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The cells need an environment to sort and transport internalized material before it reaches lysosomes for processing. Many proteins, such as lipoprotein, bind to receptors to dissociate. For this purpose, the eukaryotic cells make machinery known as endosomes. However, there are different types of endosomes depending on the activity within the vesicle. But they have other functions within both plants and animal cells. In addition to their primary function, these heterogeneous organelles can transport materials from Golgi apparatus to vacuoles. It helps in the interaction between different cell organelles. The organelles are connected directly or indirectly through transport vesicles.

What are Endosomes?

The eukaryotic system consists of intracellular sorting organelles referred to as Endosomes. The system synthesizes, regulates, and delivers proteins and lipids within the cell. It has a sorting function among the lysosomes, plasma membrane Golgi, and trans-Golgi network (TGN). Endosomes transport material such as proteins and lipids from Golgi to vacuoles through the biosynthetic and the endocytic membrane transport pathways. 

Therefore, it can be considered an intermediate compartment that receives plasma membrane proteins. Protein molecules are internalized by endocytosis from the plasma membrane. After sorting, it recycles again from the plasma membrane. Fast recycling pathways recycle the materials at recycling endosomes for rapid recycling. 

Structure of Endosomes 

A model has been proposed for the endosomes present in the plant cells. They have a trans-Golgi network with a multivesicular body. Plant endosomes also have an additional type known as recycling endosomes. It is assumed that the Golgi network matures to form the multivesicular body. There are two compartments present in the endosomes, namely early and late endosomes. Early endosomes are present at the cell periphery, while late endosomes can be more profound in the cytoplasm. It has a plasma membrane that collects recycled products. The invagination of the plasma membrane engulfs internalized materials to form vesicles.

Types of Endosomes 

The following are the three different types of endosomes.

• Early endosomes – It is a system of vesicles and tubules bound to plasma membranes. The compartment receives materials being internalized. Within the cell’s periphery, they contain the receptor’s ligand complex. Early endosomes have a pH lower than 6 for uncoupling ligands and receptors. The acidity is present due to Vacuolar Proton-ATPase. Molecules are sorted here to form vesicles. But if Vacuolar Proton-ATPase is inhibited, it will block vesicles trafficking. Early endosomes consist of tubules that have a little higher pH than vesicles. The contents from the tubular part are transported to recycling endosomes. 

• Recycling endosomes – As the name suggests, recycling endosomes are for recycling materials. These endosomes are concentrated in the microtubule. Rab11a and Rab4 regulate the recycling compartment. It helps in the recycling of proteins, transferrin and integrin receptors. 

• Late endosomes – Late endosomes, an irregular vesicle, receive the recycled material. It is present near the Golgi complex to receive lysosomal enzymes from it. Later, the enzymes degrade ligands.

Functions of Endosomes

Endosomes play a role in transportation along with other processes. These include:

• Transportation of Materials – The endomembrane system synthesizes macromolecules such as protein and lipids. In plants, the molecules in the endocytic pathway pass through the trans-Golgi network. The pathway also plays a role in recycling transport. It is made for the degradation of cellular material in lysosomes.

• Uptake of Nutrients – In humans, Low-density lipoprotein is taken up by the LDL receptors present on the cell’s plasma membrane. The early endosomes do the delivery of this vesicle. LDL consists of cholesterol and acts as a transporter of it. When the vesicle is formed, it detaches from the receptors to get released into the lysosomes or vacuoles.

• Sorting signal in the neurons – Early endosomes exist in neurons’ presynaptic and postsynaptic membranes. They play a significant role in sorting the signs on the neuron transmembrane. In the late endosome, packaged protein is transported. But, proteins such as transmembrane receptors can also return to the plasma membrane for recycling. Some neurons recycle endosomes as an intermediate point. 

The whole process taken into action by endosomes is known as endocytosis. It can be of two types: phagocytosis or cellular eating and pinocytosis or cellular drinking.

Conclusion 

In this article, we studied the eukaryotic system and the structure and functions of endosomes. It is important to note that endosomes help internalize and transport materials. To disassociate, many proteins, such as lipoprotein, bind to receptors. Endosomes, a type of machinery made by eukaryotic cells, serve this job. However, depending on the activity within the vesicle, multiple types of endosomes exist. However, they serve additional purposes in both plant and animal cells. These heterogeneous organelles can carry materials from the Golgi apparatus to vacuoles in addition to their primary role. It aids in the interaction of several cell organelles. Transport vesicles connect the organelles, either directly or indirectly.