What makes endoplasmic reticulum smooth?
The lack of ribosomes on the surface.
The smooth endoplasmic reticulum (SER) is an organelle that is involved in the production of steroid hormones and (phospho)lipids.
The SER is called smooth because it is compared to the rough endoplasmic reticulum (RER). The RER is involved in protein production and folding; for this it requires ribosomes that are associated with the RER membrane, giving it a 'rough' appearance. The SER doesn't require ribosomes for its task and is therefore smooth.
The smooth endoplasmic reticulum lacks ribosomes on its outer surface and hence is smooth.
The smooth endoplasmic reticulum forms a continuous system with the rough portion.
In the liver cell, it makes a tubular network that encompasses large portions of the cytoplasm. These fine tubules are present in regions rich in glycogen.
Smooth endoplasmic reticulum is termed smooth to distinguish it from rough endoplasmic reticulum where ribosomes are scattered on the surface of the membrane.