(c) the chemical elements that make up biological molecules

The idea

Biological molecules are built from a small set of chemical elements. The bulk of every carbohydrate, lipid, protein and nucleic acid is carbon, hydrogen and oxygen; proteins and nucleic acids additionally contain nitrogen, and nucleic acids also contain phosphorus. Sulphur appears in some amino acids, so it is present in certain proteins. A useful check is to link each element to a molecule class: C, H and O in all four classes; N in proteins and nucleic acids; P in nucleic acids (and in phospholipids); S in some proteins. In a multiple-choice question you may be asked which element is absent from a named class, or which class contains a named element. Read the option set carefully, eliminate options that add an element to a class that lacks it, and confirm the remaining choice against the class you were given.

What earns the marks

• Carbon, hydrogen and oxygen are present in carbohydrates, lipids, proteins and nucleic acids.
• Nitrogen is present in proteins and nucleic acids but is not a component of carbohydrates or triglycerides.
• Phosphorus is present in nucleic acids and in phospholipids.
• Sulphur is present in some amino acids and therefore in some proteins.
• Correctly matching an element to the class of biological molecule it occurs in, and rejecting an option that assigns an element to a class that lacks it.

Common mistakes and how to put them right

• Assuming all biological molecules contain nitrogen.

  Put it right: Carbohydrates and triglycerides contain only carbon, hydrogen and oxygen.

• Thinking phosphorus is a component of proteins.

  Put it right: Phosphorus is characteristic of nucleic acids and phospholipids, not of amino acids.

• Treating sulphur as universal in proteins.

  Put it right: Sulphur occurs only in the amino acids that contain it, such as cysteine and methionine, so only some proteins contain it.

Exam tips

• Build a quick grid of molecule class against C, H, O, N, P, S and rehearse it before the exam.
• When an option adds an element to a class, test it against a specific example such as glucose or a triglyceride.
• Eliminate two clearly wrong options first, then decide between the remaining pair using the element you are most confident about.