The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins. Since the RER is engaged in modifying proteins (such as enzymes, for example...The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins. Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, the RER is abundant in cells that secrete proteins. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells.
