Robert Lue: As we have all experienced, we live in a sea of chaos, noise, and entropy.
The plasma membrane is a physical boundary
between the highly organized inside of the cell
and the chaotic external environment.
So even though there is this important need
to separate what is living from non-living,
the plasma membrane must also allow for a dynamic interplay and exchange
between the cell and its surroundings.
A plasma membrane is a feature common to every cell.
It surrounds every cell, and retains the contents of the cell.
The cell's membrane is made up of mostly lipids and proteins.
Lipids are fats like oil or butter, and the most common type
of lipid in a plasma membrane is called a phospholipid.
Phospholipids have both hydrophilic or water-loving, and hydrophobic or water
avoiding, regions in a single molecule.
Specifically their phosphate heads are water-loving,
while their too long hydrocarbon tails are water-avoiding.
Hydrophilic groups form favorable interactions
with water via their polar or charge groups.
They therefore dissolve readily in aqueous solutions.
Hydrophobic groups avoid water due to their uncharged, non-polar
hydrocarbons.
The interaction between water and the hydrocarbon tails of a phospholipid
is therefore energetically unfavorable.
And they tend to cluster together in an aqueous solution
to minimize their interaction with water.
Phospholipids hydrophilic phosphate heads
tend to partly dissolve in water, while their hydrocarbon tails pack amongst
themselves to avoid exposure to water.
The plasma membrane is a phospholipid bilayer,
in which phospholipids arrange themselves
so that the hydrophobic tails of the lipids face toward each other,
and are isolated from the surrounding aqueous medium.
While the hydrophilic head regions associate together
and face the intracellular and extracellular faces of the cell.
Hydrophobic interactions are the major driving force
in the lipid bilayer formation.
When dissolved in water, phospholipids self-assemble into a bilayer structure.
It is remarkable that something as organized as the plasma membrane
is primarily driven by spontaneous self-assembly.
It is not made by somehow being put together on a factory floor
before being delivered to the edge of the cell.
This is one of the nicest ways to underscore
the importance of self-assembly phenomena, the ability
of a well-organized and quite complex structure
to arise simply based on the chemistry of individual components.
In a lipid bilayer, there is also dynamic diffusion of the lipids
and other membrane components within the two-dimensional plane of the bilayer.
This lateral diffusion gives the membrane its fluid structure.
A fluorescence photobleaching experiment readily proves this concept,
showing that the fluorescence from a particular region of a fluorescently
labeled membrane can recover after the region is bleached by a laser beam.
Fluorescent membrane lipids or other components diffuse into the region,
and bleached membrane lipids or components
diffuse out of the region over time.
This is where the fluorescence recovery occurs.
And this is why the method is called FRAP or FRAP,
Fluorescence Recovery After Photobleaching.
Biological membranes are composed of more than just phospholipids.
They also contain many different types of membrane proteins
that are associated in different ways with a lipid bilayer.
These membrane proteins carry out many critical functions,
facilitating the dynamic processes that occur at the membrane.
In most mammalian cell's membrane proteins constitute 50% of the membrane
by mass.
There are some exceptions, such as the myelinated membranes of some neurons,
in which case lipids constitute 80% of the membrane by mass.
Membrane proteins can be classified into two groups in terms
of their association with the bilayer, integral membrane proteins
and peripheral membrane proteins.
Integral membrane proteins are tightly bound
to membrane lipids via hydrophobic forces,
and can only be separated from the membrane under denaturing conditions.
While some of the integral membrane proteins
are embedded in only one leaflet of the bilayer,
some others span both leaflets of the bilayer
and are called transmembrane proteins.
Because these proteins are designed to have large regions buried
in the hydrophobic core of the phospholipid bilayer,
they tend to be very insoluble in water, and tend
to lose their structure upon purification
from their membrane environments.
Peripheral proteins, on the other hand, interact
with the one surface of the membrane.
They can be attached to the surface of a membrane
via either electrostatic interactions or hydrogen bonds with lipid head groups.
Alternatively, they can also associate with a different integral membrane
protein.
Peripheral proteins are typically water soluble,
and can be separated from the membrane under mild conditions
that denature neither the protein nor the membrane.
One example of a peripheral membrane protein
is cytochrome c, which we will cover in more detail in coming lessons.
The wonderfully dynamic phospholipid and protein composed membrane
provides all cells with an outer boundary,
while also mediating the exchange of materials
between the interior of the cell and its external environment.
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