Which cell structure is responsible for transporting proteins and materials within the cell?

The endoplasmic reticulum (ER) is the correct answer because it plays a crucial role in transporting proteins and materials within the cell. The ER comes in two forms: rough and smooth. The rough ER is studded with ribosomes, which are the sites of protein synthesis. After proteins are synthesized on the rough ER, they are then folded and modified. The smooth ER, on the other hand, is involved in lipid synthesis and the detoxification of harmful metabolic byproducts.

Once the proteins are processed, the ER helps in their transport to different parts of the cell or to the Golgi complex for further modifications and sorting. This structural and functional characteristic makes the ER vital for maintaining cellular organization and ensuring that proteins and materials reach their correct destinations.

In contrast, the Golgi complex is primarily involved in modifying, sorting, and packaging proteins but does not directly transport them throughout the cell. The nucleus is associated with housing genetic material and regulating gene expression rather than transporting materials. The cell membrane acts as a barrier and gateway for the cell but does not serve the function of transporting substances internally within the cell.