Question

In certain amino acids, which element is present in addition to carbon, hydrogen, nitrogen and oxygen?

• A. Phosphorus
• B. Zinc
• C. Calcium
• D. Sulphur

Hint:

Cysteine and methionine are the amino acids whose side chains contain sulphur in addition to the usual C, H, N and O.

Answer 1

The Correct Option is D

Step 1: Recall the basic elements in every amino acid.
Every amino acid has an amino group, a carboxyl group, a hydrogen atom, and a side chain attached to a central carbon atom. Because of this, carbon, hydrogen, nitrogen and oxygen are always present in an amino acid.

Step 2: Recall which amino acids carry an extra element.
A small group of amino acids has a side chain that also contains sulphur. The two clearest examples are cysteine, whose side chain ends in a thiol group, written as -SH, and methionine, whose side chain contains a sulphur atom linked between two carbon groups.

Step 3: Rule out the other options.
Phosphorus is not a normal part of amino acid structure. It shows up later, when certain amino acid side chains, like serine or tyrosine, get phosphorylated as part of cell signalling, but that is an added modification, not a building block of the amino acid itself.
Zinc and calcium are minerals the body needs for enzyme activity, bone strength, and nerve or muscle function, but neither one is ever built into the actual structure of an amino acid.

Step 4: Final conclusion.
The extra element found in certain amino acids, alongside carbon, hydrogen, nitrogen and oxygen, is sulphur.
\[ \boxed{\text{Sulphur}} \]