THE CELL CYTOPLASM/ENDOPLASMIC RETICULUM
The endoplasmic reticulum (ER) is a membrane system in the cytosol (the fluid component of the cytoplasm). The membranes of the ER are phospholipids, composed of C, H, O, and P. These membranes contain enzymes (proteins composed primarily of C, H, O, and N) that carry out lipid and protein synthesis. The ER can transport materials within the cell and is a site of protein and lipid synthesis. Proteins synthesized in the ER are made by ribosomes, utilizing the protein pattern coded in messenger RNA. ER with ribosomes is called rough ER. Glycoproteins are an example of the proteins produced and modified in the ER. These glycoproteins, which have carbohydrates added to them, are packaged in membranous sacs called vesicles. These vesicles travel through the cytosol to a Golgi complex.