lecture 21 - structure and function of the cell membrane Flashcards
thickness of membrane
8nm
lipid bilayer structure described as? constitutes?
fluid mosaic. 50% lipids, 50% proteins (“floating in lipid sea”)
what holds proteins and lipids together?
hydrogen bonds
3 types of lipids in the bilayer
cholesterol and glycolipids scattered amongst double layer phospholipids (75%)
amphipathic
phospholipids have both a polar and a non-polar region
tails of phospholipids form?
hydrophobic core
lipids can move around in their respective __________ _________
membrane leaflet
do lipids flip flop between leaflets or nah?
very rarely (basically no) this allows leaflets to be assymetric in what they constitute.
longer lipid tail length =
less fluid membrane
more double bonds =
more fluid membrane (more space between lipids)
more cholesterol =
less fluid membrane
integral proteins
extend INTO or completely ACROSS the membrane (transmembrane protein)
peripheral proteins
associated with but NOT EMBEDDED into the membrane. Only on the surface. may be bonded to integral proteins. e.g. cytoskeleton proteins
how do integral membranes stay anchored in membrane?
hydrophobic regions span the hydrophobic core (non-polar amino acids coiled into alpha helices). the hydrophilic regions interact with the aqueous solution.
how is the maintenance of concentration gradients allowed?
by the cell membrane controlling what comes in and what doesn’t
functions of membrane proteins?
- receptors
- cell identity markers
- linkers
- enzymes
- channels
- transporters
organisation of of molecules in the membrane determines?
selective permeability
3 things lipid bilayer permeable to (alone)…
- non-polar, uncharged molecules (e.g. oxygen, nitrogen, benzene)
- lipid soluble molecules (e.g. steroids, vitamins)
- small, uncharged polar molecules (e.g. water, urea)
2 things lipid bilayer impermeable too… (due to hydrophobic core
- large uncharged polar molecules (e.g. glucose)
- Ions Na+, K+, Cl-, Ca2+, H+
diffusion
high to low areas of concentration. down concentration gradient. due to particles KE
ways to decrease rate of diffusion
lower temperature, larger molecule, smaller concentration difference, less surface area, larger distance
rate of diffusion limits cell size to…
20μm
how is O2 gradient maintained?
used up as quickly as it is provided
movement of ions influenced by…
electrochemical gradient = both concentration gradient and electrical gradient together
difference in concentrations across membrane enabled by?
selective permeability
membrane potential =
differences in charged ions across membrane forming electrical gradient. acts as a ‘capacitor’
how much energy is required to maintain electrical and chemical gradients?
~30% of resting potential
what do these electrical and chemical gradients represent?
stored energy
osmosis =
net diffusion of water across a selectively permeable membrane, from a region of high water conc. to lower water conc.
can osmosis still occur if membrane is more permeable to other CERTAIN solutes?
NO e.g. more permeable to Na+/K+ in neurons than water.. no osmosis
osmotic pressure =
opposing, hydrostatic force, preventing osmosis from equilibrating the osmotic gradient
permeability to water (Pw) =
permeability through lipid bilayer (Pd) + permeability through water channel (Pf)
what causes cells to have different permeability to water?
different aquaporin isoforms
permeability through lipid bilayer (Pd) is…
- small
- mercury insensitive
- temp. dependent (fluidity)
permeability through water channel (Pf)
- large
- sensitive to Mercury
- temp. dependent
- mediated by aquaporins
how many isoforms of aquaporins?
9
osmolarity of blood =
280 mOsmol (isosmotic with interstitial fluid)
same osmolarity =
isosmotic
solution has higher osmolarity =
hyperosmotic
solution has lower osmolarity =
hyposmotic
Tonicity is?
effect a solution has on cell volume. Dependent on permeability of membrane to solute
are osmolarity and tonicity the same thing?
NO! (remember blood lab g)
isotonic =
no change in cell volume
hypotonic solution =
higher concentration inside cell. cell swells and lyses (bursts)
hypertonic solution =
higher concentraion outside cell. cell shrinks = CRENATION
osmotic pressure can be opposed by?
hydrostatic pressure
