diffusion and osmosis Flashcards
what are the mechanisms by which a substance can cross a membrane
- diffusion
- osmosis
- transport
describe ways in which small molecules can cross cell membranes
- passive diffusion
- aqueous diffusion
- faciliated diffusion
- active transport
- endocytosis
what is passive diffusion
diffusion directly through the lipid or through aquous pores formed by aquaporins that transverse the lipid bilayer - many lipid soluble molecules cross the cell membrane this way
works for lipophilic solutes - they have a diffusion coefficient which allows them to “swim” through membrane pores
what is aqueous diffusion
diffusion via a channel that transverses the plasma membrane - does NOT require energy, but does require a concentration gradient (osmosis)
what is facilitated diffusion
diffusion via specialised carrier proteins that bind the drug on one side of the bind molecule on one side of the membrane then change conformation and release on the other side - does NOT require energy, but does require a concentration gradient
what is active transport
transport via specialised carrier proteins that require energy (ATP) and can moves molecules AGAINST their concentration graident
what is endocytosis
invagination of a part of the membrane - the molecule is encase in a small vesicle then ‘release’ inside the cell
define diffusion
a net movement from a region of greater concentration to a region of lesser concentration
what must polar molecules have to cross the cell membrane
a hydration shell
rate of diffusion is ____ of diffusion of other substances but related to ____
independant
- physical proterties pf solute and solute movement (size, elec charge)
- temperature
- electric field
what does diffusion of solutes across membranes depend on?
- membrane itself (selective barrier properties)
- concentration gradient across the membrane barrier
- permeability of solute
why do drugs only act on certain tissues
because each tissue has a different permeability - this allows for drugs to only act on their target tissues/organs
what does ficks law state
rate of diffusion of a solute across a medium is proportional to the thickness of the medium, the cross-sectional area and the concentration graident across the medium
what is the diffusion coefficient
permeability/sqrt(molecular weight)
how does cross-sectional area effect diffusion
increased area available for permeation = increased diffusion rate
