Shifting between three temperature-dependent states (solid, liquid, gas), water is a fluid over a range of temperatures commonly found on or near Earth's surface. Water is internally polarized (its molecules are electrically charged); it dissolves, transports, and gives shape to many organic substances; and it moderates its surroundings by storing a great deal of heat. All of which is of more than academic interest because of the profound impact such qualities have on life forms of every sort.

Everyone knows how slowly water heats up. And those who have burned their tongues on steaming coffee know how readily, once hot, it shares its heat. Water is said to have a high specific heat, meaning that its molecules are so attracted to one another, it takes a strong outside influence to get them to release their hold. Heat is a measure (in calories or joules) of molecular motion, temperature is a measure (in degrees) of molecular velocity or speed. When we heat water by adding energy to it, we make its molecules more active, forcing them to move faster until, at the boiling point, they rise into the air as a gas. Conversely, when we cool water by withdrawing energy from it, we make its molecules less active, causing them to move slower until, at the freezing point, they solidify as crystals of ice.

That single fact, water's resistance to change due to the mutual attraction between its molecules, allows organisms, which are largely made of water, to maintain steady internal temperatures in settings which run alternately hot and cold. If water did not have that property, Earthlings would be at the mercy of their surroundings, overheating in summer sunlight, freezing on winter nights, finding it impossible to survive in the most temperate climate, or even in the ocean, which would be as thermally fickle as air.

But water does have the property of holding heat, so Earthlings containing large amounts of water (as every species does) can survive for a time everywhere from the equator to the poles, the depths of the ocean to the peak of Everest. Blood and sap do not readily boil nor turn to ice. That, in survival terms, has made all the difference.

Water expands in volume as it nears the freezing point, causing ice to float because it displaces a greater amount of water than it itself contains. As a result, streams and ponds freeze top-down rather than bottom-up. Which is fortunate for overwintering aquatic species, allowing them to survive in above-freezing conditions below the ice-bound surface.

Water has a high heat of evaporation, which means it must absorb a great deal of heat to turn from a liquid to a gas. As a result, evaporation is a cooling process, enabling many species to maintain an equable temperature even in direct sunlight (Wessells and Hopson 1988, 37).

Another property of water stemming from the ready bonding of its molecules is the tendency of small quantities to form drops. Because of the arrangement of its atoms, a water molecule is electrically charged at different parts of its anatomy. As a fluid, water molecules line up pole-to-pole (negative to positive, positive to negative), arranging themselves in a fluid lattice as a kind of liquid crystal which, if lasting less than a billionth of a second before realigning, causes water in small amounts to take definite shape. Forming as many internal bonds as it can at any moment, water tends to assume shapes with small surface areas for a given volume--that is, to form drops.

Which seems both obvious and trivial, until the effect of that tendency on other substances is considered. Fats, oils, and waxes do not dissolve in water because their molecules are not electrically charged. Instead, the attraction between water molecules rounds up such substances and forces them into minimal shapes which take up as little space as possible. Because water is self-organizing, it imposes regularity on them as well, pressing them into droplets and thin films. The miraculous nature of that influence is revealed by the fact that cell walls and membranes are composed of oily lipids suspended in a watery environment. That is, the interaction between lipids and water causes cells to form and maintain their shapes, promoting the subdivision of life into units capable of carrying out essential processes undisturbed by the turmoil beyond their walls. Removed from a watery context, cell walls rupture, destroying their internal milieus as havens of life.

Substances that dissolve in water share the benefit of its distribution. Body fluids convey soluble nutrients to every cell, and conduct cell wastes to sites where they can be removed. In the plant world, carbohydrates, organic nitrogen compounds, and hormones are delivered by water. Because glucose dissolves in water, cells can use it as a basic fuel supplying energy to run the processes of life. It is no accident the essential ionic elements (including sodium, potassium, chlorine, calcium, and magnesium) are soluble in water. Every cell is bathed by a solution of such nutrients, inviting it to replenish the stock of materials consumed in its daily affairs.

Life is shaped by water, stabilized by water, and fed by water through the mediating influence of watersheds. As aquifers, lakes, streams, wetlands, springs, and seeps manage water on an environmental scale, so the organs and vessels in plants and animals perform the same tasks on an individual scale in individual members of living species. Transpiration, the transport of water to, and evaporation from, the leaves of plants, results in a flow on the order of 100 gallons of water a day in a single tree. That flow supplies water where it is needed to support photosynthesis in leaf cells, perhaps sixty or ninety feet above the ground where roots tap into the larger flow within the soil below. The vessels in a plant are the capillaries of a watershed. So, too, are the vessels in animals, completing the circuit from sky to earth, to living beings, to cells, and back to the sky again as vapor given off by the outer membranes of life. All life takes part in the exchange of water between sky and earth, every animal and plant, whether representing one cell or a hundred trillion. We participate in the water cycle through the watersheds in which we live. The unity and integrity of the system as a whole depends on the flow and distribution of water in each organism. In a larger sense, those watersheds are an essential part of us. Without them, we couldn't live. (Adapted from Perrin 1994.)