Mitochondria : Definition, discovery, origin, structure and function

The word Mitochondria is formed from the Greek word mitos meaning thread and chondrion meaning grain. Mitochondria means filamentous grains. The energy producing organelles located in the cytoplasm are called mitochondria. It is called Power House as it provides the necessary energy for cells.

 

Discovery and naming of mitochondria

In 1850, the Swiss scientist A.V. Kolliker noticed tiny organelles in insect muscle cells and named them sarcosomes. He is called the discoverer of mitochondria. In 1882 W. Fleming observed filamentous mitochondria and named them Fila. In 1890, Altman named it Bioplast. Mitochondria was named by Carl Benda in 1898. In 1904, Frederick Mavis was the first to observe mitochondria in the plant cell Nymphaea alba. In 1957 scientist Philip Siekevitz coined the term Power House of the cell. Modern scientists call mitochondria semi-autonomous cells. They also called it electrosome. Mitochondria are also known as chondriosomes and plasmosomes. Mitochondria of birds’ flight muscles are called sarcosomes.

 

All true cells contain mitochondria. Stem cells, progenitor cells, and red blood cells do not contain mitochondria.

In shape it is round, rod, oval, star or coil shape.

Mitochondria make up 20% of cell volume. Mitochondria are typically 3-40 microns in length and 0.2-2.0 microns in diameter. Rod mitochondria are 9 microns in length and 0.5 microns in width. Circular mitochondria are 40-70 microns in length. Circular mitochondria are 0.2-2.0 microns in diameter.

 

Number of mitochondria

Each cell has an average of 300-400 mitochondria. Advanced plant cells contain 100-3000 mitochondria. Liver and kidney cells contain 1000 or more mitochondria. Human sperm has 20 mitochondria. A sea urchin egg has 140,000 mitochondria, an amphibian egg has 300,000, and an Amoeba has 500,000. Mammalian red blood cells do not contain mitochondria.

Origin of Mitochondria

There are three theories about the origin of mitochondria.

1. De novo origin: According to this theory, amino acids and lipids combine to form mitochondria. This doctrine is not accepted at present.
2. Cell membrane or endoplasmic origin: According to scientist Morison (1966), vesicles are formed from endoplasmic reticulum or cell membrane and mitochondria are formed from vesicles. This doctrine is now defunct.
3. Mitochondrial origin: According to this theory, during cell division, old mitochondria divide to form new mitochondria. The new mitochondria quickly change shape and move to other parts of the cell. It grows rapidly and divides to increase the number of cells to meet more energy.

Structure of Mitochondria

1. Membrane: Each mitochondrion is covered by a bilayered membrane. Outer cover and inner cover. The outer layer is called the outer layer and the inner layer is called the inner layer. The outer coat is smooth and the inner coat is rough. Each coat is 40–60 Å thick. The outer end of the mitochondrial membrane is called the H-face and the inner end is called the C-face. The outer membrane contains porin proteins and acts as a transporter. The space between the two membranes is called the perimitochondrial space. The distance of perimitochondrial space is 6-8 nm. It is composed of lipoproteins. It gives the mitochondria specific shape and protects them from external injury.
2. Chambers: Mitochondria have two types of chambers. Outer chamber and inner chamber. The compartment between the outer and inner envelope is called the outer compartment and the compartment between the inner envelope is called the endocompartment (70 Å wide). It is filled with various chemicals.
3. Matrix: The jelly-like substance inside the cell is called matrix. It is dense, homogenous and full of enzymes and co-enzymes.
4. CRISTIE: The inner membrane of the mitochondria folds inwards to form a number of finger-like extensions. It’s called Christy. The space between each cristae is called the intercristi space.
5. Oxysome or F1 Cells: Mitochondria have numerous fine granules like tennis bats on the inner envelope. These are called oxisomes or F1 cells. It consists of head, stem and base. There are two types of oxisomes. Sabrintak and Abrintak. The oxisomes which have a stalk are called stalked oxisomes and the oxisomes which do not have a stalk are called unstalked oxisomes. It has ATP-ase enzyme in its head.
6. ETS: Electron Transport System is abbreviated as ETS. It is called ETC (Electron Transport Chain). Christy has ETS on him. It transfers electrons from one place to another.
7. ATP-Synthases: All substances that synthesize ATP are called ATP-Synthases. Christy has spherical or oval ATP-synthases. It produces ATP.
8. mDNA: Each mitochondrion contains circular double-stranded DNA. These DNAs have their own characteristics. It is called mitochondrial DNA. It is self-reproducing. Human mDNA contains 37 genes.
9. Ribosomes: Mitochondria contain 70S ribosomes. It is called mitoribosomes. It synthesizes enzymes for cells.

 

Chemical structure of mitochondria

1. Protein: About 65% of the dry weight of mitochondria is protein. It contains both soluble and insoluble proteins.
2. Lipids: Mitochondria contain about 35% lipids. Of this, 90% are phospholipids and 10% are fatty acids and carotenoids. It contains glycerides 29%, lecithin and cephalin 4% and cholesterol 2%. Its inner membrane contains phospholipid called cardiolipin.
3. Pigments: Contains small amounts of carotenoids.
4. Nucleic acid: It contains DNA and RNA as nucleic acids. Each mitochondrion contains about 200 pieces of DNA. It contains 5% RNA.
5. Enzymes: It contains about 100 types of enzymes and co-enzymes.

 

Function of Mitochondria

1. Respiration Process: Krebs cycle and electron transport system of respiration process are completed in mitochondria. It is called mitochondrial respiration as it takes place in the mitochondria.
2. Energy production: ATP is produced in mitochondria. ATP is the source of energy.
3. Protein production: Mitochondria make proteins for the cell.
4. Nucleic acid production: It makes DNA and RNA. It plays a role in heredity.
5. Blood cell and hormone production: It helps in blood cell and hormone production.
6. Contains enzymes: It contains enzymes and co-enzymes required for respiration.
7. Ion balance: They maintain the correct concentration of calcium ions in different parts of the cell.
8. Metabolism: It metabolizes neurotransmitters and cholesterol or fat.
9. Control of apoptosis: Mitochondria control cell apoptosis.
10. Heredity: Mitochondria help in the formation of sperm and egg.
11. Energy regulation: It regulates the storage and release of energy in cells. ADP is converted into ATP and stored in the body by creating high energy bonds.
12. Cation storage: Mitochondria store Ca2+, S2+, Fe2+, Mn2+ etc.
13. Heat production: Mitochondria help in the production of excess heat in hibernating animals.
14. Production of chemical compounds: Mitochondria are capable of creating, breaking down and producing substances necessary for the cell. It produces compounds like cytochrome, ferridoxine, hemoglobin, chlorophyll, steroids, alkaloids etc.
15. Biosynthesis: It causes biosynthesis of iron and steroids in cells.
16. Transport: It is capable of active transport of calcium, potassium etc.
17. Disinfection: It participates in ammonia disinfection.
18. Pathogenesis: Mutations can occur in mitochondrial DNA that cause mitochondrial disorders. 100 such disorders are known. In old age, diseases like Parkinson’s, Alzheimer’s, type-1 diabetes etc. can occur. Proper health is dependent on the structure and function of mitochondria.