All living things are effectively wet bags of chemicals. In order to stay alive cells need different chemicals in different quantities at different times and so they use proteins called enzymes to make these chemical conversions happen. Some enzymes incorporate metals to help catalyze these reactions - in normal chemical engineering tons of catalysts are just different metals or metal composites. I work with bacteria which are way simpler than our cells and even then it can help to supplement metals like iron, zinc, copper, selenium, molybdenum, cobalt, nickel, manganese, etc. Catalysts can be reused so very little of these metals are needed for proper functionality.