Cholesterol's Importance to the Cell Membrane
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 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.
References
1. Alberts, et al., Molecular Biology of the Cell: Fourth Edition, New York: Garland Science, 2002, p. 588.
2. ibid, 589F.
3. ibid, 589F.
4. ibid, 588.
5. ibid, 588.
6. ibid, 589-590.
7. ibid, 590.