Water is essential to life, without water life on earth would not exist. Water is a major part of cells, forming between 70 and 95% of the mass of the cell. This means that we are made from approximately 80% water by mass and some soft bodied creatures such as jellyfish are made of up to 96% water and this is their cause of transparency. Water also provides an environment for organisms to live in, 75% of the earth is covered in water. .
Water is a simple molecule yet fundamental to life. Water is therefore extremely abundant, and biologically it has great importance both inside cells, and externally, for example as a habitat.
Water has been called the 'universal solvent', as more substances can dissolve in it that any other solvent. Often in living organisms, substances must be solution, and water is the solvent which makes this possible. For example, plants can only obtain mineral salts in solution, and human digestion will only dissolve soluble foods, so large starch molecules must be broken down in soluble sugars, such as glucose. The crucial reactions of metabolism take place in the protoplasm with the materials in solution. Water is also crucial in gas exchange as this requires a moist surface, since gas exchange takes place in solution. In mammals, the alveoli in the lungs are moist with water, to allow gas exchange, and many plants have moist surfaces in their leaves (the mesophyll cells) for gas exchange. The use of water as a solvent in humans is very extensive. A main function is to transport materials around the body to where they are needed. Blood plasma, tissue fluid and lymph are all mainly water and are used to dissolve a wide range of substances which can then be easily transported. It dilutes waste products in order to safely remove them from the body such as ammonia and urea. Water is also used to split up molecules by adding water.