Which organelle is primarily responsible for protein synthesis?

Ribosomes are the organelles primarily responsible for protein synthesis. They can be found floating freely in the cytosol or bound to the rough endoplasmic reticulum, which assists them in synthesizing proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. Ribosomes translate messenger RNA (mRNA) into polypeptide chains, effectively constructing proteins by linking together amino acids in the order specified by the mRNA sequence. This fundamental role makes ribosomes essential for cellular functions, as proteins are vital for a myriad of processes, including structure, enzymes, signaling, and metabolic pathways.

In contrast, while the nucleus harbors the cell's genetic information and is involved in the transcription of DNA to mRNA, it does not directly participate in the assembly of proteins. Mitochondria are mainly known for their role in energy production through cellular respiration and metabolism. The smooth endoplasmic reticulum is involved in the synthesis of lipids and detoxification processes rather than protein synthesis.