Biology · Biomolecules · NEET
Take any living tissue like a piece of liver or a vegetable and grind it in trichloroacetic acid (Cl3CCOOH). Strain the thick slurry through cheesecloth. The liquid that passes through is the filtrate, also called the acid-soluble pool. It holds thousands of small organic molecules with molecular weight from 18 to about 800 daltons. It roughly shows the composition of the cytoplasm.
The acid-insoluble fraction is the retentate, the part that stays back on the cloth after straining. NCERT says it has only four types of organic compounds: proteins, nucleic acids, polysaccharides and lipids. These are the macromolecules of the cell. So while the soluble pool has thousands of small molecules, the insoluble part has just these four big classes.
Trichloroacetic acid is used to grind the tissue because it breaks open the cells and makes the big macromolecules clump together (precipitate) so they do not dissolve. The small molecules stay dissolved and pass through as the filtrate. This clean split is how scientists separate micromolecules from macromolecules for chemical analysis.
Lipids are not truly large macromolecules, yet they end up in the acid-insoluble fraction. Reason: lipids are part of cell membranes. When you grind the tissue, membranes break into tiny closed pieces called vesicles. These vesicles are not water-soluble, so they get trapped with the retentate. That is why lipids sit in the macromolecular (acid-insoluble) fraction even though their own molecular weight is small.
NCERT states the acid-soluble pool roughly represents the cytoplasmic composition. The macromolecules from the cytoplasm and organelles become the acid-insoluble fraction. Together, both fractions represent the whole tissue or cell.
Acid-soluble pool compounds: molecular weight 18 to around 800 daltons (micromolecules, below 1000 Da). Acid-insoluble macromolecules: molecular weight in the range of ten thousand daltons and above, except lipids which are smaller but still land here because of membrane vesicles.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The acid-soluble pool has far more variety, thousands of different small organic molecules. The acid-insoluble fraction has only four classes of macromolecules.
Yes. The compounds of the acid-soluble pool have molecular weight below 1000 daltons and are called micromolecules. The acid-insoluble fraction holds the macromolecules.
Filtrate = acid-soluble pool (passes through the cloth). Retentate = acid-insoluble fraction (stays back on the cloth).
Strictly no. NCERT says lipids are not true macromolecules because their molecular weight is small, but they appear in the acid-insoluble (macromolecular) fraction because membrane vesicles get trapped there.
