Living organisms need water to survive. All oxygen-dependent organisms need water to aid in the respiration process; some organisms, such as fish, cannot breathe outside its presence, while other organisms need water to help break down food molecules or generate energy during the respiration process. According to Chemistry for Biologists, water is also used to help regulate metabolism and dissolve compounds going into or out of the body.
Chemistry for Biologists says there are four basic uses for water in living organisms: as a solvent, as a temperature buffer, as a metabolite, and as a living environment.
As a solvent, water dissolves molecules such as sugar; as a temperature buffer, it helps regulate body temperature; as a metabolite, it regulates important chemical reactions within the body; and as a living environment, it enables organisms such as fish to breathe.
Water attracts both positive and negative ions, because of the nature of the chemical bonds in water. Thus, positive ions are attracted to the oxygen in water, while negative ions are attracted to the hydrogen. This allows water to dissolve compounds important for survival, such as glucose gleaned from ingesting food.
Temperature regulation is vital for chemical reactions important to cellular activity, such as cellular respiration. Enzymes, or proteins that act as catalysts to start chemical reactions, are heat-sensitive and will operate only at specific temperatures.
According to Chemistry for Biologists, water has a relatively high specific heat capacity, meaning it takes a lot of heat to raise its temperature. Thus, water absorbs much heat without the temperature of the organism being raised. This prevents enzymes from becoming overheated and failing to function.
Cells work via chemical reactions. The sum total of chemical reactions within an organism is called metabolism. Water is a metabolite, or a chemical involved in reactions. It is necessary for the continued survival of both plants and animals.
In plants, water aids in photosynthesis, the process by which plants convert sunlight into food. During the first stage of photosynthesis, water is split into hydrogen and oxygen atoms. Oxygen is released into the atmosphere, while hydrogen is used in the rest of the chemical reaction to produce glucose to feed the plant.
In animals, water aids in respiration. Water is used to split adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and phosphoric acid. Cellular energy is released as a byproduct of this process. Water formation from oxygen and depleted hydrogen also moves waste products out of the body after the respiration cycle is complete.
Water-based organisms such as fish require water to breathe. According to the Northeast Fisheries Science Center, these organisms directly breathe oxygen dissolved in water. Without a water supply, they could not access oxygen and would suffocate.
Water also helps insulate the living environment for these organisms. Chemistry for Biologists says that water is kept warm for fish during the winter months by ice on the surface of the water.