I never thought I'd see a white blood cell with a gun, but here we are!
I liked the aesthetics and the especially art a lot. The gameplay is quite fresh and overall, and the music fits well. I do have a couple of nitpicks though; I'd sometimes confuse the direction that I was going because of the front and back of the mitochondria looking very similar, especially in high speeds (this can be fixed with a pointing arrow), a high-score display would be nice addition because the ATP quickly goes down when moving fast, and adding a sound effect for getting energy would go a long way to make the game feel more juicy.
Overall, it was a really nice experience to play your game. This is the highest score I managed to screenshot:
