To be selectively permeable means to allow some substances to pass through while blocking others, based on specific criteria such as size, charge, or chemical properties. In biology, this usually refers to cell membranes, which control what enters and leaves the cell so the cell can maintain a stable internal environment.

Key idea

A selectively permeable barrier “chooses” what can cross it instead of letting everything through freely. For example, a cell membrane may let water and certain gases move across easily but require special transport proteins or energy for ions and larger molecules.

Why it matters in cells

Selective permeability lets cells take in needed nutrients and remove wastes without losing important molecules or letting in harmful ones. This control helps the cell keep conditions inside relatively constant (homeostasis), even when the surroundings change.