"ricin inhibits the functioning of ribosomes. what part of protein synthesis would be affected by ricin? what effect would this have on a cell's ability to synthesize proteins? "

Ricin comes under the category of functionally related toxins, in a combination known as RIPs (ribosome inactivating proteins). This disables ribosomes and stops the process of protein synthesis.  

They directly associate with and inactivate the ribosomes or modify the factors taking part in the process of translation, generally the elongation step. These proteins bring about depurination of adenine at position 4324 in the 28 S rRNA.  

This further inhibits the generation of a critical-stem-loop configuration to which the elongation factor is considered to combine at the time of the translocation step of translation. The ultimate outcome of this activity is the complete inhibition of cellular translation.