Passive transport

Passive transport is the movement of molecules across the plasma membrane down the concentration gradient without the input of energy.

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) .