The structure that has ribosomes attached to it and helps transport proteins throughout the cell is the endoplasmic reticulum (ER).

Here's why:

* Ribosomes: Ribosomes are the sites of protein synthesis. The rough endoplasmic reticulum (RER) is studded with ribosomes, giving it its "rough" appearance.

* Protein Transport: The ER acts as a network of interconnected membranes that fold and modify newly synthesized proteins. It also packages proteins into vesicles that can then be transported to other parts of the cell, such as the Golgi apparatus or the cell membrane.

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