Every living organism is built from matter, and the behavior of that matter is governed by chemistry. The oxygen we breathe, the water in our cells, the proteins that perform cellular work, and the DNA that stores genetic information all depend on interactions among tiny particles.
Three ideas provide a useful foundation for understanding this chemistry: atoms, ions, and molecules. An atom is the basic unit of an element. An ion is an atom or group of atoms with an electrical charge. A molecule consists of two or more atoms held together by chemical bonds.
These distinctions may seem elementary, but they explain much of what happens in living systems. Cells depend on particular atoms in particular proportions, on molecules forming and breaking apart, and on charged particles moving across membranes or participating in chemical reactions. Understanding those relationships makes topics such as metabolism, nerve signaling, acid-base balance, and genetics much easier to follow.
Atoms are the building blocks of matter
An atom is the smallest unit of an element that retains the chemical properties of that element. Elements are substances defined by the number of protons in their atoms.
An atom has a dense central nucleus containing positively charged protons and electrically neutral neutrons. Negatively charged electrons occupy regions around the nucleus. The number of protons determines the element: every carbon atom has six protons, every oxygen atom has eight, and every hydrogen atom has one.
Atoms are extremely small, but their internal structure matters because electrons determine how atoms interact chemically. In ordinary chemical reactions, atoms do not usually gain or lose protons. Instead, their electrons are rearranged, shared, or transferred.
For example, a neutral oxygen atom has eight protons and eight electrons. If it gains two electrons, it becomes an oxygen ion with a charge of −2. The nucleus has not changed; only the number of electrons has.
The elements most important to life
Living organisms contain many elements, but a relatively small group accounts for most of the matter in cells. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are especially important.
Carbon is central to biological chemistry because it can form stable bonds with many other atoms, including other carbon atoms. This allows carbon to serve as the framework for an enormous variety of molecules, including carbohydrates, lipids, proteins, and nucleic acids.
Hydrogen and oxygen are abundant in water and occur throughout biological molecules. Nitrogen is essential in proteins and nucleic acids. Phosphorus is important in nucleic acids and molecules involved in cellular energy transfer. Sulfur occurs in certain amino acids and contributes to the structure of some proteins.
Other elements are needed in smaller amounts but remain biologically essential. Calcium, sodium, potassium, magnesium, chlorine, and iron, for example, participate in processes ranging from bone formation and muscle function to electrical signaling and oxygen transport.
Isotopes change the nucleus, not the element
Atoms of the same element can have different numbers of neutrons. These variants are called isotopes.
Because the number of protons remains the same, isotopes are still atoms of the same element. Carbon-12 and carbon-14, for instance, both contain six protons, but they have different numbers of neutrons.
Some isotopes are stable, while others are radioactive and spontaneously change into different forms. Radioactive isotopes have important uses in biology and medicine, including certain methods for tracing chemical processes and producing diagnostic or therapeutic materials.
Isotopes illustrate an important distinction: the identity of an element comes from its protons, while its isotope depends on its neutrons.
Ions are electrically charged particles
A neutral atom contains equal numbers of protons and electrons. An ion has an unequal number because it has gained or lost electrons.
An atom that loses electrons becomes positively charged and is called a cation. An atom that gains electrons becomes negatively charged and is called an anion.
Sodium provides a familiar example. A neutral sodium atom has 11 protons and 11 electrons. If it loses one electron, it becomes a sodium ion, Na⁺, with 11 protons but only 10 electrons.
Chlorine behaves differently. A neutral chlorine atom has 17 protons and 17 electrons. If it gains one electron, it becomes chloride, Cl⁻, with 18 electrons.
These oppositely charged ions attract one another. The attraction between Na⁺ and Cl⁻ helps form sodium chloride, the familiar compound in table salt.
Why ions matter so much in biology
Ions are not merely components of salts. They are essential participants in living processes.
Sodium (Na⁺) and potassium (K⁺) help establish electrical gradients across cell membranes. Those gradients are fundamental to nerve impulses and many other forms of cellular signaling.
Calcium (Ca²⁺) serves both structural and signaling roles. It contributes to bones and teeth and acts as an important intracellular signal involved in processes such as muscle contraction and secretion.
Magnesium (Mg²⁺) interacts with molecules involved in cellular chemistry, including ATP and nucleic acids, and is important for the activity of many enzymes.
Chloride (Cl⁻) contributes to electrical balance and fluid regulation and is also involved in maintaining the appropriate chemical environment within cells and tissues.
The concentrations of these ions are tightly regulated. Cells cannot simply allow ions to move wherever they will. Membranes and specialized proteins control ion movement, using some of the cell’s energy to maintain concentration differences.
Molecules form when atoms bond
A molecule is a group of two or more atoms held together by chemical bonds. The atoms may be identical, as in molecular oxygen (O₂), or different, as in water (H₂O).
Chemical bonds arise from interactions involving electrons. Two major types are particularly important in biological chemistry: covalent bonds and ionic interactions.
In a covalent bond, atoms share electrons. The bonds within water, proteins, carbohydrates, fats, and DNA are largely covalent. Covalent bonds give biological molecules stable structures while still allowing those molecules to participate in chemical reactions.
An ionic bond is better understood as an electrostatic attraction between oppositely charged ions. In a solid such as sodium chloride, positively and negatively charged ions form an extended crystal structure rather than existing primarily as separate NaCl molecules. In water, the ions can separate and become surrounded by water molecules.
This distinction is useful because the word “bond” can describe different kinds of chemical attraction, and those differences affect how substances behave in cells.
