Endoplasmic Reticulum Definition, Structure, and Function

In this endoplasmic reticulum definition, structure, and function post we have briefly explained about endoplasmic reticulum in animal cell definition, structure, ultra structure, functions and importance.

Endoplasmic Reticulum in Animal Cell

Endoplasmic reticulum in animal cell is a network of membranes inside a cell through which proteins and other molecules move. Proteins are assembled at organelles called ribosomes. When proteins are destined to be part of the cell membrane or exported from the cell, the ribosomes assembling them attach to the endoplasmic reticulum, giving it a rough appearance. Smooth endoplasmic reticulum lacks ribosomes and helps synthesize and concentrate various substances needed by the cell.

History

Early cytologists held that some sort of supporting network or cytoskeleton was present in the cells. It was given various names Nissil substance, ergastoplasm, basophilic bodies, etc. In 1945, Porter, Claude and Fullman with the help of electron microscope noted a delicate membranous network in the cytoplasm. It was later called endoplasmic reticulum in animal cell (ER) by Keith Porter in 1953. The ER originally seemed to be confined to the endoplasm of the cell, hence its name.

Structure

Endoplasmic Reticulum

Endoplasmic Reticulum Definition, Structure, and Function

In eukaryotic cells endoplasmic reticulum in animal cell is generally the largest membrane which forms extensive system of intercommunicating membranous sacs or channels. It represents 30 to 60% of total membrane in a cell. The membrane of endoplasmic reticulum in animal cell may or may not have ribosomes attached to their outer membrane.

Accordingly these are classified as rough (RER) or smooth endoplasmic reticulum (SER). Rough endoplasmic reticulum is characterized by the presence of ribosomes of about 150Å in diameter and rich in protein and RNA. Smooth endoplasmic reticulum lacks ribosomes. It comprises three types of elements: cisternae, tubules and vesicles.

Cisternae

These are flattened, unbranched, sac like elements with about 40-50µm in diameter. They lie in stacks (piles) parallel to but interconnected with one another. They are separated from one another by cytosolic spaces. The small granular structures called the ribosomes may or may not be present on the surface of cisternae.

Tubules

These are irregular, branching elements, which form a network along with other elements. They are about 50-100µm in diameter, and are often free of ribosomes.

Vesicles

These are oval, vacuole like elements, about 25-500µm in diameter. They often occur isolated in the cytoplasmic matrix. They are also free of ribosomes. A fluid called the endoplasmic matrix is present in the lumen of ER. All the elements of ER freely communicate with one another.

Ultra structure

The membrane bounding the cisternae, tubules and vacuoles of the ER is similar to the cell membrane. It is 50-60Å thick. The membranes of endoplasmic reticulum in animal cell are composed of two layers of phospholipids molecules sandwiched by two layers of protein molecules like other membranes in the cell.

The ER membrane has a relatively high protein/lipid ratio. It is continuous with the cell membrane, Golgi membranes and outer membrane of the nuclear envelope. Certain cisternae open out by pores in the cell membrane. The lumen acts as a passage for the secretary products.

About 30-40 different enzymes are associated with the ER for the various synthetic activities. These may be located on the cytoplasmic surface or luminal surface or both. Membrane bound endoplasmic reticulum in animal cell spaces varies in shape and sizes in different cell types.

Endoplasmic Reticulum in Animal Cell Ultra structure

Smooth Endoplasmic Reticulum

Ribosomes are absent on the walls of ER and so it appears smooth and hence called smooth or agranular ER. It mainly occurs as tubular forms.

The tubules forms irregular lattices and measures about 500-1000Å in diameter. Smooth ER is commonly found in the cells involved in the synthesis of steroids or lipids i.e. non-protein type of synthesis (Christensen and Fawcett, 1961) such as adrenal or sebaceous glands, gonadial interstitial cells. 

Certain cells with carbohydrate metabolism (e.g. liver cells), impulse conduction (e.g. muscle cells), with pigment production (e.g., retinal pigment cell) and electrolyte excretion (e.g., chloride cells of fish gills) are also have more of SER in them.

