Which structure is directly involved in protein synthesis?

Ribosomes are the structures directly involved in protein synthesis. They play a crucial role in translating messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins. During this process, ribosomes facilitate the binding of transfer RNA (tRNA), which carries amino acids to the ribosome. The ribosome moves along the mRNA strand and catalyzes the formation of peptide bonds between the amino acids, effectively constructing a protein. This function is essential for cellular activities, growth, and repair, making ribosomes vital to the overall functioning of the cell.

In contrast, chloroplasts primarily function in photosynthesis, lysosomes are involved in breaking down waste materials and cellular debris, and the cell membrane serves as a protective barrier that regulates what enters and exits the cell. Each of these structures has important roles, but only ribosomes are directly responsible for synthesizing proteins.