Endoplasmic reticulum is of two types [a] Rough endoplasmic reticulum: Ribosomes are attached on the surface of rough endoplasmic reticulum by ribophorin proteins. Ribosomes are the site of protein synthesis. [b] Smooth endoplasmic reticulum: This type of endoplasmic reticulum possess smooth wall because the ribosomes are not attached to the surface. SER performs synthesis of lipids (phospholipid, cholesterol etc.) and steroid hormones.