Biological Molecule

Biological molecules are the large molecules that cells build from smaller units. Cambridge O Level Biology (5090) Chapter 4 asks three separate questions about each family. Which elements? Carbohydrates and lipids contain carbon, hydrogen and oxygen; proteins also contain nitrogen; DNA also contains nitrogen and phosphorus. Which building blocks? Glucose builds starch, cellulose and glycogen; amino acids build proteins; fatty acids and glycerol build lipids; nucleotides build DNA. Which role? Starch and glycogen store energy, cellulose forms plant cell walls, proteins work as enzymes and as structural material, lipids store energy, insulate and build membranes, and DNA carries genetic information.

A food test is a laboratory procedure in which a reagent is added to a food sample and a change in colour or appearance shows whether a particular biological molecule is present. Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a hot-water bath, turns from blue through green, yellow and orange to brick-red with glucose or maltose, and a coloured precipitate forms. Biuret reagents turn from blue to lilac or purple with protein. In the ethanol emulsion test a clear mixture turns into a milky-white emulsion with lipid. Every test is qualitative: it shows presence at a detectable level, not an amount.