Mitochondria were first observed by Altman (1894) as filamentous structures and he called them as bioblasts, but Brenda (1897) called them as mitochondria.† Unlike chloroplasts they are found in almost all eukaryotic organisms.† They are very important energy rich ATP producing organelles and make it available for other biological process in the form of energy rich compounds like ATP and NADH2 and NADPH2.† It also directs fatty acid oxidation products for ATP synthesis.
EM of mitochondria- showing inner membrane bond Elementary particles involved in ATP production; http://lifesci.dls.rutgers.edu/
Elementary particles (ATP synthases); http://biology4isc.weebly.com/
Mitochondrial DNA circular; www.visualphotos.com
Mitochondria in Sieve cells.(M)
Vicia faba; C, callose; CC, companion cell; CP, frolicsome; CW, cell wall; ER, endoplasmic reticulum; M, mitochondria; N, nucleus; P and Pl, plastids; P-proteins-dispersive and nondispersive, PP, parietal protein; PPU, pore plasmodesma unit; SE, sieve element; SP, sieve plate; V, vacuole. From Knoblauch and Van Bel (1998) Sieve tubes in action. Plant Cell 10, 35-50. Copyright American Society of Plant Biologistswww.public.wsu.edu
Using specific stains or fluorescent dyes, mitochondia can be observed cytologically.† In the presence of janus green they look like small rod shaped, greenish blue structures.† The shape of mitochondria is never constant; at one moment they look like small bacterial shaped or tubular shaped structures of 0-.5 mm to 7 mm and in few moments later, they appear as long filaments or vesicles.† A movie on mitochondria, pictured to show its variability in structure with time, it is amazing to observe how the mitochondria divide and fuse with one another and divide - a process continuum, perhaps in all organisms. Thus they exhibit polymorphism in their shape and size.
Another more recent discovery about the structure of mitochondria is the close association that they can have with the endoplasmic reticulum (ER), called the mitochondria-associated ER membrane or MAM.Mitochondria store signalling molecules like Ca2+; Mitochondria can also participate in cell signaling through the reactive oxygen species (ROS) that are created during metabolism. http://frombenchtobedside.wordpress.com/
Mitochondria are involved in regulating the cell cycle and play a key role in the apoptosis pathway, a type of programmed cell death. And just for the sake of completeness: some parts of the steroid hormone, heme synthesis and urea cycle pathways take place in mitochondria. http://frombenchtobedside.wordpress.com/
The number of mitochondria found in cells varies from cell type to cell type, it varies from species to species.† Even physiological state of the cell determines the number. Trypanosomes contain only one mitochondria.† Yeast cells under glucose repression posses just one or two mitochondria per cell.† But a liver or meristems in roots contain 500-1600 mitochondria per cell.† In some amphibian oocytes which is very active, there can be 10, 0000 or more mitochondria.† The cell that requires more energy contains more number of mitochondria, ex: flight muscle cells in insects, heart cells have more mitochondria than their intestinal cells.
Even the intracellular distribution and orientation of mitochondria depends upon the structures involved in a particulars function.† For example, in flight muscle cells, in the vicinity of centrosomes, mitochondria are arranged radially and mitochondria are packed longitudinally.† At the basal granules of flagella they are aggregated at the base; otherwise they are randomly distributed in the cytoplasm.
Probably mitochondrion is the only cell organelle whose structure has been studied in greater details.† Like any other cell organelles, it is also bounded by membranes.† The outer membrane is 6 nm thick and the length varies from cell to cell. Structurally mitochondrion consists of two membranes, the outer and the inner membranes and they are separated from each other with perimitochondrial space. The outer membrane can be striped of from the inner membrane by dgitonin treatment and such mitochondria are called mitoplasts.
The outer membrane can be easily identified by its marker enzyme called monoamine oxidase.† The outer surface of the outer membrane appears to be smooth but careful observations reveal the presence of many aggregated granular structures which are believed to be enzymes responsible for glycolytic process.† The perimitochondrial space is filled with a fluid in which, enzymes like Adenosyl cyclases Cyt.c and others are found.
The inner membrane shows structural complexity which is unique for this organelle.† It is folded into a number of membranous crests called cristae.† They may be oriented transversely or longitudinally.† But the number of cristae varies and it depends upon the functional state of the cell.† For example, mitochondria found in flight muscle cells of insects contain 10,000 or more cristae, but in resting cells the number of cristae in mitochondria is few. The increase in the number of cristae increases the surface area of reaction which helps in the production of more of ATPs.
Though cristae provide a kind of compartmentation to mitochondrial chamber, it is not total.† The perimitochondrial space is continuous in the cristae folds.† In spite of both membranes being distinct, they are in contact with each other at least at 70-100 sites.† Recent studies, involving mitochondrial contraction and expansion due to changes in energy levels of mitochondria, have demonstrated that most of the mitochondria exhibit several contact zones; where at the cytoplasmic surface a cluster of ribosomal complexes are found.† It is speculated that it is through this region that some of polypeptides synthesized in the cytosol are transported into mitochondia.† Probably these regions are specialized structures involved in the transportation of components from cytoplasm into mitochondria.† Now it is known that there are specific transporters of nuclear coded proteins are transported into mitochondria through such transporters.† The outer membrane has Tom and the inner membrane has Tim.† These have been characterized.† Basically these contain a receptor protein and channel proteins.
