Why Living Organisms Need Water
If carbon is the building block of life, then water is the glue. According to the American Museum of Natural History, humans are about 60 percent water by weight, and about 99 percent of molecules in cells are water. The importance of water does not stop at an organism's skin, however. The blue planet's surface is 71 percent water, while only 3 percent of it is fresh. It turns out water has some special properties that make it essential for life on Earth.
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Cells
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Water is known as the "universal solvent," according to the textbook "Biology" by Campbell and Reese. Water is the medium of exchange and transport in living things. With the exception of lipids (fats), most materials that must be digested and transported through a living organism are dissolved in water. Water diffuses across a cell membrane depending upon the concentration of dissolved molecules on either side of the membrane. This is the all-important process known as osmosis. The process of osmosis enables living cells to maintain the proper balance of nutrients while also eliminating wastes. Water is also liquid at the range of temperatures that predominate on Earth, facilitating its function as a solvent.
Hydrogen Bonds
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Water is a polar molecule, which means its hydrogen end has a slight positive (+) charge and its oxygen end has a slight negative (--) charge. This helps the formation of hydrogen bonds, which are relatively weak bonds caused by electromagnetic forces between positive and negative ends of polar molecules. Hydrogen bonds are important in many organic molecules, but between water molecules they give water some of its most unique properties, such as cohesion and a high specific heat.
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Capillarity
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Water molecules have cohesion (they cling to each other) and adhesion (they cling to other molecules). Cohesion of water molecules at the surface of a liquid causes surface tension. Together, cohesion and adhesion account for capillarity, the tendency of a liquid to move upward through a narrow space against the pull of gravity. Capillarity is the property that facilitates the movement of water to the tops of very tall trees, such as redwoods.
Neutral pH
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Water has a slight tendency to break down into hydrogen (H+) and hydroxide (OH-) ions. Water has a neutral pH of 7, meaning neither acid nor alkaline. The pH inside most cells is between 6.5 and 7.5. It is within this range that many vital cellular reactions can take place at optimum speed.
Heat Buffer
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Water has a high specific heat, that is, it can absorb a lot of heat energy compared to other substances before its temperature increases. This property makes water the ideal substance to ensure that both the external and internal environments of living things stay relatively constant. It also contributes to the effectiveness of water as a coolant, because water absorbs a lot of heat energy from its surroundings to change from the liquid to gas state. For example, the evaporation of sweat feels cool because the water is absorbing heat from the body.
Ice Floats
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Water is the only element that becomes less dense in the solid state, because ice molecules spread out into a lattice network. Thus, ice floats and often only the tops of lakes freeze, which insulates plant and animal life below the ice through the winter. Without this property, lakes and rivers would almost always freeze solid at freezing temperatures.
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References
- Photo Credit Clean water and water bubbles in blue image by Suto Norbert from Fotolia.com
