Which structure is primarily responsible for protein synthesis in the cell?

Ribosomes are the critical structures responsible for protein synthesis in the cell. They are the molecular machines that translate messenger RNA (mRNA) into amino acid sequences, which then fold into functional proteins. Ribosomes can be found free-floating in the cytoplasm or bound to the endoplasmic reticulum, which is known as rough ER due to its ribosome-studded appearance. This process of translation involves reading the sequence of codons in the mRNA and matching them with the appropriate transfer RNA (tRNA) molecules that carry specific amino acids, allowing the ribosome to string these amino acids together in the correct order to form a polypeptide chain.

The other structures listed have distinct functions unrelated to protein synthesis. Lysosomes are primarily involved in breaking down waste materials and cellular debris through hydrolytic enzymes. Peroxisomes contain enzymes that detoxify harmful byproducts of metabolism, such as hydrogen peroxide. Vesicles serve as transport containers within the cell, moving materials to specific locations, but they do not play a direct role in the synthesis of proteins. Therefore, ribosomes are unequivocally the central players in the process of protein synthesis.