Which of the following statements is true?

Question

(a) Extracellular signal molecules that are hydrophilic must bind to a cell-surface receptor so as to signal a target cell to change its behavior.
(b) To function, all extracellular signal molecules must be transported by their receptor across the plasma membrane into the cytosol.
(c) A cell-surface receptor capable of binding only one type of signal molecule can mediate only one kind of cell response.
(d) Any foreign substance that binds to a receptor for a normal signal molecule will always induce the same response that is produced by that signal molecule on the same cell type.

Answer 1

Only choice (a) is true. A hydrophilic molecule cannot diffuse across the membrane and it can therefore only affect a cell if it binds to a cell-surface receptor [choice (a)]. Most signal molecules remain bound to the extracellular domain of the receptor, whereas the intracellular domain mediates signal transduction; although many signal molecules are endocytosed with their receptor, they remain inside membrane-bounded compartments and are therefore not transported into the cytosol [choice (b)]. A cell-surface receptor capable of binding only one type of signal molecule can stimulate more than one kind of cell response, depending on the types of intracellular signaling pathway it activates [choice (c)]. Foreign substances that bind to a receptor for a normal signal molecule can sometimes induce the same response as the natural signal molecule, but in other cases they can block the binding of the natural signal molecule without activating the receptor [choice (d)].