Ribosomes are composed of specialized RNA (rRNA) and protein. Ribosomes are where amino acids are assembled into proteins in the cell. In other words, the translation of mRNA sequence into protein sequence occurs on the ribosomes. For this reason, ribosomes are centrally important for gene expression. Most cellular proteins are manufactured on ribosomes free floating in the cytoplasm, while secretory proteins and most membrane proteins are synthesized on the endoplasmic reticulum. Over the years, the MCAT has found a number of ways to see if students have this basic knowledge. Most cellular proteins are synthesized free floating in the cytoplasm, while secretory and membrane proteins are synthesized on the rough ER.