What does it mean for a membrane to be selectively permeable?

A selectively permeable membrane is one that allows certain substances to pass through while restricting others. This property is crucial for maintaining the internal environment of a cell, as it enables the cell to control the movement of ions, nutrients, and waste products.

In the context of cell membranes, selectively permeable means that small nonpolar molecules, like oxygen and carbon dioxide, can typically diffuse freely across the lipid bilayer. However, larger or polar molecules, such as glucose or ions, require specific transport mechanisms to facilitate their movement across the membrane. This selectively allows the cell to regulate its internal conditions and respond appropriately to changes in the environment, an essential part of maintaining homeostasis.

Other responses do not accurately capture the concept of selective permeability, as some imply unrestricted passage for all molecules or focus on the size of the molecules without considering the necessity of specific transport proteins for various substances.