Duo is talking biology here. My dictionary tells me a prison cell is a 'celda'
Cell phone= celular Prison cell= celda Biological cell= célula (Wow, my first comment that isn't a joke. Hope this helps! Unless you were joking in which case this is embarrassing for me.)
It is talking about animal cells because this subject is about nature
Why does nobody care about the plants? Plants also have cells, you know? :^)
How would you say cells are interesting? Would it be: Células son interesantes. Or do you still need the Las?