Macromolecules are extremely large molecules that form the fundamental structural and functional basis of all living organisms. These compounds are built from thousands of smaller atoms linked together by covalent bonds, primarily featuring carbon skeletons. They are necessary for life processes like growth, metabolism, and reproduction. These four major classes of organic compounds make up the majority of a cell’s dry mass, giving cells their shape and carrying out their diverse activities.
How Macromolecules Are Constructed
The vast majority of macromolecules are constructed through a repetitive process involving smaller subunits. These smaller building blocks are known as monomers, and they link together to form long chains called polymers. The chemical process responsible for building these chains is termed dehydration synthesis, which means “to put together while losing water.” During this reaction, monomers release a molecule of water (\(\text{H}_2\text{O}\)) and form a new covalent bond between the subunits.
The building process requires an investment of energy to form the new bonds. Conversely, polymers are broken down into their constituent monomers through a process called hydrolysis. Hydrolysis reverses the building reaction by inserting a molecule of water across the bond, effectively splitting the polymer chain. Both the construction (dehydration synthesis) and the destruction (hydrolysis) of macromolecules are regulated and sped up by specific protein molecules known as enzymes.
Carbohydrates: Fuel and Structure
Carbohydrates serve two primary roles: immediate energy sources and structural components. The monomers of carbohydrates are simple sugars called monosaccharides, such as glucose and fructose. These simple sugars can be linked together to form larger structures like disaccharides (two units) or polysaccharides (many units).
Glucose is the most accessible form of energy, used by cells to power cellular functions. Organisms store excess glucose in large polysaccharide chains, such as starch in plants and glycogen in animals. Carbohydrates also provide structural support, demonstrated by cellulose, which forms the rigid cell walls of plants. Chitin, found in the exoskeletons of insects and the cell walls of fungi, is another structural polysaccharide that gives strength and protection.
Lipids: Energy Storage and Barriers
Lipids represent a diverse group of compounds defined by their physical property of being largely hydrophobic, or water-fearing. Unlike other macromolecules, lipids lack a repeating monomeric unit. Instead, many common lipids, like fats, are constructed from a glycerol molecule and fatty acid chains.
The three main types of lipids perform distinct functions. Triglycerides, composed of one glycerol and three fatty acids, function primarily as long-term energy storage, holding more than twice the energy per gram compared to carbohydrates. Phospholipids are the structural components of all cell membranes; their amphiphilic nature allows them to spontaneously form the double-layered barrier separating the cell’s interior from its environment. Sterols, including cholesterol and certain hormones, are involved in cell signaling and maintaining membrane fluidity.
Proteins: The Workhorses of the Cell
Proteins are the most functionally and structurally diverse class of macromolecules, carrying out the majority of tasks required for life. The monomers of proteins are amino acids; 20 different types are commonly found in biological systems. Amino acids link together via peptide bonds to form long chains called polypeptides, which represent the protein’s primary structure—a specific linear sequence.
A protein’s function is entirely dependent on its precise three-dimensional shape, which is determined by the amino acid sequence. The chain folds into distinct secondary structures, such as the alpha-helix or beta-pleated sheet, stabilized by hydrogen bonds. Further folding, driven by interactions between amino acid side chains, creates the specific 3D functional form known as the tertiary structure.
Proteins serve as enzymes, acting as biological catalysts that speed up nearly all chemical reactions within a cell. Other roles include structural support, such as the collagen found in connective tissues, and defense, carried out by antibodies. They also facilitate transport, with examples like hemoglobin carrying oxygen in the blood.
Nucleic Acids: Information Storage
Nucleic acids are the specialized macromolecules responsible for storing, transmitting, and expressing genetic information within a cell. Their monomers are nucleotides, each consisting of a five-carbon sugar, a phosphate group, and a nitrogen-containing base. The sequence of these bases forms the genetic code that dictates the traits and functions of an organism.
The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA serves as the long-term hereditary blueprint, holding the instructions required for building and maintaining the organism. RNA functions primarily to translate the genetic information stored in DNA into proteins, acting as an intermediary molecule. A related nucleotide, adenosine triphosphate (ATP), also plays a role as the primary energy currency for immediate use.