Proteins made on the rough ER include those that will be released, or secreted, from the cell as well as many membrane proteins and proteins destined for lysosomes and other specialized locations within the cell. Rough ER is abundant in cells that produce large amounts of protein for export. Other cellular proteins are made on “free” ribosomes, which are not attached to membranes.

The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on its surface. In many cells, the smooth ER contains collections of enzymes that perform specialized tasks, including the synthesis of membrane lipids and the detoxification of drugs. Liver cells, which play a key role in detoxifying drugs, often contain large amounts of smooth ER.

Golgi Apparatus In eukaryotic cells, proteins produced in the rough ER move next into an organelle called the Golgi apparatus, which appears as a stack of flattened membranes. As proteins leave the rough ER, molecular “address tags” get them to the right destinations. As these tags are “read” by the cell, the proteins are bundled into tiny vesicles that bud from the ER and carry them to the Golgi apparatus. The Golgi apparatus modifies, sorts, and packages proteins and other materials from the endoplasmic reticulum for storage in the cell or release outside the cell. The Golgi apparatus is somewhat like a customization shop, where the finishing touches are put on proteins before they are ready to leave the “factory.” From the Golgi apparatus, proteins are “shipped” to their final destination inside or outside the cell.

In Your Notebook Make a flowchart that shows how proteins are assembled in a cell.

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