What characterizes selective permeability in cell membranes?

Selective permeability in cell membranes refers to the ability of the membrane to regulate the passage of substances in and out of the cell. This characteristic is crucial for maintaining homeostasis and for the function of the cell.

The correct answer indicates that the cell membrane permits certain molecules to pass while restricting others. This selective process occurs due to the specific structure of the membrane, which is composed primarily of a phospholipid bilayer embedded with proteins. These proteins play key roles as channels or transporters that facilitate the movement of specific ions, nutrients, and waste products based on size, charge, and solubility.

By allowing specific substances to enter or exit, the cell membrane helps maintain an internal environment that is conducive to the cell's needs, whether it involves the uptake of nutrients or the expulsion of waste.

The other options do not accurately represent the nature of selective permeability. Allowing all substances to pass freely would disregard the regulatory function of the membrane, while only permitting water to pass ignores the diverse range of substances, such as ions and larger molecules, that can also be selectively transported. Lastly, preventing any substances from entering the cell contradicts the fundamental needs of a living cell to interact with its environment for nutrients and communication.