Summary
Highlights
Lipids are a diverse group of compounds including fats, oils, fatty acids, triglycerides, phospholipids, steroids, waxes, terpenes, and prostaglandins. They are primarily nonpolar and hydrophobic due to their carbon-hydrogen bonds, meaning they do not mix well with water. Lipids are essential for long-term energy storage, with triglycerides storing more than double the energy of carbohydrates. They also form the cellular membrane and provide thermal insulation and protection in the body.
Fatty acids are long hydrocarbon chains with a carboxylic acid functional group at one end. The carboxylic acid part is polar (hydrophilic), while the hydrocarbon tail is nonpolar (hydrophobic). Saturated fatty acids have no double bonds and are solid at room temperature (e.g., butter). Unsaturated fatty acids contain double bonds, causing kinks in their structure, and are typically liquid at room temperature (e.g., vegetable oils). Trans fatty acids differ from cis fatty acids in the configuration of hydrogen atoms around the double bond.
Triglycerides are formed by reacting one glycerol molecule with three fatty acid molecules through a dehydration synthesis reaction, releasing three water molecules. This process creates a very effective molecule for long-term energy storage. Conversely, a hydrolysis reaction (adding water) can break down triglycerides back into glycerol and fatty acids. Triglycerides are non-polar and do not hydrogen bond with water.
Phospholipids consist of a phosphate group, a glycerol molecule, and two fatty acid chains. They also contain nitrogen and phosphorus. Phospholipids are crucial components of the cell membrane, forming a bilayer. They have a polar (hydrophilic) head, which faces water, and two nonpolar (hydrophobic) tails, which avoid water. The kink in one of the fatty acid tails is due to the presence of a double bond in an unsaturated fatty acid.
Steroids are recognized by their characteristic four fused rings. Examples include cholesterol, which maintains cell membrane fluidity; estradiol, a primary female sex hormone; testosterone, a primary male sex hormone; and cortisol, a steroid hormone that reduces inflammation. Estradiol and testosterone share chemical similarities but differ in specific functional groups, such as the aromatic ring in estradiol. Cortisol, when used as medication, is called hydrocortisone.
Waxes are nonpolar lipids with very long alkyl chains and various functional groups. They are characterized by high melting points and are solid at room temperature. Examples include beeswax, plant waxes (for hydration regulation), and paraffin wax used in candles. Waxes do not mix well with water due to their hydrophobic nature.
Terpenes are lipids built from isoprene units, which have five carbon atoms and two double bonds. Terpenes typically have carbon atoms in multiples of five. Examples include mercine and limonene (found in citrus fruits) and beta-carotene (found in carrots). Beta-carotene, with 40 carbon atoms, is composed of eight isoprene units.
Eicosanoids are a category of lipids that include prostaglandins, thromboxanes, and leukotrienes. Prostaglandins have a five-carbon ring and 20 carbon atoms; they act as vasodilators and inhibit platelet aggregation. Thromboxanes have a six-membered ether-containing ring and promote platelet aggregation and act as vasoconstrictors. Leukotrienes, with four double bonds (three conjugated), serve as inflammatory mediators. All eicosanoids contain a carboxylic acid functional group.