Lipids are organic molecules found in all living organisms that are largely insoluble in water. This insolubility stems from their predominantly non-polar structure, causing them to repel water molecules. They serve fundamental roles in biology, most notably as an efficient form of long-term energy storage. Lipids are also the primary building blocks that form the structural boundaries of all cells, known as cellular membranes.
Identifying the Two Structural Components
The structure of many biologically relevant lipids, such as phospholipids and triglycerides, revolves around two components: a backbone molecule and long hydrocarbon chains. The backbone is typically a small molecule like glycerol, which serves as the attachment point. The hydrocarbon chains, which are sequences of carbon and hydrogen atoms, are called fatty acids. This architecture creates a molecule with a dual nature, making it “amphipathic.”
Amphipathic molecules have two distinct ends with opposing interactions with water: one part is polar and attracts water, while the other is non-polar and repels water. This characteristic allows lipids to organize spontaneously into complex structures like cell membranes in an aqueous environment. The components exhibiting these opposing properties are the fatty acid tails and the polar head group.
The Hydrophobic Fatty Acid Tails
The fatty acid tails constitute the non-polar, water-fearing portion of the lipid. These tails are chains of hydrocarbons, typically 12 to 24 carbon atoms in length. Because carbon and hydrogen atoms share electrons almost equally, the chains have no significant electrical charge, making them hydrophobic and causing them to avoid contact with water.
The physical properties of a lipid are influenced by the structure of these fatty acid chains. Chains are classified as either saturated or unsaturated based on the type of bonds between their carbon atoms.
Saturated fatty acids have only single bonds, allowing the chains to remain straight and pack tightly together. This typically results in fats that are solid at room temperature.
Unsaturated fatty acids contain one or more double bonds along the chain. Each double bond introduces a bend or “kink” into the tail structure. These kinks prevent the chains from packing closely, which lowers the melting point of the lipid. Consequently, lipids high in unsaturated fatty acids, such as vegetable oils, remain liquid at room temperature.
The Polar Head Group
The polar head group represents the water-loving, or hydrophilic, component of the lipid. This group is attached to the glycerol backbone opposite the fatty acid tails. In phospholipids, the main lipids in cell membranes, this head contains a phosphate group.
The phosphate group carries a negative charge and is often modified by the addition of other small, polar molecules. This charge and the presence of polar atoms allow the head group to form favorable interactions and hydrogen bonds with water. Because of this strong attraction, the head group readily faces the aqueous environment both inside and outside the cell.
The combination of the hydrophilic head and the hydrophobic tails drives the formation of the lipid bilayer. In an aqueous environment, the heads orient outward toward the water, while the tails spontaneously tuck inward to shield themselves. This self-assembling structure forms the stable, two-layered boundary that defines the cell and controls the passage of substances across the membrane.