Passive transport is the movement of molecules across the plasma membrane down the concentration gradient without the input of energy, therefore the concentration determines the direction and rate of transport across the membrane.
There are two types of passive transport:
- Simple diffusion - Non-polar, water, small, hydrophobic molecules can cross the membrane by simple diffusion
- Facilitated diffusion - Many molecules (polar, charged, large, and hydrophilic) can not cross the plasma membrane, so the passage of these molecules is facilitated by membrane transport proteins (channel or carrier proteins) . An advantage of facilitated diffusion is that there is potential for movement across the membrane in either direction of the concentration gradient.
A carrier that transports only one substrate during facilitated diffusion is called a uniporter.