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Biological Molecules: Question 6

Syllabus 2.3

Multiple choice AS 1 mark

Which statement correctly describes the general structure shared by all amino acids?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

The general structure of an amino acid

Every amino acid is built around a central carbon atom, often called the alpha carbon, to which four different groups are attached:

  • An amino group (-NH2).
  • A carboxyl group (-COOH).
  • A single hydrogen atom (-H).
  • A variable R group (side chain).

The amino group, carboxyl group and hydrogen atom are identical in every amino acid. It is the R group that differs between the twenty amino acids found in proteins, giving each one different chemical properties (for example, whether it is polar, non-polar, acidic or basic), which in turn influences how a polypeptide chain folds.

Why the other options are wrong

  • B wrongly gives two carboxyl groups and no hydrogen atom; a real amino acid has one amino group, one carboxyl group, one hydrogen atom and one R group.
  • C wrongly includes a phosphate group, confusing the structure of an amino acid with that of a nucleotide (which contains a phosphate group, a pentose sugar and a nitrogenous base).
  • D wrongly omits both the carboxyl group and the hydrogen atom, leaving an incomplete structure.

Final answer

A, a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.