31. Be able to explain the difference between a cell-surface receptor and an intracellular receptor. Be able to provide examples of each. 32. Be able to describe how nitrous oxide triggers smooth muscle contraction in response to acetylcholine. 33. Understand the pathway in which steroid hormones activate a cellular response. 34. Be able to list and briefly describe the three types of cell-surface receptors. 35. Why is signaling by GTP-binding proteins and phosphorylation considered molecular switches?.