2b (cell membranes) Flashcards
role of the cell membrane
acts as a barrier between cell and the environment and controls what enters and leaves the cell
what does fluid mosaic structure mean?
it shows how phospholipid molecules form a continuous bilayer
it is fluid as phospholipids are flexible and constantly moving
it is mosaic because proteins imbedded vary in shape/ size/ pattern
what does cholesterol do in membranes?
gives stability
makes membranes less fluid (bind to phospholipid tails so restrict movement)
prevents leakage of water and dissolved ions
maintain shape of animal cells
cold temp- membrane likely to fracture but cholesterol prevents it
hot temp- stop it from becoming too fluid
what do glycoproteins do in membranes?
recognition sites
help cells attach to form tissues
what do glycolipids do in membranes?
recognition sites
maintain stability of membrane
help attach cells
cell surface receptors
what are protein receptor sites?
allow other proteins to attach due to complimentary shape
send signals
what do channel proteins do?
allow water soluble ions to diffuse by facilitated diffusion
what do carrier proteins do?
bind to ions/molecules to change their shape- active transport
does a low temp increase or decrease permeability? why?
increase as ice crystals may form and pierce the membranes
but
rigid as phospholipids dont have much energy
permeability of a membrane at 0-40 degrees
partially permeable as phospholipids can move. increases as phospholipids gain more energy
does a high temp increase or decrease permeability of membrane?
increase as bilayer begins to break down
water moves in which puts pressure on membrane
channel and carrier proteins deform so cant control what enters/leaves and permeability increases
what is simple diffusion and examples of things that do this
molecules passing straight through the membrane
eg oxygen and carbon dioxide
what is diffusion?
net movement of particles from area of higher concentration to an area of lower concentration. passive
what is facilitated diffusion and examples of things that do this
large/charged particles diffusing through carrier/channel proteins
still down conc gradient
eg larger molecules (amino acids, glucose), charged particles (ions, polar molecules)
why cant larger molecules simply diffuse?
why cant charged particles simply diffuse?
too big so diffuse slowly
they are water soluble and centre centre of bilayer is hydrophobic so diffuse slowly
carrier proteins job and how they do this
move large molecules across membranes
1) large molecule attaches (diff protein for diff molecule)
2) protein changes shape
3) released on opposite side of membrane
channel proteins job
form pores in membrane for charged particles to diffuse through
diff protein for diff particle
what does rate of simple diffusion depend on?
conc gradient- higher means faster but slows over time as reach equilibrium
thickness of exchange surface- thinner means faster (shorter path to travel)
surface area- larger means faster eg microvilli increase sa by about 600x so more particles exchanged in same time
what does rate of facilitated diffusion depend on?
conc gradient- higher means faster until equilibrium reached
number of channel/ carrier proteins- once all in use, rate cant increase so more proteins means faster
what are aquaporins?
channel proteins that allow facilitated diffusion of water through cell membranes. some kidney cells have lots
they allow cell to reabsorb a lot of water otherwise excreted
what is osmosis?
(net) movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane
what is water potential? units?
potential/ likelihood of water molecules to diffuse into or out of a solution
KPa
does pure water have a high or low water potential? what happens as you add more solute?
highest water potential
it decreases/ becomes more negative
what is isotonic?
two solutions having the same water potential
what is hypertonic?
total molecular conc of dissolved solute particles is greater than that of another cell
what is hypotonic?
total molecular conc of dissolved solute particles is less than that of another solution
what does the rate of osmosis depend on?
water potential gradient- higher means faster
thickness of exchange surface- thinner means faster
surface area- larger means faster
what happens to a cell in a hypotonic solution? what is turgor?
net movement is into cell so cells w/o walls lyse
plant cells have walls so prevent breaking- turgor pressure is pressure pushing against wall
haemolysis- rbc busrts
what happens to cells in hypertonic solutions? what is plasmolysis?
water inside the cell flows out so cells lose water and cant divide/ function
plasmolysis is protosplast (everything in cell appart from wall) shrinking away from cell wall can be reversed
cells w/o cell walls shrink resulting in dehydrated cells can be reversed
what is active transport?
movement of particles from an area of lower concentration to an area of higher concentration using energy
similarity between facilitated diffusion and active transport
both use carrier proteins
differences between active transport and facilitated diffusion
against/ down conc gradient
requires energy (ATP)/ doesn’t
what are cotransporters?
type of carrier protein that binds 2 molecules at a time
the conc gradient of one is used to move other against its own conc gradient
factors affecting rate of active transport
speed of individual carrier proteins
number of carrier proteins present
rate of respiration/ availability of ATP
why does cotransport take place?
diffusion creates an equilibrium so not all available glucose/ amino acids would be able to pass out of body
what is cotransport?
indirect active transport
glucose and amino acid molecules being drawn into cells with sodium ions
describe the sodium-potassium pump
1) Na ions actively transport out of epithelial by sodium potassium pump (carrier protein)
2) maintains higher conc in lumen than the epithelial cells
3) Na ions diffuse into epithelial cell through carrier protein and carry glucose/ amino acids with them. Na diffuses, glucose/ amino acids go against conc gradient
4) glucose/ amino acids pass to blood plasma by facilitated diffusion using carrier proteins
