Biological Molecules, Enzymes and ATP

Macromolecule Assembly, Carbohydrates, and Lipids

Cells build complex structures by joining smaller molecular units together. The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Monomers are single, repeating chemical subunits that join together to form large chains called polymers. To build these polymers, cells use dehydration synthesis, a reaction that links monomers together by removing a molecule of water to form a covalent bond. When a cell needs to disassemble these structures, it carries out hydrolysis, which breaks the covalent bonds between monomers within a polymer by the addition of water.

Carbohydrates are essential cellular fuels and structural components that contain carbon, hydrogen, and oxygen. Many monosaccharides, which are the monomer building blocks of carbohydrates, display a carbon-to-hydrogen-to-oxygen ratio of approximately 1:2:1, with glucose serving as a primary example. When two monosaccharides are linked by a glycosidic bond, they form a disaccharide. For instance, linking glucose and fructose yields sucrose, whereas combining glucose and galactose produces lactose. Monomers can also assemble into long chains called polysaccharides. Starch acts as the main storage polysaccharide in plants to hold glucose for later use, while glycogen is a branched polysaccharide that serves as the primary glucose storage molecule in animals. Plants also assemble cellulose, a structural polysaccharide found in plant cell walls composed of glucose units connected by beta glycosidic bonds.

Lipids form a diverse group of largely hydrophobic molecules that are rich in carbon and hydrogen. One major type of lipid is the triglyceride, which consists of one glycerol molecule bonded to three fatty acids by ester linkages. The nature of these fatty acid chains determines the physical properties of the lipid. Saturated fatty acids have no carbon, carbon double bonds, and fats rich in long-chain saturated fatty acids tend to be solid at room temperature. In contrast, unsaturated fatty acids contain one or more carbon, carbon double bonds, and fats rich in cis-unsaturated fatty acids are typically liquid at room temperature.

Other lipids perform critical architectural and regulatory roles. Glycerophospholipids, a common type of phospholipid, feature a glycerol backbone attached to two fatty acid tails and a phosphate-containing head. Because their heads are hydrophilic and their tails are hydrophobic, phospholipids are amphipathic molecules that assemble to form cell membranes. Another distinct class of lipids is the steroids, which are characterized by a carbon skeleton composed of four fused rings. Cholesterol is a steroid embedded directly in animal cell membranes, where it helps maintain membrane fluidity.