Energy

Autotrophs make their own food and heterotrophs must consume food from an outside source. ATP is the energy source of cells.

Cell Specialization

Animal cells and plant cells.  Fungal cells and bacteria cells.  Turns out cell are even more diverse than that.  A unicellular organism, like a typical bacterium, grows and operates alone.  It may hang out with other cells, but they are all on their own.  Some organisms however, are multicellular, and in more complex organisms these [...]