Biological molecules

Biology · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The main chemicals of life are carbohydrates, lipids (fats and oils), proteins and DNA. All of them contain carbon, hydrogen and oxygen. What you must remember is which extra elements each one has: proteins also contain nitrogen, and DNA contains nitrogen and phosphorus.

These are large molecules built by joining many small units. Starch, glycogen and cellulose are all long chains of glucose. Proteins are chains of amino acids. A lipid is made from fatty acids and glycerol. DNA is made from nucleotides.

Each type of molecule has its own chemical test with a colour change. Learn the starting colour and the positive colour for each.

Rules to remember

Elements: carbohydrates C, H, O; lipids C, H, O; proteins C, H, O, N (some also S); DNA C, H, O, N, P.
Starch, cellulose and glycogen are made from glucose; proteins from amino acids; lipids from fatty acids and glycerol; DNA from nucleotides.
Starch: add iodine solution. Orange-brown turns blue-black.
Reducing sugars (glucose, maltose): add Benedict's solution and heat in a water bath. Blue turns green, yellow, orange, then brick red as the amount of sugar increases.
Protein: biuret test. Blue turns purple.
Lipids: shake with ethanol, then pour into water. A cloudy white emulsion forms.

Common mistake

Forgetting that Benedict's solution must be heated. Without heating in a water bath it stays blue even when glucose is present. It is the only one of the four tests that needs heat.

Worked example

A drink gives these results: iodine solution stays orange-brown; Benedict's solution turns brick red on heating; the biuret test turns purple; the ethanol test stays clear. Which nutrients does it contain?

Iodine stayed orange-brown, so there is no starch.
Benedict's turned brick red, so a reducing sugar such as glucose is present in a large amount.
Biuret turned purple, so protein is present.
No cloudy white emulsion, so there are no lipids.

Answer: Reducing sugar and protein only.

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