SMOOTH ENDOPLASMIC RETICULUM: CHARACTERISTICS AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The smooth endoplasmic reticulum is a membranous cellular organelle present in eukaryotic cells. In most cells it is found in small proportions. Historically, the endoplasmic reticulum has been divided into smooth and rough. This classification is based on the presence or absence of ribosomes in the membranes.

The smooth does not have these structures attached to its membranes and is composed of a network of saccules and tubules connected to each other and distributed throughout the cell interior. This network is extensive and is considered the largest cellular organelle

This organelle is responsible for lipid biosynthesis, in contrast to the rough endoplasmic reticulum, whose main function is the synthesis and processing of proteins. It can be seen in the cell as a tubular network connected to each other, with a more irregular appearance compared to the rough endoplasmic reticulum.

This structure was observed for the first time in 1945 by researchers Keith Porter, Albert Claude and Ernest Fullam.

General characteristics

The smooth endoplasmic reticulum is a type of disordered network-shaped reticulum of tubules that lacks ribosomes. Its main function is the synthesis of structural membrane lipids in eukaryotic cells and of hormones. Likewise, it participates in calcium homeostasis and cell detoxification reactions.

Enzymatically, the smooth endoplasmic reticulum is more versatile than the rough one, allowing it to perform a greater number of functions.

Not all cells have an identical and homogeneous smooth endoplasmic reticulum. In fact, in most cells these regions are quite sparse and the differentiation between the smooth and the rough reticulum is not really very clear.

The ratio of smooth to rough depends on cell type and function. In some cases both types of reticulum do not occupy physically separate regions, with small areas free of ribosomes and other covered areas.

Location

In cells where lipid metabolism is active, the smooth endoplasmic reticulum is very abundant.

Examples of this are the cells of the liver, the adrenal cortex, neurons, muscle cells, the ovaries, the testes and the sebaceous glands. The cells involved in the synthesis of hormones have large compartments of smooth reticulum, where the enzymes to synthesize these lipids are found.

Structure

The smooth and rough endoplasmic reticulum form a continuous structure and are a single compartment. The reticulum membrane is integrated with the nuclear membrane.

The structure of the reticulum is quite complex because there are several domains in a continuous lumen (without compartments), separated by a single membrane. The following areas can be distinguished: the nuclear envelope, the peripheral reticulum and the interconnected tubular network.

The historical division of the reticulum includes the rough and the smooth. However, this separation is the subject of heated debate among scientists. Cisternae have ribosomes in their structure and therefore the reticulum is considered rough. In contrast, the tubules lack these organelles and for this reason this reticulum is called smooth.

The smooth endoplasmic reticulum is more intricate than the rough one. The latter has a more granular texture, thanks to the presence of ribosomes.

The typical shape of the smooth endoplasmic reticulum is a polygonal network in the form of tubules. These structures are complex and have a high number of branches, giving them a sponge-like appearance.

In certain tissues grown in the laboratory, the smooth endoplasmic reticulum clusters into stacked sets of cisterns. They can be distributed throughout the cytoplasm or aligned with the nuclear envelope.

Features

The smooth endoplasmic reticulum is primarily responsible for lipid synthesis, calcium storage, and cell detoxification, especially in liver cells. In contrast, in the rough, biosynthesis and modification of proteins occurs. Each of the mentioned functions is explained in detail below:

Lipid biosynthesis

The smooth endoplasmic reticulum is the main compartment in which lipids are synthesized. Due to their lipid nature, these compounds cannot be synthesized in an aqueous environment, such as the cell cytosol. Its synthesis must be carried out in association with already existing membranes.

These biomolecules are the basis of all biological membranes, which are made up of three fundamental types of lipids: phospholipids, glycolipids and cholesterol. The main structural components of membranes are phospholipids.

Phospholipids

These are amphipathic molecules; they have a polar (hydrophilic) head and a nonpolar (hydrobolic) carbon chain. It is a glycerol molecule linked to fatty acids and a phosphate group.

