Mitochondria





What is Mitochondria?

The rod-like, filamentous, spherical or oval, double membrane bound cytoplasmic organelles of eukaryotic cells, which are concerned with cellular respiration, are called mitochondria. The name mitochondria were introduced by Benda (1897) for these organelles.




Where the Mitochondria is found?

Mitochondria are found in all living cells except the prokaryotic ones. However, the bacterial cells possess mesosomes as a substitute of mitochondria. The mature mammalian RBCs do not contain mitochondria. Generally, these are uniformly distributed throughout the cytoplasm and they move (change their position) singly or in groups. In certain cases, they remain accrued at a particular part of the cell. For example, during cell division, they accumulate around the spindle; in muscle fibers, they are located close to the contractile elements; in adipose tissue cells, due to excessive fat deposition, mitochondria along with other organelles are pushed towards the periphery of the cells; in renal tubular cells, these are present near the basement membrane. Generally, mitochondria gather in greater numbers in that part of a cell where energy production is needed.




Number of Mitochondria

The number of mitochondria varies from cell to cell; plant cells contain fewer mitochondria than animal cells. The number of mitochondria in a cell is generally proportional to its energy requirement. The cells concerned with active processes such as secretion, contraction etc., contain large number of mitochondria. Conversely, those cells which do not require much energy (e.g., squamous or stratified epithelial cells) possess smaller number of mitochondria. The cell of Microsterias, which is a unicellular alga, contains only one mitochondrion whereas the cell of the giant protozoa called Chaos chaos possesses largest number of mitochondria, about 500,000. In a cell of higher plant or animal, the number may range between a few to several thousands. So the number of mitochondria is not equal in all types of cell.

Origin of Mitochondria

Mitochondria originate from the pre-existing mitochondria within the cell. However, it is believed that mitochondria may also originate from plasma membrane or endoplasmic reticulum or nuclear membrane. The life span of mitochondria is only 4 to 10 days; obviously, they are continuously formed and destroyed within the cell cytoplasm.



Shape and Size of Mitochondria

Mitochondria vary in shape and size. Typical mitochondria are generally rod-shaped, having lengh 1-4 µm and breadth 0.2-1.5 µm. In some cases, these may be spherical or oval or filamentous. Mitochondria are closed sac-like structures covered by two membranes, each of which is a trilamellar lipoprotein (P-L-P) unit membrane very similar to the plasma membrane. These two membranes, called outer membrane and inner membrane, form two chambers in mitochondria. m and breadth 0.2-1.5 µm. In some cases, these may be spherical or oval or filamentous. Mitochondria are closed sac-like structures covered by two membranes, each of which is a trilamellar lipoprotein (P-L-P) unit membrane very similar to the plasma membrane. These two membranes, called outer membrane and inner membrane, form two chambers in mitochondria. m and breadth 0.2-1.5 urn. In some cases, these may be spherical or oval or filamentous.
Mitochondria are closed sac-like structures covered by two membranes, each of which is a trilamellar lipoprotein (P-L-P) unit membrane very similar to the plasma membrane. These two membranes, called outer membrane and inner membrane, form two chambers in mitochondria.




Chemical Composition of Mitochondria

Mitochondria are composed of protein (60-70%), lipid (25-30%) and RNA (0.5%). Majority of proteins are enzymes while the remaining ones are structural proteins. Recently it has been discovered that the mitochondrial matrix contains ribosomes of relatively smaller size called mitoribosomes, DNA threads and some granules ; one type of granule contains calcium phosphate.

Functions of Mitochondria

Due to the presence of various metabolic enzymes in them, mitochondria function as one of the most important sites of cellular metabolism. Metabolic processes occurring in mitochondria, those are Catabolic processes and Anabolic processes.




Catabolic processes

Majority of the mitochondrial enzymes are concerned with final oxidation of foodstuff in cellular respiration. Mitochondria are the sites of TCA cycle, electron transport system (or biological oxidation) and oxidative phosphorylation. Mitochondria also carry out the oxidation of fatty acids (P-oxidation) and amino acids. In this context, it should be mentioned that TCA cycle, P-oxidation of fatty acids and oxidation of amino acids take place in the mitochondrial matrix. Electron transport and ATP production by oxidative phosphorylation take place in the cristae; the enzymes for electron transport are present in the inner membrane of cristae and those for ATP synthesis.

Anabolic processes

Mitochondria carry out two types of anabolic processes-(i) synthesis of fatty acids and (ii) production of DNA, RNA and some proteins. The proteins synthesized in mitochondria are mainly enzymes, which catalyze the metabolic reactions occurring in it.

CLICK FOR NEXT ►►

Click For ►