Endoplasmic Reticulum

The endoplasmic reticulum makes up 10% of the cell's volume and surounds the nucleus. There are two types of ER: smooth and rough. The smooth ER contains no ribosomes on its membrane, hence the name smooth. This is because the main purpose of the smooth ER is to synthesize lipids and decrease toxicity of the cell. In contrast the rough ER has ribosomes scattered along the membrane because the main purpose of the rough ER is to synthesize, process, and export protiens. In terms of the relationship of the ER to ribosomes, a ribosome will recgonize a translation initation sequence of an mRNA and begin synthesizing the protein. This complex will then bind a tranlocon located on the membrane of the ER. This channel protein will direct the newly synthesized protein into the lumen of the ER. The protein is then processed and can possibly be retained in the lumen, secreted into the cytoplasm, or released in a vesicle.