1.6.8      Mitochondria

·         Mitochondria (Gr. Mitos, thread; chondrion, granule) are organelles found in the cytoplasm of plants and animals.

·         Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope. They were first seen by Kollicker in 1850 in muscles and he called them ‘sarcosomes’.

·         Mesosome of prokaryotes (bacteria) is analogous to mitochondrion in eukaryotes.

·         Flemming (1882) described these organelles as ‘filia’.

·         Altmann (1890) observed these structures and name them ‘bioblasts’.

·         Benda (1898) stained these organelles with crystal violet and renamed them ‘mitochondria’.

·         Michaelis (1900) used Janus Green B as a vital stain to observe mitochondria in living cells.

·         Bensley and Hoerr (1934) isolated mitochondria for biochemical studies by differential centrifugation.

·         Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.

·         The mitochondria divide by fission.

·         Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.

·         Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell’s energy by the process of oxidative phosphorylation, utilizing oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP.

·         Mitochondria multiply by splitting in two. Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through the process of photosynthesis, which utilizes solar energy to generate carbohydrates and oxygen from carbon dioxide and water.

·         Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell.

·         Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, strongly supporting the evolutionary theory of endo symbiosis; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.

1.6.8.1 Morphology

·         The shape of mitochondria is highly variable, ranges from short rod-shape to elongated filamentous form. Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0μm (average 0.5μm) and length 1.0-4.1μm.

·         Mitochondria show variable shape, so it shows pleomorphism.

·         The size of mitochondria is variable, they generally measure about 0.6 to 2 µm in diameter.

·         Mitochondria have an average length of 5 to 10 µm .

·         Mitochondria are not found in prokaryotes and mature human red blood cells.

·         The number of mitochondria per cell is variable depending on the physiological activity of the cells.

·         The number correlates with the metabolic activity of the cell.

·         Mitochondria will be found in abundance where there is a maximum activity in the body.

·         A small number of mitochondria generally indicates cells of low metabolic activity.

·         A single mitochondrion is found in Microsterias, a unicellular green algae.

·         Plant cells have fewer mitochondria than animal cells.

1.6.8.2 Ultrastructure

A mitochondrion is enclosed by a double membrane envelope composed of lipid and protein

·         The two membranes are separated by a narrow fluid-filled space called the outer compartment.

·         The out membrane is smooth; it is more permeable to small molecules, contains some enzymes but is poorer in proteins.

·         The inner membrane surrounds a central cavity of matrix (inner compartment) filed with a fluid.

·         The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins.

·         The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area.

·         Crista causes an increase in the total surface area.

·         Crista bears subunits called oxysomes (elementary particles, F₁ particles, electron transport particles, inner membrane subunits or Fernandez Moran particles).

·         The two membranes have their own specific enzymes associated with the mitochondrial function.

·         Disruption of mitochondria yields membrane fragments which are able to synthesis ATP.

·         Mitochondria contain electron transport systems aggregated into compact association. F₁ particle is connected by a cylindrical stalk to a base piece or F₀ subunit.

·         The F₀-F₁ combination functions as ATP synthetase, an enzyme that catalyses ATP synthesis.

·         In F₁ particles of mitochondria, ATP is generated.

1.6.8.3 Chemical Composition

·         Chemically, mitochondria consist of protein 70% (dry weight) and lipids 25-30%.

·         Of the lipid component, 90% is phospholipid and 10% carotenoids, cholesterol, vitamin E and other traces.

·         Mitochondria contain 0.5% of RNA and traces of DNA.

·         Mitochondrial DNA comprises about 1% of total cell DNA.

·         Mitochondria contain enzymes for oxidation, phosphorylation and electron transfer.

·         Out of total enzymes present in the cell, mitochondria alone has 70%.

·         Mitochondria are rich in respiratory catabolic enzymes.

1.6.8.4 Functions of Mitochondria

·         Mitochondrion is known as the ‘power house of cell’; the energy is released in it through cellular respiration. Within the cell, the site of respiration is mitochondria.

·         The organelles in which aerobic respiration occurs in a cell are mitochondria.

·         Mitochondria are present in the aerobic organism only.

·         If a cell is living in an environment without oxygen, it need not have mitochondria.

·         Mitochondria are considered as storehouse of energy or centres of energy liberation.

·         In a cell, the energy in the form of ATP is formed in mitochondria.

·         A common and easily available source of energy in living cells is ATP.

·         After respiration the conversion of energy is mainly in the form of ATP.

·         The cell organelle which has electron transport system is mitochondrion.

·         Oxidative phosphorylation is the formation of ATP in respiration.

·         The site of respiration or oxidative phosphorylation is mitochondria.

·         The end product of glycolysis is pyruvic acid when enters mitochondria and takes part in Krebs’ cycle in the mitochondrial matrix.

·         Enzymes connected with Krebs’ cycle are packed in mitochondria.