2.4: What are Lipids?
Lipids are a group of structurally and functionally diverse organic compounds that are insoluble in water. Certain classes of lipids, such as fats, phospholipids, and steroids are crucial to all living organisms. They function as structural components of cellular membranes, energy reservoirs, and signaling molecules.
Lipids are a Diverse Group of Hydrophobic Molecules
Lipids are structurally and functionally diverse group of hydrocarbons. Hydrocarbons are chemical compounds that consist 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. The individual nonpolar bonds impart an overall nonpolar characteristic to the hydrocarbon compound. Additionally, nonpolar compounds are hydrophobic, or “water-hating.” This means they do not form hydrogen bonds with water molecules, rendering them nearly insoluble in water.
Depending on the chemical composition, lipids can be divided into different classes. The biologically important classes of lipids are fats, phospholipids, and steroids.
Fat Is a Triester of Fatty Acids and Glycerol
The hydrocarbon backbone of fat has three carbon atoms. Each carbon carries a hydroxyl (–OH) group, making it glycerol. To form a fat, each of the hydroxyl groups of glycerol is linked to a fatty acid. 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). Fat is an ester of glycerol and three fatty acids; hence it is also referred to as triglyceride. The three constituent fatty acids can be identical or different and are usually 12-18 carbons long.
Saturated Versus Unsaturated Fats
Fats are either saturated or unsaturated depending on the presence or absence of double bonds in the hydrocarbon chains of their fatty acids. If a fatty acid chain does not have double bonds between the carbon atoms, the individual carbon atoms bind a maximum number of hydrogens. Such a fatty acid is completely saturated with hydrogen, and is called a saturated fatty acid. On the other hand, if the fatty acid contains one or more double bonded carbon atoms, the fatty acid is called unsaturated fatty acid.
Fats that contain all saturated fatty acids are called saturated fats. Fats obtained from animal sources, for instance, butter, milk, cheese, and lard, are mostly saturated. Fats from fish or plant sources are often unsaturated, like olive oil, peanut oil, and cod liver oil. The absence of double bonds in the hydrocarbon chains of saturated fatty acids, making them flexible. The flexible fatty acid chains can pack tightly with each other; hence saturated fats are mostly solid at room temperature.
Most naturally occurring unsaturated fatty acids are in “cis” conformation, meaning that the hydrogen atoms adjacent to the carbon-oxygen double bond are on the same side. The presence of cis-double bonds causes a bend in the hydrocarbon chain which makes the long hydrocarbon chain less flexible and difficult to pack. As a consequence, most unsaturated fatty acids are liquid at room temperature.
Fats are a long-term energy reservoir in many organisms. If the need arises, the organism breaks down fats to produce energy. In animals, fat provides cushioning around vital organs, and a subcutaneous layer of fat insulates the body from external temperatures.
Phospholipids Are an Integral Part of Cellular Membranes
Phospholipids are critical to the cell as they are major constituents of cell membranes. Phospholipids are structurally similar to fats but contain only two fatty acids linked to glycerol instead of three. The fatty acid residues can be saturated or unsaturated. In phospholipids, the third hydroxyl group of glycerol is linked to a negatively charged phosphate group.
An additional functional group attached to the phosphate group can lead to diverse chemical properties of phospholipids. Most common additives are small polar groups like choline or serine.
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. Such phospholipid bilayer forms the cell membrane in all living organisms. It compartmentalizes the fluids on 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. Steroids are composed of four carbon rings that are fused to each other. Steroids vary amongst each other based on the chemical groups attached to the carbon rings. Although steroids are structurally different, they are hydrophobic and insoluble in water. Steroids reduce the fluidity of the cell membrane. They also function as signaling molecules within the cell. Cholesterol is the most common steroid and is synthesized by the liver. It is present in the cell membrane and is a precursor of sex hormones in animals.