Life is built from ordinary matter, but living systems use that matter in extraordinarily organized ways. Every cell, from a bacterium to a human neuron, is made of atoms. Those atoms form molecules, molecules interact through chemical bonds, and certain molecules become the proteins, nucleic acids, carbohydrates, lipids, and other substances that allow cells to grow, reproduce, communicate, and maintain themselves.
Understanding the chemical basis of life therefore begins at a very small scale. The behavior of atoms determines how molecules form; molecular structure determines how molecules interact; and those interactions make biological processes possible.
Atoms provide the raw material of life
An atom is the smallest unit of an element that retains that element’s chemical identity. Each atom has a dense nucleus containing protons and neutrons, surrounded by electrons.
Protons carry a positive electrical charge, neutrons have no charge, and electrons carry a negative charge. The number of protons in an atom’s nucleus defines which element it is. A carbon atom always has six protons; an oxygen atom has eight; a hydrogen atom has one.
Electrons are especially important in biology because they determine how atoms interact chemically. They occupy regions around the nucleus called orbitals, and the electrons in the outermost occupied energy level—the valence electrons—play the largest role in chemical bonding.
The elements that make up living organisms are not all equally abundant or equally important. Six elements account for most of the mass of living matter: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Other elements, including calcium, sodium, potassium, chlorine, magnesium, and iron, are also essential, often in smaller amounts.
The importance of these elements comes from their chemical properties. Carbon, for example, can form four stable covalent bonds and can connect to other carbon atoms, producing chains, branches, and rings. This versatility makes carbon the central element of biological chemistry.
Chemical bonds turn atoms into molecules
Atoms rarely exist in isolation inside living organisms. They are held together in molecules by chemical bonds, which arise largely from interactions involving valence electrons.
Covalent bonds
A covalent bond forms when atoms share electrons. The atoms in a water molecule, for example, are connected by covalent bonds between oxygen and hydrogen.
Covalent bonds can be single, double, or, in some molecules, triple bonds, depending on how many pairs of electrons are shared. These differences affect the shape, strength, and reactivity of molecules.
Carbon’s ability to form several covalent bonds with other carbon atoms is particularly important. It allows biological molecules to have complex three-dimensional structures rather than being limited to simple, repetitive arrangements.
Ionic interactions
An ionic bond results from the attraction between oppositely charged ions. An atom may gain or lose electrons and become electrically charged. For example, sodium can lose an electron to become a positively charged sodium ion, while chlorine can gain one to become a negatively charged chloride ion.
In biological systems, ionic attractions often occur between charged groups within or between molecules. They can help stabilize the structures of proteins and other biomolecules, although they behave differently in water than they do in a dry crystal.
Hydrogen bonds
A hydrogen bond is a relatively weak attraction involving a hydrogen atom that is covalently bonded to an electronegative atom, commonly oxygen or nitrogen, and another electronegative atom nearby.
Individually, hydrogen bonds are much weaker than covalent bonds, but they are extremely important in biology because many can act together. They help give water its distinctive properties and help stabilize the structures of DNA, proteins, and other biological molecules.
Polarity gives molecules different chemical personalities
The electrons in a covalent bond are not always shared equally. When one atom attracts the shared electrons more strongly than another, the bond becomes polar.
Oxygen, for instance, attracts electrons more strongly than hydrogen. As a result, the oxygen end of a water molecule has a partial negative charge, while the hydrogen ends have partial positive charges. Water is therefore a polar molecule.
Molecular polarity strongly influences how substances behave in water. Polar and charged substances tend to interact favorably with water, while many nonpolar substances do not.
This distinction helps explain why biological molecules organize themselves in particular ways. It also contributes to the behavior of cell membranes, in which molecules with water-attracting and water-avoiding regions arrange themselves into a stable barrier.
Water is the central chemical environment of cells
Water is not simply a substance that happens to be abundant in organisms. Its chemical properties make it an unusually effective medium for life.
Because water is polar, it can surround and interact with ions and many polar molecules. Substances that interact readily with water are often described as hydrophilic, meaning “water-loving.” Substances that do not mix well with water are hydrophobic, or “water-fearing.”
Water molecules also form extensive hydrogen-bond networks. This contributes to water’s ability to resist rapid temperature changes, its relatively high boiling point for such a small molecule, and its role as a solvent.
Many biological reactions take place in aqueous environments. Dissolved molecules can move, collide, change partners, and participate in chemical reactions. The behavior of water therefore shapes virtually every level of cellular chemistry.
Carbon creates the framework of biological molecules
Carbon occupies a special position in biochemistry because it can form four covalent bonds and bond stably with many other elements, including hydrogen, oxygen, nitrogen, phosphorus, and sulfur.
Carbon atoms can form straight chains, branched structures, and rings. They can also form single and double bonds. These possibilities generate an enormous variety of molecular structures.
A molecule’s structure is not merely its physical arrangement. Structure determines which other molecules it can interact with and which chemical reactions it can undergo. Two molecules can contain the same types and numbers of atoms yet differ in how those atoms are arranged, producing different chemical properties.
This relationship between structure and function is one of the central principles of biology.
The major classes of biomolecules
Most of the molecules associated with cellular structure and function fall into four broad classes: carbohydrates, lipids, proteins, and nucleic acids. These groups differ substantially in structure and function, but they are chemically interconnected.
| Biomolecule | Main building components | Important roles |
|---|---|---|
| Carbohydrates | Simple sugars and related units | Energy, energy storage, structural materials, cell recognition |
| Lipids | Hydrophobic components such as fatty acids and glycerol in many lipids | Membranes, energy storage, signaling, insulation |
| Proteins | Amino acids | Catalysis, structure, transport, movement, regulation, defense |
| Nucleic acids | Nucleotides | Storage, transmission, and expression of genetic information |
The categories are useful, but biological molecules do not always fit neatly into a single functional box. Some molecules contain features associated with more than one class, and their functions depend on their precise structures.
