How do we know how cells function 
In previous lessons, you have learned that all living cells share the same mechanism. They were evolved from an ancestral cell. What it means is that if we know how this cell of one species functions this way, it is likely the other cells of other species function the same. Some cells are easier to study. They reproduce fast, show high adaptation to the environment, being transparent, etc. so it is generally a good idea to focus on these species. At the moment, efforts are being spent mostly on E. coli, yeasts, worms, flies, mice and human.