The essential molecule that comprises an average of 60% of any living organisms' cells is water. Water is a molecule made up of two hydrogen atoms and one oxygen atom. Water is an essential part that makes up the matrix of the inside and outside membranes of, for example, tissues in the human body. Water, to a certain degree, also helps maintain balance of electrolytes and solutes that go in and out of the cell. It also maintains hydrostatic pressure in the circulatory system of the body via the balance of Starling Forces. These are just a few examples of the importance of water and hence why living things need so much of them. This is also why water is one of the first things astronauts look for in planets to determine if life is possible there or if there was life before.
Other important elements that make living things are nitrogen (N) and calcium (Ca). To serve as a reference point, we can look at the human body. The amount of nitrogen (N) in the body is about 3.2%. Calcium (Ca) on the other hand, is about 1.5%