins (concanavalin A), drugs (vinblastin, curcumin), pigments (carotenoid, anthocyanin), and polymeric substances (rubber, gums). Memory hook: "A-Ter-Tox-Lec-Dru-Pig" = Alkaloid, Terpenoid, Toxin, Lectin, Drug, Pigment.
NCERT Table 9.3 classes of secondary metabolites with their standard examples. Note the red Toxins box (abrin, ricin) is separate from the yellow Drugs box (vinblastin, curcumin) — the exact pairing NEET tests.
Your doubts, answered
Is ricin a toxin or a drug in NCERT Table 9.3?
Ricin is a TOXIN, not a drug. NCERT lists abrin and ricin together under the class 'Toxins'. The 'Drugs' class contains vinblastin and curcumin. NEET 2021 marked 'Drugs - Ricin' as the incorrect pair for exactly this reason. Do not confuse them.
Which class does Concanavalin A belong to?
Concanavalin A is a LECTIN. NCERT gives it as the single example of the 'Lectins' class in Table 9.3. It is not an alkaloid, not an essential oil, and not a pigment. NEET 2019 asked this directly and the answer was 'a lectin'.
Is rubber a secondary metabolite?
Yes. Rubber and gums are listed as 'Polymeric substances' among secondary metabolites in NCERT. But amino acids and glucose (sugars) are PRIMARY metabolites, not secondary. NEET 2021 tested this: 'Amino acids, glucose' was the pair that is NOT a secondary metabolite.
Are morphine, codeine, nicotine, strychnine and caffeine all alkaloids?
Yes, all five are alkaloids and all are secondary metabolites. NCERT names morphine and codeine in Table 9.3. Nicotine, strychnine and caffeine are alkaloids produced by plants mainly for DEFENCE against herbivores (NEET 2020 answer).
What is the difference between a terpenoid example and an alkaloid example?
Terpenoids include monoterpenes and diterpenes (built from isoprene units, often smell-related). Alkaloids include morphine and codeine (nitrogen-containing, often bitter, drug-like). In NEET matching questions: Terpenoides pairs with Diterpenes, Alkaloids pairs with Codeine.
Is curcumin a pigment or a drug?
NCERT classes curcumin under 'Drugs' (along with vinblastin), even though curcumin is coloured. Pigments in Table 9.3 are carotenoid and anthocyanin. NEET 2023 used curcumin as a drug example and lecithin as the odd one out (lecithin is a primary-metabolite lipid).
⚠️ The NEET trap ✗ Ricin is a drug because it is used in research and medicine. ✓ Ricin is a TOXIN in NCERT Table 9.3 (paired with abrin). The 'Drugs' class holds only vinblastin and curcumin. NEET 2021 flagged 'Drugs - Ricin' as the incorrect pair. 🧠 Toxin = Abrin + Ricin. Drug = Vinblastin + Curcumin. Never swap ricin into drugs.
Real NEET questions
NEET 2023 Phase 2
Match List-I with List-II. List-I: (A) Terpenoides (B) Lectins (C) Alkaloids (D) Toxins. List-II: (I) Codeine (II) Diterpenes (III) Ricin (IV) Concanavalin A. Choose the correct answer:
A · (A)-(II), (B)-(III), (C)-(I), (D)-(IV)
B · (A)-(II), (B)-(IV), (C)-(I), (D)-(III) ✓
C · (A)-(II), (B)-(IV), (C)-(III), (D)-(I)
D · (A)-(II), (B)-(I), (C)-(IV), (D)-(III)
Solution: From NCERT Table 9.3: Terpenoids include diterpenes (A-II), lectins are shown by Concanavalin A (B-IV), alkaloids include codeine (C-I), and toxins include ricin (D-III). Only option B has all four correct.
NEET 2021
Identify the incorrect pair.
A · Lectins - Concanavalin A
B · Drugs - Ricin ✓
C · Alkaloids - Codeine
D · Toxin - Abrin
Solution: Ricin is a toxin (with abrin), not a drug. The drugs listed in NCERT are vinblastin and curcumin. So 'Drugs - Ricin' is the wrong pair; the other three are correct.
NEET 2019
Concanavalin A is
A · an alkaloid
B · an essential oil
C · a lectin ✓
D · a pigment
Solution: NCERT Table 9.3 lists Concanavalin A as the example of the 'Lectins' class of secondary metabolites. So it is a lectin, not an alkaloid, essential oil, or pigment.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They have no direct role in the plant's own growth or physiology. Many act in DEFENCE (alkaloids like nicotine, strychnine, caffeine deter herbivores) and some have ecological importance. Humans use many for medicine and industry.
Is caffeine a secondary metabolite?
Yes. Caffeine is an alkaloid, and alkaloids are secondary metabolites. NEET 2020 grouped nicotine, strychnine and caffeine as alkaloids produced by plants for defence action.
Are amino acids and glucose secondary metabolites?
No. Amino acids and glucose (sugars) are PRIMARY metabolites with clear roles in normal physiology. This is a common NEET trap (NEET 2021).
What is the odd one out: anthocyanin, lecithin, curcumin, morphine?
Lecithin. It is a phospholipid (primary-metabolite membrane lipid). Anthocyanin (pigment), curcumin (drug) and morphine (alkaloid) are all secondary metabolites (NEET 2023).
