Smooth Endoplasmic Reticulum: Definition, Structure and Functions of this Membranous Organelle

It is a membranous organelle found in most eukaryotic cells.

It is a subset of the endomembrane system of the endoplasmic reticulum. Its main functions are the synthesis of lipids, steroid hormones, the detoxification of harmful metabolic by-products, and the storage and metabolism of calcium ions within the cell.

The smooth endoplasmic reticulum is more tubular than the rough endoplasmic reticulum  and forms a subcompartment of the interconnecting network of the reticulum.

The smooth endoplasmic reticulum is distinguished from the other parts of the endoplasmic reticulum by the absence of membrane-bound ribosomes. This organelle is also morphologically distinct, often made up of tubular structures called cisternae.

This type of endoplasmic reticulum is prominent in liver cells that process harmful chemicals, in cells of  the endocrine system  , such as those of the adrenal glands that produce steroid hormones, and in excitable cells, such as neurons and muscle cells that They use  Ca  2+ signaling  .

Structure of the smooth endoplasmic reticulum

The smooth endoplasmic reticulum is composed primarily of three-dimensional polygonal networks of tubules called cisternae. They are approximately 50 nm in diameter in mammals and 30 nm in diameter in yeast.

The large curvature of these structures needs to be stabilized by many proteins, including reticulons, DP1, and proteins that enhance receptor expression.

These proteins fold the membrane through structural elements that bind in the lipid bilayer or make up the membrane through oligomerization.

The presence of these proteins seems crucial for the existence of tubular cisterns since their suppression or suppression leads to an excess of flat sac-like structures in the endoplasmic reticulum and an almost complete absence of tubules.

The smooth endoplasmic reticulum is also a dynamic structure, with new tubules sprouting from the sides of the existing structures. The extent of the smooth endoplasmic reticulum network depends on the actin and microtubule cytoskeleton of the cell.

The structure of the smooth endoplasmic reticulum is of particular importance in two types of cells in the human body: muscle cells and neurons.

The presence of an extensive endoplasmic reticulum network throughout the neuron is closely related to its interaction with actin and microtubules, and the organelle forms a continuous network throughout the cell. It is present in small dendritic spines, along the narrow axon, and extends across the synapse.

At the synapse, the smooth endoplasmic reticulum is often associated with mitochondria. Even when the cytoskeleton depolymerizes and the reticulum network of tubules undergoes major morphological changes, the association between mitochondria and the smooth endoplasmic reticulum remains intact.

In muscle cells, the smooth endoplasmic reticulum is called the sarcoplasmic reticulum and is an important site for the storage of calcium ions.

The image shows a skeletal muscle, with the sarcoplasmic reticulum colored blue. Together with special structures in the plasma membrane of the muscle cell (T tubules), the sarcoplasmic reticulum plays an important role in the contraction of muscle fibers.

Functions of the smooth endoplasmic reticulum

The smooth endoplasmic reticulum is important in the synthesis of lipids, such as cholesterol and phospholipids, which form all the membranes of the body.

In addition, it is important for the synthesis and secretion of steroid hormones from cholesterol and other lipid precursors. In addition, it is involved in carbohydrate metabolism.

The dynamic nature of the smooth endoplasmic reticulum is particularly important in the liver as it detoxifies a number of substances and makes them easy to remove from the body.

For example, when there is a sudden and dramatic increase in the amount of some lipid-soluble drugs in the body, the smooth endoplasmic reticulum of hepatocytes in the liver metabolizes them to water-soluble compounds so that they can be excreted in the urine.

To do this, the smooth endoplasmic reticulum network of a hepatocyte can almost double in size and then return to its original shape and size after the chemical assault has been neutralized.