Endoplasmic Reticulum: Definition, Types, Examples, Diagram, Function

What is the Endoplasmic Reticulum?

The endoplasmic reticulum is defined as a complex network of membranous tubules and sacs in the cytoplasm of eukaryotic cells. The ER can be further divided into two regions. The rough ER is studded with ribosomes and mainly engaged in the synthesis of proteins. The smooth ER is associated with lipid synthesis. It sends the newly synthesized proteins and lipids within the cell to their right destinations within the cell or out of it.

Discovery of Endoplasmic Reticulum

The discovery of endoplasmic reticulum dates back to the 1940s, with the help of an electron microscope. The more significant contributions are observed to have been made first by Keith Porter, Albert Claude, and Ernest Fullam. Their work on this area of cell structure led to the initial periods of knowing about this component of cells.

Structure of Endoplasmic Reticulum

The structure of the endoplasmic reticulum is described below:

Membranous Network and Lumen

• The endoplasmic reticulum is a single, continuous membrane system composed of cisternae, tubules, and vesicles.

• Cisternae are long, flat, unbranched sac-like structures with ribosomes attached to them.

• Vesicles are round or ovoid structures having a diameter. They are abundantly present in pancreatic cells.

• Tubules are irregular branched structures. They are abundant in cells which actively synthesise cholesterol and glycerides, for

• The lumen of ER is continuous with the lumen of the nuclear envelope. Together, they constitute more than 10%.

Ribosomes on RER

• The rough ER is tubular and contains ribosomes that are studded over the surface and are, therefore, involved in protein synthesis.

• Cells which secrete synthesised proteins have mostly RER, such as pancreatic beta cells.

Smooth Surface of SER

• The smooth ER is often dilated and convoluted and does not have ribosomes.

• The cells that are involved in lipid synthesis, calcium signalling, and serve as sites of contact for other organelles, possess tubular SER. For example, adrenal, liver, and muscle cells.

Types of Endoplasmic Reticulum

The endoplasmic Reticulum is divided into two types:

Rough Endoplasmic Reticulum (RER)

• The surface of the RER is rough because of ribosomes scattered over its cytoplasmic aspect.

• Protein synthesis takes place in these ribosomes.

• The RER consists of a series of flattened membrane sacs, called cisterns, with connecting tubules.

• This supports the effective processing of proteins destined for secretion, insertion into the membrane, or targeting lysosomes.

Smooth Endoplasmic Reticulum (SER)

• The smooth endoplasmic reticulum appears smooth because there are no ribosomal subunits overlaying its cytoplasmic surface.

• The principal functions are lipid synthesis, carbohydrate metabolism, drug detoxification, and calcium ion storage.

• In muscle cells, the SER is differentiated further as the sarcoplasmic reticulum.

Functions of Endoplasmic Reticulum

Certain functions of the endplasmic reticulum (RER and SER) are discussed :

Functions RER

• The RER is an important component of the protein synthesis system because ribosomes decode mRNA molecules into polypeptides on its surface.

• The nascent proteins are discharged into the lumen of the RER for folding and a number of quality control processes to ensure their accuracy in conformation.

• The RER also participates in the system of glycosylation, a process where carbohydrate groups are added to proteins to ensure the stability and functionality of proteins.

Functions of SER

• The main function of smooth endoplasmic reticulum (SER) is to synthesise lipids which form part of the cell membrane.

• It is also involved in the carbohydrate metabolism that converts glucose-6-phosphate to glucose.

• The SER is involved with detoxifying drugs and poisons because it chemically changes them into more water-soluble compounds that can be excreted.

• The SER is also a store for calcium ions that, on its release into the cytoplasm, function to turn on various cellular processes, including muscle contraction.

Dynamics and Regulation of ER

The dynamic activity of the endoplasmic reticulum in view of maintaining cellular homeostasis is thoroughly regulated.

Unfolded Protein Response (UPR)

• The unfolded protein response or UPR results are due to ER stress arising from misfolded or unfolded proteins accumulating in the lumen of the ER.

• In normal function restoration, this compound blocks the translation of proteins, degrades misfolded proteins, and upregulates the signal pathways.

• It is an important response in the maintenance of health and activity of the cell and prevents diseases caused by misfolded proteins.

ER Stress and Disease Link

• Prolonged ER stress leads to apoptosis of the cells, contributing to diseases like Alzheimer’s, Parkinson’s, diabetes, and cancer.

• ER stress is also linked to inflammatory and metabolic disorders.

Biogenesis and Maintenance of ER

The biogenesis of the ER is traced back to early cell development. The ER is developed and altered through genetic control by specific genes and proteins. These drive the synthesis of the membrane and its further expansion. Due to the dynamic properties of ER development, its structure and function are moulded based on the cell's physiological demands.

The ER is maintained by the continuous reorganisations of the membrane, which are orchestrated through selective autophagy, known as reticulophagy and the ERAD pathway. ERAD makes sure that the organelle is continually robust and efficient to meet the requirements of the cell.

Applications and Relevance in Medicine & Biotechnology

The use of the ER is much greater in the field of medicine and biotechnology. It helps in understanding the pathological mechanisms of neurodegenerative diseases and diabetes, where stress in the endoplasmic reticulum and misfolded proteins are very important.

Knowledge about ER functions and its stress-related responses will help design possible drugs and genetic engineering to restore or repair homeostatic imbalances. In cellular and molecular biology, the ER is an extremely important part because understanding the synthesis of proteins and lipids, their intracellular transport, and the intricate regulation of cell functions is very crucial.

Endoplasmic Reticulum NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure of ER (Cisternae, vesicles, tubules)

• Functions of RER

• Functions of SER

Practice Questions for NEET

Q1. Nuclear envelope is a derivative of:

1. Microtubules

2. Rough endoplasmic reticulum

3. Smooth endoplasmic reticulum

4. Membrane of Golgi complex

Correct answer: 2) Rough endoplasmic reticulum

Explanation:

During telophase, the final stage of cell division, the nuclear envelope is reassembled around the separated sets of chromosomes. This process involves the rough endoplasmic reticulum (RER), which produces vesicles containing nuclear envelope components. These vesicles fuse to form the nuclear envelope, enclosing the chromosomes within the nucleus. This restoration of the nuclear envelope marks the re-establishment of the nucleus, allowing the cell to transition back to its interphase state.

Hence, the correct answer is option 2) Rough Endoplasmic Reticulum.

Q2. A major site for synthesis of lipids is

1. RER

2. SER

3. Nucleoplasm

4. Cytoplasm

Correct answer: 2) SER

Explanation:

SER (Smooth Endoplasmic Reticulum) is the major site for the synthesis of lipids.

Hence, the correct answer is option 2) SER.

Q3. ER divides the intercellular space into two distinct compartments, luminal ______ and extra luminal ______ compartments.

1. Cytoplasm, inside ER

2. Inside ER, cytoplasm

3. Nucleus, cytoplasm

4. Inside ER, nucleus

Correct answer: 2) Inside ER, cytoplasm

Explanation:

The endoplasmic reticulum is a network of membranous tubules within the cytoplasm of a eukaryotic cell. It is occupied close to the outer nuclear membrane. It usually has ribosomes attached and is involved in protein and lipid synthesis. The endoplasmic reticulum divides the intracellular space into two distinct compartments, viz. luminal and extra-luminal.

Hence, the correct answer is option 2) Inside ER, cytoplasm.

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