What is metabolism ? Can metabolism considered as a defining characteristic of living organism ? 

Explanation

All living organisms are made of chemicals.
These chemicals belonging to various classes, sizes, functions etc. are constantly being made and changed into some of other biomolecules.
These conversions are chemical reactions or metabolic reactions.
There are thousands of metabolic reactions occuring simultaneously inside all living organisms.
All plants, animals, fungi and microbes exhibit metabolism.
The sum total of all chemical reactions occuring in our body is metabolism.
Metabolic reactions can be demonstrated outside the body in cell-free systems.
An isolated metabolic reaction outside the body of an organism, performed in a test tube is neither living nor non-living.
Hence, while metabolism is defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things but surely living reactions.