Biomolecules are the massive organic compounds that form the machinery and structure of all living cells. These large molecules are responsible for storing genetic information, catalyzing chemical reactions, providing structural support, and supplying necessary energy. Understanding the composition of life requires looking past the complexity of these final structures, such as DNA or cellulose. The core answer to what they are made of lies in their basic elemental makeup and the repetitive, modular way they are chemically assembled.
The Essential Atomic Ingredients
While the periodic table contains over one hundred elements, the architecture of life relies almost entirely on a select few. Biomolecules are overwhelmingly constructed from only six elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S). These six elements, often summarized by the acronym CHNOPS, form the covalent combinations that make up nearly all biological compounds.
Carbon is the foundational element for organic life because of its unique bonding capacity. A single carbon atom possesses four valence electrons, allowing it to form four stable covalent bonds with other atoms, including other carbon atoms. This property enables carbon to create long, complex chains, branched structures, and rings, forming the molecular skeletons of large biological molecules.
Nitrogen is a primary component of amino acids and the nitrogenous bases found in genetic material. Sulfur is limited primarily to certain amino acids like cysteine and methionine. Phosphorus is incorporated into the backbone structure of nucleic acids and is also a component of phospholipids, which form cell membranes.
Structural Organization: Monomers and Polymers
The large size of biomolecules means they follow a systematic, modular pattern of construction. Most of these massive compounds are classified as polymers, which are long chains composed of many smaller, repeating units. These smaller building blocks are known as monomers, and they are linked together via covalent bonds to form the larger structure.
The process of constructing a polymer from individual monomers is called dehydration synthesis. During this reaction, a covalent bond forms between two monomers, and a molecule of water is simultaneously removed as a byproduct. This removal of a hydrogen atom from one monomer and a hydroxyl group from the other allows the two units to link together chemically.
Conversely, polymers are broken down into their constituent monomers through a process called hydrolysis. The word hydrolysis literally means “to split water.” A water molecule is inserted across the covalent bond linking the monomers, effectively splitting the bond and returning the molecule to its smaller, individual units.
Dehydration synthesis requires energy to form the new bond, while hydrolysis releases energy by breaking the existing bond. This pattern of building and breaking down polymers through the addition or removal of water is consistent across the major classes of biomolecules. The diversity in biological structures arises from the order and combination of the specific types of monomers that are linked together.
The Building Blocks of Life: The Four Major Classes
The final composition of biomolecules is defined by which specific monomers are used to create the four overarching classes of biological molecules. These classes are:
- Carbohydrates
- Lipids
- Proteins
- Nucleic acids
Proteins
Proteins are large, complex polymers built from monomers called amino acids. There are 20 common types of amino acids that can be linked in countless combinations to form a polypeptide chain. Each amino acid shares a fundamental structure, featuring a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain known as the R-group. The R-group differentiates the 20 amino acids, giving each unique chemical properties that dictate the final three-dimensional folding of the protein.
Amino acids link together via a peptide bond, which is the specific covalent bond formed during the dehydration synthesis reaction between the carboxyl group of one amino acid and the amino group of the next. The sequential order of these amino acid monomers in the chain establishes the protein’s primary structure. Proteins can range significantly in size, with some containing fewer than 50 amino acids.
Nucleic Acids
The molecules responsible for carrying genetic information, Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA), are polymers constructed from monomers called nucleotides. Every nucleotide is composed of three distinct parts: a five-carbon sugar (either deoxyribose in DNA or ribose in RNA), a phosphate group, and a nitrogen-containing base. The sugar and phosphate groups link together to form the long backbone of the nucleic acid strand.
These monomers are linked by phosphodiester bonds that connect the phosphate of one nucleotide to the sugar of the next, creating a long, directional chain. Carbon, Hydrogen, Oxygen, and Nitrogen are present in the sugar and base components, with Phosphorus being unique to the phosphate group. This composition allows nucleic acids to store information in the sequence of their nitrogenous bases.
Carbohydrates
Carbohydrates include sugars and starches, and their polymeric forms are built from monomers known as monosaccharides, or simple sugars. The most common monosaccharide is glucose, which typically exists as a ring structure in cellular environments. Carbohydrates are composed primarily of Carbon, Hydrogen, and Oxygen atoms, often in a ratio of approximately one carbon atom to two hydrogen atoms to one oxygen atom.
Monosaccharides link together to form larger polymers like starch, glycogen, and cellulose, which function in energy storage and structural support. The covalent bond linking two monosaccharides is known as a glycosidic bond. The final shape of the carbohydrate polymer, whether linear or branched, is determined by how the glycosidic bonds are formed between the sugar units.
Lipids
Lipids, which include fats, oils, and waxes, differ fundamentally from the other three classes because they are generally not true polymers built from a chain of repeating monomers. Instead, many lipids, such as triglycerides, are assembled from smaller components: three long hydrocarbon chains called fatty acids attached to a single glycerol molecule. The defining characteristic of lipids is their non-polar nature, meaning they are hydrophobic, or water-fearing. This composition is why lipids are efficient at long-term energy storage and are used to form the phospholipid bilayers that make up cell membranes.