Endoplasmic Reticulum in Animal Cell Definition, Structure, and Function

Rough Endoplasmic Reticulum

It is characterized by the presence of ribosomes on the surface of reticulum and so it is also known as granular ER. It is in the form of flattened cisternae with the width of 400-500Å.

RER occurs largely in the cells that are actively involved in the synthesis of proteins such as enzymes (e.g. pancreatic cells, plasma cells and liver cells) or mucus (goblet cells).

In exocrine cells of pancreas, RER consists of reticular sheets and fenestrated cisternae in the basal region of the cell. These cisternae measures about 5-10 micron in length and their groups are 400-1000Å in diameter.

In apical region of the cells, granular reticulum occurs in the form of vesicles. Granular and agranular ER are in continuity of their membranes in the regions of contact.

Functions

Functions of SER

1. The Smooth endoplasmic reticulum in animal cell provides surface for the synthesis of fatty acids, phospholipids, glycolipids, steroids and visual pigments.

2. The Smooth endoplasmic reticulum in animal cell carries enzymes for glycogen metabolism in liver cells. Glycogen granules are attached in larger numbers to the outside of the SER’s membranes in liver cells.

3. The Smooth endoplasmic reticulum in animal cell has enzymes that are involved in the detoxification in the liver, i.e., converts harmful materials such as carcinogens and pesticides, into harmless ones for excretion by the cell.

4. The Smooth endoplasmic reticulum in animal cell produces Golgi apparatus, lysosomes, micro bodies and vacuoles. The proteins shift from RER through SER to Golgi apparatus for further processing.

5. The sarcoplasmic reticulum in skeletal muscle cells release Ca2+ ions to cause contraction and absorbs Ca2+ ions to bring about relaxation.

6. The Smooth endoplasmic reticulum in animal cell membranes carry out the initial reactions in the oxidation of fats.

Functions of RER

1. The rough endoplasmic reticulum in animal cell provides a large surface for the attachment of ribosomes.

2. The rough endoplasmic reticulum in animal cell offers extensive surface on which protein synthesis can be conveniently carried on by ribosomes. 

3. The proteins in ER lumen are processed and get enclosed in spherical membrane bound vesicles which get pinch off from the ER. 

4. The rough endoplasmic reticulum in animal cell gives rise to the smooth ER by loss of ribosomes.

5. The rough endoplasmic reticulum in animal cell forms nuclear envelope around daughter cells in cell division.

6. The process of linking sugars to proteins to form glycoproteins starts in the rough endoplasmic reticulum in animal cell and is completed in Golgi apparatus.

Importance

1. The endoplasmic reticulum in animal cell facilitates transport of materials from one part of the cell to another thus forming the cell’s circulatory system.

2. Tubular extension, called desmotubule, extends through plasmodesmata to make endoplasmic reticulum in animal cell continuous in the two adjacent plant cells.

3. The endoplasmic reticulum in animal cell acts as an intracellular supporting framework, the cytoskeleton that also maintains the form of the cell.

4. It keeps the cell organelles properly stationed and distributed in relation to one another.

5. The endoplasmic reticulum in animal cell offers extensive surface for the synthesis of a variety of materials.

6. The endoplasmic reticulum in animal cell provides space for temporary storage of synthetic products such as proteins and glycogen.

7. The endoplasmic reticulum in animal cell helps in the exchange of materials between the cytoplasm and the nucleus.

8. A variety of enzymes is located in the endoplasmic reticulum in animal cell membranes to catalyze the biochemical reactions.

Further Readings

Reference

  1. https://byjus.com/biology/endoplasmic-reticulum/
  2. https://www.britannica.com/science/endoplasmic-reticulum
  3. https://www.genome.gov/genetics-glossary/Endoplasmic-Reticulum-Smooth
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4700099/

Endoplasmic Reticulum Definition, Structure, and Function