What structure is responsible for finishing and packaging proteins for export from the cell?

The Golgi apparatus is the cellular structure responsible for finishing and packaging proteins before they are exported from the cell. This organelle modifies proteins that have been synthesized in the rough endoplasmic reticulum, adding functional groups or sorting them into vesicles that will transport them to their designated locations. The Golgi apparatus essentially acts as a processing center where proteins are refined, packaged, and tagged for delivery either to the cell membrane for secretion or to other areas within the cell.

In contrast, the nucleus is primarily involved in storing genetic information and controlling cellular activities, including gene expression, but it does not play a direct role in the processing or packaging of proteins. Ribosomes are the sites of protein synthesis where amino acids are assembled into polypeptides, but they do not modify or package these proteins. Mitochondria are mainly responsible for energy production through aerobic respiration and do not have a role in protein processing or export. Thus, while all these organelles are essential for cellular function, the Golgi apparatus specifically fulfills the role of finishing and packaging proteins for export.