Carbohydrates do more than provide energy
Carbohydrates include sugars and molecules built from sugar units. Monosaccharides, such as glucose, are relatively simple carbohydrates. Two or more sugar units can be linked to form larger carbohydrates.
Cells can use sugars as sources of chemical energy. Through metabolic pathways, the chemical energy stored in their bonds can ultimately be captured in forms the cell can use to drive other processes.
Carbohydrates also have structural and informational roles. Long carbohydrate polymers can form strong biological materials, while carbohydrate groups attached to proteins and lipids can help cells recognize and interact with one another.
Thus, the common idea that carbohydrates are simply “energy molecules” describes only part of their biological importance.
Lipids organize membranes and store energy
Lipids are a diverse group of molecules united more by their physical behavior than by one single chemical structure. Many are poorly soluble in water and contain substantial nonpolar regions.
Triglycerides, for example, are major forms of stored chemical energy in animals and many other organisms. They consist of glycerol linked to three fatty acids. Because their hydrocarbon-rich structures contain many energy-rich carbon-hydrogen bonds and are stored with relatively little associated water, they are efficient long-term energy stores.
Phospholipids have a different role. They contain a water-interacting region and hydrophobic regions. In an aqueous environment, they can spontaneously organize into a lipid bilayer, with hydrophobic portions shielded from water and hydrophilic portions exposed to it.
This bilayer forms the basic structural framework of cell membranes. The membrane is not an inert wall; it is a dynamic environment containing proteins and other molecules that control transport, communication, and many chemical processes.
Proteins are molecular machines
Proteins are polymers made from amino acids. Cells use a set of amino acids to build proteins, linking them together through peptide bonds.
A protein’s amino-acid sequence is its primary structure, but that sequence is only the beginning. Interactions among the amino acids cause the chain to fold into specific three-dimensional arrangements. These structures determine what the protein can do.
Some proteins act as enzymes, biological catalysts that accelerate chemical reactions without being consumed by the reactions themselves. Others form structural materials, transport substances, receive signals, generate movement, regulate cellular activity, or participate in immune defense.
The importance of protein structure becomes especially clear with enzymes. An enzyme’s three-dimensional shape creates a particular chemical environment in which certain molecules can bind and react efficiently. Small changes in structure can therefore alter a protein’s function.
Protein folding is influenced by several types of interactions, including hydrogen bonds, ionic attractions, hydrophobic interactions, and covalent bonds such as disulfide bonds in appropriate environments.
Nucleic acids store and use biological information
The two major nucleic acids are DNA and RNA. Both are polymers of nucleotides.
A nucleotide contains a sugar, a phosphate group, and a nitrogen-containing base. The sequence of bases in a nucleic acid provides information.
DNA is particularly well suited for long-term information storage. Its two complementary strands are held together in part by hydrogen bonds between specific pairs of bases. Because each strand contains information that corresponds to the other, DNA can be copied with high fidelity.
RNA performs several roles in cells. It can carry information from DNA, form part of molecular machines involved in protein production, and in some cases act directly as a catalyst or regulatory molecule.
The flow of information from DNA to RNA to protein is central to how genetic information is expressed, although biological systems contain additional layers of regulation and exceptions to this simplified description.
Chemical reactions keep cells alive
A cell is not chemically static. Thousands of reactions occur within it, involving the breakdown, construction, modification, and transfer of molecules.
The complete network of chemical reactions in an organism is called its metabolism. Metabolic pathways often consist of sequences of reactions in which the product of one reaction becomes the starting material for another.
Some reactions release usable energy; others require an input of energy. Cells manage these processes through molecules that transfer energy and chemical groups between reactions.
ATP (adenosine triphosphate) is especially important as a short-term energy-transfer molecule. Energy released by certain metabolic reactions can be used to produce ATP, while ATP breakdown can help drive processes that require energy, such as active transport, movement, and the synthesis of cellular molecules.
Enzymes coordinate much of this chemistry. Without catalysts, many reactions essential to life would occur too slowly under cellular conditions.
Acids, bases, and pH matter because chemistry depends on charge
The acidity of a solution is commonly described by its pH, a scale related to the concentration of hydrogen ions. Lower pH values indicate greater acidity, while higher values indicate greater basicity.
Biological molecules often contain chemical groups that can gain or lose hydrogen ions. Their electrical charges can therefore change with pH. Because charge influences molecular interactions and protein structure, changes in pH can strongly affect biological processes.
Cells maintain their internal chemical conditions within controlled ranges. Buffers help resist sudden changes in pH by taking up or releasing hydrogen ions as conditions change.
This control illustrates an important feature of biological chemistry: cells do not merely contain chemicals. They actively regulate their chemical environment.
From molecules to living systems
The chemical basis of life is ultimately a story of organization.
Atoms provide the elements. Chemical bonds connect atoms into molecules. The properties of those molecules determine how they interact. Some molecules assemble into larger structures such as proteins, nucleic acids, and membranes. Those structures participate in networks of chemical reactions and information transfer. Together, these organized interactions allow cells to maintain themselves, respond to their surroundings, reproduce, and evolve.
Life does not require a special category of matter separate from chemistry. Instead, living systems arise from ordinary chemical elements arranged and regulated with extraordinary complexity. Understanding that progression—from atoms and electrons to bonds, molecules, biomolecules, and cellular processes—is the foundation for understanding how biology works at every larger scale.
