Mitochondria : Definition, Structure and Functions

Definition

The word mitochondria come from two Greek words, mito meaning thread and chondrion meaning granule. It has double membrane enclosed rounded or rod-like or filamentous bodies which generate chemical energy in the form of ATP. It is scattered throughout the cytoplasm in most of the cells.

History of Mitochondria

Scientist Kolliker (1850) first observed mitochondria in the striated muscle. Flemming (1882) named it as fila. Rechard Altmann (1897) referred to the mitochondria as Bioplasts. Carl Benda (1897) first called these organelles as mitochondria.

Distribution and occurrence 

It is not found in the prokaryotic cell and matured circulated RBC. Generally they are evenly distributed in the cytoplasm. It is also found in the base of the proximal convoluted tubules of nephron. The number of mitochondria present in the cell depends on its activities. Plant cell contains less number of mitochondria than animal cell. A normal liver cell may contain 1000-1600 mitochondria while some oocytes may contain more than 300000 mitochondria. On the other hand, Microasterias algae contain only one mitochondrion

Structure of Mitochondria

Mitochondria have generally spherical or rod-shaped or filamentous structures. It is about 3.0-7.0 µm in length and 0.2-2.0 µm in diameter. It usually consists of two unit membranes, two chambers, mitochondrial matrix and mitochondrial particles. Every mitochondria is bounded by the double membranes, the outer membrane and inner membrane. Each membrane is made up of lipo-protein and it is about 60 Angstrom thick. The outer membrane is smoothed and covers the mitochondria but the inner membrane remains folded inward at various points to form a number of incomplete partitions which are known as the cristae or mitochondrialis cristae. The two membranes remain 60-80 Angstroms apart from each other. The space between the two membranes is filled up with fluid.

Diagrammatically structure of mitochondria

Diagrammatically Structure of Mitochondrion

Chambers: It has two chambers, outer and inner chambers. The outer chamber is the space between the outer and inner membranes which is filled with watery fluid. The inner chamber is covered by the inner membrane. The inner chamber contains mitochondrial matrix.

Mitochondrial matrix: The inner chamber is filled with a relatively dense proteinaceous material usually called mitochondrial matrix. This matrix contains dense granules, ribosomes and mitochondrial DNA. The enzyme of the Krebs cycle is located in the matrix.

Mitochondrial particles: The cristae that project into the matrix are usually incomplete septa or ridge. The cristae are covered by mushroom-like particles of 85 Angstrom. These particles are known as F1-Particles or Fernandez-Moran subunits or elementary particles. Each particle consists of three parts; head, stem or stalk and base. The particles are spaced about 100 Angstrom interval on the cristae.

Detail structure of cristae

Detail Structure of Cristae

Functions of Mitochondria

  • It acts as a power house- Different enzymes present in the mitochondria help in oxidative phosphorylation and is thus site for formation of adenosine triphosphate (ATP). ATP is high energy-rich substrates which supply 95% energy to the cell.
  • Cell Respiration- Mitochondria are the respiratory organelles of the cell. Cell respiration includes glycolysis, oxidation of pyruvic acid, Krebs cycle and oxidative phosphorylation.
  • Oxidation of fat- Fatty acids of fats are oxidized at their β-position in the mitochondria.
  • Syntheses of lipid- A set of enzymes that control synthesis of phospholipids are present in the mitochondria. These enzymes help in the formation of the phospholipids.
  • Syntheses of protein- The mitochondrial matrix contain DNA which helps to synthesize RNA. This RNA and ribosome help for the formation of protein.
  • Mitochondria help to maintain proper concentration of calcium ions in the cell.
  • It helps to build certain parts of blood and hormones such as testosterone and estrogen.
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