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.
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.
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.
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