Water: An Essential Component to Make Life Happen

The origin and maintenance of life on earth depend critically upon water. Water is the most abundant of all compounds in cells, forming 60% to 90% of most living organisms. Water has several extraordinary properties that explain its essential role in living systems and their origin. These properties result largely from hydrogen bonds that form between its molecules.

Water has a high specific heat capacity: 1 calorie is required to elevate the temperature of 1g of water 1° C, a higher thermal capacity than any other liquid except ammonia. Much of this heat energy is used to rupture some hydrogen bonds in addition to increasing the kinetic energy (molecular movement), and thus the temperature, of the water. Water’s high thermal capacity greatly moderates environmental temperature changes, thereby protecting living organisms from extreme thermal fluctuation.

Water also has a high heat of vaporization, requiring more than 500 calories to convert 1 g of liquid water to water vapor. All hydrogen bonds between a water molecule and its neighbors must be ruptured before that water molecule can escape the surface and enter the air. For terrestrial animals (and plants), cooling produced by evaporation of water is important for expelling excess heat.

Another property of water important for life is its unique density behavior during changes of temperature. Most liquids become denser with decreasing temperature. Water, however, reaches its maximum density at 4° C while still a liquid, then becomes less dense with further cooling. Therefore, ice floats rather than sinking to the bottoms of lakes and ponds. If ice were denser than liquid water, bodies of water would freeze solid from the bottom upward in winter and might not melt completely in summer. Such conditions would severely limit aquatic life. In ice, water molecules form an extensive, open, crystal-like network supported by hydrogen bonds that connect all molecules. The molecules in this lattice are farther apart, and thus less dense, than in liquid water at 4° C.

Water is an excellent solvent. Salts dissolve more extensively in water than in any other solvent. This property results from the dipolar nature of water, which causes it to orient around charged particles dissolved in it. When, for example, crystalline NaCl dissolves in water, the Na and Cl ions separate. The negative zones of the water dipoles attract the Na ions while the positive zones attract the Cl ions. This orientation keeps the ions separated, promoting their dissociation. Solvents lacking this dipolar character are less effective at keeping the ions separated. Binding of water to dissolved protein molecules is essential to the proper functioning of many proteins.

Water also participates in many chemical reactions in living organisms. Many compounds are split into smaller pieces by the addition of a molecule of water, a process called hydrolysis. Likewise, larger compounds may be synthesized from smaller components by the reverse of hydrolysis, called condensation reaction.

                                         R-R+H2O→R-OH+H-R

                                        R-OH+H-R→R-R+H2O

Because water is critical to the support of life, the continuing search for extraterrestrial life usually begins with a search for water. Plans for a human outpost on the moon likewise depend upon finding water there. As we write, NASA is planning to crash a space probe into the moon in 2009 in a search for ice; the moon’s south pole is a prime candidate for a human outpost if ice is found there.

Chemical evolution in the prebiotic environment produced simple organic compounds that ultimately formed the building blocks of living cells. The term “organic” refers broadly to compounds that contain carbon. Many also contain hydrogen, oxygen, nitrogen, sulfur, phosphorus, salts, and other elements. Carbon has a great ability to bond with other carbon atoms in chains of varying lengths and configurations. Carbon-to-carbon combinations introduce the possibility of enormous complexity and variety into molecular structure. More than a million organic compounds are known. We review the kinds of organic molecules found in living systems, followed by further discussion of their origins in earth’s primitive reducing atmosphere.

---

External Helpful Links 

• Biological Roles of Water: Why is water necessary for life? by Harvard University.

• Why is water so important for life as we know it? by Astrobiology at NASA.