Endocytosis and exocytosis

Entry and exit of small uncharged molecules occur freely through the semipermeable cell membrane, the process of simple diffusion. Passive movement occurs from areas of higher concentration to areas of lower concentration. Transportation of larger molecules by this method is not always possible. Macromolecules in the form of polysaccharides and proteins cannot proceed so easily and require a bulk transport system for transportation. 

Endocytosis and exocytosis are two mechanisms of the bulk transport system in eukaryotes that performs this task. Both these processes require a large amount of energy in the form of ATP and are called active transport systems. 

Below are the 3.5 active transport endocytosis and exocytosis power notes, which will help us better understand the topic.

Active transport and vesicle formation 

How do endocytosis and exocytosis move large molecules? 

Vesicles are membranous sacs composed of a lipid bilayer. The cell membrane is also made up of a lipid bilayer. Bulk transport by exocytosis and endocytosis requires a large amount of energy in the form of ATP. During active transport, binding of the lipid layer of cell membrane and vesicle occur that facilitates bulk transport into and out of the cell.

Endocytosis

Proposed by Du Duve in 1963, endocytosis is a process by which a cell takes up a substance from outside by surrounding it with vesicles. These include nutrients that support cells and pathogens that swallow and destroy immune cells. 

Endocytosis is a cellular process in which a substance is introduced into a cell. The internalised material is surrounded by the cell membrane region, which then forms a bladder containing the intracellularly budded and captured material. 

Here, a portion of the cell membrane folds around extracellular fluid and various molecules and microorganisms. The resulting vesicles break and are carried inside the cell. Processes included in endocytosis are:

Phagocytosis

Phagocytosis is also known as “cell eating”, as through this process, larger particles with a diameter of more than 0.5 um, are ingested and eliminated from the cell. 

Microorganisms, bacteria, and toxic compounds are eliminated through this process; eg, primary phagocytes responsible for infectious organisms are macrophages and neutrophils.

Pinocytosis

Pinocytosis is also known as “cell drinking”, as, through this process, the cells ingest fluid and smaller molecules of 0.5- 5um diameter. A specialised type of pinocytosis is receptor-mediated endocytosis, when macromolecules and receptors bind on the plasma membrane. For example, the small intestine has microvilli that absorb nutrients from the gastrointestinal tract.

Purpose of endocytosis

1. Lipids and proteins are essential for a cell that cannot be ingested by simple diffusion. Ingestion of nutrients by active transport aid in growth, repair and functioning.
2. Protection of the body against harmful pathogens. The immune system identifies and engulfs bacteria and harmful organisms that may otherwise be a danger to the body.
3. The cycle of regeneration and degeneration goes on throughout our life. Old, damaged cells may hinder the functioning of normal cells and are eliminated by endocytosis. 

Exocytosis

Exocytosis is a form of active and mass transport in which cells transport molecules (such as neurotransmitters and proteins) extracellularly. 

Exocytosis occurs through a secretory portal of the protoplasmic membrane called the porosome. Permanent, cup-shaped lipoprotein structures on the protoplasmic membrane of the cell, where secretory vesicles temporarily dock and fuse to release the intracellular contents from the cell. 

Exocytosis and its corresponding endocytosis are used in all cells because they are large polar molecules that cannot passively cross the hydrophobic part of the cell membrane.

Processes included in exocytosis are:

Constitutive exocytosis

This pathway is used for major molecules travelling across the plasma membrane. After the process of exocytosis, there are three pathways:

1. Full vesicle fusion: where the plasma membrane incorporates the exocytic vesicles.
2. Kiss and run pathway: where some vesicles pass into the cell after releasing their content.
3. Kiss and stay pathways: where vesicles remain in the remembrane and are used multiple times.

Regulated exocytosis

• This pathway is regulated by hormones and neurotransmitters secreted by the endoplasmic reticulum.
• The golgi bodies modify these secreted substances.
• This control and regulation occur by extracellular signals, resulting in the membrane’s depolarisation.

Purpose of exocytosis

1. To maintain haemostasis within the cell, toxins and waste products are removed by exocytosis.
2. The hormones and neurotransmitters secreted facilitate cellular communication and are delivered to other cells where they are needed.
3. During endocytosis, lipids and proteins from the plasma membrane create vesicles. During the expulsion of waste materials, the lipids and proteins are replenished and facilitate cell membrane repair, growth and migration.

Conclusion 

The 3.5 active transport endocytosis and exocytosis power notes have brought to our knowledge that endocytosis and exocytosis move large molecules. Both mediate a cell’s proper functioning, and disruption of even one can hamper the body internally and externally.