Here's why:
* Rough ER: This part of the ER is studded with ribosomes, which are the protein-making machinery of the cell. Proteins made by ribosomes on the rough ER are folded and modified as they move through its channels.
* Smooth ER: This part of the ER lacks ribosomes and is involved in lipid and steroid synthesis, detoxification, and calcium storage.
Other cell parts involved in transport:
* Golgi apparatus: After leaving the ER, proteins are often transported to the Golgi apparatus, where they are further modified, sorted, and packaged into vesicles for transport to other parts of the cell or for secretion outside the cell.
* Vesicles: These small membrane-bound sacs bud off from the ER and Golgi apparatus and act like delivery trucks, transporting materials within the cell or to the cell's exterior.
So, while the ER is the main "highway" for protein and other molecule transport within the cell, it works together with other organelles to ensure efficient and accurate movement of materials.
