Biomolecules are organic molecules produced by living organisms that are fundamental to biological processes, providing the structure, energy, and regulatory mechanisms necessary for life. These substances are primarily built around a carbon skeleton, allowing for the formation of the large, complex molecules characteristic of living systems. Though water is the most abundant compound in a cell, organic biomolecules are the functional components that dictate the cell’s form and activity. Their chemical composition centers on a few elements, mainly carbon, hydrogen, and oxygen, but also includes nitrogen, sulfur, and phosphorus.
The Foundation: Monomers and Polymers
The complexity of most biomolecules is achieved through the assembly of smaller, repeating units called monomers. These monomers link together to create long chains called polymers, or macromolecules. This construction process is a chemical reaction called dehydration synthesis, also known as a condensation reaction.
During dehydration synthesis, a covalent bond forms between two monomers, releasing a molecule of water as a byproduct. This reaction is repeated to construct large polymers like starches, proteins, and nucleic acids. The reverse process, called hydrolysis, uses a molecule of water to break the covalent bond between monomers, separating the polymer chain back into its individual units.
Building large structures from small components allows life to achieve enormous diversity using a limited set of chemical reactions. Carbon, hydrogen, and oxygen form the backbone of these structures, providing stability and potential for structural variation. The specific bonds linking the monomers define the properties of the resulting macromolecule.
Energy Storage and Structural Components (Carbohydrates and Lipids)
Carbohydrates function primarily as immediate energy sources and structural support materials. Simple sugars, or monosaccharides like glucose, are the basic monomers that fuel cellular respiration. Monosaccharides link together through glycosidic bonds via dehydration synthesis to form larger carbohydrates.
Disaccharides, such as sucrose, form when two monosaccharides join, while polysaccharides are long, complex chains. Starch and glycogen are examples of polysaccharides used for energy storage in plants and animals, respectively. Cellulose, a primary polysaccharide, forms the tough structural component of plant cell walls, providing rigidity and protection.
Lipids, commonly known as fats, oils, and waxes, are a diverse group of non-polar molecules defined by their hydrophobic nature. They function as highly efficient, long-term energy storage molecules, containing more than twice the energy per gram compared to carbohydrates. The most common storage form is the triglyceride, which consists of a glycerol molecule bonded to three fatty acid chains.
Lipids are also primary components of all cellular membranes as phospholipids. A phospholipid is an amphipathic molecule, having a hydrophilic, phosphate-containing head and two hydrophobic fatty acid tails. In an aqueous environment, these molecules spontaneously arrange into a double layer, or bilayer, forming the barrier that separates the cell from its surroundings.
Catalysis and Genetic Instruction (Proteins and Nucleic Acids)
Proteins are the most functionally diverse class of biomolecules, serving roles in virtually every cellular process. They are polymers constructed from amino acid monomers, joined by a specific covalent bond known as a peptide bond. This bond forms between the carboxyl group of one amino acid and the amino group of the next, creating a long polypeptide chain.
The specific sequence of amino acids determines how the protein folds into a precise three-dimensional shape. This structure dictates the protein’s function, which includes acting as structural supports or functioning as enzymes. Enzymes are proteins that perform catalysis, accelerating chemical reactions without being consumed in the process.
Nucleic acids, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), function as the information storage and transfer molecules of the cell. Their monomers are nucleotides, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base. Nucleotides link together to form the long strands of DNA and RNA.
The sugar and phosphate components of adjacent nucleotides form the backbone of the strand through a phosphodiester bond. DNA stores the complete genetic blueprint in its double-helix structure, while RNA molecules translate this blueprint into functional proteins.