Endocytosis

If the cell wants to live and grow and wants to increase its number they must obtain nutrients and other substances from their surrounding environment.

Most of these substances enter the cell through the cell membrane by two process
1. Diffusion
2. Active transport

 

bacteria cells

Diffusion is the movement of molecules due to concentration gradients. Diffusion involves simple movement caused by the random motion of molecules either through the membrane pores or the lipid matrix of the membrane.

Active transport is the movement of molecules through the membrane by proteins structure that penetrates all the way through the membrane.

Large molecules enter the cell through the cell membrane by a specialized function known is endocytosis.

Endocytosis is a biological process in which substances are brought into the cell. The material that is to go inside is surrounded by a membrane which then buds off. These buds off inside the cell form vesicles that contain the ingested material.  According to the types of transport, it is active transport. Endocytosis includes two main pathways that are phagocytosis and pinocytosis.

History

The term endocytosis was proposed by De Due in 1963. Phagocytosis was discovered in 1882.

Endocytosis pathways

There are four pathways for endocytosis that are
1. Receptor-mediated
2.  Caveolae
3.  Pinocytosis
4.  Phagocytosis

Clathrin-mediated endocytosis

They are also known as receptor-mediated endocytosis. It is mediated by production small vesicles that have a clathrin protein coat. These vesicles are found in all the cells in the body and form domains of the cell membrane that are known as clathrin-coated pits. These clathrin-coated pits concentrate large extracellular molecules that have different receptors for clathrin-mediated endocytosis of ligands.

Caveolae

These are non-clathrin-coated cell membrane buds that are present in many cells but not in all cells. They consist of caveolin proteins that are cholesterol-binding proteins. Caveolae are cave shaped pits in the membrane. Caveolae are abundant in smooth muscles, fibroblast, and endothelial cells.

Pinocytosis

Pinocytosis also termed as bulk phase pinocytosis is a type of endocytosis in which small particles in extracellular fluid are brought to the cell through the plasma membrane investigation that results in the suspension of particles in the vesicles inside the cell. These particles fuse with the lysosomes and the lysosomes then hydrolyzed the particle. The process of pinocytosis requires energy in the form of ATP. Pinocytosis is divided into various categories on the basis of molecular mechanisms and the fate of internalized molecules. In some cases, pinocytosis is a constructive process while in some it is a receptor-mediated process.

In human fat droplets can goes inside the cell through the process of pinocytosis. In the process of pinocytosis, the plasma membrane extends and folds around the extracellular material and forms a pouch. This pouch the pinches off and creates internalized vesicles inside the cells. The vesicles that are generated through pinocytosis are much smaller than the vesicles form by the phagocytosis. These vesicles then fuse with the lysosomes and the lysosomes digest the ingested particle. This process requires ATP.

Pinocytosis clear extracellular fluid and is non-specific in the substances that it transports. Pinocytosis works as phagocytosis but the difference is that phagocytosis is specific while pinocytosis is non-specific in the substances that it transports. Another difference is that phagocytosis engulfs unbroken food while pinocytosis engulfs broken-down foods.

Non-specific absorptive pinocytosis
It is a form of endocytosis in which small particles are taken in by a cell by splitting vesicles from the cell surface.

Phagocytosis

It is a process in which the cell engulfs large particles by the use of its plasma membrane. In the immune system, phagocytosis is the major process to remove pathogens. The materials that are ingested are then digested in the phagosome.

Examples of objects that may be phagocytized are
1. Bacteria
2.  Dead tissue cells
3.  Small mineral particles etc.

History of phagocytosis

Phagocytosis was first discovered by William Osler and later studied by Elle Metchnikoff. Metchnikoff also gives the name phagocytosis.

Phagocytosis in the immune system

In innate immune defense, phagocytosis is the major process and it is the first process that responds to an infection. There are many initiating branches of the adaptive immune responses and phagocytosis is one of them.  Although every cell in the body is capable of phagocytosis there are some cells that perform phagocytosis as part of their main function. The cells that perform this process as a part of their main function are called professional phagocytes. Professional phagocytosis cells are

1.  Neutrophils
2.  Macrophages
3.  Monocytes
4.  Dendritic cells
5.  Osteoclast
6.  Eosinophils

The role of neutrophils is rapid migration in case of infection and patrolling Blood. Neutrophils have a direct microbicidal effect and they are efficient in intracellular killing. The microbicidal effect of neutrophils is due to granules present in them that contain enzymes and other molecules such as collagenase, a serine protease, lactoferrin, and antibiotic proteins.

Macrophages leave the cardiovascular system and migrate to the tissues where they settled and form a resting barrier. These cells initiate phagocytosis by various receptors that are mannose receptors, scavenger receptors, and complement receptors, etc. These cells continuous phagocytosis by forming new lysosomes.

Dendritic cells also reside in tissues and engulf pathogens through a type of endocytosis named as phagocytosis.

Phagocytosis and apoptosis

In apoptosis, the cell dies these dead cells need to be taken up into tissues by macrophages. The process of taking up the dead cells into the surrounding tissues is known as efferocytosis. Defects in the clearance of the optic cell are due to the impaired phagocytosis of macrophages. If the apoptotic cells clearance is disturbed then these cells accumulate and cause autoimmune disorders.  Phagocytosis has a potential in the treatment of autoimmune disorders.

The principal component of the endocytosis pathways

The cells contain membrane compartments that internalize molecules from the cell membrane and recycle them back to the surface or degrade them.  The main component of the endocytosis pathways are
1. Early endosomes
2.  Late endosomes
3.  Lysosomes

Early endosomes
It is located in the periphery of the cells and receives vesicles from the cell surface. They have a tubulo-vesicular structure and mildly acidic pH.

Late endosomes
It is the second component of the endocytic pathway that receives the endocytosed material from early endosomes, trans-Golgi network, and phagosomes. These endosomes contain protein characteristic of nucleosomes, mitochondria, and messenger RNA.

Lysosomes
It is the last compartment and its main function is to breakdown cellular waste products, fats, carbohydrates, proteins, and other molecules into simpler compounds.