Facilitated Diffusion
- Certain substances cannot diffuse directly through the phospholipid bilayer of cell membranes.
- They require assistance (facilitation) to get across the membrane
- These include:
- Large polar molecules such as glucose and amino acids
- Ions such as sodium ions (Na+) and chloride ions (Cl-)
- These substances can only cross the phospholipid bilayer with the help of certain proteins
- This form of diffusion is known as facilitated diffusion
- There are two types of proteins that enable facilitated diffusion:
- Channel proteins
- Carrier proteins
- They are highly specific (they only allow one type of molecule or ion to pass through)
Channel Proteins
- Channel proteins are water-filled pores
- They allow charged substances (eg. ions) to diffuse through the cell membrane
- The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
- This allows the channel protein to control the exchange of ions
Channel Protein Diagram
A channel protein (open and closed)
Carrier proteins
- Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
- This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
- The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
- Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
- Carrier proteins are also necessary for the active transport of substances as discussed in 2.6 Membrane Transport
Carrier Protein Action During Facilitated Diffusion
A carrier protein changing shape during facilitated diffusion
Exam Tip
Remember – the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein (for example because the molecule is charged and cannot pass directly through the phospholipid bilayer) this is facilitated diffusion.
