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In this article we will discuss about:- 1. Ultrastructure of Mitochondria 2. Function of Mitochondria 3. Reproduction.
Ultrastructure of Mitochondria:
In 1953, Palade and Sjostrand independently described the ultrastructure of mitochondria. Mitochondria are bounded by an envelope consisting of two concentric membranes, the outer and inner membranes. The space between the two membranes is called inter-membrane space.
A number of invaginations occur in the inner membrane; they are called cristae (Fig. 2.21). The space on the interior of the inner membrane is called matrix.
Outer Membrane:
The outer mitochondrial membrane has high permeability to molecules such as sugars, salts, coenzymes and nucleotides etc. It has many similarities with the ER but differs from it in some respects, e.g., mono-amine-oxidase is present in the mitochondrial outer membrane but not in ER.
On the other hand, the enzyme glucose-6-phosphatase is absent from the mitochondrial outer membrane but is present in ER. The mitochondrial outer membrane contains a number of enzymes and proteins (Table 2.6).
Inter-Membrane Space:
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The inter-membrane space is divided into two regions:
(1) Peripheral space and
(2) Intracristal space (Fig. 2.21).
Large flattened cristae are connected to the inner membrane by small tubes called peduculi cristae which are few nanometers in diameter. The inter-membrane space has several enzymes of which “adenylate kinase” is the chief one (Table 2.6). This enzyme transfers one phosphate group from ATP to AMP to produce two molecules of ADP.
Inner Membrane:
The inner mitochondrial membrane invaginates inside the matrix; the invaginations are called cristae (Fig. 2.21). This membrane has a high ratio of protein to lipid. “Knobs” or “spheres” of 8-9 nm diameter are spaced 10 nm apart on the cristae membranes. These knobs contain F1 proteins and ATPase responsible for phosphorylation. They are joined to the cristae by 3 nm long stalks called “F0“. The F0-F1 ATPase complex” is called ATP synthase. The inner membrane contains large number of proteins which are involved in electron transfer (respiratory chain) and oxidative phosphorylation (Table 2.6). The respiratory chain is located within the inner membrane, and consists of pyridine nucleotides, within the inner membrane, and consists of pyridine nucleotides, flavoproteins, cytochromes, iron-sulphur proteins and quinones.
Besides its role in electron transfer, and phosphorylation, the inner membrane is also the site for certain other enzymatic pathways, such as, steroid (hormone) metabolism.
Matrix:
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The interior of mitochondrion is called matrix (Fig. 2.21). It has granular appearance in electron micrographs. Some large granules ranging from 30 nm to several hundred nanometers in diameter are also present in the matrix. The matrix contains enzymes and factors for Krebs cycle, pyruvate dehydrogenase and the enzymes involved in β-oxidation of fatty acids. (Table 2.6).
However, succinate dehydrogenase is present in the inner membrane instead of matrix; this enzyme catalyses the direct transfer of electrons from succinate to the electron transfer chain.
The enzyme pyruvate dehydrogenase converts pyruvate to acetyl-Coenzyme A (acetyl-CoA) which enters the Krebs cycle. Besides above, matrix also contains DNA, RNA, ribosomes and proteins involved in protein and nucleic acid syntheses.
Function of Mitochondria:
Mitochondria is regarded as the power house of the cell as it is the site of respiration. The general formula for glucose oxidation is,
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C6H12O6 + 6O2 ———-> 6CO2 + 6H2O + 686 kcal …(2.3)
Glucose is degraded into two pyruvate molecules through glycolysis which occurs in the cell sap (cytosol). Further steps in oxidation of pyruvate take place in the mitochondria. Pyruvate is converted to acetyl-Coenzyme A (acetyl-CoA) which is then metabolised through the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle.
In this cycle, energy is liberated and CO2 is produced. Some of the released energy is used to produce ATP, while a major part is conserved in the form of reduced coenzymes NADH and FADH2 (FAD = flavinadenine dinucleotide). The energy conserved in NADH and FADH2 is released by re-oxidizing them into NAD+ and FAD, respectively; the energy so obtained is utilized to produce ATP (oxidative phosphorylation).
This process occurs in different steps in a strict sequence called electron transfer chain or respiratory chain located in the cristae. The electrons are finally transferred to oxygen, and H2O is produced at the end of this chain. The carriers of electrons are organized into three complexes, viz., I, III, and IV, and the sequence of electron transfer is as follows.
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COMPLEX I (NADH ——> FMN group of NADH dehydrogenase ——> iron-sulphurcentre ——> ubiquinone) ——> COMPLEX III (ubiquinone ——> cytochrome b ——> cytochrome c1 ——> cytochrome C) ——> COMPLEX IV (cytochrome C——> cytochrome a ——> cytochrome a3) ——> Oxygen.
There is another complex (Complex II) which transfers electrons from succinate (produced by Krebs cycle) to ubiquinone. At last O2 is reduced to water, as the following reaction.
O2 + 4e– + 4H+ —> 2H2O…(2.4)
In complete oxidation of one glucose molecule, 6 molecules of oxygen are utilized resulting in the production of 6 carbon dioxide and 6 water molecules; in addition, energy is released (see formula 2-3). The maximum number of ATP molecules produced during complete oxidation of one glucose molecule is 36 (Table 2.7).
Reproduction in Mitochondria:
Mitochondria originate by growth and division of pre-existing mitochondria. Their development requires the presence of oxygen. In the absence of O2, yeast mitochondria are replaced by “pro-mitochondria” which are double-membrane vesicles without cristae. In the presence of O2, cristae and other components of mitochondria develop so that pro-mitochondria convert into mitochondria.
