Mitochondria of Eukaryotic Cell: History, Structure and Function

Synonyms:

Chondriosomes, Sarcosome, Plastosome, Fachsinophilic granules, Bioblast, Power houses, ATP mills, Storage batteries cellular furnace, biochemical machine, cell within a cell, or enuosymbiont in cell.

Definition:

Mitochondria are the filamentous, self-duplicating, double membranous cytoplasmic organelles of eukaryotic cells which are concerned with cellular respiration.

Occurrence:

They are the energy transducing organelle found in all aerobic eukaryotic cells. But in mature mammalian RBC mitochondria are lost secondarily. They are also absent in prokaryotic cells where mesosomes act as a substitute of mitochondria.

History:

Kolliker (1850):

First discovered mitochondria as granular structures in insect striated flight muscles and called as sarcosomes.

Altmann (1894):

He called them asbioblast.

Benda (1897):

First coined the term mitochondria.

Meves (1904):

First noticed the presence of mitochondria in plant cells of Nymphaea.

Kingsbury (1912):

First suggested that mitochondria are the sites for cellular respiration.

Kennedy and Lehninger (1948- 50):

He showed that TCA cycle, oxidative phosphorylation and fatty acid oxidation took place in mitochondria.

Number:

The number of mitochondria varies from cell to cell; plant cells contain fewer than animal cells. The number of mitochondria in a cell is generally proportional to its energy requirement. The Trypanosoma, Chlorella and Microsterias contain 1 mitochondrion per cell, but the number is 25 in human sperm cell, 300-400 – in a kidney cell, 500-1000 – in a hepatic cell, 50,000 – giant amoeba (Chmn chaos), 30000 -300000 – in oocytes of sea urchins and 5,00,000 -in flight muscle cell.

Shape and Size:

Mitochondria vary in shape and size. Typical mitochondria are generally rod shaped, having length 1-4 /µm and breadth 0.2-1.5/µm. In some cases, these may be spherical or oval or filamentous (up to 12µ.m long). All mitochondria of a cell are collective called as condriome and constitutes about 25% of the cell volume. Mitochondria appear yellowish due to riboflavin and rich in Mn. The life span of mitochondria is only 5-10 days. They are continuously produced from the pre-existing mitochondria within the cell and destroyed within the cells.

Ultrastructure:

Each mitochondrion is bounded by a mitochondrial envelope and encloses two chambers or compartments within it.

(a) Mitochondrial envelope:

It consists to two membranes called outer membrane and inner membrane (each 60-75 A thick). Both the membranes come in contact with each other at several places called adhesion sites or contact zones. The outer membrane is smooth but porous due to the presence of integral proteins called porins. It contains 40% lipids and 60% proteins. The mitochondrion excluding the outer membrane is called mitoplast.

The inner membrane is semipermeable. It is highly convoluted to form a series of in-folding called cristae or mitochondrial crests. Each crista encloses intracristal spaces which is continuous with the outer chamber. The cristae greatly increase the surface areas of inner membrane. The inner membrane consists of 75% proteins and 25% lipids. It is rich in enzymes of respirators chain and a variety of transport protein is.

(b) Mitochondrial chambers:

In between two membranes a narrow space (about 6-10 nm wide, present called outer chamber or inter-membrane space. The central wider space enclosed by the inner membrane is called inner chamber or mitochondrial matrix. The outer chamber is filled with a watery fluid and contains enzymes like adenylate kinase and nucleoside diphosphokinase.

The matrix is filled with a homogenous, granular, dense, jelly like material. It contains-circular DNAs (2-6 copies). Mitoribosomes, granules of inorganic salts, enzymes for the citric acid cycle (TCA cycle) and for the oxidation of pyruvate and fatty acids.

(c) Oxysomes:

The matrix side of the inner membrane and cristae bear numerous tennis racket like particles present called oxysomes. They are also known as elementary particles, Parson’s particle, Fernandez-Moran particle, F₀F₁-particles, F₀F₁-ATPase, H₊ – ATPase, ATP synthetase or ATP synthase. A mitochondrion contains about 104 -105 oxysomes regularly placed at the intervals of l0 nm. Oxysomes comprise about 15% of the total inner membrane protein.

Each oxysome is a multi-polypeptide complex consists of 3 parts:

(i) Head piece or F₁ particle or soluble ATPase.

(ii) Base or F₀ subunit.

(iii) A stalk that connects F₁ subunit with the F₀ subunit.

Functions of Mitochondria:

Mitochondria are associated with the following functions:

(i) They are the main seat of cellular respiration, a process involving the release of energy from organic molecules (such as glucose) and its transfer to molecules of ATP (Adenosine triphosphate), the chief immediate source of chemical energy for all eukaryotic cells. On this account, the mitochondria are often described as the “power houses”, or “storage batteries” or “ATP mills” or “cellular furnace” of the cell. Mitochondria tend to assemble where energy is required.

(ii) They provide intermediates for the synthesis of important biomolecules, such as chlorophyll, cytochromes, steroids etc.

(iii) Some amino acids are also formed in mitochondria.

(iv) Mitochondria regulate the calcium ion concentration in the cell by storing and releasing Ca^{2 +}as and when required. The calcium ions in turn regulate many biochemical activities in the cell.

(v) They help in β oxidation of Fatty acids.