Get complete information on Mitochondria (Power Houses)

Mitochondria were disccovered by Kolliker (1880) in striated muscles of insects and given the present name by Benda (1897). Mitochondria were called sacromeres by Kolliker (1880), flia by Flemming (1882) and bioblasts by Altman (1890). Other names are chondriosomes, chondriomites and plastochondria. Chondriome includes all the mitochondria of a cell. They can be stained by Janus Green. Mitochondria are absent from anaerobic cells and RBCs. A single mitochondrion occurs in Microsterias, Trypanosoma and chlorella.

The number is 5,00,000 per cell in flight muscles. They are 1 -10 nm long and 0.5 -1.0 nm broad with spherical to cylindrical shape. Each mitochondrion is covered by a double membrane envelope. Intermembrane shape of outer chamber is present in between the two membranes. Innner memberane is thrown into various types of folds called cristae (cristae mitochondriale, Palade, 1952). The inner membrane bears oxysomes (10M0⁵) and Electron Transport Chains (flavoprotein, cyt.b, cyt. c, cyt. a, cyt. a,). Oxysomes take part in ATP synthesis (oxidative phosphorylation).

They are, therefore, functional units of mitochondria. Each oxysome (= elementary particle = EP = F_o – F₁ or Fernandes Moran particle, Fernandes-Moran, 1961) has a flat base of F_o subunit, a stalk and a head of F, subunit.

The inner chamber or matrix of a mitochondria contains DNA (Nass and Afzelius, 1965), RNA, ribosomes (70 s), lipids, granules and enzymes of Kreb’s cycle. DNA can be circular and linear. It is naked or without association with histones. Mitochondria are semi-autonomous. They multiply by division (Luck & Rich. 1964).

As mitochondria are centres of Kreb’s cycle or respiration, they liberate the maximum amount of energy. They are, therefore, called ‘power houses’ of the cell (Seeckevitz). In prokaryotes, where mitochondria are absent, the site for oxidative phosphorylation and electron transport including dehydrogenases is plasma membrane.

Ordinarily mitochondria are evenly distributed in the cytoplasm. In columnar or prismatic cells, mitochondria are oriented parallel to the long axis of the cell. Mitochondria may move freely in some cells, carrying ATP wherever required, or may be localized in a particular region. Movement is less in animals than in plants. Often movements are rhythmical.

In one respect the mitochondrion is a part of the eucaryotic cell, in another, it is an independent organism. Mitochondrial genetic code differs from both ‘nuclear’ and bacterial codes in that the triplet UGA codes for tryptophan and is not a stop codon. Other codons may also have different meanings; even between mitochondria from different organisms Mitochondria of most C₃ plants of temperate latitudes do not engage in aerobic respiration during daylight, this being restricted to the hours of darkness. Instead these plants engage in the seemingly wasteful process of photorespiration,which is greatly reduced or absent is C₄ plants of tropical region.

Plastids:

Plastids (Haeckel, 1866) are semiautonomous cell organelles which are surrounded by double membrane envelope, take part in storage and synthesis of organic compounds which occur in plants and some protistans. ‘Plastids develop from colourless precussors called ‘proplastids’. Plastids are of three main types (Schimper, 1883). Leucoplasts, chromoplasts, chloroplasts.

(i) Leucoplasts:

They are colourless plastids of various shapes. These are found in storage organs where light is not available, e.g. underground stem and roots. They are found in parenchymatous cells, embryonic cells, sex cells and meristernatic cells. These get specialised into:

(a) Amyloplasts: Large plastids containing starch.

(b) Elaioplasts (oleosomes). They store fats and are found in seeds.

(c) Proteinoplast or Aleuroplasts. They store protein and are found in seeds.

(ii) Chromoplasts:

These plastids are coloured other than green due to presence of carotenoids but lack chlorophylls, e.g. red chillies, tomatoes. Chromoplasts attract animals and insect to flowers and fruit for performing pollination and fruit dispersal. They are responsible for the bright colours of not only fruits & flowers but also leaves and vegetables.

