(05) Structure and Function of Cell Membrane Flashcards
model used to describe membranes
fluid mosaic model
sea of lipids, proteins float like icebergs
composition of PM
50% lipid, 50% protein
Name the three types of lipids involved in the plasma membrane
Cholesterol / Glycolipids scattered amongst double row of phospholipid (75%) molecules
molecular architecture of PM
made of amphipathic phospholipids
polar heads outside, non-polar tails form hydrophobic core that is highly impermeable to the passage of charged ions
factors affecting the fluidity of PM
lipid tail length (longer=more structured=less fluid)
# double bonds (more double bonds = more fluid)
amount of cholesterol (more decreases fluidity)
two types of membrane proteins + describe
integral / transmembrane: amphipathic, hydrophobic regions (usually non-polar amino acid helices) in hydrophobic bilayer core, hydrophilic ends interact w/ aq slm
N-terminus on extracellular side, C-ter,omis on cytoplasmic side
peripheral: attached to inner/outer surface
list the functions of membrane proteins
receptors
cell identity markers
linkers
enzymes
ion channels
transporter proteins
what is the lipid bilayer permeable to
Nonpolar, uncharged molecules: O2
Lipid soluble molecules: steroids, fatty acids
small uncharged polar molecules: H2O, Urea, CO2
what is the lipid bilayer impermeable to
Large uncharged polar molecules
eg. Glucose / amino acids
charged ions
what factors do the rate of diffusion depend on
greater difference in concentration = faster
higher temp = faster
surface area available for exchange
larger size of diffusing substance = slower
larger distance of diffusion = slower
describe electrochemical gradients
chemical = conc grad
electrical = ions influenced by membrane potential AS WELL AS concentration gradient
relative extracellular vs cytoplasmic concentrations of Na+
HIGH outside cell
LOW inside cell
relative extracellular vs cytoplasmic concentrations of K+
LOW outside the cell
HIGH inside cell
relative extracellular vs cytoplasmic concentrations of Cl-
same as Na+
HIGH outside, LOW inside
describe osmosis
net movement of water through selectively permeable membrane from high H2O conc to low conc
water moves to eliminate osmotic gradient
“equation” for membrane permeability to water
Pw = Pd + Pf
Permeability to water = Pd (through lipid bilayer) + Pf (through water channel)
what are water channels mediated by
aquaporins
there are 9 isoforms
cells have different Pw bc they express different isoforms
what does the permeability of a molecule depend on
size
charge
lipid solubility
Cells use about 30% of resting energy to…
maintain concentration and electrical gradients
define osmotic pressure
the hydrostatic pressure applied to oppose osmosis
(a colligative property - depends only on NUMBER and not type of particles in the solution)
How can osmolarity to calculated?
from the concentration of solution and knowledge of ionisation properties
osmolarity of body fluids
about 280 mOsmol
define tonicity
the effect a solution has on cell volume
DEPENDS ON MEMBRANE PERMEABILITY OF SOLUTE
hypotonic –> swelling / lysis (haemolysis)
hypertonic –> shrinkage (crenation)
