Biological macromolecules are the large, complex molecules that form the foundation of all living matter. These organic compounds are built primarily from carbon atoms bonded with hydrogen, oxygen, and nitrogen. They are often constructed as polymers, which are long chains assembled from repeating smaller subunits called monomers. There are four main classes of these biological molecules that together make up the majority of a cell’s mass and perform nearly all the functions necessary for life.
Carbohydrates: Energy and Structure
Carbohydrates function primarily as readily available fuel for cells and as structural components in many organisms. The simplest form is the monosaccharide, or simple sugar, with glucose being the most recognized example. These single sugar units typically have a chemical formula ratio of one carbon to two hydrogen to one oxygen atom.
Two monosaccharides can join together to form a disaccharide, such as sucrose, which is common table sugar. When hundreds or thousands of these simple sugar units link up, they form long chains known as polysaccharides. Starch is a common storage polysaccharide in plants, while glycogen serves the same purpose in animals for quick energy release.
Beyond energy storage, some polysaccharides provide structural support. Cellulose is the main component of plant cell walls, giving rigidity to wood and plant fibers. Chitin forms the hard exoskeletons of arthropods, such as insects and crustaceans.
Lipids: Storage and Membranes
Lipids are a diverse group of macromolecules defined by their characteristic property of being hydrophobic, meaning they are insoluble in water. This water-fearing quality stems from their long hydrocarbon regions. This unique structure allows them to perform functions related to energy storage, insulation, and forming barriers.
One major class is the triglyceride, composed of three fatty acids attached to a glycerol molecule. Triglycerides are the primary form of long-term energy storage in animals and are commonly referred to as fats and oils. Another type is the phospholipid, composed of two fatty acids and a phosphate-containing group attached to a glycerol backbone.
The phospholipid molecule is amphipathic, possessing both a water-loving (hydrophilic) head and a water-fearing (hydrophobic) tail. This dual nature causes phospholipids to spontaneously arrange into a phospholipid bilayer, forming the fundamental structure of all cellular membranes. A third important class of lipids is the steroids, which include cholesterol and hormones such as testosterone.
Proteins: Catalysis and Function
Proteins are the most diverse group of macromolecules, acting as the primary agents of cellular function. Their monomers are amino acids, of which there are 20 different types, each with a unique side chain. Amino acids link together via peptide bonds to form long, linear chains called polypeptides.
The chain of amino acids represents the protein’s primary structure, which dictates the subsequent levels of folding. The polypeptide folds into a precise three-dimensional conformation, forming secondary structures like alpha-helices and beta-sheets, and eventually the tertiary structure. This final shape is necessary for the protein to carry out its specific function.
Many proteins act as enzymes, which are biological catalysts that speed up nearly all chemical reactions. Other proteins provide structural support, such as collagen, or facilitate transport, like hemoglobin carrying oxygen. If a protein is exposed to extreme conditions, it can undergo denaturation, where it loses its correct three-dimensional shape and, consequently, its function.
Nucleic Acids: Genetic Information
Nucleic acids are the informational macromolecules responsible for encoding, transmitting, and expressing hereditary information. The building blocks of these molecules are nucleotides, each consisting of a phosphate group, a pentose sugar, and a nitrogenous base. Nucleotides link together to form long strands, known as polynucleotides.
The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA typically exists as a double helix and functions as the genetic blueprint, storing the complete set of instructions. The sequence of nitrogenous bases along the DNA strand constitutes the genetic code.
RNA is usually single-stranded and plays a direct role in expressing the information stored in DNA. Messenger RNA (mRNA) copies the genetic instructions from DNA, and other forms of RNA, such as transfer RNA (tRNA), are involved in the subsequent synthesis of proteins.