If you ment A living organism, then no it isn't. But I'm sure you ment in living organisms. Then yes, it is an important element found in most organisms and is used for a great range of functions. One example would be pH regulation in the blood, or any other liquid.
Oxygen burns hydrogen in the living system, releasing the energy that runs living beings.
Catalase Catalase
carbon
Well if you put magnesium with hydrochloric acid, the magnesium reacts with it and creates hydrogen and magnesium chloride. Here is the word equation... Magnesium + Hydrochloric acid --> Magnesium chloride + Hydrogen
consist of organisms and the non living things.
Oxygen burns hydrogen in the living system, releasing the energy that runs living beings.
Hydrogen
water
If you mean element-wise, living organisms contain mostly carbon, oxygen, hydrogen.
Carbon and hydrogen
Catalase Catalase
oxygen hydrogen and carbon
The predominant elements in living organisms are the four CHON elements; Carbon, Hydrogen, Oxygen and Nitrogen.
Hydrogen
Carbon, hydrogen, nitrogen and oxygen
Hydrogen
carbon, hydrogen, oxygen, nitrogen, calcium, phosphorus