Final answer:
The carboxyl group is the structural group common in biomolecules and is crucial for protein structure, making it hydrophilic and enabling peptide bond formation between amino acids.
Step-by-step explanation:
The structural group common in biomolecules, especially as a building block of proteins, is the carboxyl group. The carboxyl group is hydrophilic and plays an essential role in the structure and function of proteins. It is involved in peptide bond formation, which is the link between the carboxyl group of one amino acid and the amino group of another. This carboxyl group can ionize to release hydrogen ions from the COOH group, resulting in the negatively charged COO- group which contributes to its hydrophilic nature. Carboxyl groups are found in the amino acids that make up proteins, as well as in the fatty acids that form triglycerides and phospholipids.