The option that is NOT a function of proteins is provide barriers.
Proteins play various roles in biological systems, including:
- Making hormones (e.g., insulin)
- Speeding up reactions (as enzymes)
- Transporting substances (e.g., hemoglobin transporting oxygen)
However, providing barriers is primarily a function associated with structures like cell membranes or epithelial tissues, rather than a direct function of proteins themselves.