Biomacromolecules

When the compounds of a living tissue are sorted by size, a clear pattern emerges, and it lines up neatly with the acid-soluble versus acid-insoluble split introduced earlier.

The size cut-off

Every compound in the acid-soluble pool shares one feature: a fairly small molecular weight, roughly in the range of eighteen to about eight hundred daltons (Da). By contrast, the acid-insoluble fraction contains only four kinds of organic compound — proteins, nucleic acids, polysaccharides and lipids — and, with the single exception of lipids, these run to molecular weights of about ten thousand daltons and above.

This size difference is the basis for dividing biomolecules into two types:

• Micromolecules (often just called biomolecules): molecular weight less than about one thousand daltons; these make up the acid-soluble pool.
• Macromolecules (biomacromolecules): the compounds of the acid-insoluble fraction.

Among the insoluble compounds, all except lipids are polymers — long molecules built by repeating smaller units.

Why lipids sit with the macromolecules

This raises a puzzle. Lipids have molecular weights that do not exceed about eight hundred daltons, so by size they belong with the small molecules — yet they end up in the acid-insoluble, macromolecular fraction. Why?

The reason is not chemical size but physical organisation. Inside a cell, lipids are not only present as free molecules; they are also assembled into structures such as the cell membrane and other membranes. When the tissue is ground up, these membranes are torn apart and the fragments reseal into tiny closed sacs called vesicles, which are not soluble in water. Because they are insoluble, the vesicles are retained along with the acid-insoluble pool and therefore appear in the macromolecular fraction. Strictly speaking, then, lipids are not true macromolecules — they simply travel with them because of the membrane pieces they form.

The overall composition of a cell

The acid-soluble pool roughly represents the composition of the cytoplasm, while the macromolecules of the cytoplasm and organelles make up the acid-insoluble fraction. Put together, the two fractions represent the entire chemical composition of a living tissue or organism. …