Mitochondria

 Mitochondria

Mitochondria is one of the most important organelles in eukaryotic cells. It is also known as the “Powerhouse of the cell” because energy is generated inside it and the cell utilizes this energy for various metabolic activities.

Discovery of Mitochondria

The credit for mitochondria discovery goes to Albert Von Killer, a Swiss biologist who was studying granular structures in insect muscle cells in 1857. Later a German histologist, in 1890, realized their extensive existence in cells and named them “bioblasts” and considered them the vital parts of cells. 

Then in 1898, a German microbiologist Carl Benda used the term “mitochondria” for them which means “threads” or “granules”.

Structure of Mitochondria

Its size is approximately 1 mm and it has a sausage-shaped structure with a little flexibility but most of the time it looks like a sausage. It is a double membrane-bounded organelle just like the nucleus.

Outer membrane 

The outer membrane of mitochondria is smooth. It gives mitochondria a sausage appearance. It is more permeable as it allows more molecules to come inside and outside it. Its permeability is due to the presence of special types of proteins which are called porins. These are hollow proteins and allow many smaller molecules to pass through them from the cytoplasm.

Inner membrane

The inner membrane is folded in such a way that it is invaginated towards the inside. It accedes only selective molecules to move through it. However, some small molecules like water, carbon dioxide and oxygen can easily pass through it.

The inner membrane contains invaded proteins that perform energy production mechanisms such as electron transport chain or oxidative phosphorylation.

The internal space of the inner membrane is called a matrix which contains ribosomes, mitochondrial DNA and different enzymes, ions and molecules. The finger-like folded regions of the inner membrane are known as cristae due to which the surface area of the inner membrane increases.

The mitochondrial DNA is round just like microbial DNA and it comes from the mother. The mitochondrial DNA synthesizes 15% of its proteins by itself.  The size of mitochondrial ribosomes is 70S as of microbial ribosomes.

Based on these evidences, Lynn Margulis assumed a theory, also known as endosymbiotic theory in 1967 which stated that mitochondria and chloroplasts in today’s eukaryotic cells were once separate prokaryotic microbes. This theory explains that once a eukaryotic cell engulfed a microbe and made this microbe a part of its body. This microbe was now mitochondria and it started providing energy to the eukaryotic cell and the cell provided it a space to live. So a symbiotic relationship was built between them. 

Inter Membrane Space

The space between the outer and inner membranes of mitochondria is called inter-membrane space. This space has a high concentration of hydrogen ions. 

Abundance of Mitochondria in Specific Organs

Since mitochondria are the powerhouse of cells, the cells that need more energy have more mitochondria. For example, in liver and muscle cells, the number of mitochondria is higher as compared to other cells. The number of mitochondria also increases when a person starts doing exercise because of the rise in energy requirements so the mitochondria start dividing by the fission process and increase their numbering.

Functions of Mitochondria

Apoptosis

Sometimes, when bacteria attack a cell or cancer spreads in a particular area of the body, the nucleus receives the signal and sends it to the mitochondria. Mitochondria release cytochrome C proteins which activate caspases to initiate the eating of different parts of the cell so that the infection may not spread to the other parts of the body. In this way, mitochondria have a major function in apoptosis.

Storehouse of calcium

There are only two places in the cells that act as the storehouse of calcium. Mitochondria is one of them while the other one is smooth endoplasmic reticulum. This calcium is necessary for signal transduction mechanism, muscle contraction and performing functions as cofactor, etc.

Detoxification of Ammonia

Ammonia is a toxic chemical that is produced during the breakdown of proteins inside mitochondria. Mitochondria run a urea cycle in association with cytoplasm and convert ammonia into urea which is less toxic and water soluble and can be easily expelled out from the body. So, mitochondria help in the detoxification of ammonia.

Energy Production

Our cells perform all metabolic activities of the body by the utilization of energy. This energy is created within our cells in mitochondria by cellular respirattion mechanism.

In mitochondria, the food is converted into chemical energy called ATP that the cells use for various metabolic activities. ATP is generated at the intermembrane of mitochondria by oxidative phosphorylation, involving several membrane protein complexes. Nutrients provide high energy amounts in the form of NADH which are used by protein complexes to pump protons from the matrix to the inter-membrane space. This continuous pumping creates a proton gradient, where the positively charged protons are attracted towards the negatively charged matrix. When the protons re-enter the matrix through the ATP synthase complex, they catalyze the production of ATP.