Water is far more than a substance organisms need to drink. It is the medium in which most of the chemistry of life takes place, a major component of cells, a transport fluid, a temperature regulator, and a crucial participant in countless biochemical reactions. From the movement of nutrients through blood to the folding of proteins and the activity of enzymes, life depends on water’s unusual chemical properties.
The reason water can perform so many roles comes from its molecular structure. A water molecule consists of two hydrogen atoms bonded to one oxygen atom, but the electrons in those bonds are not shared equally. Oxygen attracts electrons more strongly than hydrogen, giving the oxygen end of the molecule a slight negative charge and the hydrogen ends slight positive charges. Water is therefore a polar molecule.
That polarity allows water molecules to form hydrogen bonds with one another and interact readily with many other polar or charged substances. These interactions account for much of water’s importance in biology.
Water is an excellent solvent
One of water’s most important biological properties is its ability to dissolve a wide range of substances. A solvent is a substance that dissolves other substances, called solutes.
Water is particularly effective at dissolving ionic compounds and polar molecules. For example, when table salt enters water, its sodium and chloride ions become surrounded by water molecules. The partially negative oxygen atoms orient toward sodium ions, while the partially positive hydrogen atoms orient toward chloride ions. This interaction helps separate and stabilize the ions in solution.
Water also dissolves many biological molecules, including sugars and numerous amino acids. This makes aqueous solutions the setting for much of cellular chemistry.
Not everything dissolves in water. Nonpolar substances such as many fats and oils interact poorly with water. This apparent limitation is itself biologically important: the tendency of nonpolar molecules to avoid contact with water helps drive the formation of cell membranes and influences the three-dimensional structures of proteins.
Water enables biochemical reactions
Cells are chemical systems, and many of their reactions occur in water. Dissolved molecules can move through the cell, encounter one another, and interact. Enzymes, which accelerate specific chemical reactions, generally operate in an aqueous environment.
Water can also be a reactant rather than merely the surrounding medium. In hydrolysis, water is used to break a chemical bond. Digestion provides familiar examples: large biological molecules can be broken into smaller components through hydrolysis.
The reverse process, in which molecules are joined while water is removed, commonly occurs during the formation of biological polymers. Thus, water is directly involved in the construction and breakdown of many of the molecules that make life possible.
Water has unusual thermal properties
Water helps organisms resist rapid changes in temperature because a relatively large amount of heat is required to raise its temperature. This property is related to the hydrogen bonds between water molecules.
Water also absorbs substantial amounts of heat when it changes from liquid to vapor. Evaporation therefore provides an effective cooling mechanism. When sweat evaporates from human skin, for example, energy is removed from the body along with the escaping water molecules.
Water’s thermal behavior also contributes to environmental stability. Large bodies of water can absorb and release considerable heat without changing temperature as rapidly as many other substances would. This moderates temperature changes in aquatic environments and influences climate and weather.
Hydrogen bonding gives water cohesion
Water molecules do not behave as independent particles. Their polarity allows them to form temporary hydrogen bonds with neighboring water molecules. The resulting attraction is called cohesion.
Cohesion contributes to water’s relatively high surface tension. At the surface of water, molecules are pulled toward other water molecules, producing a surface that can resist small disturbances.
Cohesion is also important in plants. Water evaporating from leaves can contribute to an upward pull on the continuous column of water within the plant’s vascular tissue. Together with adhesion—the attraction between water and other surfaces—this helps support the movement of water through plants.
Water behaves differently near surfaces
Water can adhere to other polar materials as well as cohere to itself. Adhesion is the attraction between unlike substances.
The balance between cohesion and adhesion helps explain capillary action, the tendency of water to move through narrow spaces. In plants, interactions between water, the walls of narrow vascular tubes, and neighboring water molecules contribute to the movement of water from roots toward leaves.
These effects become especially important at small scales, where surface interactions can have a large influence on how fluids move.
