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Approximately what is the percentage in mass of water of the human body? Is this percentage expected to be larger in the adult or in the elder?

Approximately 65% of the human individual mass is water. The brain, for example, has around 90% of water in mass, the muscles, 85%, and the bones have between 25 and 40% of water.

Younger individuals have proportionally more water in mass than elder individuals.

What are the main biological functions of water?

Water is the fundamental solvent for chemical reactions of the living beings, it is the main means of substance transportation in the cell and between cells and tissues and it is responsible for the maintenance of adequate temperature for the functioning of the organism. Water is also the reagent or the product of many biochemical reactions, like photosynthesis, cellular respiration, peptide bond for protein formation, etc.

Water has key participation in organic reactions. What are examples of two types of organic reactions in which water is respectively incorporated or liberated in the products of these reactions?

Photosynthesis is a biochemical process in which water is incorporated into organic molecules. In the reaction the hydrogen atoms from water go to the produced glucose and the oxygen atoms from water form the molecular oxygen liberated: carbon dioxide + water + light = glucose + molecular oxygen. Aerobic respiration is an example of biochemical reaction in which water is produced: glucose + molecular oxygen = carbon dioxide + water.