Water is one of the most important substances in biology because it can dissolve an unusually wide range of molecules and ions. This property makes water more than a passive ingredient in living organisms: it provides the chemical environment in which many of life’s essential reactions occur.
Water is especially effective as a solvent because of its molecular structure. A water molecule has an uneven distribution of electrical charge, allowing it to interact strongly with charged particles and with other polar molecules. At the same time, water can participate in hydrogen bonding, helping it surround, separate, and transport many substances.
These properties help explain why cells are largely water-based and why water is central to processes ranging from metabolism and nutrient transport to the structure of proteins and DNA.
What makes water a good solvent?
A solvent is a substance capable of dissolving other substances, called solutes, to form a solution. Water is often called a polar solvent because its molecules have an uneven distribution of electrical charge.
A water molecule consists of two hydrogen atoms bonded to one oxygen atom. Oxygen attracts the shared electrons more strongly than hydrogen does, giving the oxygen end of the molecule a slight negative charge and the hydrogen ends slight positive charges. The molecule as a whole is electrically neutral, but its charges are arranged unevenly.
This polarity allows water molecules to attract ions and other polar molecules. When a substance dissolves, interactions between its particles and the solvent can pull those particles apart and keep them dispersed throughout the solution.
For example, when table salt dissolves in water, the ionic solid contains positively charged sodium ions and negatively charged chloride ions. Water molecules orient themselves around these ions: the oxygen end is attracted to sodium, while the hydrogen ends are attracted to chloride. These interactions help separate the ions from the crystal and keep them surrounded by water molecules.
Water dissolves many substances important to cells
Water’s solvent properties are particularly valuable because many biologically important substances are ionic or polar.
Ions such as sodium, potassium, calcium, chloride, and phosphate interact readily with water. These dissolved ions are essential for processes such as nerve signaling, muscle contraction, maintaining fluid balance, and regulating cellular activity.
Many small polar molecules also dissolve well in water. Sugars, for instance, contain several polar groups that can form favorable interactions with water. As a result, glucose and other water-soluble nutrients can be transported through aqueous body fluids and made available to cells.
Water is therefore well suited to serve as the medium through which dissolved substances move. Blood plasma, the fluid surrounding cells, and the interior of cells all contain water in which numerous substances are dissolved.
Hydrogen bonding adds another important advantage
Water’s polarity also allows its molecules to form hydrogen bonds with one another and with other suitable molecules. A hydrogen bond is a relatively weak attraction involving a hydrogen atom associated with one electronegative atom and another electronegative atom nearby.
Individually, hydrogen bonds are much weaker than covalent chemical bonds. But large numbers of them can have major effects on biological molecules and on the physical behavior of water.
Hydrogen bonding helps water interact with molecules containing groups such as hydroxyl and carbonyl groups. These interactions contribute to the ability of water to dissolve many carbohydrates, amino acids, and other polar compounds.
Hydrogen bonding also gives water unusual physical properties that support life, including a relatively high boiling point and strong cohesion between water molecules. Those properties are related to, but distinct from, water’s ability to act as a solvent.
Water does not dissolve everything
Calling water an excellent solvent does not mean that it dissolves every substance.
Nonpolar substances, such as many oils and hydrocarbons, do not interact favorably with water. Their molecules lack the charged or strongly polar regions that would allow them to form comparable interactions with water. As a result, many nonpolar substances tend to separate from water rather than dissolve in it.
This limitation is itself important to biology.
The tendency of nonpolar substances to avoid water contributes to the hydrophobic effect, which helps drive the organization of biological molecules. In cell membranes, for example, phospholipids arrange themselves so that their water-attracting portions face aqueous surroundings while their water-avoiding portions are shielded from water. This behavior is fundamental to the formation of the membrane structures that separate cells from their surroundings.
So water’s solvent behavior does not merely help substances dissolve. It also helps determine how molecules organize themselves when they are in an aqueous environment.
Why this matters for chemical reactions in living things
Most of the chemistry of a cell takes place in an aqueous environment. Dissolving reactants in water allows them to move and encounter one another, while water itself can participate directly in many reactions.
One important example is hydrolysis, a reaction in which water helps break a chemical bond. Hydrolysis is involved in the digestion and breakdown of many biological molecules, including proteins, carbohydrates, and nucleic acids.
Water also participates in the opposite type of process. Cells frequently build larger molecules by joining smaller molecules through reactions that remove water, often called condensation or dehydration reactions.
Water therefore serves both as the medium surrounding biological chemistry and, in many cases, as a reactant or product of that chemistry.
Water helps biological molecules maintain their functional shapes
The interaction between water and biological molecules is not limited to simple dissolution. Water strongly influences the three-dimensional structures of proteins, nucleic acids, and other cellular components.
Proteins contain both water-attracting and water-avoiding chemical groups. In an aqueous environment, these groups interact with their surroundings in different ways, helping determine how a protein folds into its functional shape. The hydrophobic effect is an important part of this process, alongside hydrogen bonding, ionic interactions, and other forces.
DNA is also shaped by its aqueous environment. Its charged phosphate groups interact readily with water, while hydrogen bonding and other molecular interactions help stabilize its structure.
Because the behavior of biological molecules depends heavily on their interactions with water, changing the surrounding conditions can alter molecular structure and function.
Water provides a medium for transport
An effective solvent is also useful for moving dissolved substances from one place to another.
In the human body, water is the main component of blood plasma and other body fluids. Dissolved nutrients, ions, gases, hormones, metabolic waste products, and other substances can be transported through these fluids.
At the cellular level, water-based solutions allow substances to diffuse through aqueous environments and interact with cellular structures. In plants, water helps transport dissolved minerals and other substances through vascular tissues.
This transport function follows directly from water’s ability to keep many charged and polar substances dispersed in solution.
Water’s solvent properties support the chemistry of life
Water is an excellent solvent for life because its molecular polarity, hydrogen bonding, and resulting interactions with ions and polar molecules create a versatile chemical environment. It can dissolve and transport many substances that organisms need, provide the medium for countless chemical reactions, and influence the structures and behavior of biological molecules.
Its limitations are equally significant. Water does not readily dissolve many nonpolar substances, and that incompatibility helps produce structures such as cell membranes and contributes to the folding and organization of biological molecules.
The importance of water, then, is not simply that it can dissolve a lot of material. Its particular pattern of molecular interactions makes it an exceptionally useful environment for the organized, dynamic chemistry on which living systems depend.
