
Advanced cells have specialized organelles within, such as mitochondria (so named because they look like tiny threads). Inside the mitochondria are the molecular machines which convert the chemical energy we eat as food into high energy forms that power many of the chemical reactions upon which life depends.
We have learned that mitochondria are the remains of bacteria that once were parasites inside their host cells. They have long since lost the machinery that would enable them to live on their own. Somewhere along the line they lost most (but not all) of their original DNA and turned into the batteries that power life.
Life is not an object. It is a force, like gravity and electromagnetism. Life is a property of an assembly of molecules. During the life of a cell, virtually all its components can disassemble and be replaced with new ones. Life transcends its components.