Endomembrane System

Introduction

An endomembrane system is a group of membranes and organelles in eukaryotic cells that work together for the modification, packaging, and transportation of lipids and proteins. It includes various parts like a nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, endosomes, and cell membranes. An organelle refers to a cell’s various structures that undertake the performance of a particular function. The Endomembrane system is a system of membranes within a cell that fulfil the role of being a single functional and developmental unit. This is a system that is characterised by membranous components. Mitochondria and/or chloroplasts are not a part of this system.

Nuclear Envelope

The nucleus is one of the most prominent organelles in a eukaryotic cell in the endomembrane system. It is the large, membrane-bound structure that is characterised by the genetic material (DNA). The organisation of this genetic material may take place in chromosomes.

Its biological membrane is called by experts as the nuclear envelope of the nuclear membrane. The nuclear envelope is formed by two layers of lipid. There are holes in this envelope known as “nuclear pores”.

This is where the initiation of the protein synthesis takes place. The copying of the DNA segment, that carries the code for a particular protein, takes place via the process called transcription.

Endoplasmic reticulum (ER)

The endoplasmic reticulum (ER) serves the role of being the first site of the secretory pathway in the endomembrane system. Take note that the outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum.

There are two types of endoplasmic reticulum in existence which are as follows:

•     Rough endoplasmic reticulum 
•     Smooth endoplasmic reticulum 

The primary difference between the two is pertaining to ribosomes. In Rough endoplasmic reticulum, the ribosomes are attached to its surface, whereas in smooth endoplasmic reticulum there are no ribosomes.

Golgi Apparatus

The Golgi apparatus or Golgi complex or Golgi bodies refer to an organelle that is made up of cisternae. Cisternae in turn refer to flattened membrane sacs that contain fluid. In animal cells, the Golgi cisternae are connected by microtubules. In plant cells, the Golgi cisternae are connected by actin. 

The connection of the Golgi cisternae is not direct to the nuclear envelope. This is in contrast to the endoplasmic reticulum, as mentioned in the endomembrane system study material. 

The primary function of the Golgi apparatus is to serve the role of being the packaging centre of the cell. For example, the sorting of the proteins coming from the ER takes place in it. Thereafter, their tagging takes place by the apparatus to their destination sites.

Vesicles

A vesicle, generally speaking, is a small sac. Inside the cell, the vesicles refer to any bubble-like structures which are responsible for the storage and transportation of the cell products within the cell. Its contents are separated from the cytosol by at least one lipid layer.

There are different vesicles within the cells. The ER vesicles, for instance, are the transport vesicles that are responsible for the translocation of the protein cargo to the cis face of the Golgi apparatus. Another transport vesicle is the Golgi vesicle, which buds off from the Golgi apparatus to transport its cargo either internally or externally or by secretion.

Lysosomes are vesicles that facilitate the digestion of metabolic wastes. Another example of vesicles is vacuoles, whose function mostly involves osmoregulation. 

Endosomes

Endosomes are membrane-bound cytoplasmic structures through which molecules pass en route to the lysosome for “digestion”. These molecules are taken in via the process of endocytosis. The endosomes are single-membrane in nature just like the lysosomes.

There are various types of endocytosis which are as follows:

Phagocytosis: This involves the “cell-eating” in which the taking in of the larger particles takes place.

Pinocytosis: This involves “cell drinking” in which the taking in of the smaller particles takes place.

Receptor-mediated endocytosis: This involves the taking in of particles through the utilisation of receptors, like clathrin.

Cell Membrane

The cell membrane is a membrane that surrounds the protoplasm. Protoplasm is the cell’s living component. Both eukaryotic and prokaryotic cells have a cell membrane that facilitates the separation of the protoplasm from the outside environment. Each cell membrane is formed by two layers of phospholipids.

Conclusion

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together. This results in the modification, packaging, and transportation of lipids and proteins. There are various parts in the endomembrane system study material like a nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, endosomes, and cell membranes. This whole system is characterised by membranous components. Mitochondria and chloroplasts do not fall within the purview of this system.