Final answer:
Plasma membranes are selectively permeable, which allows them to regulate the passage of substances, permitting some while restricting others based on size, polarity, and other factors due to their amphipathic nature.
Step-by-step explanation:
Plasma membranes are specialized structures that encase cells, allowing them to maintain a distinct internal environment. The characteristic that enables plasma membranes to allow certain substances to pass through while blocking others is known as selective permeability. This essential feature means that plasma membranes are selectively permeable - they enable some substances to pass freely, typically those that are small and nonpolar, but restrict or prevent the passage of others, especially larger or charged molecules. For instance, lipid-soluble materials and small gases like oxygen and carbon dioxide can pass through the hydrophobic lipid core of the membrane with ease, while polar substances and ions may require specific transport proteins or other mechanisms to assist their movement across the barrier.
Due to the amphipathic nature of plasma membranes, having both hydrophilic and hydrophobic regions, the flow of substances across the membrane can be highly regulated. The capacity for selective permeability is vital for cellular function, as it allows cells to uptake necessary nutrients, expel waste products, and maintain a balance of electrolytes, all crucial for sustaining life processes.