Vesicles in the Cell

Vesicles are small sacs like a bubble that contains material that is further transported to different parts of the cells and outside the cell. They perform various functions that are important for the survival of the cells. Vesicles are present in both eukaryotic and prokaryotic cells. 

These structures mostly originate from the Golgi apparatus and endoplasmic reticulum, and in some cells, endocytosis of cell membrane forms these tiny structures.

Structures of vesicles

Vesicles contain two types of materials.

• Fluids
• Gases

The material is enclosed within the membrane composed of phospholipids. The hydrophilic heads and hydrophobic tails are arranged to form a lipid bilayer. It makes it easy for them to cross other cell membranes for a smooth transportation process. 

Functions of the vesicles

Different types of vesicles perform different functions for the survival of the cells. The most important function is removing the waste material from the cells. Sometimes some toxic substances accumulate in the cell. It is, therefore, necessary for their removal, or it may lead to cell death. 

Some of the vesicles, like lysosomes, are important for their phagocytic activity to remove dead or worn-out cells from the body of organisms. The major material transported by the cells include:

• Proteins
• Waste material
• Gases
• Hormones
• Enzymes
• Neurotransmitters

These materials are essential for different human body systems like the digestive and nervous systems, etc.

Vesicles vs Vacuoles

The vesicles are usually confused with the vacuoles that are other structures present in the cell. Both of them are involved in the storage and transport of substances. The main difference between these two is their size.

The vacuoles are larger than the vesicles. The membrane of vesicles can diffuse with the membranes of other structures within the cell to accomplish their function. On the other hand, vacuoles are unable to do so. Vacuoles are just considered as a type of vesicle.

Types of Vesicles 

Vesicles have many types based on their function and the location where their function is needed. These are:

Transport vesicles

Transport vesicles transport material like proteins etc., to various cell parts. Ribosomes are the protein factories of the cell that are either present in a free state in the cytoplasm or fixed state as being attached to the rough endoplasmic reticulum.

These proteins are needed in all parts of the cell. Transport vesicles carry them from there to the Golgi apparatus for further modification. Then, they are transported to their needed place.

Secretory vesicles

Secretory vesicles transport materials like waste products and hormones outside the cells. They are essential for disposing of toxic substances. They play an important role in the coordination of different body cells. The types of secretory vesicles are:

Synaptic vesicles

Synaptic vesicles are located at neurons on their presynaptic portions. Their function is the storage and transmission of neurotransmitters. A signal is received quickly diffuses with the cell membrane and presents the neurotransmitter to the receptor molecules on the other nerve cell for detection.

Hormone containing vesicles

Hormones are produced within the different structures of the cell that are stored in the secretory vesicles and secreted to other structures. For example, many protein digestive enzymes are transported to the stomach.

Membrane vesicles

Membrane vesicles (MVs) are released mainly by prokaryotes, fungi, and other parasitic organisms. These structures contain toxic substances or signal molecules. They provide pathogenicity to the cells and protect them from the host immune responses. These toxic substances can also kill other microbes around.

Vacuoles

• Vacuoles are present in just eukaryotic cells for the osmotic balance of their cells. A large central vacuole is present in the plants that keep the cell turgid.
• In animals, many vacuoles are present. They store fluids, mainly water, and some nutrients. 
• In freshwater organisms, contractile vacuoles collect extra liquid and nitrogenous water from the protoplasm and secrete them outside the cell to prevent the bursting of the cells.

Lysosomes

• Lysosomes are involved in the accomplishment of the digestion process through phagocytosis and utilization of food particles in the cell. Endocytosis facilitates the formation of food vacuole to carry food particles inside the cell.
• Lysosomes, containing digestive enzymes, fuse with them and digest the food particle to the form usable by the cell.
• The essential function of lysosomes is autophagy. Autophagy is the destruction of old and damaged cell structures. Lysosomes fuse with these structures and digest them using different enzymes. 

Peroxisomes

• Peroxisomes are called so because of the hydrogen peroxidase production due to the breakdown of different substances like fatty acids and amino acids.
• They contain more than 40 enzymes involved in different biochemical processes.
• The hydrogen peroxide produced in the peroxisomes is toxic. So, it is necessary to convert it into a less toxic form.
• The peroxisomes also contain a catalase enzyme that converts hydrogen peroxide to water and oxygen molecules. 
• One amazing feature of peroxisomes is their self-replicating ability. They increase their size and divide further into two organelles. 

Extracellular vesicles

• Scientists researched over many to specify some of the functions of the extracellular vesicles.
• These vesicles are present outside the cells and are important for communicating one cell with another, as the quorum sensing in bacteria.
• They also carry material from one cell to another.
• More research is needed to understand their complete functions and the reasons for their formation.

Final Words

Vesicles are important for the survival of both prokaryotic and eukaryotic cells. They are needed for every system of the body. Four main vesicles within the cells are:

• Transport vesicles
• Secretory vesicles
• Lysosomes
• Extracellular vesicles

Certain functions are specified for each type of vesicle and are crucial for the cells.

Frequently Asked Questions

References

1. https://www.medicalnewstoday.com/articles/vesicle#summary 
2. https://microbenotes.com/vesicles-structure-types-and-functions/ 
3. https://www.ncbi.nlm.nih.gov/books/NBK9930/ 
4. Alberts, Bruce, Johnson, Alexander, Lewis, Julian, Raff, Martin, Roberts, Keith, and Walter, Peter (2008). Molecular Biology of the Cell (Fifth Edition), (Garland Science, New York), p. 789
5. https://en.wikipedia.org/wiki/Vesicle_(biology_and_chemistry) 
6. https://www.nature.com/articles/s41598-020-65826-z