Cell Organelles | Endoplasmic Reticulum

Endoplasmic Reticulum (ER)- Function, Structure and Type

Recent Electron Microscope studies on Eukaryotic Cell reveal the presence of the network of the reticulum of tiny tubular structures scattered in the cytoplasm called the Endoplasmic Reticulum (ER).

Endoplasmic Reticulum is the part of the Endo-membrane system and is specific to the Eukaryotes.

ER sometimes continues with the Outer Membrane of the Nucleus and may end at the Cytoplasm.

Thus dividing the Intra-cellular space (inside the cell) into two compartments-

  1. Luminal (Inside ER)
  2. Extra Luminal (Outside ER also called as Cytoplasm)

Ribosomes might be present on the surface of the Endoplasmic Reticulum. This type of ER is called Rough Endoplasmic Reticulum (RER).

Components of ER

This big organelle is made of 3 Components.

Cisternae

This is the flat and unbranched units of ER arranged in the form of stacks.

But they are often connected with the help of small tubules.

Ribosomes are often seen on the Cisternae. Thus they are actively involved in Protein Synthesis.

Tubules

They are irregular, often branched tubes bounded by the membrane. Tubules may be free or sometimes associated with the Cisternae.

Ribosomes are not present on the Tubules.

Vesicle

A vesicle is a small oval sac-like structure that is bounded by a membrane.

Types of Endoplasmic Reticulum

Based on their appearance they are of 2 types.

RER (Rough Endoplasmic Reticulum)-

They appear Rough under a Microscope due to the presence of the Ribosomes on their surface.

SER (Smooth Endoplasmic Reticulum)

They appear Smooth and Shiny under a microscope.

Ribosomes are absent on SER.

Difference

RERSER
1. Granular1. Agranular
2. Rough2. Smooth
3. Ribosome is present3. Ribosome is absent
4. Actively engaged in Protein synthesis4. Involved in Lipid synthesis
5. Continues with the outer membrane of the Nucleus5. Present randomly in the cytoplasm
6. Mainly consist of Cisternae6. Mainly consist of the tubules
7. More developed in those cells that are actively engaged in the synthesis of Protein7. More developed in those cells that are actively engaged in the synthesis of Lipid
Eg- Liver, Pancreas, etcEg- Liver, Adrenal Cortex, Gonads, etc

Structure of the Endoplasmic Reticulum

The general structure of the endoplasmic reticulum is a network of membranes called cisternae. These sac-like structures are held together by the cyto-tubules.

Some tubules may associate with the Cisternae (or sometime freely) to form the SER.

The phospholipid membrane encloses the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol.

The membrane of the ER alone comprises of the 50 percent of the total membrane of the cell.

Due this large giant membranous structure. The cell (cytosol) is divided into 2 parts-

  1. Fluid inside the ER, called as the Luminal space (Lumen)- Takes up about of 10 % of the entire cell.
  2. Fluid outside the ER, called as the Extra-luminal space or Cytoplasm.

The functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids but vary between ER and cell type and cell function.

The quantity of both rough and smooth endoplasmic reticulum in a cell can slowly interchange from one type to the other, depending on the changing metabolic activities of the cell.

Transformation can include embedding of new proteins in membrane as well as structural changes. Changes in protein content may occur without noticeable structural changes.

Functions of the Endoplasmic Reticulum

1. Mechanical Support-

Microfilaments, Microtubules, and ER form the endoskeleton of the cell.

Which helps in maintain the shape of the cell.

2. Intra-cellular exchange (within a cell)

ER forms the intracellular conducting system.

So, transport of the materials in the cytoplasm from one place to another may occur through the ER.

In some places, the ER is often connected to the Plasma Membrane. So, the ER can secrete material outside of the cell.

3. Protein Synthesis-

As we know it is performed by the RER, as Ribosomes are present on its surface.

4. Lipid Synthesis-

Lipids (cholesterol and phospholipids) are synthesized by the agranular portion of the ER (Smooth ER) or (SER).

5. Cellular Metabolism

The Membranes of the reticulum provides an increased surface for the metabolic activities within the cytoplasm.

6. Formation of the Nuclear Membrane during cell division.

Fragmented vesicles of disintegrated nuclear membrane and ER elements arrange themselves around the chromosome to form a new nuclear membrane during cell division.

7. Formation of the Lysosomes, Golgi Body & some Micro bodies.

8. Detoxification-

SER is concerned with the detoxification of drugs, pollutants, and steroids. Cytochrome P450 in ER act as an enzyme that functions in the detoxification of drugs and other toxins.

9. ER provides the precursor of secretory material to the Golgi Body.

(ER secrete a substance which is later modified and secreted by the Golgi body).

Note– The majority of the membranous organelle is formed by ER.

Occurrence of the Endoplasmic Reticulum

Though the ER is found in each Eukaryotic cell. But their occurrence and type differ from cell to cell.

As we already know the hugely found in the cell that is engaged in Protein synthesis (RER) and Lipid Synthesis (SER).

RER is mainly found in-

  • Liver
  • Pancreas
  • Goblet Cells

SER is mainly found in-

  • Adipose tissue
  • Interstitial cells (found in testis- also called Leydig cells)
  • Muscles
  • Glycogen storing Liver cells
  • Adrenal Cortex
  • Gonads (Testis and Ovary)

Special types (Modification)

Based on the occurrence ER had modified themselves into a special type.

Microsome

Broken pieces of RER are called Microsome.

They are often used for the study of protein synthesis.

Nassal’s Granule

The RER of Neuron is called Nassal’s Granule.

Sarcoplasmic Reticulum

ER of the Muscle cell is called Sarcoplasmic Reticulum.

Also, the term “Sarco” is used as a suffix for muscle-related terms.

Their function is to store the Ca+2 ions for the muscular movements.

Important Points

  • A large subunit of the Ribosomes is attached to the ER with the help of a special protein called ‘Ribo-phorin’.
  • In Gonads, SER forms the Steroids (Estrogen in females and testosterone in males). Steroids are made up of lipids.
  • Glycogenesis is the synthesis of Glycogen from Glucose. It occurs mainly in Liver cells and somewhat in muscle cells.

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