Mitochondria

Introduction

Mitochondrion (Mitochondria; singular) is one of the highly helpful cell organelles found in most of the eukaryotes. They are mainly responsible for anabolic processes such as cellular respiration, protein synthesis for tissue repair, and cell metabolism. However, one of the fundamental function that mitochondria perform is the synthesis of energy. Mitochondria help to release energy in the form of  ATP hence it is branded as the powerhouse of a cell. In the year 1890, Richard Altman has identified the mitochondrion and he called them bioblasts.  Later on, in the year 1898, Carl Benda has coined the term mitochondria which is a Greek term, mito (thread) and kondros (granule). Another biologist, Philip Skiekevitz has identified the mitochondria as” the powerhouse of the cell' in 1957. This article is designed to explain the structure and functions of human mitochondria. 

 

 

Structure of mitochondria

Our mitochondria measure about 0.75 and 3 μm² but they vary among different species. The structure of a mitochondrion is divided into

  1. Outer membrane

  2. Intermembrane space

  3. Inner mitochondrial membrane,                                                       

  4. Cristae

  5. Mitochondrial matrix

  6. Mitochondrial DNA

  7. Mitochondrial ribosomes

 

 

 

Outer membrane

The outer mitochondrial membrane measures 60 to 75 angstroms (Å) in thickness. It is a bilayer containing both the protein and phospholipids at the ratio of 1:1. The membrane proteins located on the outer portion are known as portions. The outer membrane also contains enzymes required for the elongation of fatty acids, oxidation of epinephrine, and the degradation of tryptophan during ATP production. The most common enzymes found in the outer membrane are monoamine oxidase, rotenone-insensitive NADH-cytochrome and fatty acid Co-A ligase. The membrane helps to produce the endoplasmic reticulum (ER) membrane responsible for the formation of MAM (mitochondria-associated ER-membrane).  MAM is necessary for the ER-mitochondria calcium signalling as well as the transport of lipids between the ER and mitochondria.

 

 

Intermembrane space

Intermembrane space is also known as perimitochondrial space located in between the outer and the inner membranes of mitochondria. The 2 membranes help in the formation of 2 aqueous compartments,  intermembrane space (IMS) and the matrix. The porins present on the outer membrane allow smaller proteins ( 5000 Daltons or lesser ) and a few ions into the IMS.

 

 

Inner membrane

The inner mitochondrial membrane comprises proteins involved in 5 different functions. Furthermore, it also has 151 different polypeptides with the protein-to-phospholipid ratio of  3:1. It accommodates about 20% of the total protein present in the mitochondrion. It is also a very important membrane assisting in the electron transport chain necessary for their aerobic respiration.

 

 

Cristae

Cristae are the rough folds formed out of the shrinkage of the inner membrane of the mitochondria. Shrinkage takes place with the help of a process called compartmentalization. The number of folds in the cristae vary between individual species and they help in increasing the surface area of the membrane to provide adequate space for energy production.

 

 

Mitochondrial Matrix

Matrix is a sticky ground substance within which many enzymes float. It is an essential component of the cell as it provides space and the support necessary for the production of ATP-energy through aerobic respiration. The energy is released through the citric acid cycle that takes place with the help of enzymes such as acyl-CoA, pyruvate, acetyl-CoA, citrate, isocitrate, α-ketoglutarate, succinyl-CoA, fumarate, succinate, L-malate, and oxaloacetate. Mitochondrial matrix contains 70 different ribosomes which synthesize proteins. Furthermore, the matrix is richly embedded with many ions hence it helps in the ionic balance.

 

 

Mitochondrial DNA (mtDNA or mDNA)

It is the additional DNA located in mitochondria. Although DNA is found inside the nucleolus, mtDNA provides the necessary energy for the mitochondrial functions alone. Human mitochondrial DNA was the first genome to encode.

 

 

Mitochondrial ribosomes

The mitochondrial ribosomes are also known as mitoribosomes. A mitoribosome is a protein complex active in mitochondria and functions as a riboprotein for translating mitochondrial mRNAs encoded in mtDNA. Mitoribosomes, like cytoplasmic ribosomes, consist of two subunits — large (mtLSU) and small (mt-SSU).

 

 

Function of Mitochondria

  1. The mitochondrion is the major organelle responsible for the regulation of the metabolic activities of the cell.

  2. They aid in the growth and repair of cells. Systematic and programmed death of the cells that is mediated by mitochondria is called apoptosis. It is an essential phenomenon of a cell as it replaces the old cells by younger ones.

  3. Mitochondrion helps in detoxifying the ammonia present in the liver cells.

  4. Mitochondrion helps in the formation of hormones like testosterone and estrogen which are essential for reproduction.

  5. Some studies say that, to some extent, mitochondria help in the formation of blood cells 

  6. They help in maintaining the calcium balance by retaining the major proportion within the cells.

  7. Mitochondrion participates in cellular differentiation, cell signalling,  controlling the cell cycle and cell growth.

  8. Thermoregulation or heat balance mechanism is also aided by the mitochondrion.

 

 

Process of energy formation( ATP)

The process of formation of ATPs with the help of proteins and enzymes is called the citric acid cycle. The mitochondrial matrix contains a lot of fluid and enzymes necessary for the aerobic respiration. Proteins embedded in the inner membrane help to synthesize energy by combining themselves with many enzymes. During the process, glucose (C6H12O6) is utilized to release the energy along with water (H2O) and carbon dioxide (CO2) molecules. The process also needs o2-oxygen.

 

 

Further reading

1.  

Structure and functions of plant cell

2.

Differences between DNA and RNA                

3.

Human sperms

4 .       

Cell cycle and cell division

 

 

Questions

  1. Why do we call mitochondria as a powerhouse of a cell?

  2. Explain the structure of mitochondria

  3. List the functions of mitochondria.

 

 



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