Endoplasmic Reticulum: Structure, Types & Functions

Endoplasmic Reticulum is a network of tubular membranes that is found in the eukaryotic cell. It is a type of organelle composed of membranous sheets that start near the nucleus and stretch throughout the cell.

• The endoplasmic reticulum makes and secretes many of the products created by a cell and packages.
• It is one of the important organelles within the cell.
• The membrane forms the enclosed network in the shape of a sac called cisternae.
• The endoplasmic reticulum tends to share proteins and is involved in similar types of activities.
• The entire structure joins the outer layer of the nuclear membrane.
• It is divided into two categories, namely rough endoplasmic reticulum and smooth endoplasmic reticulum.
• The Golgi body, mRNA, tRNA and ribosomes work with ER for the functioning of a cell. 

Read More: Cell Membrane and Plasma Membrane

Key Terms: Endoplasmic Reticulum, Nucleus, Cells, Rough Endoplasmic Reticulum, Smooth Endoplasmic Reticulum, Golgi Body, Sacs, Tubules

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What is the Endoplasmic Reticulum?

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Endoplasmic reticulum is a network of flattened sacs and tubules in the cytoplasm. In the word, endoplasmic reticulum endo means inside and plasmic means cytoplasm, whereas reticulum stands for network.

• The endoplasmic reticulum is a large network of membrane-bound tubes and sheets. 
• It looks like a bag (vesicles). 
• Endoplasmic reticulum membrane is similar to the plasma membrane in structure. 
• In animal cells, it typically accounts for more than half of the cell's membranous material. 
• The two forms of Endoplasmic Reticulum are Rough ER and Smooth ER. 
• They are distinguished by differences in specific morphological and functional properties.

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Structure of Endoplasmic Reticulum

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The structure of the endoplasmic reticulum occurs in three forms. A thick membrane surrounds all three endoplasmic reticulum structures. Protein molecules form the outer and inner thick layers of the membrane, while phospholipids form the two middle thin and translucent layers. The plasma membrane, nuclear membrane, and Golgi complex membrane are all continuous in the endoplasmic reticulum. The three forms of endoplasmic reticulum are as follows:

Cisternal Form

Cistern Form is a long, flattered and unbranched unit arranged in the form of stacks.

Read More: Multicellular Organism 

Vesicular Form

The vesicular form is an oval membrane-bound structure which is used as the entry gateway for extracellular substances.

Tubular Form

It is an irregular, branched tube bounded by a membrane.

Read More: Globular and Fibrous Protein

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Types of Endoplasmic Reticulum

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Endoplasmic Reticulum is divided into two categories, which are provided in detail below.

Rough Endoplasmic Reticulum 

The rough endoplasmic reticulum looks rough under a microscope; hence, it is called the rough endoplasmic reticulum. It has ribosomes attached to its wall. Rough Endoplasmic Reticulum (RER) is a protein synthesis and packaging enzyme that is also listed in the ribosomes section.

• The ER membrane is roughed up by ribosomes attached to it. 
• Due to the direct relationship between the perinuclear space and the ER lumen, molecules can travel across both membranes.
• Protein synthesis begins when mRNA travels from the nucleus to a ribosome on the RER's surface.
• The amino acid chain is driven into the RER's cisternal space as the ribosome produces it. 
• The RER clamps off a vesicle when protein synthesis is finished. 
• Proteins, hormones, and other chemicals are synthesized and secreted in the liver.
• Hormones and other substances are synthesized and secreted in the glands.
• Rough ER is seen in cells that synthesize proteins, such as hepatocytes.

Read More: Lipoproteins

Smooth Endoplasmic Reticulum

The Smooth Endoplasmic Reticulum has no ribosomes. Due to the absence of ribosomes, it appears smooth. The smooth endoplasmic reticulum is unrelated to ribosomes and plays a different role.

• The smooth ER helps in the synthesis of lipids like phospholipids and cholesterol.
• These lipids are needed for producing new membranes of cells. 
• In some cell types, Smooth ER is involved in the production of steroid hormones from cholesterol.
• In liver cells, it aids in the detoxification of drugs and harmful chemicals.  
• The liver synthesizes and secretes proteins, hormones, and other chemicals.
• The glands synthesize and secrete hormones and other substances.

Read More: Macromolecule

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Functions of Endoplasmic Reticulum:

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The functions of Endoplasmic Reticulum are as follows:

• It is largely responsible for transporting proteins and carbohydrates to other organelles.
• The protein is transferred to lysosomes, the Golgi apparatus, and the plasma membrane.
• Endoplasmic Reticulum increases the surface area for cellular reactions.
• They contribute to the development of the nuclear membrane during cell division.
• It is crucial in the formation of the skeletal structure.
• It is essential for the synthesis of proteins, lipids, glycogen, and other steroid hormones.
• Proteins and fats manufactured help in building the cell membrane called membrane biogenesis. 
• The endoplasmic reticulum functions as a transport channel. 
• It also provides a cytoplasmic framework to cells.

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Things to Remember

• Endoplasmic Reticulum is a large organelle composed of tubules and membranous sheets.
• It will start its operation near the nucleus and stretch throughout the cell.
• It is divided into two parts: Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum. 
• Rough Endoplasmic Reticulum is defined as the endoplasmic reticulum (ER) that has ribosomes attached to it. 
• Smooth Endoplasmic Reticulum is defined as an ER that does not have ribosomes attached to it.
• Rough endoplasmic reticulum proteins are intended for use outside the cell.
• The Endoplasmic Reticulum helps in the building of the cell membrane, called membrane biogenesis.