Living things need water in their cells. Water is the universal solvent - most of the chemicals a cell needs is dissolved and transported in water. Water is an environment where chemical reactions can occur and water is needed in many of those reactions - like the hydrolysis reaction (needed in the break down of food molecules). If a cell becomes dehydrated chemical reactions slow down. Chemical balance of the cell is changed as waste materials are not removed fast enough, food is not broken down, etc. Water also helps regulate the heat that can build up in a cell during chemical reactions. A dehydrated cell = increase in cell temp. Water content of a cell also affects the pH of a cell environment. Certain reactions occur better at certain pH. Homeostasis is maintaining a balance with the the environment. Without water the balance is upset.
