Lipids are hydrocarbon-containing molecules that make up the structure and function of living cells. Lipids include fats, oils, waxes, certain vitamins (such as vitamins A, D, E, and K), hormones, and most cell membranes that are not composed of proteins. Lipids are insoluble in water because they are non-polar, but are therefore soluble in non-polar solvents such as chloroform.
What are lipids made of? Lipids are primarily composed of hydrocarbons in their most reduced form, making them an excellent form of energy storage. When metabolized, hydrocarbons are oxidized to release large amounts of energy. The type of lipid found in fat cells for this purpose is triglyceride, an ester formed from glycerol and three fatty acids.
Where do lipids come from? Excess carbohydrates in the diet are converted to triglycerides, which involve acetyl-CoA in the synthesis of fatty acids in a process called lipogenesis, which occurs in the endoplasmic reticulum. In animals and fungi, a single multifunctional protein handles most of these processes, while bacteria utilize multiple separate enzymes. Certain types of unsaturated fatty acids cannot be synthesized in mammalian cells and must therefore be consumed as part of the diet, such as omega-3s.
Acetyl-CoA is also involved in the mevalonate pathway and is responsible for the production of a wide range of isoprenoids, which include important lipids such as cholesterol and steroid hormones.
Hydrolyzable and non-hydrolyzable lipids Lipids containing ester functional groups can be hydrolyzed in water, including neutral fats, waxes, phospholipids and glycolipids. Fats and oils consist of triglycerides, which consist of glycerol (1,2,3-trihydroxypropane) and 3 fatty acids. Triglycerides are present in the blood and are stored in fat cells. Complete hydrolysis of triacylglycerol yields three fatty acids and one glycerol molecule. Non-hydrolyzable lipids lack such functional groups, including steroids and fat-soluble vitamins (alpha, beta, beta, and potassium).
Fatty acid Fatty acids are long-chain carboxylic acids (usually 16 or more carbon atoms) that may or may not contain carbon-carbon double bonds. The number of carbon atoms is almost always an even number, usually unbranched. Oleic acid is the most abundant fatty acid in nature.
Unsaturated and saturated fatty acids Fatty acids without carbon-carbon double bonds are called saturated fatty acids. Those with two or more double bonds are called polyunsaturated. Oleic acid is monounsaturated because it has a double bond. Saturated fats are usually solid and derived from animals, while unsaturated fats are liquid and usually derived from plants.
Unsaturated fats exhibit a special geometry that prevents molecules from packing as efficiently as in saturated molecules, causing them to tend to exist as liquids rather than solids. Therefore, the boiling point of unsaturated fats is lower than that of saturated fats.
Synthesis and function of lipids in the body Lipids are utilized directly or synthesized from dietary fats. There are many biosynthetic pathways in the body to break down and synthesize lipids. The main biological functions of lipids include storing energy, as lipids can be broken down to generate large amounts of energy. Lipids also form the structural components of cell membranes and form various messenger and signaling molecules in the body.