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PSL201Y1 Lecture Notes - Extracellular Fluid, Carbohydrate, Endosome

Course Code
Michelle French

of 5
PSL300 Lecture 1
Cell membrane
Membrane Permeability
D epends on :
molec size
lipid solubility (ex. H20-soluble molec can’t pass)
Membrane = permeable to a substance if the substance can cross by
any means
Permeable to:
Lipid-soluble molec (ex. Fat)
so diffuse thru readily
I mpermeable (won’t let thru) to:
organic anions
large molec
anything strongly polarized ex. H20
need prot Channels or Carriers to cross
Structure Fluid-Mosaic Model
Phospholipid B ilayer
polar head (inner+outer surface):
phosphate (hydrophilic)
choline/serine or other amino acids
nonpolar (interior):
2 long fatty acid chains (hydrophobic)
1 = saturated: straight
1 = polyunsaturated: bends@kink
determines membrane properties
cholesterol embedded btwn the chains
splits into layers in freeze fracture electron microscopy
Matrix of long carbohydrate filaments fuzzy external surface
Fill extracellular space btwn cells slowing diffusion
Anchored to membrane glycolipids + glycoproteins
“glyco-” as prefix since anchored to glycocalyx
often contain hyaluronic acid, chondroitin sulfate
net (-)ve charge polarized sequesters/attracts Ca2+
important for cell identification (ex. immune responses), cell
Membrane Fluidity
Determined by lipid composition
Essential for working protein carriers and channels (allowing shape
changes for function)
Reduced by aging and poor diet
ex. Cholesterol leaky lets things thru
ex. Lack of unsaturated fats
ex. Wrong proportion of amino+fatty acids in membrane
C holesterol :
stabilizes membrane
prevents diffusion of polar molecules across it
Membrane Proteins (embedded)
structural support for anchoring to cytoskeleton in cell interior
Receptors for signaling molec in metabolic processes
Enzymes/ion pumps for active transport
Membrane-Spanning Proteins :
Cross membrane multiple times (most commonly 6 or 7)
Helical embedded portions
Loops on outer surfaces – often attached to carb chains
NH2 at one end, COOH at the other
Transmembrane Channels
For diffusion of select molec/ions
4-5 prot subunits fit snuggly together
central pore created thru membrane
“Pore Loops” of the prot molec dangle inside the channel
physical properties (from amino acids & charges in the loop)
selective shape of loops
selectivity filter, ex. for Na+ only
4th segment usually +ly charged susceptible to electrical fields
across membrane
Channels specific for H20, small nonionic molec (glycerol,
urea w/o H20)
H20 follows concentration gradient to low conc area
Narrow part w/ diameter~4 Å (~size of H20 molec)
Size exclusion
Pore specificity
Diffusion single filed
3-5 times slower than bulk water diffusion;
ex. In a glass of water (great difference)
Selectivity filter made of pore loops dangling into hoop, loops
stick up
Gated Channels
not always open, can change shape
generally not kept perpetually open
The prot components switch between 2 shapes (open pore
and blocked pore)
Factors determining shape :
1) binding of chemical agent
2) voltage across the membrane (potential difference, if pore
shifted up or down)
Ex. V oltage gated K +
channel :
if charge added to the membrane itself pushes down thru membrane on
the prot closes cytoplasm pore so K+ can no longer diffuse thru
Ex. N ormal gated :
normally open, K+ can go thru thru selectivity filter
Transporters (carriers)
Facilitated diffusion for select molec w/o use of ATP so energy
is solely from moving down/with the conc grad
ex. Large prots, Glucose, other sugars, polar molec
competition for transporters saturation is possible, depends
how fast substances diffuse across
Example transporters:
‘GLUT 1’ in brain
Choline transporter
Nucleic Acid Precursor transporter
Diff transporters for neutral, basic, acidic amino acids
M echanism :
part toggle-switch/flipping of core helices
molec attached to binding site on one side switch the
conformation released to the other side upon
release, switches to original conformation
Toggle-switch model:
↑ = Uniporter – transports 1 molec at a time
For this example, needs low concentration on outside since
goes with/down conc gradient
Co-transport/Secondary Active Transport
Specifically for Secondary Active Transport:
For sym-,anti- porters, when a substance is carried
against/up its conc grad w/o ATP catabolism
the movement of the substance against it’s conc grad is
done simultaneously
requires binding of more than 1 substance to carrier prot
powered by kinetic energy of the substance(s) moving down
its chemical(conc) gradient
some substances may move against/up the gradient due
to another moving down its
Types :
Symporter: molec transported in the same direction
Antiporter/Counter-transporter: carrier protein exchanges 1
molec for another (transport in opposite directions)
Ex. Cerebral Na+-K+-2Cl- co-transporter: energy from
only one of the sources
Ex. Capillaries, Kidney: can go in dif directions
Ex. Na+- Ca2+: exchange removes Ca2+ from heart
Na+ - H+ exchange: secretes H+ in kidney
Above example :
(both are also Secondary Active Transport)
Sym: all move into cell
Na+, Cl- move down their grads allows Dop to move
against/up its grad Dop gets concentrated in the cell
energy provided by Na+ and Cl- moving w/ their grads
Anti: transport direction is not the same for all
Na+, Cl-, K+ move down their grads allows Serotonin
to move against its grad
Above example of Secondary Active Transport :
Using the Symporter, glucose gets “free ride” w/ Na+ along
the Na+ grad to get inside
All kinetic energy provided by Na+ grad