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Why Does Cholesterol Lower Membrane Permeability?

By Christy Callahan ; Updated August 14, 2017

Cholesterol is a lipid, which is a waxy, fat-like substance produced by the liver and obtained through eating certain foods. Although you may only think of cholesterol in terms of heart disease and strokes, it is actually an essential part of your overall structure and health. Cholesterol is an important part of your cell membranes and the formation of certain substances in your body. Without cholesterol, your sex hormones would deplete and your cell's structure would weaken.

Cellular Transport

The cells in your body have their own tasks to perform. Some have a limited amount of functions while others may serve multiple purposes. To perform these tasks, your cells require energy. One of the most important players in your cell's energy is oxygen. Your need to breathe is actually your cells' requirement for oxygen. Molecules, such as oxygen, sodium or potassium, and solutes need to pass through the cell membrane for the cell to function. Passage, or permeability, is regulated by proteins, lipoproteins and cholesterol within the cell's membrane, which maintains a certain amount of fluidity, a state known as liquid crystal. According to the Journal of General Physiology, a membrane that is less fluid will not be as permeable to fluids or other molecules.

Cell Membrane Structure

Your body's cells may be small, but their overall importance is extremely vast. Cellular structure begins with the membrane, which is composed of two layers of phospholipids, protein and cholesterol. The phospholipids have two ends, one end that likes water, hydrophilic, and another that does not, or hydrophobic. The hydrophobic ends face each other, with the hydrophilic ends pointing outward. Proteins are scattered throughout the phospholipid layers, as are cholesterol molecules. According to Clinton Community College, cholesterol is present in the membrane in almost equal amounts to phospholipids, and plays a large role in membrane permeability.

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Function

The cell membrane functions to provide structure, fluidity and protection to the inner cell structures. It allows for the passage of certain molecules into and out of the cell. Cholesterol itself is embedded in the membrane and adds structure and a bit of solidity to the permeable membrane. Without cholesterol, more fluids and molecules could leak into and out of the cell, which could potentially interfere with its function. Cholesterol also keeps the membrane from transitioning into a crystal state at lower temperatures, allowing the cell to maintain some of its fluidity.

Membrane Cholesterol

According to the Biochemistry of Metabolism website, cholesterol's biochemical structure has a rigid ring system and a short branched hydrocarbon tail. Within a cell membrane, it is oriented so that its hydroxyl group is pointed outward, while its hydrophobic ring system is within the membrane, with the phospholipid's fatty acid tails. The fatty acid heads of the phospholipids form hydrogen bonds with cholesterol's hydroxyl group. The rigidity of cholesterol decreases the mobility of the hydrocarbon tails of the phospholipids. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states.

Evidence

In a January 2008 study published in the Journal of General Physiology, Dr. John Mathai and colleagues studied the permeability of cell membranes in an attempt to more accurately pinpoint permeability factors. Although previously thought to be affected by membrane thickness, the researchers discovered that water permeability more strongly correlates with the area of lipids within the membrane itself. Cholesterol molecules played a major role in water permeability; adding cholesterol decreased the membranes' permeability to water.

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