For a protein hormone to influence the workings of a cell, what is the essential requirement?

• A. The hormone must be present in high concentrations within the cell's interior.
• B. The hormone must bind to specific protein receptors on the cell's plasma membrane.
• C. The cell must produce its own hormones to interact with the incoming hormone.
• D. The hormone must undergo a chemical reaction within the cell's cytoplasm.

Answer: (B)