Cell & Molecular Biology

Cholesterol's
Importance to the Cell Membrane

July,
2005

by
Chris Masterjohn

Cholesterol
is Abundant in Cell Membranes

Cholesterol
is found in every cell of your body. It is especially abundant in the membranes
of these cells, where it helps maintain the integrity of these membranes, and
plays a role in facilitating cell signaling-- meaning the ability of your cells
to communicate with each other so you function as a human, rather than a pile of
cells.

Molecule
for molecule, cholesterol can make up nearly half of the cell
membrane.1 Since it is smaller and weighs less than other molecules
in the cell membrane, it makes up a lesser proportion of the cell membrane's
mass, usually roughly 20 percent.2

Cholesterol
is also present in membranes of organelles inside the cells, although it usually
makes up a smaller proportion of the membrane. For example, the mitochondrion,
the so-called "power-house" of the cell, contains only three percent cholesterol
by mass, and the endoplasmic reticulum, which is involved in making and
modifying proteins, is six percent cholesterol by mass. 3

Cholesterol
Maintains the Integrity of the Cell Membrane

Surrounding
each of our cells is a membrane called the plasma membrane. The plasma
membrane is a continuous double-layer of phospholipids, interweaved with
cholesterol and proteins. Phospholipids are composed of two fatty acids
attached to a phosphate compound as a head.

The
phosphate head is water-soluble, also called "hydrophilic" (water-loving), and
the fatty-acids are water-insoluble, or "hydrophobic" (water-fearing). Since
outside the cell is a water-containing, or aqueous, environment, and inside the
cell is also aqueous, the phosphate heads of the phospholipids face both the
cell's inside and the environment outside the cell, while the fatty acids face
the inside of the membrane.

The
membrane is fluid, and the molecules are always moving. It has about the same
consistency as olive oil.

Cholesterol
is an amphipathic molecule, meaning, like phospholipids, it contains a
hydrophilic anda hydrophobic portion. Cholesterol's hydroxyl (OH) group
aligns with the phosphate heads of the phospholipids. The remaining portion of
it tucks into the fatty acid portion of the membrane.

Because
of the way cholesterol is shaped, part of the steroid ring (the four hydrocarbon
rings in between the hydroxyl group and the hydrocarbon "tail") is closely
attracted to part of the fatty acid chain on the nearest phospholipid. This
helps slightly immobilize the outer surface of the membrane and make it less
soluble to very small water-soluble molecules that could otherwise pass through
more easily.4

Without
cholesterol, cell membranes would be too fluid, not firm enough, and too
permeable to some molecules. In other words, it keeps the membrane from turning
to mush.

Cholesterol
Helps Maintain the Fluidity of Cell Membranes

While
cholesterol adds firmness and integrity to the plasma membrane and prevents it
from becoming overly fluid, it also helps maintain its fluidity.

At
the high concentrations it is found in our cell's plasma membranes (close to 50
percent, molecule for molecule) cholesterol helps separate the phospholipids so
that the fatty acid chains can't come together and cyrstallize.5

Therefore,
cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the
consistency of the cell membrane.

Cholesterol
Helps Secure Important Proteins in the Membrane

The
plasma membrane contains many proteins that perform important functions like
channeling or pumping substances into and out of the cell, attaching to other
cells, forming borders to keep other proteins in one specific part of the cell,
communicating with nearby cells, or responding to endocrine hormones from
far-away cells.

Because
certain proteins' size or shape requires a thicker phospholipid bed to sit in,
and because certain proteins need to stick together to function properly, the
fluidity of the cell membrane, where the molecules are constantly moving
randomly, could pose a problem.

Fortunately,
the plasma membrane contains many lipid rafts where proteins are secured.
A lipid raft contains high concentrations of cholesterol and
sphingolipids-- a type of phospholipid-- containing longer and more
saturated fatty acid tails.

Because
the fatty acids are longer and more saturated (straighter), they aggregate more,
which cholesterol also helps. That part of the membrane is also thicker, making
it ideal for accommodating certain proteins.6

Since
the fatty acids in lipid rafts are longer, the phospholipids also move in sync
with the phospholipids on the other side of the membrane.

In
the rest of the membrane, the phospholipids on one side of the membrane move
independently of those on the other.7

By
stabilizing certain proteins together in lipid rafts, cholesterol is important
to helping these proteins maintain their function.

This
could range from forming blood clots or thinning blood, to allowing sugar into
your cells, to burning fat, to regulating calcium in your blood, and literally
includes, in some way, most of the functions in your body, although which
proteins exist in lipid rafts and which do not is still being researched.

It
is the proteins, after all, by which cells communicate with one another. If
cells didn't communicate with one another, you and I would be a large pile of
unrelated cells rather than the individuals that we are.