| The endoplasmic reticulum (ER) is a feature of all [[Eukaryotic cells|eukaryotic cells]], thats main functions are its role in [[Lipid|lipid]] and [[Protein|protein]] biosynthesis and as a store for [[Calcium|calcium]] within the cell. It is a network of internal membranes called cisternae which are continuous with the outer membrane <ref>Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p723</ref>. The ER can be rough ([[Rough endoplasmic reticulum|rough endoplasmic reticulum]]) in which [[Ribosomes|ribosomes]] coat the surface of the ER <ref>Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p724</ref> which synthesize [[Proteins|proteins]] in the ER. In mammalian cells, synthesis of [[Proteins|proteins]] is [[Co-translational|co-translational]], which means [[Proteins|proteins]] are captured from the [[Cytosol|cytosol]] by the ER and imported into it before they are fully synthesized <ref>Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p724</ref>. [[Cells|Cells]] that produce a lot of specific [[Proteins|proteins]] to be secreted have an abundance of [[Rough ER|rough ER]] <ref name="null">Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p376</ref>. Modifications of proteins to be secreted such as [[Glycosylation|glycosylation]] or formation of [[Disulphide bonds|disulphide bonds]] take place in the lumen of the ER <ref name="null">Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p376</ref>. The ER can also be smooth ([[Smooth endoplasmic reticulum|smooth endoplasmic reticulum]]) which lack a coating of [[Ribosomes|ribosomes]]. Transport vesicles bud off from the smooth ER, which carry the newly synthesized [[Proteins|proteins]] and [[Lipids|lipids]] and transport their contents to the [[Golgi apparatus|Golgi apparatus]] <ref name="null">Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p725</ref>. Here they are modified further, before being transported to their final destination <ref name="null">Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p15</ref>. As the ER is also a store of [[Calcium|calcium]], the ER contains a [[Calcium pump|calcium pump]] which transports [[Calcium|calcium]] [[Ions|ions]] from the [[Cytosol|cytosol]] into the ER lumen <ref name="null">Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p725</ref>. Cells which require rapid responses to extracellular signals, such as muscle cells, require transport of [[Calcium|calcium]] [[Ions|ions]] into the [[Cytosol|cytosol]] from the ER (which is the uptaken back into the ER.) This is why muscle cells have many modified smooth ER, named the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]], so that [[Calcium|calcium]] is available to trigger [[Myofibril|myofibril]] contraction during muscle contraction <ref>Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p726</ref>.<br>
| | See [[Endoplasmic Reticulum]] |