Movement Across Membranes

Diffusion

Spontaneous movement of molecules from areas of high to low concentration.

Osmotically Active Solutes

Molecules that cannot diffuse across a biological membrane.

Osmosis

Movement of water across a semi-permeable membrane responding to solute concentrations.

Osmosis Water Direction

Water moves from the lowest solute concentration to the highest.

Uniport

Carries only one type of molecule or ion across a membrane.

Symport

Carries two molecules or ions in the same direction.

Antiport

Carries two molecules or ions in opposite directions.

Facilitated Diffusion and Ion Channels

Transport down a concentration gradient without requiring energy.

Active Transport

Transport across a membrane that requires an expenditure of ATP.

Co-Transport

Linked active transport involving a pump and symport/antiport.