Diffusion and facilitated diffusion

Diffusion

the net movement of a substance from a region of its higher concentration to a region of its lower concentration (passive), due to the random movement of particles (KE)

Diffusion will continue until…

equilibrium

Facilitated diffusion

the passive movement of large, polar, or charged molecules down a concentration gradient across a cell membrane via specific transmembrane proteins, without using ATP

Molecules that can move by simple diffusion are …

• small (fit between the phospholipid)

• non-polar (able to interact with the non-polar tails of the phospholipids)

Rate of diffusion depends on…

• Concentration gradient

• Temperature

• Surface area

• Properties of substance involved

factors affecting diffusion…

• steepness of concentration gradient

• temperature

• surface area

• properties of molecules and ions

substances that can’t diffuse with simple diffusion

• large, polar molecules

• ions

types of transport protein that enable facilitated diffusion…

• channel proteins

• carrier proteins

• They are highly specific, meaning that they only allow one type of molecule or ion to pass through

channel proteins

• pores that extend through the membrane from one side to another

• all charged substances (eg ions) to diffuse through the membrane

• gated: 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

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 via carrier proteins will occur down a concentration gradient