Final answer:
Plasma membranes are selectively permeable, allowing some substances to cross while preventing others. This selectivity is crucial for the cell's survival.
Step-by-step explanation:
Plasma membranes are selectively permeable, which means they allow some substances to cross from one side of the membrane to the other while preventing others from doing so. This selectivity is essential for the cell's survival. Without it, harmful materials could enter the cell, and essential materials could leave, leading to the cell's destruction.
For example, ions like sodium and potassium move across the plasma membrane through specific transport proteins. These proteins bind to the ions and facilitate their passage. On the other hand, larger molecules like proteins cannot freely diffuse across the membrane and require other mechanisms, such as endocytosis or facilitated diffusion, to enter or leave the cell.