Why do lipids end up in the acid-insoluble pool if they are not macromolecules? NCERT's grinding experiment confuses me
Diya Menon11th Sample
4 Sept 2026 · 1 min read
ahh I'm at the first page of Biomolecules and already confused 😅 NCERT describes this experiment where you take a living tissue (like a vegetable or a piece of liver), grind it in trichloroacetic acid with a mortar and pestle, and strain the slurry through cheesecloth. You get two portions: the filtrate and the retentate.
I understood: filtrate = acid-soluble pool, which has thousands of small organic compounds (micromolecules, molecular weight about 18 to 800 Da), like amino acids, sugars, nucleotides. Retentate = acid-insoluble fraction, which has macromolecules: proteins, nucleic acids, polysaccharides.
But NCERT says lipids come in the acid-insoluble fraction too, even though their molecular weight is not more than 800 Da, so technically they aren't macromolecules. I really don't understand why that is. How can small molecules be in the "insoluble" part?
Also, how are the cell's contents related to these two pools, like does the filtrate = cytoplasm? A simple explanation would be perfect.
7 answers & replies
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[+][–]Nisha Verma12thSample· 1mo ago
When you grind the tissue, the cell membrane and other membranous structures (organelle membranes) break into pieces and these fragments seal up into vesicles. Those vesicles don't dissolve in the acid, so they get stuck in the retentate. The vesicles are made of lipids (plus proteins), so lipids appear in the acid-insoluble pool, even though the lipid molecules themselves are small. So it's not that lipids are macromolecules, it's that membrane lipids end up as insoluble vesicles during grinding.
[+][–]Diya Menon11thSample· 1mo agoAuthor
ahh vesicles! That explains it, they aren't dissolving even though the molecules are small. Thank you nisha!
[+][–]Farhan ShaikhDropperSample· 1mo ago
Lipids: any molecule that's soluble in acid goes to the filtrate, which includes proteins, since proteins are soluble in acid too when they are small enough? Anyway, the acid soluble pool has all organic molecules with low molecular weight, including small proteins and peptides. Lipids are fats so they fall in the insoluble part.
[+][–]Rahul DubeyDropperSample· 1mo ago
Farhan, that isn't right. Proteins, being large macromolecules, stay in the retentate (acid-insoluble) with polysaccharides and nucleic acids. NCERT says the acid-insoluble fraction has only four types of organic compounds: proteins, nucleic acids, polysaccharides and lipids. The acid-soluble pool has the micromolecules (amino acids, sugars, nucleotides, and so on). Size is what decides it, not "acid reaction" in the chemical sense. Peptides? Small ones may be soluble but not what NCERT is after.
[+][–]Farhan ShaikhDropperSample· 1mo ago
yeah true, my bad. I mixed solubility with size. Proteins stay in the insoluble pool.
[+][–]Ishita GoyalDropperSample· 1mo ago
The cell link, to answer your last question. NCERT says the acid-soluble pool roughly represents the cytoplasmic composition (small molecules floating in the cytosol), whereas the macromolecules from the cytoplasm and the organelles go into the acid-insoluble fraction. Together, the two represent the composition of living tissues or cells. And in terms of elements, an analysis of the tissue shows carbon, hydrogen, oxygen and nitrogen as major elements, along with minerals found in the ash when you burn the tissue.
[+][–]Zoya Khan12thSample· 1mo ago
Numbers to remember: micromolecules have molecular weight from about 18 to 800 Da; macromolecules are generally in the order of 10,000 Da and above. Lipids are in between (they don't go over 800), so the NCERT line "not strictly macromolecules" is a tricky one for assertion-reason. Most four-option questions are asking: "which of these is NOT a macromolecule but appears in acid-insoluble fraction?", the answer: lipids.
