There are two types of ER. Rough ER has large numbers of ribosomes attached to it and is where new proteins are assembled in the cell (see protein translation). Proteins made on the rough ER's ribosomes end up in other organelles or are sent out of the cell to function elsewhere in the body. A few examples of proteins that leave the cell (called secreted proteins) are antibodies, insulin, digestive enzymes, and many hormones. (Proteins made on free-floating ribosomes, by contrast, stay in the cytosol.)
Smooth ER has no ribosomes associated with it and has a very different function: it specializes in synthesizing lipids and also contains enzymes that break down harmful substances. Most cell types have very little smooth ER, but some cells, such as those in the liver, which are responsible for neutralizing toxins – contain lots of it.
Related category• CELL BIOLOGY
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