Ultrastructure Of Mitochondrion Mitochondria were first discovered by Kollikar, Altmann and Flemming (1880). The name mitochondria was given by Benda (1897). Hageboom (1948) demonostrated them to be power houses of Cell. Altmann called them as Bioplast. Mitochondria are the cell organelles which occur in cytoplasm of all eukaryotic organisms. Mitochondria are concerned with aerobic type of respiration. They are absent in prokaryotes like bacteria, cyanobacteria which respire anaerobically. Ultrastructure of Mitochondria : 1. Mitochondria are microscopic, colourless, polymorphic cell organelles (varies in size and shape). 2. They may be spherical, filamentous, rod-shaped, oval, club shaped etc. 3. They measures about 0.5 to 1 p in diameter and 1 to 5i in length / long. 4. The number of mitochondria varies in different cell types but constant for a particular cell. Metabolically active cell contains more number of mitochondria. Dormant and inactive cells have very few mitochondria. They are largest in number in cardiac muscle cells followed by liver cells and nerve cells in animals and in meristematic cells in - plants. Structure: Mitochondrial membranes or envelope : 1. Each mitochondria is bounded by two unit membranes called as outer and inner membrane. 2. Each membrane is 40-60 A' in thickness. 3. They are made up of lipoprotein. 4. Outer and inner membranes are separated from each other by intermembrane space or outer chamber or outer space or perimitochondrial -space. 5. Outer chamber is 40 - 70 A in width and filled with watery, hyaline, homogeneous fluid. 6. The outer membrane is smooth and continuous, permeable to small molecules or solutes. 7. The inner membrane is rough, selectively permeable and infolded into number of finger like projections called Cristae or crests. They increase surface area of inner membrane for enzymatic activity.

8. The space bounded by inner membrane is called as inner chamber or innermembrane space. 9. Each crista encloses a narrow space called intracristal space, it is continuous with outer chamber. 10. The side of inner membrane facing the matrix is called M-side (inner matrix) while the side facing outer chamber is C-side. (cytosol) 11. Outer membrane possess porins or minute pores or protein lined channels for passage of low molecular weight substances. They make outer membrane more permeable. Matrix : 1. inner chamber is filled with colourless, granular matrix of proteins and lipids. 2. The matrix contains 70s type of ribosomes called mitoribosomes, 2-6 circular, naked double stranded DNA molecules, called mitochondrial DNA and RNAs. 3. It contains enzymes necessary for Krebs cycle, oil globules and reserve food particles. 4. The matrix also consist of insoluble inorganic salts like Mg++ and Ca++.

Fig. : (A) A mitochondrion in Section

Fig: Ultrastructure of a mitochondrion Elementary particles or oxysomes : 1. On the M-f ace of inner membrane, number of microscopic, fine stalked particles are present called elementary particles/ f1 particles / Oxysomes / Subunits of Fernandez Moran/ electron transport particles (ETP)/ respiratory assemblies / inner membrane spheres! f0-f1 particles etc. 2. Each f1 particle consist of three parts base, stalk and head and placed at the interval of 100A° or 10 nm. i) Base piece (F0 subunit) : It is embedded in inner mitochondrial membrane. It is rectangular and 115 x 45 A' in size. It functions as proton channel or tunnel. ii) Stalk (F5 - F6 subunit) : It is about 50 A° in length, spherical head is connected to base piece by cylindrical short stalk. iii) Head piece (F1 - subunit) : It is spherical and is about 75-100 A' in diameter. It contains the -enzymes like ATP synthetase or ATPase which controls ATP synthesis, hence, they are also called as ATP particles. 3. They project into matrix and contains electron acceptors, enzymes, co-enyzmes required for ATP synthesis during ETS. Functions of mitochondria : 1. Mitochondria provides site for aerobic respiration. 2. Kreb's cycle takes place in matrix and ETS takes place on F1 particles. 3. Generation of ATP molecules takes place on F1 particles or oxysomes through oxidative phosphorylation during respiration.

4. Besides ATP heat is also generated that maintains body temperature i.e. Thermiogenesis. 5. During Kreb's cycle and ETS, organic substrates are completely oxidized with the release of energy. They are also called as power house of the cell. 6. It is a unit of extranuclear inheritance i.e. mitochondrial DNA control characters like male sterility, disease and herbicide resistance etc. which are transferred from female parent called as maternal inheritance. 7. Mitochondria can store and release calcium and regulate its concentration in cell. 8. It brings about synthesis of substances like haeme of haemoglobin and myoglobin. 9. Intermediate products of respiration are used for synthesis of chlorophyll pigments, steroids etc. Mitochondria acts as power houses of cell : 1. Mitochondria is the site of respiration. 2. It produces the energy rich ATP molecules during the oxidation of glucose by using oxidative energy. 3. The energy rich ATP molecules are readily supplied for various activities of the cells. 4. As, mitochondria produces, stores and supplies - biological energy, hence called as power houses of cells (by Seekevitz). Semiautonomous nature of mitochondria : In mitochondria matrix, 2-6 circular coils of DNA are present. Because of which mitochondria undergo self replication. It can form some of the required proteins, hence called semiautonomous. The remaining proteins and enzymes are obtained from cytoplasm where they are produced under the control of nuclear DNA. (Mitochondria can be seen only when stained with a special stain 'Janus green B (Michaelis 1898). Mitochondria becomes greenish blue while surrounding cytoplasm becomes colourless.)