We can have a better handle on what happens inside a given cell than the complex web of cause and effect that happens between populations of different kinds of cells and their environment.
What goes on inside your gut is still more of a mystery than what goes on inside your cells.
http://en.wikipedia.org/wiki/Cellular_respiration
http://en.wikipedia.org/wiki/Electron_transport_chain
One of the fundamental biochemical pathways and required reading for entry level life-sciences degree programs.