Biomolecules are organic molecules produced by living organisms that are fundamental for all cellular processes. These substances are the building blocks, the fuel, and the instructions necessary for a cell to function, grow, and reproduce. These large molecules, often called macromolecules, enable living things to extract energy, maintain structure, transmit hereditary information, and carry out countless chemical reactions. They are grouped into four major classes, each performing a distinct role in the biological system.
Carbohydrates
Carbohydrates are commonly known as sugars or saccharides, serving primarily as the body’s immediate energy source. Their molecular structure typically follows a ratio of one carbon atom to one water molecule, represented as \((\text{CH}_2\text{O})_n\). The simplest form is a monosaccharide, such as glucose, which is the universal fuel molecule that cells break down to produce adenosine triphosphate (ATP) energy.
Polysaccharides are long chains of monosaccharides that function as energy storage or structural components. Animals store excess glucose in the form of glycogen, a highly branched molecule predominantly found in liver and muscle cells. Plants store energy as starch, which is a mixture of amylose and amylopectin.
Carbohydrates also perform structural roles, such as cellulose, which forms the rigid cell walls of plants and is one of the most abundant organic compounds on Earth. Another example is chitin, a structural polysaccharide that provides the tough, protective exterior for arthropods like insects and crustaceans.
Lipids
Lipids are a diverse group of compounds defined by their hydrophobic nature, meaning they are water-insoluble. This property arises because they are largely composed of nonpolar hydrocarbon regions. This class includes fats, oils, waxes, and steroids, each serving different functions within the cell and organism.
The most recognized lipids are fats, also known as triglycerides, which consist of a glycerol molecule bonded to three fatty acid chains. This structure makes them highly effective for long-term energy storage, holding more than twice the energy per gram compared to carbohydrates.
Phospholipids are structurally similar to fats but contain only two fatty acid tails and a hydrophilic phosphate-containing head group. When placed in water, phospholipids spontaneously arrange into a bilayer, forming the fundamental structure of all cellular membranes. This bilayer acts as a selective barrier, regulating what enters and exits the cell.
Lipids also play a role in communication through steroid hormones, which are characterized by a fused four-ring carbon structure. Cholesterol is a precursor molecule necessary for the synthesis of hormones like testosterone and estrogen. Cholesterol is also incorporated into the cell membrane to help maintain its fluidity across different temperatures.
Proteins
Proteins are the most functionally and structurally diverse biomolecules, often described as the workforce of the cell. They are polymers constructed from building blocks called amino acids, of which there are 20 common types found in living organisms. These amino acids are linked together by peptide bonds to form long, unbranched chains called polypeptides.
The function of a protein is entirely dependent on its intricate three-dimensional shape, achieved through folding. The sequence of amino acids dictates how the chain folds into a specific, biologically active conformation, which can be disrupted by changes in temperature or pH. This specific shape allows proteins to interact precisely with other molecules in the cell.
One of their primary roles is catalysis, where proteins known as enzymes accelerate nearly all metabolic reactions without being consumed themselves. Digestive enzymes like amylase rapidly break down large food molecules so they can be absorbed by the body. Structural proteins, such as collagen, provide tensile strength to connective tissues like skin, bone, and tendons.
Proteins are also involved in movement, with motor proteins like actin and myosin facilitating muscle contraction and cellular transport. Transport proteins, such as hemoglobin, are responsible for carrying oxygen from the lungs to tissues throughout the body. They also serve protective functions in the immune system as antibodies, which recognize and neutralize foreign invaders.
Nucleic Acids
Nucleic acids are the information-carrying molecules that provide the genetic blueprint for life and the instructions for how to build proteins. They are polymers composed of repeating monomer units called nucleotides, each consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base. This class includes two major types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA serves as the long-term, stable repository of genetic information, storing the hereditary code in its double helix structure. The sequence of nitrogenous bases—Adenine, Guanine, Cytosine, and Thymine—encodes the instructions for the cell. This stable structure makes DNA an ideal molecule for reliably passing genetic traits from one generation to the next.
RNA molecules are directly involved in the process of gene expression, acting as intermediaries that transfer and translate the genetic information stored in DNA. Messenger RNA (mRNA) carries the genetic message from the DNA in the nucleus to the ribosomes, the cell’s protein-making machinery. Transfer RNA (tRNA) brings the correct amino acids to the ribosome, where ribosomal RNA (rRNA) helps link them together to construct the specified protein.