Are most rotifers large enough to be observed with naked eye

True

Rotifers, on the other hand, can grow as large as 500 microns (1/2 mm) and can probably be seen with the naked eye, or at least with a 10x magnifying glass.

Yes. There are some cellular Organisms that are large enough to be seen with the naked eye

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.