Why did the mitochondria go up the stairs of the hospital? Also, does anybody find the ending quite strange, the biologist is on fire and in the next moment he's lighting a candle?
If this is explained in the book (which I haven't read) or I didn't pay enough attention to the movie please explain.

The biologist lighting the candle was a flashback to a memory. The most strange part of this movie's climax for me has to be this hospital's security systems, which not only can lock down the entire place one corridor at a time for no practical reason, but that it can also track Eve's movements in real-time albeit out of sync with her actual speed…pretty silly 90s B movie shenanigans going on there.
Cast In The Name Of God…Ye Not Guilty

I could be wrong, but I believe in the beginning she went to the school in order to drop off his birthday/anniversary present and then died shortly after. So I think the ending was just a flashback to how he spent his birthday/anniversary, and what drove him to do what he did.
"Dyin' is the day worth livin' fer!" Captain Barbossa [POTCAWE]

I'm not really sure, but I think that the candle scene is a flashback.