ribosome is a site for protein synthesis??

to be more specific, a ribosome is not a SITE, but a one and only COMPONENT for protein synthesis. it's not usual to find ribosomes swimming freely in the cytosol

so ribosomes have to attach to a surface--that is the rough ER. This means that the rough ER (not ribosomes) is the SITE for protein synthesis. So to make a long story short, without ribosomes, the rough ER CANNOT make proteins