Which structure in the cell is responsible for producing protein?

The ribosome is the cellular structure primarily responsible for protein synthesis. Ribosomes can be found floating freely within the cytoplasm or attached to the endoplasmic reticulum, forming what is known as rough ER. They translate messenger RNA (mRNA) into polypeptide chains by facilitating the assembly of amino acids in the sequence dictated by the mRNA. This process is crucial for the expression of genes, leading to the production of proteins that perform various functions in the cell.

The Golgi apparatus plays a key role in modifying, sorting, and packaging proteins that have been synthesized in the ribosomes, but it does not produce proteins itself. The nucleus contains the DNA that encodes the instructions for building proteins and is involved in transcription, where mRNA is synthesized from DNA. However, it does not directly synthesize proteins. Mitochondria are primarily known for their role in energy production through cellular respiration and do not play a direct role in protein synthesis. Thus, among the options, ribosomes are distinctly the structures responsible for directly producing proteins.