2.6: What are Lipids?
Lipids function as structural components of cellular membranes, in addition to acting as energy reservoirs and signaling molecules. They are thus crucial to all living organisms. The three biologically important classes of lipids are triglycerides, phospholipids, and steroids.
Non-Polar and Hydrophobic Characteristics of Lipids
Lipids are a structurally and functionally diverse group of hydrocarbons—compounds consisting of carbon and hydrogen atoms. The carbon-carbon and carbon-hydrogen bonds are nonpolar, which means that the electrons between the atoms are shared equally. These nonpolar bonds impart an overall nonpolar characteristic to lipids. Additionally, lipids are hydrophobic or “water-hating” in nature. This means they do not form hydrogen bonds with water molecules, rendering them nearly insoluble in water but soluble in organic solvents, such as chloroform or benzene.
Triglycerides — A Triester of Fatty Acids and Glycerol
The hydrocarbon backbone of triglycerides has a glycerol molecule with three hydroxyl (-OH) groups, each linked to a fatty acid chain. A fatty acid is a long hydrocarbon chain with a carboxyl group (–COOH) at one end. The carboxyl group of the fatty acid and the hydroxyl group of the glycerol form a stable bond with the release of a water molecule. The resulting molecule is called an ester (–COOR). Therefore, a triglyceride is a triester of glycerol and three fatty acids. The three constituent fatty acids can be identical or different and are usually 12–18 carbons long. They are either saturated or unsaturated depending on the presence or absence of double bonds in the hydrocarbon chains.
Phospholipids are an Integral Part of Cellular Membranes
Phospholipids are critical to the cell as they form the major constituents of cellular membranes. They are structurally similar to triglycerides but contain only two fatty acids—saturated or unsaturated—linked to a glycerol moeity. The third hydroxyl group of glycerol is linked to a negatively charged phosphate group.
Addition of another functional group, such as smaller polar molecules like choline or a serine residue, to the phosphate group results in diverse chemical properties of phospholipids.
Phospholipids are amphipathic molecules, meaning they have parts that are hydrophobic and others that are hydrophilic or “water-loving.” When phospholipids are added to water, they spontaneously form a bilayer, a thin film that is two phospholipid molecules thick. This self-organization takes place because the polar heads are attracted to water, while the hydrophobic fatty acids are buried in the center of the layer to evade contact with water. This phospholipid bilayer forms the cell membrane in all living organisms. It compartmentalizes the fluids in the interior and exterior of the cell. Embedded in the bilayer are proteins and steroids, another class of lipids. Additional phospholipid bilayers may further compartmentalize the interior of the eukaryotic cell, for instance, the lysosome and endoplasmic reticulum.
Steroids Consist of a Four Ring Structure
Steroids are another biologically important class of lipids composed of four carbon rings. They vary amongst each other based on the chemical groups attached to the carbon rings. Although steroids are structurally different from triglycerides and phospholipids, they are also hydrophobic and insoluble in water. They alter the fluidity of the cell membrane and also function as signaling molecules within the cell. Cholesterol is the most common steroid and is synthesized in the liver. It is present in the cell membrane and serves as the precursor of sex hormones in animals.
