Which structure is responsible for transporting proteins within a cell?

The Golgi complex, also known as the Golgi apparatus, is the primary structure responsible for transporting proteins within a cell. It plays a crucial role in the post-translational modification, sorting, and packaging of proteins synthesized in the endoplasmic reticulum (ER) before they are sent to their respective destinations inside or outside the cell.

Proteins produced in the rough ER are transported in vesicles to the Golgi complex, where they undergo modifications such as glycosylation and phosphorylation. Once processed, the Golgi apparatus directs the proteins to various locations, which can include secretion out of the cell, incorporation into the cell membrane, or delivery to lysosomes.

In contrast, the nucleus contains the cell's genetic material and is primarily involved in processes like transcription and replication of DNA. The cell wall provides structural support and protection but does not play a role in protein transport. The nucleolus, located within the nucleus, is involved in ribosome production rather than in the transport of proteins. Therefore, the Golgi complex is integral to the cellular transport system, making it the correct answer.