Consider what kind of energy-generating catabolic processes require mitochondria. Also, what kind of ATP-generating catabolism occurs in the cytosol? Somehow, a cell needs ATP energy. If it can’t get it from the mitochondrial processes, then it must generate that energy from cytosolic processes. What does that leave?

This assumes that the mitochondrion-free cells you are observing are nucleated. If they are prokaryotes, all bets are off. Prokaryotes do some outrageous metabolic tricks with only a plasma membrane to set up potential gradients.

There is also a very common cell type in all mammals that lacks mitochondria. Though it’s energy politics are bit different from the mainstream anyway…

Think about blood and one of its main functions!

Biohazard, you would bring up the case of a eukaryotic cell that has no nucleus. Just when I think I’ve written something tidy, the real biology appears…

Heh, I thought you just wanted to concentrate on nucleated cells and I was thinking that maybe I should mention also these anucleated ones, just in case the original poster would be interested about them as well 🙂