Molecules can be simple or extraordinarily complex
A molecule can be as simple as O₂ or H₂O, or it can contain thousands or millions of atoms.
Water is small but indispensable. Its polarity allows water molecules to interact strongly with one another and with many other substances. This makes water an effective solvent for numerous biological compounds and helps give cells their characteristic chemical environment.
Glucose, a simple sugar, contains carbon, hydrogen, and oxygen. Cells can break it down through metabolic pathways and capture some of the released energy in chemically useful forms.
Proteins are much larger molecules assembled from amino acids. Their three-dimensional structures allow them to act as enzymes, receptors, transporters, structural components, and signaling molecules.
DNA is a large molecule built from repeating nucleotide units. Its chemical structure allows cells to store and copy hereditary information.
Chemical bonds give biological molecules their properties
The behavior of a biological molecule depends not just on which atoms it contains, but on how those atoms are connected and arranged in three dimensions.
Consider two molecules that contain the same kinds of atoms. If those atoms are connected differently, the molecules can have different properties. Even small changes in molecular structure can alter how a molecule interacts with enzymes, receptors, membranes, or other molecules.
Covalent bonds provide the basic framework, but weaker interactions also matter. Hydrogen bonds, for example, are attractions involving hydrogen and electronegative atoms such as oxygen or nitrogen. Individually, hydrogen bonds are weaker than typical covalent bonds, but many of them together can strongly influence biological structures.
Hydrogen bonding helps stabilize the shapes of proteins and contributes to the structure of DNA. It also affects the unusual properties of water.
Other noncovalent interactions, including attractions between charged groups and interactions involving nonpolar regions of molecules, help biological molecules fold and associate with one another. These interactions are constantly forming and breaking, allowing cells to build structures that are stable without being rigidly permanent.
Water creates the chemical environment of the cell
Water deserves special attention because most cellular chemistry occurs in an aqueous environment.
A water molecule has two hydrogen atoms covalently bonded to one oxygen atom. The electrons are not shared equally, giving the oxygen end a partial negative charge and the hydrogen ends partial positive charges. Water is therefore a polar molecule.
This polarity allows water molecules to form hydrogen bonds with one another and to interact with ions and other polar substances. Many salts dissolve in water because their ions are stabilized by surrounding water molecules.
Water also helps determine which biological molecules interact readily with one another. Substances that interact well with water are generally described as hydrophilic, while substances that do not mix readily with water are hydrophobic.
This difference is crucial for cell membranes. Many membrane molecules contain both water-attracting and water-avoiding regions, causing them to organize into structures that separate the watery interior of a cell from its surroundings.
Molecules can react and become different molecules
Life depends on continual chemical change. In a chemical reaction, atoms are rearranged as chemical bonds are broken, formed, or both.
The atoms themselves are not destroyed or created by ordinary chemical reactions. Instead, the same atoms appear in new combinations.
When cells break down glucose, for example, its atoms are redistributed through a series of reactions that produce carbon dioxide and water while transferring energy into forms the cell can use. Other reactions assemble small molecules into larger ones, such as linking amino acids to form proteins.
Enzymes make these reactions practical under the relatively mild conditions found in cells. An enzyme is a biological catalyst: it speeds up a chemical reaction without being consumed by the reaction itself. Enzymes do this by providing a molecular environment that makes particular reactions easier to carry out.
The precise shape and chemical properties of an enzyme’s active site determine which molecules it can interact with and which reactions it can promote.
Atoms, ions, and molecules work together in cells
The categories are useful, but living chemistry does not keep them in separate boxes. A single biological process can involve all three.
During nerve signaling, for example, proteins in the cell membrane control the movement of sodium and potassium ions. The movement of those charged particles changes the electrical properties of the membrane. That electrical change can then influence other proteins and trigger further cellular events.
During energy metabolism, enzymes—large molecules made of atoms linked by covalent bonds—catalyze reactions involving smaller molecules. Ions and other charged groups can influence the reactions, while water participates directly in many chemical transformations.
The same principle appears in DNA chemistry. DNA is a molecule whose atoms are connected through covalent bonds, but its structure also depends on hydrogen bonding and other noncovalent interactions. Ions in the surrounding solution can affect the behavior of charged groups within and around biological molecules.
pH describes hydrogen-ion concentration
Another important connection between ions and living systems is pH, a measure related to the concentration of hydrogen ions in a solution.
An acidic solution has a relatively high concentration of hydrogen ions, while a basic solution has a relatively low hydrogen-ion concentration compared with an acidic solution. The pH scale is logarithmic, so a change of one pH unit corresponds to a tenfold change in hydrogen-ion concentration.
Biological systems are sensitive to pH because changing hydrogen-ion concentration can alter the electrical charges of chemical groups on molecules. That can change protein structure, enzyme activity, and other chemical interactions.
Cells therefore regulate their internal chemical environment carefully. Buffers help resist sudden changes in pH by taking up or releasing hydrogen ions as chemical conditions change.
From atoms to living systems
The chemistry of life emerges from interactions across several levels of organization. Atoms provide the elements. Electron arrangements determine how those atoms interact. Chemical bonds connect atoms into molecules and larger structures. Ions create electrical and chemical gradients. Molecules interact through precise combinations of strong and weak forces. Together, these processes produce the organized chemistry that allows cells to grow, communicate, obtain energy, reproduce, and respond to their surroundings.
Understanding atoms, ions, and molecules is therefore more than memorizing definitions. It provides a framework for seeing how the physical properties of matter become the chemical processes of life.
