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STRUCTURE AND FUNCTION OF MITOCHONDRIA

Published in Cell Biology
Tuesday, 06 June 2017 19:18
DEFINITION OF MITOCHONDRIA:
 
Mitochondria are center for cellular respiration. It converts chemical energy into kinetic energy.
 
Information About Mitochondria
 
  1. In 1857 Kolliker observed mitochondria and called them as sarco-somes.
  2. Flemming called them as Fila.
  3. Altmann in 1890 called them as Bioplasts.
  4. Benda gave the name mito-chondria.
  5. Porter & Palade described their electron microscopic structure.
  6. Mitochondria are present in all eukaryotic cells.
 
Size: The length varies from 1.5 to 10 microns. The smallest mitochondrion is seen in yeast. It is 1 mill micron in length. The width is .5 to .7 microns (Oocyte of amphibian show 20 to 40 microns length mitochondria.)
 
Shape: They are filamentous or they may show rod, spherical, or thread like structures.
 
Number: In Micro monas only one mitochondrion is present. In a liver cell 1000 to 1600 mitochondria are present. The highest number of mitochondria are seen in the cell of flight muscles. However cells of green plants contain less number of mitochondria. In red blood corpuscles of mammals and other higher animals mitochondria are absent.
 
Structure Of Mitochondria: The mitochondria is covered by 2 layers. It shows outer membrane and inner membrane. In each mitochondrion 2 chambers are present.
 
  1. Outer chamber
  2. Inner chamber.
 
Outer membrane: It is a continuous membrane which covers and protects the mitochondrion. !t separates the mitochondrion from the cytosol. It is permeable.
 
Inner membrane: It is projected in wards as a cristae. The membrane shows two faces. The outer face "C" face (Cytosol face) and inner face is called "M" face (Matrix face). On this M-face a number of knob like structures are present.
 
Outer chamber: It is the space between outer and inner membranes. It is 60 to 80A°. It is filled with a fluid.
 
Inner chamber: It is called mitochondrial matrix. It is filled with jel like substance. It contains many enzyme systems. In 1963 "Nass" observed the presence of DNA molecule in the mitochondria.
 
Variations of cristae: Normally the cristae are perpendicular but various arrangements can be noticed.
 
Parallel cristae: In the nerve cells, and cells of striated muscles the mitochondria will show parallel cristae.
 
Concentric cristae: In the spermatogonia of man the mitochondria will show concentric cristae.
 
Haphazard arrangement: In choas-choas the mitochondria will show irregular arrangement of cristae
 
Spherical cristae: In the spermatocytes the cristae are spherical.
 
Mitochondria without cristae: Very rarely the inner membrane is smooth. It will not show cristae.
 
Reduced cristae: In the cells of opossum testis the aristae are reduced in the mitochondria.
 
Chemical composition: Mitochondria contain 73% of proteins, 25 to 30% of lipids, 5% of RNA and small amount of DNA. The enzyme complexes are more. The lipids contain 90% phospholipids, cholesterol, carotenoids etc.
 
Enzymes: In 1969 Lehninger gave the account of enzymes in mitochondria.
 
1) Enzymes of the outer membrane of mitochondria
a) Mono mine oxidase enzyme.
b) Fatty acid activating enzymes.
 
2) Enzymes of the outer chamber of mitochondria
1) Adenylate kinase.
2) Neuckocyte diphosphokinase.
 
3) Enzymes of the inner membrane
In the inner membrane electron transport enzymes are present. They are cytochromes, flavor proteins, dehydroginases etc.
a) ATP synthetase oxidase.
b) Carnitine fatty acid acyltransferase etc., enzymes are present. Enzymes of matrix : These enzymes systems bebng to krebs cycle and fatty acid cycle.
a) Fumarase.
b) Aconitase.
c) Citrate synthatase etc
 
Elementary particles: On the inner membrane of the mitochondrion stalked particles are present. They are called F, or elementary particles. They are equidistantly placed. Each particle shows a stalk and head piece. The stalk is 50A° in length. The head is 100A° in diameter. The distance between 2 particles is 100A°. The head piece contains enzymes of ATP-ase system and bring oxidative phosphorelation. Hence they are called oxysomes.
 
