What is Smooth Endoplasmic Reticulum?

In eukaryotes, smooth endoplasmic reticulum is found. smooth endoplasmic reticulum is a type of endoplasmic reticulum, in which ribosomes are absent and in structure is tubular.

Regulation of calcium level, removal of toxins from drug, metabolism of sugars are the function of smooth endoplasmic reticulum. It is a vital organ with various functions and comprises of a chain of tubules which are the cisternae.

Within the cell membrane and the nuclear envelope, the endoplasmic reticulum extends. There are two types of ER; smooth and rough ER. rER is located in protein generating cells.

Smooth endoplasmic reticulum is found in fatty acid cells and steroid hormones. Example of rER are plasma cells, liver cells, pancreatic cells and goblet cells.

Smooth endoplasmic reticulum is found in leucocytes, interstitial cells, heart’s conducting fibers, adipose cells and others.

Smooth Endoplasmic Reticulum Discovery

In 1902, smooth endoplasmic reticulum was first time seen in light microscope by a group of scientist and one of them was Charles Garnier, before that an important organelle like ER was not even discovered. Although it had to be proved whether ER actually subsisted, took around 50 years of wait, after the arrival of electron microscopy.

Inside the cytoplasm it appeared like netted structure and was thus called ER and its function, structure, working was all identified by 1960. It played a role in the protein synthesis was realized by Gunter Blobel around 1971.

Smooth Endoplasmic Reticulum Structure

As it does not possess ribosomes its surface is smooth, hence the name smooth endoplasmic reticulum. It is present in the cytoplasm and is linked to the nuclear envelope. Inside the reticulum is the net like structure in which tubules are present along with vesicles in the cytoplasm. As there are folding present in the ER, which increases the surface area so that the enzymes can act.

Lumen is present inside the smooth endoplasmic reticulum, which is encapsulated by a membrane of phospholipid. The tubules branches with the other tubules resulting in the formation of a 3D reticulum, however they may also exist in spiral shape in cells generating hormones.

Types of Smooth Endoplasmic Reticulum

Sarcoplasmic Reticulum: A type of smooth endoplasmic reticulum is the sarcoplasmic reticulum which is found in cells of muscle, whose function is to regulate the homeostasis of calcium. In structure they are tubular in shape surrounding the muscle and gather around the myofibril, in which is the cuff like structure of SR.

The functions are it controls the calcium concentrations, while the muscles are contracting. In structure there exist a net like structure in which are tubules spreading throughout the muscle, surrounds the myofibrils. Longitudinal and junctional sarcoplasmic reticulum are its domains.

The similarity in both the domains is that there is an arrangement of myofibrils which orient themselves with the sarcomeres. Tubules are in linear fashion and are connected to each other, surrounding the myofibril is called as longitudinal SR and the area where these tubules end are sac like structures called the terminal cisterna.

Smooth endoplasmic reticulum is the point of entry to the SR and the exit points are the ryanodine receptors along with cisterna. During the contraction of muscle, calcium is taken out from the SR and moves to the ryanodine reception, thus allowing the contraction to take place.

Then again, the calcium is reached to SR with the help of smooth endoplasmic reticulum during relaxation. Thus, acting like a pump and transporting calcium from cytoplasm to the SR and vice-versa.

Smooth Endoplasmic Reticulum Location

All types of eukaryotic cell possess smooth endoplasmic reticulum except mature RBC, embryo cells and ova. Hepatocytes, striated muscle cell, gonadal and sebaceous cell contain huge amount of smooth endoplasmic reticulum.

Smooth Endoplasmic Reticulum Function

Production of lipid, intracellular calcium regulation, drug detoxification, metabolism of sugar are some of the functions. Sebaceous cell, gonadal cell, hepatocytes contain huge amount of smooth endoplasmic reticulum. Although the amount of cholesterol produced by ER is quite minimal but homeostasis maintenance is the function of smooth endoplasmic reticulum. Movement of molecule to Golgi complex from ER is also another function of smooth endoplasmic reticulum.

i. Lipid Synthesis

The sites where membrane comes in close proximity to other organelles such as plasma membrane, lysosomes, chloroplast, Golgi apparatus and lysosomes and others are the MCS also called the membrane contact sites. Thus, transfer of molecules happen through them. For the production of lipid, in huge amount enzyme are present within the ER to carry out the synthesis.

