1. Smooth endoplasmic reticulum (agrannular form). The outer surface of the membranes of such endoplasmic reticulum is smooth and does not bear ribosomes. The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

2. Rough endoplasmic reticulum (grannular from). bears ribosomes on its surface. Due to the presence of ribosomes the outer surface appears to be grannular or rough. Rough endoplasmic is continuos with the outer membrane of the nucleus. It is found in cells actively engaged in protein synthesis e.g. cells of pancreas, plasma cells, liver cells, goblet cells etc.