What is the role of cholestrol in plasma membrane

Cholesterol is required to build and maintain cell membranes; it regulates membrane permeability and fluidity over a wide range of temperatures.

The hydroxyl group on cholesterol interacts with the polar head groups of the membrane phospho- and sphingolipids, while the bulky steroid and the hydrocarbon chain is embedded in the membrane, alongside the nonpolar fatty acid chains of the other lipids.

Cholesterol also reduces the permeability of the plasma membrane to hydrogen ions (protons) and sodium ions.

Recently, cholesterol has also been implicated in cell signaling processes, where it has been suggested that it assists in the formation of lipid rafts in the plasma membrane.

Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including the caveolae-dependent endocytosis and clathrin-dependent endocytosis.

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 and a 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.