BioCoach Activity

Concept 3: Selective Permeability of Membranes

Cell membranes are selectively permeable. Some solutes cross the membrane freely, some cross with assistance, and others do not cross at all.

A few lipophilic substances move freely across the cell membrane by passive diffusion. Most small molecules or ions require the assistance of specific protein carriers to transport them across the membrane. Large molecules do not cross intact cell membranes, except in certain special cases.