Lipids are a group of biological molecules that all share two distinct characteristics: they are insoluble in water, and they are rich in energy due to the number of carbon-hydrogen bonds. Lipids perform a number of functions, including serving as a way to store energy, providing fuel, acting as a signal molecule and being a membrane component. There are three main types of lipids found in living organisms.
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Phospholipids consist of two fatty acid chains, a phosphate group and a glycerol group. Phospholipids are characterized by their hydrophobic--not soluble on water--tails, formed by the fatty acid chains and hydrophilic--soluble in water--heads. This makes them ideal for being the main molecule found in cellular membranes.
Each cell, which is the basic unit of life, in a living organism contains genetic material and structures to help it carry out its own specialized function. In order to protect its contents, cells have membranes. In addition to providing protection, these membranes regulate the movement of water, nutrients and wastes into and out of the cell, as described by toxicologist Anthony Carpi of John Jay College
To form a cellular membrane, phospholipids form a bilayer with the hydrophilic heads facing outward, allowing the cell to interact with its environment. The hydrophobic tails form an inner core that helps to regulate the flow of proteins, nutrients, water and waste into the cell.
Glycolipids are lipid molecules that contain a sugar unit, such as glucose or galactose. Glycolipids can be simple, such as cerebroside, containing only one sugar molecule, or more complex. Complex glycolipids, such as gangliosides, found in nerve cells, can contain a chain consisting of 7 sugar molecules, as noted by the book "Biochemistry," by Jeremy M. Berg, et al.
Glycolipids are always oriented with their sugar molecule on the outside of the membrane. This enables them to serve as a marker for cellular recognition. In addition, glycolipids provide energy for cells to use.
Cholesterol is a lipid that consists of four hydrocarbon rings attached to a hydroxyl--hydrogen bound to oxygen--group. The four-ring structure makes cholesterol a steroid, and the hydroxyl group provides it with the amphipathic property of being both hydrophilic and hydrophobic.
Cholesterol is found in the cells and bloodstream of humans. Because it is not soluble in blood, due to the hydrophobic portion, it must be carried to cells with the help of lipoproteins. Although too much cholesterol can be bad for the body, cholesterol is an important molecule. It is needed to form cell membranes, and it is the precursor for other steroids, such as testosterone and estradiol.