Movement of materials across a biological membrane may occur either actively or passively and may involve different mechanisms depending on the permeability of the membrane to a particular material

Passive Transport

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

• Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)
  

There are three main types of passive transport:

• Simple diffusion – movement of small or lipophilic molecules that can freely cross the bilayer
  

• Osmosis – movement of water molecules across the bilayer (dependent on solute concentrations)
  

• Facilitated diffusion – movement of large or charged molecules via transmembrane proteins
  

Active Transport

Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

• Because materials are moving against the gradient, it does require the expenditure of energy (ATP hydrolysis)
  

There are two main types of active transport:

• Primary active transport – A molecule is moved against its gradient using energy from the hydrolysis of ATP
  

• Secondary active transport – A molecule is moved against its gradient coupled to another molecule moving  down an electrochemical gradient (cotransport)
  

Types of Transport