question: Why is water necessary for life?
comment: Not a biologist, I'm a chemist by trade. As far as actual chemical interaction, water is a great solvent, i.e. breaks down compounds for easier use, think of salt and sugar easily dissolving in water, and this allows for easier use of the ions needed by the body for functioning. It also allows most chemical reactions of the body to take place, allows easy transport of the nutrients you ingest (i.e. broken down chemicals), and simply put its the only thing we know of that supports life. There are better solvents out there for breaking down chemicals but you would not want them in your body as they are lethal to most (if not all) organic life. Granted some of the reactions are assisted by enzymes and other mechanisms in the body, but water still allows for all the necessary reactions to take place.
passages: Section::::Model.
   There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism. For example, sodium, potassium and chloride are common chemicals found in small amounts in most waters, and these elements play a role in body metabolism. Other elements such as fluoride, while arguably beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
  
  Section::::Applications.:Medical use.
  
  Section::::Applications.:Medical use.:Effects of illness.
   There is no life without water. It has been described as the "universal solvent" for its ability to dissolve many substances, and as the "solvent of life". Water is the only common substance to exist as a solid, liquid, and gas under conditions normal to life on Earth. The Nobel Prize winner Albert Szent-Györgyi referred to water as the "mater und matrix": the mother and womb of life.
   in a 2004 interview with Tom Morrow of The San Diego Union-Tribune, Holloway states "our water protects DNA. It acts as a sort of anti-aging process. Heck, it may even be the fountain of youth.”
   Extraterrestrial liquid water
  
  Extraterrestrial liquid water (from the Latin words: "extra" ["outside of, beyond"] and "terrestris" ["of or belonging to Earth"]) is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential for extraterrestrial life.
   Properties of water
  
  Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "universal solvent" and the "solvent of life". It is the most abundant substance on Earth and the only common substance to exist as a solid, liquid, and gas on Earth's surface. It is also the third most abundant molecule in the universe.
   Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue. This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the "solvent of life". It is the only common substance to exist as a solid, liquid, and gas in normal terrestrial conditions.
  
  Section::::Chemical and physical properties.:States.
   Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.
   Water is fundamental to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO in the process (cellular respiration).
   Water is the most abundant substance on Earth and also the third most abundant molecule in the universe, after and . 0.23 ppm of the earth's mass is water and 97.39% of the global water volume of 1.38 km is found in the oceans.
  
  Section::::Reactions.
  
  Section::::Reactions.:Acid-base reactions.
   From a biological standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.
   Section::::Biochemical basis.
  
  Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. Life based on ammonia (rather than water) has been suggested as an alternative, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.
   Drinking water
  
  Drinking water, also known as potable water, is water that is safe to drink or to use for food preparation. The amount of drinking water required to maintain good health varies, and depends on physical activity level, age, health-related issues, and environmental conditions. Americans, on average, drink one litre of water per day and 95% drink less than three litres per day. For those who work in a hot climate, up to 16 litres a day may be required. Liquid water, along with air pressure, nutrients, and solar energy, is essential for life.
   Earth surface waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.
   This behavior is thought to have been developed millions of years ago when plants began their journey onto dry land. While this migration led to much easier consumption of CO, it greatly reduced the amount of water readily available to the plants. Thus, strong evolutionary pressure was put on the ability to find more water.
  
  Section::::Mechanism.
   With oceanic water covering 71% of its surface, Earth is the only planet known to have stable bodies of liquid water on its surface, and liquid water is essential to all known life forms on Earth. The presence of water on the surface of Earth is a product of its atmospheric pressure and a stable orbit in the Sun's circumstellar habitable zone, though the origin of Earth's water remains unknown.
   Water is a basic necessity of life. Since 2/3 of the Earth is covered by water, the Earth is also called the blue planet and the watery planet. Hydrosphere plays an important role in the existence of the atmosphere in its present form. Oceans are important in this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of Mercury. Later the gases hydrogen and helium were expelled from the atmosphere. The gases and water vapor released as the Earth cooled became its present atmosphere. Other gases and water vapor released by volcanoes also entered the atmosphere. As the Earth cooled the water vapor in the atmosphere condensed and fell as rain. The atmosphere cooled further as atmospheric carbon dioxide dissolved in to rain water. In turn this further caused the water vapor to condense and fall as rain. This rain water filled the depressions on the Earth's surface and formed the oceans. It is estimated that this occurred about 4000 million years ago. The first life forms began in the oceans. These organisms did not breathe oxygen. Later, when cyanobacteria evolved, the process of conversion of carbon dioxide into food and oxygen began. As a result, Earth's atmosphere has a distinctly different composition from that of other planets and allowed for life to evolve on Earth.
   Plants share with animals the problems of obtaining water but, unlike in animals, the loss of water in plants is crucial to create a driving force to move nutrients from the soil to tissues. Certain plants have evolved methods of water conservation.
   In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic minerals generally enter surface water and ground water via storm water runoff or through the Earth's crust. Treatment processes also lead to the presence of some minerals. Examples include calcium, zinc, manganese, phosphate, fluoride and sodium compounds. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake.
   Section::::Aquatic organisms.
  
  Water is a critical issue for the survival of all living organisms. Some can use salt water but many organisms including the great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals, especially desert rodents, appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, and they also have mechanisms to conserve water to the maximum degree.
   Unlike other analogous hydrides of the oxygen family, water is primarily a liquid under standard conditions due to hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at timescales faster than 200 femtoseconds (2×10 seconds).
  
  However, these bonds are strong enough to create many of the peculiar properties of water, some of which make it integral to life.
  
  Section::::Physical properties.:Water, ice, and vapor.
   Global biogeochemical cycles are critical to life, most notably those of water, oxygen, carbon, nitrogen and phosphorus.
  
  BULLET::::- The nitrogen cycle is the transformation of nitrogen and nitrogen-containing compounds in nature. It is a cycle which includes gaseous components.
  
  BULLET::::- The water cycle, is the continuous movement of water on, above, and below the surface of the Earth. Water can change states among liquid, vapour, and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go.
   Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons:
  
  BULLET::::1. It constitutes 80%-95% of the plant's protoplasm.
  
  BULLET::::2. It is essential for photosynthesis.
  
  BULLET::::3. It is the solvent in which nutrients are carried to, into and throughout the plant.
  
  BULLET::::4. It provides the turgidity by which the plant keeps itself in proper position.
   Section::::Definitions.:Biology.
  
  Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This characteristic exhibits all or most of the following traits:
  
  BULLET::::1. Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
  
  BULLET::::2. Organization: being structurally composed of one or more cells – the basic units of life
   Organisms involved in water purification
  
  Most organisms involved in water purification originate from the waste, wastewater or water stream itself or arrive as resting spore of some form from the atmosphere. In a very few cases, mostly associated with constructed wetlands, specific organisms are planted to maximise the efficiency of the process.
  
  Section::::Role of biota.
  