What does it mean when a substance is described as impermeable?

When a substance is described as impermeable, it means that it does not permit the passage of a substance or substances through it. This property is significant in various biological contexts, such as cell membranes, where certain materials must be kept out to maintain cellular integrity or to regulate the internal environment.

For instance, in biological membranes, being impermeable to certain substances helps protect the cell and maintain homeostasis. This impermeability can be crucial for preventing harmful substances from entering while still allowing essential molecules to be selectively transported.

In contrast, some materials or structures are selectively permeable, allowing specific substances to enter or exit while blocking others. This distinction highlights the importance of impermeability in maintaining separation and control within biological systems.