DS had to do this for science in middle school.
We made a thick sheet cake, and cut it out to the shape of the cell, then frosted it. I then bought candy in the shapes of the ribosomes, lysosomes nucleus etc.
We used licorice, chocolates, jelly beans etc. It came out really cute and we made it big enough so the whole class could have a piece!
It was quite expensive to buy all that different shaped candy. There were quite a few shapes, but it was fun!