Many different types of molecules in the body, such as fats, waxes, and fat-soluble vitamins, fall into the category of "lipids." Energy storage is the most common of the many functions of lipids, though they can also provide cellular structure or act as signaling molecules. Other important but less common functions of lipids in the body include enzyme activation, molecular transportation, and metabolism. Humans must consume lipids as a part of their diets because of the nutrients they contain and because some fats are required in order to store fat-soluble vitamins. They are also important because lipid bilayers are used to moderate what is able to enter a cell and what is not.
One of the major functions of lipids in the body is energy storage because triglycerides and other similar molecules, which contain substantial lipid components, have a very high energy content. When the body is in need of stored energy, hormone signals initiate a biochemical process that breaks down the molecules into a usable form. Lipids are also valuable for energy storage because they can be stored with very little water. Carbohydrates, on the other hand, bind to water, which would result in a significantly higher mass-to-energy ratio if carbohydrates were used as the primary means of energy storage.
Maintaining cellular structure and moderating trans-membrane transportation is another of the important lipid function. The cellular membrane, composed largely of lipids, is used to separate the inner part of the cell from everything that exists outside. The membrane is made up of lipids that have both hydrophobic, or water-avoiding, and hydrophilic, or water-seeking, ends that organize into a lipid bilayer. The hydrophilic ends face toward water-filled environments inside and outside of the cell, while the hydrophobic ends stay between the inside- and outside-facing layers. This arrangement arises as a result of the hydrophobic and hydrophilic properties of the lipids, so cellular membranes are largely self-organizing.
Energy storage and cell membrane formation are the two most prominent functions of lipids in the body, but there are other functions. It is believed that lipids play an important role in cellular signaling, the process by which various biochemical processes are initiated or stopped. Additionally, many of the fat-soluble vitamins, which are lipids, serve important bodily functions, such as maintaining vision, promoting bone growth, and maintaining healthy immune function. Some lipid molecules are also used to transport other molecules across cellular membranes.