They are also of various types including:

(a) Phaeoplasts:

These plastids are present is Brown algae, Diatoms and Dino-flagellates. Brown coloured pigment Fucoxanthium is the main pigment along with chlorophyll. However chlorophyll b is replaced by chlorophyll c.

(b) Rhodoplasts:

These are found in Red algae. The red pigment R-phycoerythrin and R- phycocyanin, are present alongwith chlorophyll a, however chlorophyll b is replaced by chlorophyll d.

(c) Chromatophores of blue green algae:

These are found in blue green algae (BGA). They have C-phycocyanin, C-phycoerythrin and chlorophyll a.

(d) Chromatophores ofphotosynthetic bacteria:

These are found in purple and non-purple photosynthetic bacteria. These bacteria have purple red carotenoid pigments. Special type of chlorophylls bacteriochlorophyll and bacterioviridin are also found in these bacteria.

(iii) Chloroplasts:

(Autoplasts of Meyer, 1883) They are green plastids which take part in photosynthesis and temporary or permanent storage of starch. In green algae, chloroplasts are of various shapes and sizes but chloroplasts of higher plants are disc-shaped with diameter of 4-6 nm and thickness of 2 – 4 µm.

The number is small in algae but numerous in higher plants. Each chloroplast is surrounded by a double membrane envelope. Internally it contains matrix or stroma and thylakoids. Thylakoids (Menlee, 1961) or baggy trousers are structural elements of chloroplasts. They are membrane lined flattened sacs. At places they form stacks called grana.

A chloroplasts may have 40-60 grana with each granum having 10-100 thylakoids. Thus there is differentiation of granal and stromal (intergranal) thylakoids. Space present in a granal thylakoid is called loculus while that of stromal thylakoid is termed as Fret channel. Photosynthetic pigments are located in the membranes of thylakoids in specific areas (according to Park and Biggins, 1963) called quantasomes (20 x 10 nm). Photosynthetic pigments of higher plants include chl a, chl b, carotenes and xanthophylls. A quantasome is believed to have 230 chlorophyll (160 a + 70 b) and 50 carotenoid molecules.

The matrix or stroma contains naked DNA (Ris & Plant, circular as well as linear), RNA, ribosomes (70 s), plastoglobuli (lipid/fat globuli) and enzymes of carbon assimilation. Unlike isolated plant cells, isolated cell organelles like chloroplasts and mitochondria cannot be kept in distilled water as they would burst.

They can be maintained only in solutions of specific concentrations e.g. 0.25% sucrose. Each quantasome contains 250 molecules of chlorophyll. The evolutionary origin of chloroplasts is currently explained in terms of ‘Endosymbiosis’.

Spherosomes:

(Perner, 1953). Spherosomes or sphaerosomes are single membrane covered small spherical organelles (10.5-1.0 nm) which synthesise and store fats.

Microbodies:

They are single membrane covered small cell organelles which take part in oxidation reactions other than those of respiration. Microbodies often posses a crystalline core and granular matrix. They are two types-peroxisomes & glyoxisomes/glyoxysomes.

(i) Peroxisome:

(Rhodin, 1954). The microbodies have enzymes for peroxide biosynthesis. They include (a) Peroxide, producing oxidative enzymes like peroxidase, urate oxidase (hence uricosome), amino acid oxidase and (b) peroxide destroying enzymes like catalase. In mesophyll cells, peroxisomes interact with chloroplasts and mitochondria to take part in ‘photorespiration’ (Tolbert et at, 1969). For this they have glycolate oxidase for metabolism of glycolate. Their number can be 70-100 per mesophyll cell. Other enzymes found in dense granular matrix of peroxisome of both plant and animal cells are D-amino acid oxidase and a-hydroxy acid oxidase. Peroxisome is involved in purine metabolism and in the conversion of fat to carbohydrate.

(ii) Glyoxysomes:

(Briedenbach, 1967, Beevers, 1963). The microbodies occur only in fat rich plant cells where they take part in p- oxidation of fats and perform glyoxylate cycle. Glyoxysomes possess catalase.