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The CELL (You...just a million times smaller)

The Endoplasmic Reticulum is a maze of membranes that surrounds the nucleus. The space inside the ER is called the ER lumen. There can be two types of ERs. There is a smooth ER and a rough ER. The only difference is that the rough ER has ribosomes attached to it. When proteins are made in the ribosomes connected to the rough ER, they are transported through a tunnel into the ER. In the ER, the proteins are modified by the enzymes that the ER contains. After they have been modified, the ER ships the proteins off in a transport vesicle that buds  off the ER. The smooth ER is the site of phospholipid and fatty acid metabolism.

Model of smooth ER (ER without the ribosomes).