Water is essential for life as we know it on earth. It is used by plants and animals for basic biological processes which would be impossible without the use of water. The origin of all life can be traced back to the water in the Earth�s precambrien seas. Water is also the universal solvent. It reacts with more elements and compounds than any other substance known to man.
    Water is a polar molecule made up of on atom of hydrogen and two atoms of oxygen. It is attracted to itself by hydrogen bonds. Hydrogen bonds are weaker than covalent bonds, but collectively these bonds hold water together and give it its cohesiveness. These bonds are also very important to water�s ability to absorb heat, as without hydrogen bonds water would have a boiling point of -80 degrees C and a freezing point of -100 degrees C.
    In reality, however, water has a boiling point of 100 degrees C and a freezing point of 0 degrees C. The amount of energy needed to raise the temperature of one gram of water by one Celsius degree is called a Calorie. One Calorie is about twice as much energy as you need to warm one gram of most other fluids by the same amount. This makes water much better for regulating the temperatures of animals and the environment.
    Water also has a very high heat of vaporization. Converting one gram of cold water into ice requires 80 Calories of energy. Converting the same amount of very hot water into steam requires 540. The high amounts of energy required to change water from its liquid state make water tend to stay a fluid. The process of freezing water involves slowing down the activity of the water molecules until they contract and enter into a solid state. Once the ice is cooled down to 4 degrees or less, the hydrogen bonds no longer contract, but they become rigid and open, and the ice becomes less dense. Because the ice has become less dense, it floats on liquid water. Water freezes from the top down. Once the top freezes, it acts as an insulator, so that the water beneath it takes a very long time to cool off enough that it freezes. This also traps just enough warmth to keep marine animals alive during the winter.
    The process of turning water into steam is a different story. Because it requires the breaking of water�s hydrogen bonds, this process takes far more energy than it does to turn water into ice. The extra energy that is used in converting water into steam helps keep the overall temperature from getting too hot. In this manner water regulates the temperature of both animals when they sweat, and the earth through evaporation.
    Water affects the earth�s ecosystems in very important ways as well. When water in the earth�s saltwater bodies evaporates into the air. This water vapor then cools off, becomes liquid again, and then falls as rain or snow. The salt is left behind, and the resulting precipitation helps replenish the water in lakes, streams, rivers, and the groundwater supply. However, all of this water eventually flows down to the level of the oceans, and the cycle begins again. Because of this cyclical pattern, water is consided to be a renewable resource. However, some chemical impurities can remain with the water, even through the process of evaporation. These remain in the water and cause problems until they are either filtered out by natural or artificial processes, or until they are diluted enough that they are no longer a problem. Of all the water on the earth, only three percent is fresh. Of that three percent, only 1/3 is considered safe for consumption.
    The properties of water give it the ability to react with different elements and molecules in very interesting ways. Water�s properties allow it to be the focal point of many cellular functions, primarily because of its reactive abilities.
    Ionization is one example of these reactions. This occurs when a water molecule in a hydrogen bond with another one loses an atom of hydrogen. The remaining particle is a hydroxl ion. Micromolecules with different charges than water can cause ionization to happen as well. During the process of ionization water realeases an eaqual number of hydrogen (H+) and hydroxyl (OH-). This dissociation process involves only a few water molecules at once. The actual number is about 10-7 moles/liter).
    Acids [L. acidus, sour] are molecules that release the hydrogen ions in the dissociation process. Strong acids, such as hydrochloric, dissociate almost entirely in water. Bases are molecules that take up these extra hydrogen ions.
    Water passes through pores easily. Cells take advantage of this by having �channels� -- tiny holes in the cell membrane. These are exactly the right size that water can get through them, while larger particles are held inside.
    Osmosis [Gk. Osmo, pushing] is defined by the Sylvia Mader textbook as �the diffusion of water across a differentially permeable membrane�. This process is caused by a fluid attempting to seek equilibrium by going from a high pressure situation into a lower pressure one. This pressure that causes this operation is known as osmotic pressure.
    Another interesting state that water can be in is that of an isotonic solution. These are solutions which neither water is neither gained nor lost, and the pressure is equal on both sides of the cell membrane. When this pressure is not equal, the degree of the inequality is defined as tonicity.
    When the pressure is very unequal, so that the pressure causes water to flow inward, it is known as a hypotonic solution [hypo, less than]. The �less than� prefix refers to a solution with a lower percentage of solute, and which contains more water than the cell. The cell then swells, possibly even to the point where the cell will burst. These exploded cells are referred to as lysis. The pressure that caused them to pop in the first place is referred to turgor [L. turg, swell] pressure.
    The opposite state is referred to as a hypertonic solution [hyper, more than]. The ŒŒmore than� prefix in this word refers to a solution with a higher level of solute, and the cell contains more water than the outside solution. Therefore, a cell in a hypertonic solution tends to shrivel up like a grapefruit in the sun.
    Animals regulate the amount of water in their bodies in very individual ways, each suited for the environment in which they each live. Sharks and fish are able to live in an environment nearly saturated by salt by having a sort a immunity to it. Some sharks survive by making their blood as toxic as the surrounding water.
    Certain seaside animals as well have developed ways to keep the salt in their water from dehydrating them. Some kinds of birds and reptiles have a sort of nasal salt gland which allows them to excrete the large amounts of salt that they take in when they drink. Some mammals as well can live in highly saline environments by making their urine stronger, and having very dry fecal material.



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