Ice is less dense than liquid water
Water has another unusual property: its solid form, ice, is less dense than liquid water. As water freezes, hydrogen bonding organizes the molecules into a more open structure. The molecules occupy more space than they do in liquid water, reducing the density.
As a result, ice floats.
This matters biologically because floating ice forms an insulating layer on the surface of lakes and other bodies of water rather than sinking to the bottom. That can help keep the water beneath the ice from freezing solid, allowing aquatic organisms to survive in cold conditions.
Water supports transport throughout organisms
Because water dissolves many substances, it provides an effective medium for transporting materials.
In humans and other vertebrates, blood plasma is largely water and carries dissolved nutrients, ions, hormones, gases, and metabolic wastes. Water is also a major component of the fluid surrounding cells, allowing substances to move between blood and tissues.
Plants depend on water for transport as well. Water moving through the xylem carries dissolved mineral nutrients from roots toward stems and leaves. Sugars and other compounds move through separate vascular tissues in aqueous solutions.
Water therefore connects different parts of an organism chemically as well as physically.
Water helps maintain cell structure
Cells contain large amounts of water, both inside them and in the spaces surrounding them. Water contributes to turgor pressure in plant cells, helping keep tissues firm. When plant cells lose substantial amounts of water, they can lose this pressure and become limp.
Water also influences the behavior of proteins, membranes, and other cellular structures. Biological molecules do not exist in isolation: their shapes and interactions are strongly affected by their surrounding aqueous environment.
The interaction between water and nonpolar portions of molecules is particularly important. In cell membranes, molecules with water-attracting and water-avoiding regions spontaneously organize into structures in which hydrophobic portions are shielded from the surrounding water. This behavior is fundamental to the architecture of biological membranes.
Water is essential for maintaining chemical balance
Cells must regulate the concentrations of dissolved substances inside and outside their membranes. Water movement is central to this regulation.
When two solutions with different concentrations of dissolved substances are separated by a membrane that allows water to cross more readily than certain solutes, water can move toward the side with the higher effective concentration of solutes. This movement is called osmosis.
Osmosis is one reason cells must carefully control their internal environment. If a cell takes up too much water, it can swell and potentially rupture. If it loses too much water, it can shrink and malfunction. Cells use selectively permeable membranes and active transport mechanisms to maintain appropriate concentrations of ions and other solutes.
Water participates in photosynthesis
Water is also a direct source of atoms used in biological energy conversion. During the light-dependent reactions of photosynthesis, plants, algae, and certain bacteria use light energy to split water molecules. This process provides electrons and contributes hydrogen ions to the reactions that ultimately support the production of energy-rich molecules. Oxygen is released as a byproduct.
The oxygen accumulated in Earth’s atmosphere through oxygenic photosynthesis ultimately comes from water molecules that are split during this process.
Water is produced and consumed during metabolism
Water is not simply taken into an organism from the environment. Cellular metabolism can produce water as well.
During aerobic cellular respiration, cells use oxygen to help extract energy from fuel molecules such as glucose, ultimately producing carbon dioxide and water. At the same time, cells continually consume water in reactions such as hydrolysis.
The balance between water intake, metabolic production, and water loss therefore depends on an organism’s physiology and environment.
Why water is so effective at supporting life
Water’s biological importance does not come from one property alone. Its value comes from the way several properties work together.
Its polarity makes it a powerful solvent for many substances. Hydrogen bonding gives it cohesion, surface tension, and unusual thermal behavior. Its high heat capacity helps stabilize temperatures, while its heat of vaporization makes evaporation an effective cooling mechanism. Its interaction with nonpolar molecules helps organize membranes and biological structures. Its ability to participate directly in chemical reactions makes it a reactant as well as a medium.
These properties operate simultaneously inside every living cell. Water dissolves substances so they can move and react; surrounds proteins and nucleic acids as they assume functional structures; transports materials through organisms; buffers temperature changes; and participates directly in the chemistry through which cells obtain energy, build molecules, and break them down.
Life does not merely occur in the presence of water. For essentially every known form of life, water is woven into the physical structure and chemical processes that make living systems possible.
