In the context of cell surface receptors, what makes a cell responsive to a particular hormone?

• A. Expression of the specific receptor for that hormone on the cell surface (or appropriate cellular location) that can bind the hormone and initiate signaling
• B. The cell’s possession of any membrane protein capable of binding a wide range of hormones with equal affinity
• C. The hormone’s ability to diffuse through the plasma membrane, making receptors unnecessary for responsiveness
• D. High intracellular ATP levels, which alone determine whether the cell can respond to hormonal signals

Answer: (A)