Smooth Endoplasmic Reticulum: Structure, Functions and Diagram

Endoplasmic Reticulum is a network of membranes that is present in most of the portion of a cell’s cytoplasm. The Endoplasmic Reticulum is composed of a double membrane enclosed by luminal space and it separates the lumen from the cytosol. Endoplasmic Reticulum is of two types, that is, smooth ER and Rough ER.

Smooth Endoplasmic Reticulum, also called Smooth ER or SER, is a membrane-bound cell organelle which is present near the outer membrane of the nucleus. It is a meshwork of tube-like membrane-bound structure which is responsible for synthesis and storage of lipids. The only difference between the smooth ER and rough ER other than their functions and location is that smooth ER does not have Ribosome, a protein factory, present on its membrane. Due to the absence of Ribosome on the Smooth Endoplasmic Reticulum, it appears smooth and while Ribosome is attached on the membrane of the rough Endoplasmic Reticulum, it appears rough.

Structure of Smooth Endoplasmic Reticulum

The membranes of Smooth Endoplasmic Reticulum are highly curved and tubular, forming an interconnected system present in the cytoplasm and near to plasma membrane. According to some theories proposed by the scientists, the Smooth ER is an extension of the Rough ER which is itself an extension of the outer membrane of the nucleus. After homogenization (separation) the Rough Endoplasmic Reticulum forms the fragments of rough vesicles while in case of Smooth ER smooth-surfaced vesicles are formed.

A protein called Reticulon is present in Smooth Endoplasmic Reticulum which is responsible for the curvature and bending of the Smooth ER. The membrane of the Smooth ER does not contain Ribosome therefore it is not involved in protein synthesis and is also not a part of the endomembrane system.

Functions of Smooth Endoplasmic Reticulum

Smooth ER is responsible for synthesis and storage of lipids like cholesterol and phospholipids which are required for building a cell’s plasma membrane. Smooth ER is extensively developed in skeletal muscles, kidney tubules and steroid producing endocrine glands. In liver and heart, special structures called Glycosomes are present which are associated with the Smooth Endoplasmic Reticulum. Glycosomes are the protein moieties that corresponds to the enzymes involved in the synthesis of Glycogen.

Following is a list of some of the most important functions performed by Smooth Endoplasmic Reticulum.

• Synthesis of steroid hormones in endocrine cells of gonads and adrenal cortex

• Lipid Synthesis: Lipids are rapidly transferred to the target cell after being synthesised from by the Smooth ER.

• Sequestration of calcium ions: The regulated release of calcium ions from Smooth Endoplasmic Reticulum of skeletal and cardiac muscles regulates the contraction of the cell. Endoplasmic Reticulum of muscle cells is known as Sarcoplasmic Reticulum.

• Smooth Endoplasmic Reticulum also releases calcium ions when a cell receives nerve impulse and cause muscle contraction.

• It is also found in the smooth muscles cells of the body along with skeletal muscle cell but here it is more loosely organised compared to the smooth ER of the skeletal muscle cells.

• Detoxification: Wide range of compounds like ethanol, barbiturates and drugs are detoxified by using the enzymes of Oxygenase in the cells of the liver.
• SER is involved in the conversion of glycogen to glucose because it hosts an enzyme called glucose 6 phosphatase which is important in catalyzing the final step of glucose formation in the liver.