Passive transport
Passive transport is the movement of molecules across the plasma membrane down the concentration gradient without the input of energy[1], 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) [2]. 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.