The synthesis process occurs on the cytosol side of the endoplasmic reticulum membrane. Coenzyme A participates in the transfer of fatty acids to glycerol 3 phosphate. Thanks to an enzyme anchored in the membrane, phospholipids can be inserted into it.

The enzymes present on the cytosolic face of the reticulum membrane can catalyze the binding of different chemical groups to the hydrophilic portion of the lipid, giving rise to different compounds such as phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine or phosphatidylinositol.

As lipids are synthesized, they are added to only one side of the membrane (remembering that biological membranes are arranged as a lipid bilayer). To avoid asymmetric growth on both sides, some phospholipids must move to the other half of the membrane.

However, this process cannot occur spontaneously, since it requires the passage of the polar region of the lipid through the interior of the membrane. Flipases are enzymes that are responsible for maintaining a balance between the lipids of the bilayer.

Cholesterol

Cholesterol molecules are also synthesized in the reticulum. Structurally, this lipid is made up of four rings. It is an important component in animal plasma membranes and is also necessary for the synthesis of hormones.

Cholesterol regulates the fluidity of membranes, which is why it is so important in animal cells.

The final effect on flowability depends on cholesterol concentrations. At normal levels of cholesterol in the membranes and when the tails of the lipids that compose it are long, cholesterol acts by immobilizing them, thus reducing the fluidity of the membrane.

The effect is reversed when cholesterol levels decrease. By interacting with the lipid tails, the effect it causes is their separation, thus reducing fluidity.

Ceramides

Ceramide synthesis occurs in the endoplasmic reticulum. Ceramides are important lipid precursors (which are not derived from glycerol) for plasma membranes, such as glycolipids or sphingomyelin. This ceramide conversion occurs in the Golgi apparatus.

Lipoproteins

The smooth endoplasmic reticulum is abundant in hepatocytes (liver cells). Lipoprotein synthesis occurs in this compartment. These particles are responsible for transporting lipids to different parts of the body.

Lipid export

Lipids are exported via the secretory vesicle pathway. As biomembranes are made up of lipids, the membranes of the vesicles can fuse to them and release the content to another organelle.

Sarcoplasmic reticulum

In striated muscle cells, there is a highly specialized type of smooth endoplasmic reticulum made up of tubules called the sarcoplasmic reticulum. This compartment surrounds each myofibril. It is characterized by having calcium pumps and regulates its uptake and release. Its role is to mediate muscle contraction and relaxation.

When there are more calcium ions within the sarcoplasmic reticulum compared to the sarcoplasm, the cell is in a resting state.

Detoxification reactions

The smooth endoplasmic reticulum of liver cells participates in detoxification reactions to remove toxic compounds or drugs from the body.

Certain families of enzymes, such as cytochrome P450, catalyze different reactions that prevent the accumulation of potentially toxic metabolites. These enzymes add hydroxyl groups to "bad" molecules that are hydrophobic and found on the membrane.

Later, another type of enzyme called UDP glucuronyl transferase comes into play, which add molecules with negative charges. In this way, the compounds leave the cell, reach the blood and are eliminated in the urine. Some drugs that are synthesized in the reticulum are barbiturates and also alcohol.

Drug resistance

When high levels of toxic metabolites enter the circulation, the enzymes that participate in these detoxification reactions are triggered, increasing their concentration. Similarly, under these conditions, the smooth endoplasmic reticulum increases its surface up to two times in just a couple of days.

That is why the rate of resistance to certain drugs is increased and to achieve an effect it is necessary to consume higher doses. This resistance response is not totally specific and can lead to resistance to several drugs at the same time. In other words, the abuse of a certain drug can lead to the ineffectiveness of another.

Gluconeogenesis

Gluconeogenesis is a metabolic pathway in which glucose formation occurs from molecules other than carbohydrates.

In the smooth endoplasmic reticulum is the enzyme glucose 6 phosphatase, responsible for catalyzing the passage of glucose 6 phosphate to glucose.

Reference

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