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Lipids are primarily composed of carbon, hydrogen, and oxygen atoms arranged in hydrophobic hydrocarbon chains or rings. These structures contain fatty acids—long-chain carboxylic acids with nonpolar C-H bonds—that make lipids water-repellent. The hydrophobic nature of lipids allows them to form membranes and store energy efficiently in cells.

Triglycerides consist of three fatty acids bonded to a glycerol backbone through ester linkages, making them entirely hydrophobic. Phospholipids have two fatty acids and a phosphate-containing head group, creating a dual-nature molecule with hydrophobic tails and a hydrophilic head. This structural difference enables phospholipids to form bilayers in cell membranes.

Noncovalent attractions in biomolecules, such as hydrophobic interactions, drive lipid self-assembly in aqueous environments. Hydrophobic fatty acid tails cluster together to minimize water contact, while hydrophilic head groups orient toward water. These weak interactions allow lipids to form flexible structures like membranes and micelles essential for cellular function.

Lipids are organic molecules because they contain carbon atoms bonded to hydrogen and oxygen in covalent arrangements. Like other water organic molecules inorganic ions found in cells, lipids are built from carbon skeletons with functional groups. Their carbon-based structure classifies them as organic compounds distinct from inorganic salts and minerals.

Cholesterol and steroid lipids contain a four-ring carbon structure rather than long hydrocarbon chains found in fatty acids. Despite lacking extended chains, steroids remain hydrophobic due to their carbon-hydrogen composition. This rigid ring structure allows steroids to embed in membranes and function as signaling molecules, contrasting with the flexible chains of triglycerides.

Lipids contain functional groups including carboxyl groups in fatty acids, ester linkages in triglycerides, and phosphate groups in phospholipids. These functional groups and types of organic compounds determine lipid solubility and reactivity. The ester bonds linking fatty acids to glycerol are formed through condensation reactions, creating the characteristic structure of storage lipids.

Unlike carbohydrates with their sugar units or proteins with amino acid chains, lipids lack repeating monomer units and are not true polymers. Carbohydrate chemistry mono and polysaccharides involves glycosidic bonds between sugars, while lipids use ester linkages. Proteins form through peptide bonds between amino acids, making lipids structurally distinct as nonpolar, energy-dense molecules.