Water – The “elixir of life” on Earth, and possibly elsewhere…
No single molecule on this planet is more valuable to organisms than water. It forms the bulk of the mass of most organisms and is vital to the performance of the multitude of biochemical processes that support life at the individual, ecosystem, and planetary levels. Water’s value to humans and other life forms is no coincidence – it is the result of this molecule’s unique chemical and physical properties.
We have seen on Earth that where you have liquid water, you have life. Organisms are found in many extreme habitats, such as in hot, acidic springs in Yellowstone National Park, in the waters beneath a glacier-covered lake in Iceland, under rocks in the parched desert, and at crushing pressures in the dark, deep ocean. It is for this reason that explorations of other worlds that search for life focus on detecting liquid water, for we have seen on our planet that where you have liquid water, you have life – however extreme the conditions.
Good candidates within our solar system for the presence of liquid water (past or present) include the planet Mars and several moons of Jupiter. Multiple missions to map the surface of Mars and analyze its geological characteristics with robots have found evidence that suggests Mars may once have possessed liquid water on its surface and may currently house liquid water beneath the surface (Video 1). This makes the presence of life on the Red Planet a past or current possibility. The Galileo spacecraft (1989 – 2003) found evidence of possible liquid water on Jupiter’s moons of Europa, Ganymede, and Callisto, showing more extraterrestrial bodies where life may be a possibility. The close pairing of liquid water and living things is not altogether surprising, as water has many unique properties that are beneficial to organisms.
Water’s atomic structure enables hydrogen bonding
A water molecule (H2O) is comprised of two hydrogen atoms that are covalently bonded to one oxygen atom. This arrangement of atoms leads to a slightly positive charge by the hydrogen atoms and a slightly negative charge by the oxygen atom because the oxygen atom “pulls” the shared electrons in the chemical bond closer to it than to the hydrogen atoms (Figure 1). The presence of these partial positive and partial negative charges enables a water molecule to form hydrogen bonds with other water molecules or different molecules through the attraction of unlike charges.
Water molecules can hydrogen-bond to one another and form a lattice of interconnected molecules. At low temperatures, each water molecule has very little energy, moves very little, and forms ice – a stable lattice of bonded molecules (Figure 2). When temperatures are higher, the molecules will have more energy, will move more, and will form and break bonds with greater frequency. Over these temperatures water exists as a liquid. When water molecules have a great deal of energy at high temperatures, they break and form bonds very rapidly and can escape into the air as gaseous water vapor. When these molecules cool and their activity slows, they can condense into liquid again. Water can also go directly from a solid to a gas (from ice to water vapor) through sublimation.
Foundational Science Box: (1) Matter, elements, atoms, isotopes, and ions; (2) Chemical bonding; (3) Changes in Matter and the Law of Conservation of Matter
One Step Beyond: (1) “The Most Amazing Molecule” This module from the NSF-supported Technology-Enhanced Activity Modules for Science (TEAMS) project provides video tutorials on conducting hands-on science activities on topics in water with elementary school students. http://teams.kennesaw.edu/most-amaz-mol.html
Water’s properties make it the “universal solvent”
Water’s polar properties (some portions of the molecule positively charged, others negatively charged) enable it to dissolve a wide range of substances. Water can bind to positively and negatively charged ions, or portions of other molecules that contain a charge. When bonded water molecules surround a molecule, it dissolves into solution (Figure 3). Proteins, sugars, salts, and a host of important biological molecules dissolve in water in this manner. Due to its ability to dissolve many substances, water is sometimes referred to as the “universal solvent”. This enables water to transport materials inside the cells of organisms and to distribute elements like nitrogen, carbon, and phosphorus through their global cycles.
Water aids life by buffering temperature changes and distributing heat around the planet
The wide range of temperatures for water in its liquid state is due to the fact that it takes a lot of energy to break apart bonded water molecules. The amount of energy needed to change the temperature of a substance is described by its heat capacity. If a substance (such as water) requires large inputs of energy to change temperature, it will have a high heat capacity. If it changes temperature easily, the heat capacity will be low. Water has high heat capacity, and proves to be a useful buffer against rapid temperature changes. This is one reason we use water in radiators and other cooling capacities. The large amounts of water in the bodies of organisms help them to maintain a more constant body temperature which aids the multitude of biochemical reactions that maintain life.
When water does change from a liquid to a vapor, it takes with it large amounts of heat that is later released when it becomes liquid again. This property helps to distribute heat throughout Earth, as water is warmed at the equator, evaporates, and releases heat at higher latitudes through precipitation. Water also acts to cool organisms when they become overheated. When humans sweat, the water evaporating from the skin cools the tissues, reduces elevated temperatures inside the body, and returns body temperatures to their stable temperature range.
Water aided early life on Earth by protecting it from ultraviolet radiation
In addition to its other properties, water also shields organisms from harmful ultraviolet radiation from the sun. Today, we are afforded some protection from ultraviolet light by Earth’s ozone layer. Ozone is a molecule comprised of three bonded oxygen atoms, and it exists in high concentrations in Earth’s stratosphere. Much of the harmful ultraviolet radiation from the sun is screened out by the ozone layer and this protects organisms at the surface. This ozone layer was not present in the early Earth, however, as it built up over billions of years of oxygen generation through photosynthesis. Early life on Earth, long before the establishment of the ozone layer, is therefore hypothesized to have existed in bodies of water, where they would be offered some protection from the ultraviolet radiation bathing the surface.