רשתית אנדופלזמית חלקה

The part of the ER that lacks membrane-bound ribosomes is termed smooth endoplasmic reticulum or smooth ER.

It is located near the cell periphery like a loose mesh of tubules connected by three-way junctions spreading throughout the cytoplasm or may exist as bare patches within the rough ER.

Smooth ER has a greater surface area with lower luminal volume compared to rough ER. It forms transitional regions for vesicular transport and contact zones with other organelles.

The absence of ribosomes on the smooth ER helps in the dense localization of lipid synthesis enzymes on its membrane, making it the primary site for the synthesis of lipids, such as phospholipids and cholesterol.

Therefore, cells that secrete higher amounts of lipids than proteins, such as skin oil gland cells, have a significantly higher fraction of smooth ER than rough ER.

In some cases, smooth ER performs specialized functions. For instance, in skeletal muscle cells, a structurally modified version of the smooth ER, called the sarcoplasmic reticulum, regulates calcium ion storage and release.