What is essential to living things because many of the chemical reactions that occur in living things require it?

Oxygen, carbon dioxide (for respiration and photosynthesis), nitrogen (needed to make proteins which in turn are needed for enzymes and hormones), hydrogen (to be combined with carbon and oxygen to make organic compounds or food for living organisms), and sunlight.

Water because water acts as a solvent to chemical reactions. If we wouldn't have water maybe there wouldn't be any living things.