Hi, I just had 2 questions. 1. In multicellular organisms, are there any undifferentiated cells besides stem cells? Do they play any significant role? 2. Why is it that smooth ER does not have ribosomes attach to it but rough ER can have ribosomes come and attach? Is there a structural difference b/w the two types of ER? All I've been getting is that rough ER has ribosomes (which attach from cytosol) but why can't this happen on smooth ER? Likewise, why does the rough ER not create lipids like the smooth ER? Any help would be much appreciated!