Acid-soluble and acid-insoluble fractions.

Grind a tissue in trichloroacetic acid and strain it: each molecule drops into filtrate or retentate by size, except lipids, which ride down with membrane vesicles.

Get lecithin (~750 Da) to end up in the acid-soluble filtrate, and say which switch you had to turn off to do it.

Lipids are true macromolecules because they sit in the acid-insoluble fraction.

• Every compound in the acid-soluble pool has a molecular weight between 18 and about 800 daltons (Da).
• The acid-insoluble fraction holds only four kinds of organic compound: proteins, nucleic acids, polysaccharides and lipids. Except for lipids, these have molecular weights of ten thousand Da and more.
• Biomolecules therefore fall into two groups: micromolecules (or simply biomolecules), under one thousand Da, and macromolecules (biomacromolecules), found in the acid-insoluble fraction.
• Apart from lipids, the molecules of the insoluble fraction are polymers.
• Lipids, whose molecular weights do not go beyond 800 Da, still come down in the insoluble fraction. Grinding breaks the cell membrane and other membranes into pieces that form water-insoluble vesicles, and these separate out with the insoluble pellet. Lipids are not strictly macromolecules.
• The acid-soluble pool roughly represents the composition of the cytoplasm; the macromolecules of the cytoplasm and organelles form the acid-insoluble fraction. Together they are the whole chemical make-up of a tissue.
• Water is the most abundant chemical in living organisms. Average share of total cell mass: water 70–90%, proteins 10–15%, nucleic acids 5–7%, carbohydrates 3%, lipids 2%, ions 1%.

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