One of the most vital parts of a cell, the endoplasmic reticulum is responsible for various functions, like protein synthesis. It is found in eukaryotic plant and animal cells, and not in prokaryotic cells. However, recent researches suggest that certain types of prokaryotic cells have organelles resembling the endoplasmic reticulum.
What is Endoplasmic Reticulum?
Endoplasmic reticulum (ER) is a complex network of membranes, that is found in the form tubules, and compressed, sac-like structures called cisternae. The internal space of the organelle, called cisternal space, is interconnected and fluid-filled. The cisternal space is found in continuation with the perinuclear space - the space between the inner and outer nuclear membranes. Endoplasmic reticulum is arranged in irregular rows, along the circumference of the nucleus. It is surrounded by cytosol, the liquid part of the cytoplasm, in which the organelles are suspended. The cross-section diagram given below shows the nucleus and the perinuclear space, the rough endoplasmic reticulum with ribosomes and the smooth endoplasmic reticulum.
There are three different types of endoplasmic reticulum - rough, smooth, and sarcoplasmic reticulum. The third and the last type, called sarcoplasmic reticulum, is a type of smooth endoplasmic reticulum, which is found in smooth and striated muscles. Most of the cells consist of both rough and smooth types, which are mostly continuous in nature. Rough ER is mainly found near the nucleus and the Golgi apparatus, whereas smooth ones extend to the periphery of the cell. The rough endoplasmic reticulum is studded with ribosomes, which give the granular appearance. The smooth ER has very few or no ribosomes. In general, the concentration of smooth ER is less, when compared to that of rough ER.
Smooth Endoplasmic Reticulum - Location and Functions
As mentioned above, the two types of endoplasmic reticulum differ in appearance, location, function and concentration. The smooth ER is said to be derived from the rough type and is mostly found as a continuation of the latter. The smooth ER is like an interconnected tubular network that may extend to the periphery of the cell. It is mainly responsible for production and metabolism of lipids. Smooth endoplasmic reticulum is assigned with various functions, that may vary with different types of cells.
- The functions of smooth ER include carbohydrate metabolism, regulation of calcium ions, synthesis and metabolism of steroids and lipids, drug detoxification, storage of enzymes, steroids and ions, membrane synthesis, etc.
- In certain types of muscle cells, smooth ER (referred to as sarcoplasmic reticulum) stores calcium ions. Release of these calcium ions results in contraction of muscles.
- This cell organelle has the function of detoxification in liver cells. Smooth endoplasmic reticulum found in the liver cells contain detoxifying enzymes that can inactivate harmful toxins, like drugs and metabolic waste.
- Another function is transportation of newly synthesized lipids and proteins (from rough ER) to other locations, both inside and outside the cell. This is achieved through a process called budding, wherein small vesicles, which contain proteins, are detached from the smooth ER and are carried to other locations.
- Smooth ER is abundantly found in the cells of the reproductive organs, where they produce steroid hormones, like estrogen and testosterone.
- This cell organelle aids in converting glucose-6-phosphate to glucose, which is an important step in gluconeogenesis, the process by which glucose is made from non-carbohydrate sources. This process takes place mainly in the liver.
- In certain types of plant cells, smooth ER facilitates connection between adjacent cells through plasmodesmata.
Smooth Endoplasmic Reticulum Vs. Rough Endoplasmic Reticulum
The main difference between them lies in the appearance. The rough type has a bumpy surface, whereas the smooth ER has a smooth surface. The rough ER is composed of interconnected, sac-like structures, that look like discs arranged in irregular rows. The smooth ER is like a network of interconnected tubules.
As mentioned above, rough endoplasmic reticulum is mainly responsible for synthesis of proteins and smooth type is involved in synthesis and metabolism of lipids. Smooth ER is also a location for storage and transport.
The proportion of smooth endoplasmic reticulum is very small, when compared to that of rough ER. Smooth ER is mostly found in cells that are actively involved in synthesis and metabolism of lipids. It is prominently seen in the liver, ovaries, testes, muscles, etc.
In short, smooth endoplasmic reticulum is primarily meant for storage and transport of lipids and proteins. Though it is sparse, when compared to the rough ER, smooth one is involved in some of the vital functions of the cells.