A cell must exchange materials with its surroundings, a process controlled by the plasma membrane. Plasma membranes are selectively permeable, regulating the cell’s molecular traffic. Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass. Through the membrane rapidly and polar molecules, such as sugars, do not cross the membrane easily.
Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel. Channel proteins called aquaporins facilitate the passage of water. Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane. A transport protein is specific for the substance it moves.
Passive transport is diffusion of a substance across a membrane
• Diffusion is the tendency for molecules to spread out evenly into the available space.
• Although each molecule moves randomly, diffusion of a population of molecules may be directional.
• At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other.