diffusion and facilitated diffusion

TRANSPORT METHODS

TRANSPORT METHODS

• PROPERTIES of cell membrane and molecules determine how they’re transported in and out of cells

• SIZE of molecule and whether it’s SOLUBLE in lipids or water

• small molecules that dissolve in lipids (hydrophobic) can pass through bilayer by diffusion (e.g. O2 and CO2)

• larger lipid soluble molecules (e.g. steroid hormones) won’t pass through

• many charged particles (hydrophilic) need specific carriers and channel proteins to make it through as they are water soluble

DIFFUSION AND CELLS

• whether a substance can pass in or out of a cell by simple diffusion or not depends on its properties

• to pass through a membrane by simple diffusion a substance needs to:

  • have a conc gradient going down

  • be small

  • be non-polar (lipid soluble)

FACILITATED DIFFUSION

• substances with strong pos or neg charges (polar/water soluble) and large molecules can’t cross the membrane by simple diffusion

• they still move passively from high to low conc using proteins

• each type of protein will only allow one particular type of molecule through depending on its shape or charge

• 2 types of intrinsic proteins involved:

  • channel proteins (pores)

  • carrier proteins

FACILITATED DIFFUSION- VIA CHANNEL PROTEINS

• channel proteins are water filled pores

• water soluble ions (e.g. Na+) and polar molecules (glucose and AAs) can diffuse through these to the opposite side of the membrane

• movement through channel protein faster than carrier

• some have a gate that controls their permeability- can opened and closed

FACILITATED DIFFUSION- VIA CARRIER PROTEINS

• carrier proteins have specific binding sites (receptors)

• water soluble molecules bind to it causing protein to change shape

• causes protein to rotate so molecule can cross membrane

FACILITATED DIFFUSION- INCREASE ROFR

• increase no. of proteins

• increase concentration gradient

• increase temp

• increase pressure

SIMPLE/FACILITATED DIFFUSION-GRAPHS

• at low concs, facilitated diffusion is faster than simple

  • bc proteins in membrane allow substances to get through membrane easily

• as conc increases the no. of available proteins becomes a limiting factor- eventually they will be at their max rate and at this point the rate of diffusion stops increasing (Vmax- max velocity)