Lipids have a small molecular weight (not more than 800 Da), so by weight they are small molecules. But when you grind a tissue, the cell membranes break into tiny pieces called vesicles. These vesicles are not soluble in water, so they get retained in the acid-insoluble (macromolecular) fraction. Memory hook: lipids are small by weight but "trapped" in the big fraction because they hide inside insoluble membrane vesicles.
Lipids are small (under 800 Da) but their membrane vesicles are water-insoluble, so they are retained in the acid-insoluble (macromolecular) fraction instead of passing into the acid-soluble pool.
Your doubts, answered
If lipids are small (under 800 Da), why are they not in the acid-soluble pool?
By molecular weight, a single lipid molecule is small (18 to 800 Da), so on paper it looks like a micromolecule. But lipids do not float around freely inside cells. Many are built into membranes. When you grind the tissue, the membranes break into small closed bags called vesicles. These vesicles are not water soluble, so they stay behind in the acid-insoluble fraction instead of passing into the filtrate. This is why lipids come under the macromolecular fraction even though they are small molecules.
Are lipids real macromolecules like proteins and polysaccharides?
No. Proteins, nucleic acids and polysaccharides are true biomacromolecules because they are polymers, with molecular weights of ten thousand daltons and above. Lipids are found in the same acid-insoluble fraction, but only because their membranes are insoluble, not because they are large or polymeric. So NCERT says all four types are in the acid-insoluble fraction, but lipids are the exception that are not truly macromolecular by weight.
Why are lipids not polymers?
A polymer is made of many repeating small units (monomers) joined together, like amino acids in a protein. Lipids are esters of fatty acids with glycerol or another alcohol. They are not built by repeating the same monomer again and again, so they are not polymeric. This is why the NEET 2017 answer to 'which is not polymeric' is Lipids.
What is the acid-insoluble fraction exactly?
When you grind a living tissue in trichloroacetic acid and strain it, you get two parts. The filtrate is the acid-soluble pool (small molecules, 18 to 800 Da). The retentate that stays behind is the acid-insoluble fraction. It contains proteins, nucleic acids, polysaccharides and lipids. This retained part is called the macromolecular fraction.
⚠️ The NEET trap ✗ Lipids are in the macromolecular fraction because they have a very high molecular weight, above ten thousand daltons. ✓ Lipids have a small molecular weight, not more than 800 Da. They stay in the acid-insoluble fraction only because membrane lipids form water-insoluble vesicles when the tissue is ground. 🧠 NTA loves the statement 'Lipids are water soluble' or 'Lipids are macromolecules by weight'. Both are WRONG. Lipids are water-insoluble and small; they sit in the macro fraction by trickery of membranes, not by size.
Real NEET questions
NEET 2017
Which of the following are not polymeric?
A · Nucleic acids
B · Proteins
C · Polysaccharides
D · Lipids ✓
Solution: Nucleic acids (polymers of nucleotides), proteins (polymers of amino acids) and polysaccharides (polymers of monosaccharides) are all true polymeric biomacromolecules. Lipids are esters of fatty acids with glycerol or an alcohol; they are not built from repeating monomer units, so they are not polymeric. This is why lipids are the exception in the acid-insoluble fraction.
NEET 2026
Identify the correct statements about biomolecules. A. Lipids are generally water soluble. B. Proteins are polypeptides. C. Polysaccharides are long chains of sugars. D. Adenine and guanine are substituted pyrimidines. E. Almost all enzymes are proteins.
A · B, D and E only
B · B, C and E only ✓
C · A, B and C only
D · C, D and E only
Solution: A is wrong: lipids are generally insoluble in water (this insolubility is exactly why membrane lipids stay in the acid-insoluble fraction). D is wrong: adenine and guanine are purines, not pyrimidines. B (proteins are polypeptides), C (polysaccharides are long sugar chains) and E (almost all enzymes are proteins) are correct, so the answer is B, C and E only.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
By molecular weight, lipids are micromolecules (18 to 800 Da). But because membrane lipids form water-insoluble vesicles when tissue is ground, they are found in the acid-insoluble macromolecular fraction. So they are small molecules that sit in the macro fraction.
What is the molecular weight of lipids?
Lipids have molecular weights that do not exceed about 800 daltons, which is small. True macromolecules like proteins and polysaccharides are ten thousand daltons and above.
Why do lipids come in the acid-insoluble fraction?
Because many lipids are part of cell membranes. Grinding breaks membranes into vesicles that are not water soluble, so they are retained (not filtered out) and end up in the acid-insoluble fraction.
Are lipids true macromolecules?
No. Only proteins, nucleic acids and polysaccharides are true biomacromolecules (polymers, high molecular weight). Lipids share the fraction only due to membrane insolubility.