The inner surface of the cristae membranes ie, towards mitochondrial matrix side, is studded with a large number of knob or globular shaped particles with a conspicuous stalk like structure.† Such globular particulate are called Racker particles which have been identified as F1 particles.† They act as ATP synthetases and they are uniformly spaced at 10 nm apart.† The number of F1 particles present in each mitochondria ranges from 100 to 1000.† The cristae membranes also contain a large number of electron transporting enzymes and other oxido reductases deeply buried in the core of the lipid bilayers, but vectorially organized.† In fact, electron transporting enzymatic units constitute more than 20% of the total membrane proteins.
Mitochondrial chamber is filled with fluid called mitochondrial matrix.† A host of enzymes are present in this fluid.† The unique feature of matrix is the presence of a circular DNA duplex and all t RNAs, and mRNAs.† It also contains 70s ribosomes which are similar to that of prokaryotes.† Having the above said structures, mitochondrial is considered as a semiautonomous organelle.
The outer membrane consists of 40% lipids and 60 percent proteins.† The common liquids found are lecithin, cephalin, and cholesterol.† Among proteins, along with structural ones, the outer membrane posses a marker enzyme called monoamine oxidase.† On the contrary, the inner membrane is made up of 20% lipids and 80% proteins.† The notable lipids present are cholesterol, lecithin and cephalin and cardiolipins, the latter is present is significant amount.† Among proteins present in the inner membrane are ATP synthetase succinic dehydrogenase carrier proteins like carnitine, fatty acid acyl transferase, ATP and ADP carrier, which transport inorganic phosphates, calcium, malate, glutamate, and many others.
The electron transport enzymes, proton secreting proteins are virtually buried in the core of the inner membranes.† However, electron transporting proteins, which include oxido-reductases, are grouped into four complexes.† Each of them vectorially arranged for receiving the reduced coenzymes like FADH2 and NADH2 which are then subjected to terminal oxidation, while the outer is virtually impermeable even for simple ions.† They have to be transported across the membrane through specific carriers.
Mitochondrial matrix also consists of a wide variety of enzymes for krebís cycle, amino acid metabolism, protein metabolism, nucleic acid metabolism and fatty acid oxidation.
Mitochondrial DNA found in the matrix is found in multiple copies from 40 to 60 or more.† The DNA is in circular form and codes for 2rRNAs, all 22tRNAs and 13 mitochondrial proteins.† Mammalian mitochondria contains a circular DNA of ~16500 bases pairs, but yeast mit is 2-3 times larger than mammalian and it has been ††mapped.† ††Among 13 protein coding genes, three for 3 large polypeptides of cytochrome oxidase, one for the large subunit of cyt. B-c reductase and one of hydrophobic protein transfer protein of ATPase complex are present.† The other gene products which have not been identified are called URF1, URF2 etc.† The unique feature of mit DNA is that most of the gene products are transcribed on H strand, but L strand in also used for the production of transcripts for few tRNAs and one or two URFs.† In trypanosome only one mitochondria is present and it contains Kinetoplastida.† It has a large single maxi chromosomal DNA and hundreds of smaller min circular DNAs.
The DNA replication in mitochondria is independent or sometime synchronized with that of nuclear DNA replication.† But the mechanism of replication, though it is semiconservative, and exhibits D-loop mechanism.† Another important feature of mit DNA is all genes are tightly packed without any spacers or intervening sequences in between the genes or within the transcribing genes.† tRNA genes are located in between protein coding frames.† However, yeast mitochondria have a long DNA with redundant and intervening sequences within the coding sequences.† In plants, mitochondrial DNAs, besides having the above said genes they also have a gene for male sterility, ex: Zea mays.
The ribosomal machinery present in mitochondrial matrix is typical of prokaryotic 70s type, but in some plants the ribosomes presents in mitochondria are slightly smaller than 70s.† Nevertheless the 23s RNA, 16s rRNA and 5s RNA are present with some minor differences.† Most of the transcribing the translational factors and other enzymes found in mitochondrial matrix are coded for by nuclear genome and translated on cytosol 80s ribosomes, then they are imported into mitochondria.† From the ribosomes it is clear that mitochondrial matrix has all the components for nucleic acid & protein synthesis.
Mitochondrial matrix is also involved in citric acid cycle, where energy rich co-enzymes like FADH2 and NADH2 are produced.† They are then taken up by cristae membranes, where they are step wisely and sequentially oxidized.† During this process, the energy released is used for the synthesis of ATP.† This phenomenon is called oxidative phosphorylation.† These mitochondria are endowed with an unique structural and functional co-ordination.