On receiving a signal, these enzyme start to act and maintain homeostasis, allows the growth and differentiation to occur. Phospholipid’s synthesis safeguards the cell and its content, allows the growth, provides immunity and transfers signal along with transportation of lipid. Between the mitochondria and the ER is the MCS which produces phospholipids.

Another type of lipid are ceramides, produced in the smooth endoplasmic reticulum and are moved to Golgi apparatus. Its functions are cell adhesion, migration, signaling, cell death and cell cycle.

In the smooth endoplasmic reticulum the steroid hormones are formed, however as they cannot be stored in vesicles, hence depending on the requirement they are produced from precursors. smooth endoplasmic reticulum is abundant in testis, ova, hepatocytes and other cells.

ii. Carbohydrate Metabolism

In eukaryotes the source of energy is the carbohydrate. In varying conditions glucose is synthesized from precursors such as pyruvate, lactate, succinate which are non- carbohydrate and the process is known as gluconeogenesis. Although this is a multi-step process.

Glucose-6-Phosphate is the final compound synthesized and cannot pass through the cell and is therefore stored in the cell, thus converting glucose-6-phosphate to glucose by the enzyme present in the smooth endoplasmic reticulum which is glucose-6-phosphatase. Its function are maintaining homeostasis and glucose level and is present in liver and kidney, where the conversion takes place.

iii. Calcium Concentration Regulation

The calcium concentration is regulated by sarcoplasmic reticulum, which is a type of smooth ER. This calcium regulation takes place in the muscle cell. smooth endoplasmic reticulum is the point of entry to the SR and the exit points are the ryanodine receptors along with cisterna.

During the contraction of muscle, calcium is taken out from the SR and moves to the ryanodine reception, thus allowing the contraction to take place. Then again, the calcium is reached to SR with the help of smooth endoplasmic reticulum during relaxation. Thus, acting like a pump and transporting calcium from cytoplasm to the SR and vice-versa.

iv. Drug Detoxification

There are various enzymes present in smooth ER and they are cytochrome P450, which helps in removals of toxins from the liver. The mechanism behind it is to solubilize the drug by attaching a 0H- group so that it can be removed out of the body.

Examples of drug working on the same mechanism are barbiturates and phenobarbital. The detoxification rate can be increased by barbiturates and alcohol; however, this is not possible with small volume of doses, due to wide action of smooth endoplasmic reticulum.

Disorders Caused by Dysfunctional Smooth Endoplasmic Reticulum

i. Cytochrome P450 Oxidoreductase Deficiency

The enzyme present in the smooth endoplasmic reticulum is Cytochrome P450 and its enzyme is Cytochrome P450 reductase and its absence can result in a disorder impacting the production of steroid hormone. It would lead to improper growth, effect on the reproductive system and other issues.

Moderate symptom patient would be infertile, whereas those with severe symptom are impacted with the Antley Bixler syndrome where bones have pre-fused, ears are in lower position, and face is also flattened.

ii. Von Gierke Disease

The disorder is named so due to the person who discovered it in 1929.The disease which has an impact on the metabolism of glycogen is called as Glycogen storage disease (GSD-1). Its symptom are improper growth and blood sugar levels quite low and can be deadly too. Glucose-6-phosphatase is the enzyme which converts glucose-6-phosphate to glucose and the process is gluconeogenesis. This enzyme is found in the smooth endoplasmic reticulum.

The absence of this enzyme can lead to this disorder. In this disorder, due to absence of the enzyme, glycogen cannot be broken thus gets deposited in kidneys and liver and thus increasing the liver size. Out of 100,000 people 1 is affected by this disorder and is an autosomal recessive disorder.