Final answer:
Transport proteins provide passages that allow a cell membrane to selectively permit the movement of specific substances as needed.
Step-by-step explanation:
Passages that permit a cell membrane to be permeable as needed are called transport proteins. These proteins are integral to the function of the plasma membrane, which is selectively permeable and allows only certain substances to pass through while preventing others. Transport across cell membranes can occur passively without energy input, through processes like diffusion, osmosis, and facilitated diffusion. In contrast, active transport processes require the expenditure of energy, often in the form of ATP, to move substances against their concentration gradient.