Question

Which elements are present in all proteins?
1. Carbon
2. Hydrogen
3. Oxygen
4. Nitrogen

• A. 2 and 3
• B. 1, 2 and 4
• C. 1, 3 and 4
• D. 1, 2, 3 and 4

Hint:

Think about the parts every amino acid shares: the amino group, the carboxyl group and the carbon backbone.

Answer 1

The Correct Option is D

Step 1: Understanding the Question.
Proteins are large molecules built from smaller units called amino acids joined end to end. The question asks which elements from the numbered list, carbon, hydrogen, oxygen and nitrogen, are found in every protein without exception.

Step 2: Key Fact, the Structure of an Amino Acid.
Every amino acid has the same basic backbone: a central carbon atom attached to an amino group (\(-NH_2\)), a carboxyl group (\(-COOH\)), a hydrogen atom, and a side chain that differs between amino acids.

Step 3: Detailed Explanation.
Carbon is needed because the whole backbone and side chains of amino acids are built on carbon atoms. Hydrogen is present in the amino group, the carboxyl group and in the side chains, so it appears in every amino acid. Oxygen is part of the carboxyl group (\(-COOH\)), which every amino acid carries, so it too is always present. Nitrogen is part of the amino group (\(-NH_2\)), which again every amino acid has, so nitrogen is always present as well. Some amino acids, such as cysteine and methionine, additionally contain sulfur, but sulfur is not found in all proteins, only in the ones that use these particular amino acids. Since carbon, hydrogen, oxygen and nitrogen are all part of the amino group, the carboxyl group and the carbon backbone that every single amino acid shares, all four of these elements are present in every protein, with no exceptions.
This is why option (A), which leaves out carbon and nitrogen, is incomplete. Option (B), which leaves out oxygen, ignores the carboxyl group that every amino acid must have. Option (C), which leaves out hydrogen, is not possible since hydrogen is present in every organic molecule, including the amino and carboxyl groups.

Step 4: Final Answer.
All four elements, carbon, hydrogen, oxygen and nitrogen, are present in every protein.
\[ \boxed{1, 2, 3 \text{ and } 4} \]