A biomolecule is any molecule produced by a living organism that is essential to one or more biological processes. These molecules form the fundamental machinery and architectural components of all life forms, from single-celled bacteria to complex mammals. As organic compounds, biomolecules are universally carbon-based, meaning their structure is built upon carbon atoms covalently bonded with hydrogen, oxygen, and nitrogen, among other elements. This carbon backbone allows for the immense structural diversity necessary to carry out the thousands of unique functions required to sustain life.
The Monomer-Polymer Relationship
The vast majority of biomolecules are constructed based on a repeating architectural principle. Small, single units, called monomers, link together to form large chain-like structures known as polymers. This assembly process is driven by dehydration synthesis, or condensation, where a covalent bond forms between two monomers as a molecule of water is removed.
The reverse process, necessary for digestion and recycling materials, is called hydrolysis. Hydrolysis breaks the covalent bond between two monomers by adding a water molecule. The added water splits, with a hydroxyl group attaching to one monomer and a hydrogen atom attaching to the other, effectively separating the polymer chain into its constituent units. These two enzyme-catalyzed reactions are the fundamental chemical mechanics for building up and breaking down the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.
Energy and Structure: Carbohydrates and Lipids
Carbohydrates are primarily used for immediate energy and structural support. Simple carbohydrates, such as glucose, adhere to a general stoichiometric formula where the ratio of carbon, hydrogen, and oxygen is approximately 1:2:1. Monosaccharides, like glucose, are the direct fuel source for cellular respiration, the process that generates usable energy for the cell. These can be joined together to form disaccharides and complex polysaccharides.
Polysaccharides serve both energy storage and structural roles. Starch in plants and glycogen in animals are branched polymers of glucose used for energy storage. Cellulose, found in plant cell walls, provides rigid structural support that allows plants to stand upright. The modified carbohydrate chitin forms the hard exoskeleton or cell walls in insects and fungi.
Lipids are a diverse group of non-polar, hydrophobic molecules that include fats, oils, waxes, and steroids. Their primary functions revolve around long-term energy storage and the formation of biological membranes. Fats, such as triglycerides, store energy in their long hydrocarbon chains, making them a dense and efficient form of fuel storage. Lipids can store significantly more energy per gram compared to carbohydrates due to their lower proportion of oxygen atoms.
The most important structural lipids are phospholipids, the main components of all cellular membranes. A phospholipid is an amphipathic molecule, having both a hydrophilic phosphate head and two long hydrophobic fatty acid tails. When placed in water, these molecules spontaneously arrange into a phospholipid bilayer, forming a selectively permeable barrier that defines the cell boundary.
Information and Catalysis: Proteins and Nucleic Acids
Proteins are the most functionally diverse class of biomolecules, performing roles in defense, transport, movement, and structural support. Their building blocks are 20 different types of amino acids, which link together to form long chains called polypeptides. The specific sequence of amino acids in this chain constitutes the protein’s primary structure, determined by genetic instructions.
The polypeptide chain folds into a precise three-dimensional shape, which is directly responsible for its function. Initial folding involves secondary structures, such as the alpha-helix or the beta-pleated sheet, stabilized by hydrogen bonds between the backbone components. The overall globular shape is the tertiary structure, resulting from interactions between the amino acid side chains. Some functional proteins are comprised of multiple polypeptide chains interacting to form a quaternary structure.
A major function of proteins is catalysis, performed by enzymes. Enzymes speed up biochemical reactions by providing an active site where a specific reactant, called the substrate, can bind. This interaction lowers the activation energy required for the reaction to proceed, allowing chemical processes to occur rapidly. Since an enzyme’s function depends entirely on its three-dimensional structure, any change to the amino acid sequence can alter the folding and render the enzyme nonfunctional.
Nucleic acids, deoxyribonucleic acid (\(\text{DNA}\)) and ribonucleic acid (\(\text{RNA}\)), are the carriers of genetic information and the instructions for building proteins. The central concept of molecular biology describes the flow of information from \(\text{DNA}\) to \(\text{RNA}\) to protein. \(\text{DNA}\), found primarily in the cell nucleus, stores the hereditary blueprint as a double helix composed of nucleotide monomers.
The information stored in \(\text{DNA}\) is accessed through transcription, where a segment of \(\text{DNA}\) is copied into a messenger \(\text{RNA}\) (\(\text{mRNA}\)) molecule. The \(\text{mRNA}\) then travels to a ribosome, the cell’s protein synthesis factory. At the ribosome, translation occurs, where transfer \(\text{RNA}\) (\(\text{tRNA}\)) molecules read the \(\text{mRNA}\) sequence and deliver the corresponding amino acids. This mechanism ensures the genetic code is accurately translated into the exact amino acid sequence needed to construct a functional protein.