Respiratory chains: Respiratory chain contains a series of complex proteins and electron carriers. These electron carriers are represented by 4 complexes.
 
Complex I: It contains flavo-protein of NADH dehydroginase. It contains non-haemiron which combine with protein. It receives hydrogen from NADH.
 
Complex II: It contains flavo-protein of succinic dehydrogenase. It receives hydrogen from succinic dehydroginase.
 
Complex III: It contains Cytochrome b1, Cytochrome cr
 
Complex IV: It contains Cytochome at Cytochrome a3.
 
These complexes are connected by ubino-quinone. Cytochrome C will be present. Ubinoquinone connects complex 1,2 and 3. Cytochrome C will connect complex 384.
 
'Respiratory chain is proposed by 'Mochenan' and 'Green' and 'Baum' in 1970.
 
Biogenesis of mitochondria:
 
  1. Luck stated free existing mitochon­dria will elongate and divide and new mitochondria are formed.
  2.  Morrison stated mitochondria arise from either plasma membrane or endoplasmic reticulum.
  3. In the cytoplasm small particles may-be present they are called promitochondria they may give rise to mitochondria.
 
Mitochondrial D.N.A.: Mitochondria contain one or two molecules of D.N.A. Mitochondrial D.N.A. is circular. It is highly twisted double strand molecule. "Rabino- with" stated that mitochondrial D.N.A. contains more G and C content thatunclear D.N.A. Moleculer weight of mitochondrial D.N.A is 9 to 11 millions. This D.N.A. has the capacity of multiplication. D.N.A. polymerase enzyme is present. This D.N.A. will produce R.N.A. It is believed that it may take up the production of some proteins.
 
Origin of mitochondria
 
Mitochondria semi auto nous or prokaryotic origin:
 
In 1890 'Akmann' suggested that mitochondria and chloroplasts may be intracellular parasites of the cells which have entered the cytoplasm of eukaryotic cell. And they live as symbotonts. Hence Akmann called them as Bioplats.
 
  1. In Bacteria & Mitochondria electron transport system is present in plasma membrane and in inner membrane respectively.
  2. Bacterial plasma membrane shows mesosomes, mitochondrial crystae can be compared with them.
  3. Both bacteria & Mitochondria will show circular D.N.A.
  4. In both bacteria and mitochondria ribosomes are reported.
  5. Both bacteria and mitochondria will produce AT.P. and R.N.A. Hence we consider the mitochondria might have originated from bacterial cell. In the cell mitochondria will function as semi-autonomous body.
 
Mitochondria Functions:
 
  1. A.T.P. Synthesis: It is the power house of the cell. It brings oxidation of food. Hence Kreb's cycle reactions, electron transport system enzymes are located in mitochondria. By the oxidation of food energy is liberated in the form of A.T.P. (Oxidative phosphorelation takes place.)
  2. Yolk formation: Mitochondria are responsible for the fcri soiydk in the developing ovum Granules are formed in the matrix They oeconie large masses Mitochondrion is converted into yolk storing body.
  3. Mitochondrian sperm formation: When spermatid become? <pe'm mitochondria will form a spiral around the axial filament. This is called Neben-kem. It forms the middle piece of the sperm.
  4. Origin of new system: It is believed that some of the ceil organelles may originate from mitochondria.
  5. Heat production: In the oxidation of food ATP is released. Only 45% of the total energy is trapped in the form of ATP. The remaining 35% of ATP will come out as heat. (In birds and mammals this heat is useful for the maintenance of body temperature.).
  6. ATP released during respiration (because of mitochondria ) will take part in many biosynthetic paths of the cell.

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