On this page
The Biomolecules chapter spends one short section on primary and secondary metabolites, and then moves on to proteins and enzymes. That brevity is the problem: questions on it are mostly "which of these is a secondary metabolite?", and the answer depends on knowing NCERT's exact examples and categories. This post sets them out, explains the logic behind the split, and finishes with a sorting exercise that also uses molecular weights.
Where is this in the syllabus?
The topic is section 9.2 of NCERT's Class 11 Biology Chapter 9, Biomolecules (2026-27 reprint). The NEET (UG) 2026 syllabus lists "Chemical constituents of living cells: Biomolecules-structure and function of proteins, carbohydrates, lipids, nucleic acids" and enzymes. It does not name metabolites as a separate line, but they are part of the same NCERT chapter. The CBSE Biology curriculum for 2026-27 lists Chapter 9, Biomolecules, with the same wording. Check your exam year's documents.
One oddity: NCERT's section 9.2 says that "for reasons that are given in section 9.10" these biomolecules can be called metabolites. The rationalised chapter now ends at 9.8, Enzymes, so there is no section 9.10 to look up. Do not waste revision time hunting for it.
How does NCERT find these compounds in the first place?
Section 9.1 describes the method. Grind a living tissue in trichloroacetic acid, strain the slurry, and you get two fractions: the filtrate, or acid-soluble pool, and the retentate, or acid-insoluble fraction. Thousands of organic compounds turn up in the acid-soluble pool. NCERT gives their molecular weights as roughly 18 to 800 daltons (Da).
The acid-insoluble fraction holds only four types of compound: proteins, nucleic acids, polysaccharides and lipids. All except lipids have molecular weights of ten thousand daltons and above.
What makes a metabolite primary or secondary?
NCERT's distinction:
Primary metabolites are the compounds found in animal tissues, such as amino acids, sugars, nucleotides and fatty acids. They "have identifiable functions and play known roles in normal physiological processes".
Secondary metabolites are the thousands of other compounds found when you analyse plant, fungal and microbial cells. For these, NCERT says, we do not at the moment understand the role or functions of all of them in their host organisms. Many are useful to human welfare, and some have ecological importance.
Research has filled in some of that ecology. In a review in Phytochemistry, Wink (2003)00300-5) argued that the major secondary metabolites in a plant apparently function as defence, against herbivores, microbes, viruses or competing plants, and as signal compounds that attract pollinators or seed-dispersing animals.
Which examples does NCERT give?
Table 9.3 lists these categories and examples. Learn them as pairs:
Pigments: carotenoids, anthocyanins.
Alkaloids: morphine, codeine.
Terpenoides: monoterpenes, diterpenes.
Essential oils: lemon grass oil.
Toxins: abrin, ricin.
Lectins: concanavalin A.
Drugs: vinblastin, curcumin.
Polymeric substances: rubber, gums, cellulose.
The text adds flavonoids, antibiotics, scents, gums and spices as further examples.
Worked example: sort the list
Classify each item as a primary or secondary metabolite (as NCERT uses the terms) and predict which fraction of a tissue extract it would end up in.
Glucose (C₆H₁₂O₆, about 180 Da). Primary: a sugar with a known role in respiration. Well under 800 Da, so acid-soluble pool.
Alanine (C₃H₇NO₂, about 89 Da). Primary: an amino acid. Acid-soluble.
Sucrose (C₁₂H₂₂O₁₁, about 342 Da). A sugar, so primary. Acid-soluble.
Morphine. Secondary: an alkaloid in Table 9.3.
Ricin. Secondary: a toxin in Table 9.3.
Concanavalin A. Secondary: a lectin in Table 9.3. Do not confuse it with an enzyme.
Anthocyanin. Secondary: a pigment.
Curcumin. Secondary: listed under drugs in Table 9.3.
A membrane lipid. Lipids are small, below 800 Da, yet they end up in the acid-insoluble fraction. NCERT explains why: grinding breaks membranes into vesicles that are not water soluble, so they separate with the macromolecules. NCERT adds that lipids are "not strictly macromolecules".
Insulin. A protein; NCERT lists it in Table 9.5 as a hormone. Proteins belong to the acid-insoluble fraction.
The molar masses above come from the formulas: for glucose, 6 × 12.01 + 12 × 1.008 + 6 × 16.00 ≈ 180.
Which mistakes cost the most marks?
Calling secondary metabolites "waste". NCERT says many are useful to human welfare and some have ecological roles.
Thinking secondary metabolites occur in animal tissues in NCERT's account. The text links them to plant, fungal and microbial cells.
Putting lipids in the acid-soluble pool because they are small.
Mixing up the category for an example, such as listing ricin as a lectin or curcumin as an alkaloid. Use Table 9.3's labels.
A practice routine
Day 1: Cover the examples in Table 9.3 and recall them from the category names, then the other way round.
Day 2: Write ten mixed items on cards: primary, secondary and macromolecules. Sort them and say the fraction for each.
Day 3: Explain in three sentences why lipids appear in the acid-insoluble fraction.
Day 4: Move on to proteins and enzymes, which come next in the chapter. Our post on enzymes for NEET biology covers them.
If the chemistry of these molecules is the weak point, the Fermi Concept Library covers every concept in chemistry and biology with its own lesson, so you can revise one idea and return to the chapter.
Tonight, close the book and write Table 9.3 from memory, then mark what you missed.