In the past 20-30 years various speculative theories have been proposed from time to time to explain mitochondrial biogenesis.† Most of the views were based on cytological observations.† But now the structure, genetic composition and its functional aspects are fairly known.† Recent studies clearly indicate that mitochondria originate from the pre existing mitochondria by growth and fission.† The biogenesis of mitochondria is highly regulated and† co-ordinates with the nuclear genome. Though, more than 95% of the total mitochondrial protein are coded by nuclear genome certain factors coded for by the nuclear genome expert control over mitochondria gene expression. Similarly, some mitochondrial protein factors coded for by the mitochondrial genes are transported into nucleus where they regulate the expression of genes and gene products required for mitochondria (one report from Neurospora).
As mitochondrial genome and 70s ribosomal machinery and other transcription translating factors are more or less similar to that of prokaryotic bacteria, it is viewed by many biologists that the eukaryotic mitochondria were once bacterial cells; in course of time they have incorporated into eukaryotic cells properly for 3 billion years ago.† Since then they exist as symbionts. The similarities between bacteria and mitochondria are circular DNA, chloramphenicol sensitive ,70s ribosomes, 23s RNA, 16s RNA and 5s RNA.† Based on these observations, it is now strongly believed that mitochondria are symbiotic bacteria; now it is an accepted fact. Mitochondrial codons are not universal.† Mitochondrial DNA has been used to find out phylogeny.† It is now believed all the present 6 billion humans have derived mitochondria from few African mothers.
Mitochondria very often found associated with smooth ER andCa2+ is transported into mitochondria. www.mdpi.com;
Hypothetical models of the role of contacts between mitochondria and ER in apoptosis. The hFis1/Bap31 platform transmits the mitochondrial stress signal to the ER via the activation of procaspase-8. The cytosolic region of the ER integral membrane protein Bap31 is cleaved by activated caspase-8 to generate proapoptotic p20Bap31, which causes rapid transmission of ER calcium signals to the mitochondria via the IP3 receptor. At close ER-mitochondria contact sites, mitochondria takes up calcium into the matrix via the mitochondrial calcium channels MICU1 or LETM1. The massive influx of calcium leads to mitochondrial fission, cristae remodeling, and cytochrome release. Mfn2 is enriched in the mitochondria-associated membranes (MAM) of the endoplasmic reticulum (ER), where it interacts with Mfn1 and Mfn2 on the mitochondria to form inter-organellar bridges. Upon apoptosis signal, a BH3-only member of the Bcl-2 family, Bik, induces Ca2+ release from the ER and, in turn, induces Drp1 recruitment to the mitochondria and their fragmentation and cristae remodeling. SERCA- sarco /endoplasmic reticulum Ca2+-ATPase. MICU1, mitochondrial calcium uptake1. LETM1,leucine zipper/EF hand-containing transmembrane 1. http://www.hindawi.com/
Mitochondria is involved in various functions which are important to cell; www.edoc.hu-berlin.de
Central role of Krebís cycle that takes place in the mitochondrial matrix; www.ridge.icu.ac.jp; www.gopixpic.com
Mitochondria are also involved in urea cycle and below citric acid cycle; edoc.hu-berlin.de
Oxidative phosphorylation- Electron Transport chain
Electron transport chain leads to production of ATP; http://www.exatest.com/ www.ruf.rice.edu
The size of mitochondrial genome varies from one species to the other, human mitochondria contain ~16 Kbp and codes for13 proteins, 22tRNAs and 7 URFs. The number mitDNAs per mitochondria can be 100 to 200 and the number of mitochondria per cell varies from one to ten thousand or more.
Mitochondrial DNA rRNA genes and 13 protein coding genes and ~22 tRNA genes and 6-7 unknown URFs; www.cellbiology.med.unsw.edu.au
Replication of organelle DNA (Mitochondrial) with Replication Ori-H andOri-L and Trnascriptional start sites-Hsp and Lsp
Some genetic codes differ from universal coded, this is specific to mitochondrial codons
Mitochondrial in the process of division
Mitochondrial Protein Import:
Protein transport across outer and inner mitochondrial membranes; Protein import machinery; www.cell.com
Protein Translocation und Membrane Protein Insertion at the Inner Mitochondrial
Mitochondrial changes in neuro-degeneration. Most neurodegenerative diseases result in defects in oxidative phosphorylation, respiratory dysfunction, increased ROS production, lowered mitochondrial membrane potential, decrease in synthesis of antioxidants, mtDNA mutations, impaired protein import, increased fragmentation of mitochondria, and activation of mitophagy and apoptosis. Sources of reactive oxygen species are marked by red stars. Abbreviations: Htt, Huntingtin; Ub, ubiquitin; ΔΨ, membrane potential; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; ROS, reactive oxygen species; mtDNA, mitochondrial DNA; Aβ, amyloid beta; APP, amyloid precursor protein; TIM, translocase of inner membrane; TOM, translocase of outer membrane; SOD1, superoxide dismutase 1; VDAC, voltage dependent anion channel; ANT, adenine nucleotide translocator; Pink1, PTEN-induced putative kinase 1; αKGDH, alpha ketoglutarate dehydrogenase; cyt c, cytochrome c.;ww.hindawi.com.
For More Information Read-www.grkraj.org