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Lecture 2: Membranes
No t e b o o k:
C re at e d :

Physiology
8/15/2012 10:21 PM

U p d at e d :

8/15/2012 11:33 PM

Cell Membrane: semi-permeable membrane, made of a phospholipid bi-layer
Some organelles have their own membranes inside the cell, so there can be
specialized compartments (ex: mitochondria)
properties of cell membrane allow for homeostatic environment (aka keeps correct
[ions])
Phospholipid Bilayer: also known as glycerides (more specifically diphosphoglycerides)
glycerides are a derivative of glycerol (a sugar w/ 3 Carbons) with addition of fatty
acids
The phosphate group on the head is charged creating a hydrophilic (water soluble)
region of the molecule
The fatty acid of the tail is non-polar, creating a hydrophobic region of the molecule
having a polar and non-polar region of a molecule makes that molecule, by definition,
amphipathic
Bilayer is arranged as follows:
Hydrophilic heads on outer and inner most layer
hydrophobic tails between the head layers
cholesterol is also embedded in the membrane to stabilize the fat and
increase fluidity

Fluid Mosaic Model: membranes are mobile and fluid, molecules (such as cholesterol) are
embedded in the membrane to increase fluidity
If temperatures are too low fluidity decreases
some cells such as those in hands have specialized membranes that maintain fluidity
at temperatures lower than core temperature
Some materials such as aspirin, drinking alcohol (ethanol), water and steroid hormones can
cross lipid bilayers by simple diffusion (pass through membrane from area of high
concentration to area of low concentration; a form of Passive Transport )
...
(integral proteins span the whole
membrane others don't)
allow diffusion of particles through the channels WITH NO ATP USE
Channels often let water through as well
this is how most ions get through
Carriers
Still requires NO ATP
used for molecules too large to fit through a channel (ex: glucose, amino
acids)
...

Keep in mind the molecules are moving in the same direction
still, even though one is moving down gradient and the other
moving against gradient
Countertransport (antiport): use of integral proteins to move
molecules in the opposite direction (ex: Cl- into the cell, HCO3 - out
of the cell) using NO ATP
sometimes transport of molecules against their gradients using Protein Pumps (a form of
Active Transport; REQUIRES BREAKDOWN OF ATP)
Classic example of active transport is the Sodium-Potassium Pump (aka Na+/K+
pump)
Na+ is pumped out, K+ is pumped in, both against their concentration
gradients (meaning [Na+] is higher outside the cell and the pump is pumping
Na+ out of the cell anyway; [K+] is higher inside the cell, and the pump is
pumping K+ in anyway)
Mechanism
3 Na+ and an ATP molecule bind the pump inside the cell
ATP is broken down to ADP + inorganic phosphate, simultaneously the
pump changes conformation and the 3 Na+ are released outside
This new conformation allows 2 K+ to bind with the protein; inorganic
phosphate is then released
Conformation change back to original shape occurs, releasing the 2 K+
into the cell
SUMMARY: 3 Na+ OUT
...
K+ is just the opposite of Na+
may seem stupid, but makes you never forget
ION
NAME

Concentration Inside
cell

Concentration Outside Cell (interstitial
fluid)

Concentration in
Plasma

Na+

low

high

high

K+

high

low

low

Cl-

low

high

high

low

high

protein - high

Aquaporins: Channels that let alot of water in
Plasma Membrane is not always constant in its number of Channels/carriers (depends on
what's needed/ cell type)
endocytosis: used to take proteins back into the cell and OUT OF THE MEMBRANE
Exocytosis: is used to embed proteins INTO THE MEMBRANE

Membrane Potential
All cells have an electrical potential across their membrane
inside the cell the charge is negative
voltage is 5-100 mV
This means that the potential is negative, but what makes it negative
unequal distribution of ions (see chart)
many amino acids are negatively charged and the proteins made of them are
large and stuck inside of the cell
The cell membrane's selective permeability helps maintain this
large negatively charged organic phosphates also get stuck inside (like the
proteins)
These fixed anions attract cations that can pass through channels
which accumulate at higher and higher concentrations outside of the
cell
There is huge pressure for Na+ to go into the cell, but only few
channels to let it through (negative charge inside attracts it AND Lower
[Na+] inside)
Na+/K+ pump keeps the Na+ that does seep in balanced out to keep the cell
from depolarizing (covered later) when it doesn't want to
called an electrogenic pump because it creates the electric gradient
(pumps 3 Na+ out 2 K+ in resulting in a loss of + charge every cycle
...

Raising [K+] outside cell makes it favorable for K+ to move into the cell and make the

cell more positive (depolarize)
This creates problems with muscle contrations, notably in the heart



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