Question

1. which element is found in all proteins?

○ iron
○ phosphorus
○ sulfur
○ nitrogen

Explanation:

Proteins are made of amino acids. Each amino acid has an amino group (-NH₂) which contains nitrogen. Iron is in some proteins (like hemoglobin) but not all. Phosphorus is in nucleic acids more, not all proteins. Sulfur is in some amino acids (like cysteine) but not all proteins. Nitrogen is a key element in the peptide bonds and amino acid structure, so it's in all proteins.

Answer:

D. Nitrogen (Note: Assuming the options are labeled A - D as Iron (A), Phosphorus (B), Sulfur (C), Nitrogen (D))