“…..and We made from water every living thing? Then will they not believe?” (Quran_Al Anbiyaa’: 30)
As astronomers study newly discovered planets orbiting distant stars, they hope to find evidence of wateron these far-off celestial bodies, for water is the substance that makes possible life as we know it here on Earth. All organisms familiar to us are made mostly of water and live in an environment dominated by water. Water is the biological medium here on Earth, and possibly on other planets as well.
Three-quarters of Earth’s surface is submerged in water (Figure 3.1). Although most of this water is in liquid form, water is also present on Earth as ice and vapor. Water is the only common substance to exist in the natural environment in all three physical states of matter: solid, liquid, and gas. The abundance of water is a major reason Earth is habitable. In a classic book called The Fitness of the Environment, ecologist Lawrence Henderson highlights the importance of water to life. While acknowledging that life adapts to its environment through natural selection, Henderson emphasizes that for life to exist at all, the environment must first be a suitable abode.
Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Modern life, even terrestrial (land-dwelling) life, remains tied to water. An living organisms require water more than any other substance. Human beings, for example, can survive for quite a few weeks without food, but only a week or so without water. Molecules of water participate in many chemical reactions necessary to sustain life. Most cells are surrounded by water, and cells themselves are about 70-95% water.
What properties of the simple water molecule allow it to function as a support to all living organisms? In this article, you will learn how the structure of a water molecule allows it to interact with other molecules, including other water molecules. This ability leads to unique emergent properties that support and maintain living systems on our planet. Your objective in this chapter is to develop a conceptual understanding of how water contributes to the fitness of Earth for life.
The Polarity of water molecules results in hydrogen bonding
Water is so common that it is easy to overlook the fact that it is an exceptional substance with many extraordinary qualities. Following the theme of emergent properties, we can trace water’s unique behavior to the structure and interactions of its molecules.
Studied in isolation, the water molecule is deceptively simple. It is shaped something like a wide V, with its two hydrogen atoms joined to the oxygen atom by single covalent bonds. Because oxygen is more electronegative than hydrogen, the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen; in other words, they are polar covalent bonds (see Figure 2.13).
This unequal distribution of electrons makes water a polar molecule, meaning that the two ends of the molecule have opposite charges: The oxygen region of the molecule has a partial negative charge (&-), and the hydrogen have a partial positive charge (&+).
The anomalous properties of water arise from attractions between its polar molecules: The slightly positive hydrogen of
one molecule is attracted to the slightly negative oxygen of a nearby molecule. The two molecules are thus held together by a hydrogen bond (Figure 3.2). Vhen water is in its liquid form, its hydrogen bonds are very fragile, each about Y’Q as strong as a covalent bond. The hydrogen bonds form, break, and re·form with great frequency. Each lasts only a few trillionths of a second, but the molecules are constantly forming new hydrogen bonds with a succession of partners.
Therefore, at any instant, a substantial percentage of all the water molecules are hydrogen-bonded to their neighbors. The extraordinary qualities of water are emergent properties resulting from the hydrogen bonding that orders molecules into a higher level of structural organization.
Four Emmergent properties of water contribute to Earth’s fitness for life
We will examine four emergent properties of water that contribute to Earth’s suitability as an environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.
Direction of water movement
Water molecules stay close to each other as a result of hydrogen bonding. Although the arrangement of molecules in a sample of liquid water is constantly changing, at any given moment many of the molecules are linked by multiple hydrogen bonds. These linkages make water more structured than most other liquids. Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion.
Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants (Figure 3.3). Water from the roots reaches the leaves through a network of water-conducting cells. As water evaporates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots. Adhesion, the clinging of one substance to
another, also plays a role. Adhesion of water to cell walls by hydrogen bonds helps counter the downward pull of gravity (see Figure 3.3).
Related to cohesion is surface tension, a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than most other liquids. At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below. This makes the water behave as though coated with an invisible film. You can observe the surface tension of water by slightly overfilling a drinking glass; the water will stand above the rim. In a more biological example, some animals can stand, walk, or run on water without breaking the surface (Figure 3.4)