Which organelle is primarily responsible for synthesizing proteins?

The ribosome is the organelle primarily responsible for synthesizing proteins. Ribosomes play a critical role in the process of translation, where messenger RNA (mRNA) is read to assemble amino acids in the correct order to form a specific protein. They may be free-floating in the cytosol or attached to the endoplasmic reticulum, creating a rough ER, enhancing the efficiency of protein synthesis for those proteins destined for secretion or for use in membranes.

The ribosome's structure consists of two subunits that come together during protein synthesis, allowing the ribosome to facilitate the binding of tRNA molecules, which bring amino acids to the growing polypeptide chain. The precision of this process is vital for the production of functional proteins necessary for various cellular functions.

In contrast, other options represent roles that, while important in cellular functions, do not involve direct protein synthesis. The nucleus contains genetic material and is key in the transcription phase of gene expression, while mitochondria primarily generate ATP for the cell's energy needs. The Golgi apparatus is involved in modifying, sorting, and packaging proteins synthesized in the rough ER for delivery to their destinations.