The term lipid describes any of the compounds created by living organisms that resist reaction with water. The group is comprised of fats, hormones, oils and membranes. Fats and oils serve as energy storage and insulation, hormones as messengers between cells. Cellular membranes comprised of lipids form the outer wall of animal cells and barriers within the cell. Lipids have several physical and chemical properties that make them well suited for these functions.
Lipids that form cellular membranes are usually amphipathic. This means that one end of each lipid molecule is attracted to water and the other repels water. When the lipid molecules are submerged in water, as they are in living cells, this property automatically forces the lipids into an alignment that creates a natural water barrier. This barrier functions as the outer membrane of a cell, which allows for cell specialization and cooperation.
Understanding how the lipids repel water with one end and attract it with the other requires an understanding of the basic chemical structure of a lipid molecule and a water molecule. Water molecules are naturally polar, that is, one side has a positive charge while the other has a negative one. Lipids are formed lacking a hydrogen ion on one end, which makes them positively charged and hydrophilic, or attracted to water. The other end has balanced ions, is not charged and is, therefore, hydrophobic, or repelled by water.
In general, lipids are composed of four components: triglycerides, cholesterol, cholesterol esters and phospholipids. Triglycerides and cholesterol esters form the hydrophobic core of the lipid molecule.
Cholesterol is a lipid that has received much medical attention for a role it can play in heart disease and strokes. This lipid is generated in the body and absorbed from consumed foods; it circulates with the blood. Cholesterol is in two forms: high-density lipoprotein, or HDL, and low-density lipoprotein, or LDL. High levels of LDL in the bloodstream can easily become a health risk, as it can collect on the interior of blood vessels, along with other substances, and form plaque. This plaque causes constriction in the vessels and reduces flexibility. HDL, the good form of cholesterol, is thought to return LDL to the liver, where it can be properly processed and expelled. For this reason, a proper amount of HDL can help prevent heart disease and strokes.
It is significant that lipids are able to move freely between both water and other lipids, because they are often used as messengers within an individual cell or across an entire body. Lipids also form very dense atomic structures, so a single lipid molecule may hold several bonds that can be used to store and release chemical energy.