Final answer:
The small subunit of a prokaryotic ribosome contains one 16S rRNA molecule while the small subunit of a eukaryotic ribosome contains one 18S rRNA molecule, both being essential for the protein synthesis process in cells.
Step-by-step explanation:
The small subunit of a ribosome in prokaryotic cells (like E. coli) contains one ribosomal RNA (rRNA) molecule, specifically the 16S rRNA. In eukaryotic cells, the small subunit, designated as 40S, contains the 18S rRNA molecule. These rRNA molecules play a crucial role in the structure and function of ribosomes, which are the molecular machines responsible for protein synthesis through the process of translation. The small subunits bind to messenger RNA (mRNA) and are important for the decoding of the mRNA sequence into a polypeptide chain.