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There is one feature common to all those compounds found in the acid soluble pool. They have molecular weights ranging from 18 to around 800 daltons (Da) approximately.

The acid insoluble fraction has four types. They are proteins, nucleic acids, polysaccharides and lipids.

These classes of compounds with the exception of lipids have molecular weights in the range of ten thousand daltons and above. For this very reason, biomolecules found in living organisms are of two types. They are micro and macromolecules. Micromolecules have molecular weights less than one thousand Dalton. Macromolecules or biomacromolecules or biomolecules are found in the acid insoluble fraction.

Lipids - Not Macromolecules

Lipid's molecular weights that do not exceed 800 Da come under acid insoluble fraction. Lipids are indeed small molecular weight compounds and are arranged in structures like cell membranes and other membranes. When we grind a tissue, we are disrupting the cell structure. Cell membranes and other membranes are broken into pieces and form vesicles which are not water-soluble. Therefore, these membrane fragments get separated in the form of vesicles along with the acid insoluble pool in the macromolecular fraction.

The acid soluble pool represents roughly the cytoplasmic composition. The macromolecules from cytoplasm and organelles become the acid insoluble fraction. Together, they represent the entire chemical composition of living tissues or organisms.

In summary if we represent the chemical composition of living tissue from abundance point of view and arrange them class-wise, we observe that water is the most abundant chemical in living organisms.



% of the total cellular mass









Nucleic acids




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