Facilitated Diffusion

Facilitated diffusion: the passive movement of molecules across the cell membrane via a membrane protein.

Used by molecules that can’t cross the phospholipid bilayer freely.

Aid from channel proteins and carrier proteins

Channel proteins: specific to certain ions/molecules based on size, charge or other characteristics

Only move molecules along a concentration gradient

Carrier proteins: bind to a solute and undergo a conformational change move the solute across the membrane

Often involves energy (ATP) moves against a concentration gradient

Much slower than channel proteins

The structure of channel proteins allow them to act as selective gates, enabling the passage of specific substances while blocking others

Channel proteins can be selective based on charge, size, the binding of a solute, voltage and more

E.g. potassium channels found in nerve cells

These are an example of a voltage-gated channel - they open and shut in response to transmembrane voltage

The channel is comprised of four subunits - the inner pore contains a selectivity filler at its narrowest region that restricts passage of alternative ions