Biological macromolecules are large, organic molecules that form the foundation of all living systems. Built around chains of carbon atoms, they are necessary for structure, energy, and information storage within a cell. They include four primary classes: carbohydrates, lipids, proteins, and nucleic acids, which make up the majority of a cell’s dry mass. These compounds execute virtually every function required for an organism to survive and reproduce.
Assembly and Structure
Biological macromolecules are created using a building-block approach. Small, repeating units called monomers link together to form long chains known as polymers. This process is analogous to connecting many single beads to create a long necklace.
The chemical reaction used to build these chains is called dehydration synthesis, or a condensation reaction. During assembly, a water molecule is removed as two monomers join to form a covalent bond. Conversely, hydrolysis breaks polymers down into smaller monomer subunits. This reverse reaction requires the addition of a water molecule, which splits the bond between the monomers.
Carbohydrates
Carbohydrates serve primarily as a rapid source of fuel for the body. They are chemically characterized by a ratio of carbon, hydrogen, and oxygen atoms, often represented by the formula C(H₂O)n. The simplest form is a monosaccharide, or simple sugar, with glucose being the most well-known example for cellular energy production.
When two simple sugars link, they form a disaccharide, such as sucrose. Complex carbohydrates, or polysaccharides, are long chains of monosaccharides that function as energy storage or structural components. For example, excess glucose is stored as glycogen in the liver and muscle tissues for short-term energy reserves. Cellulose provides structural support in plant cell walls, demonstrating the dual role of carbohydrates.
Lipids
Lipids are a diverse class of molecules defined by their inability to mix with water, meaning they are largely nonpolar. They serve functions including long-term energy storage, thermal insulation, and forming cell boundaries. Triglycerides (fats and oils) are the most common form of lipid and are stored in adipose tissue as the body’s main energy reserve.
A gram of fat provides about nine calories, making it a highly condensed energy source. Phospholipids are the main structural component of the cell membrane. These molecules arrange into a double layer (bilayer) that acts as a selective barrier, regulating what enters and exits the cell. Other lipids, like cholesterol and steroid hormones, act as chemical messengers that regulate physiological processes.
Proteins
Proteins are often described as the “workhorses” of the cell due to their diverse range of functions. They are polymers constructed from smaller units called amino acids. The sequence of amino acids is known as the primary structure, and this order dictates how the protein folds into its unique three-dimensional shape.
This folding pattern, which includes secondary and tertiary structures, determines the specific function of the protein. One major role is catalysis, where proteins called enzymes speed up nearly all metabolic reactions. Other proteins provide structural support, such as collagen or keratin. Transport proteins, like hemoglobin, carry molecules such as oxygen throughout the body. Antibodies, which are also proteins, attach to foreign invaders to help clear them from the immune system.
Nucleic Acids
Nucleic acids are the macromolecules responsible for storing, transmitting, and expressing genetic information. The monomers that form these long chains are called nucleotides. Each nucleotide consists of a five-carbon sugar, a phosphate group, and a nitrogenous base.
The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA holds the complete genetic blueprint and is characterized by its stable double-helical structure. RNA is primarily involved in translating the instructions contained in DNA to direct protein assembly. This flow of information is fundamental to cellular life and heredity.