Biology water is the most abundant molecule found in living organisms. most mammals, in fact, are approximately 70% water by weight. about two-thirds of this water is present inside cells. the other one-third is present outside cells (e. g., in blood plasma or other body fluids). why is water so important to cells? question 18 options: water is stored in cells to be used when the organism gets thirsty. almost all chemical reactions in life process occur in solutions with water. the main structural component found in plasma membranes and cell walls is water.