mitochondria biogenesis and functions.pptx

mitochondria

1. Biogenesis of mitochondria ( mitochondriogenesis ) Three mechanisms have been proposed to account for the formation of mitochondria. They are ( i ) self-duplication of pre-existing mitochondria (ii) de novo origin and (iii) transfor mation of non-mitochondrial systems to mitochondria. Of these, the self-duplication hypothesis is most widely accepted now.

2. (a) Denovo origin of mitochondria This hypothesis holds that mitochondria are formed from cytoplasmic vesicles . During this, the vesicles form buds which are soon covered by membrane . The buds grow in size, undergo internal compartmentalisation by the formation of cristae , and transform to mitochondria. The current view is that mitochondria and chloroplasts are never formed de novo , but are formed from pre-existing ones.

3. (b) Transformation of non-mitochondrial systems to mitochondria According to this hypothesis, mitochondria are formed by the infolding of plasma membrane and ER , or by the delamination of nuclear membrane . Though the origin of mitochondria from plasma membrane and ER has been observed in rat liver cells and the nerve fibres of cray fish, it is not the case with most organisms .

4. (c) Self-duplication of mitochondria It is generally held that mitochondria (and chloroplasts) are semi-autonomous cell organelles, capable of self duplication by fission . They contain DNA, RNAs, ribosomes and the complete set of molecular machinery for DNA duplication, genetic transcription, and genetic translation or protein synthesis. Still then, their biogenesis depends essentially on an integrated activity of nuclear (chromosomal) genes and their own genes .

5. Evolution of mitochondria Endosymbiont hypothesis Endosymbiont hypothesis is the concept that the energy-transducing cell organelles of eukaryotes, namely mitochondria and chloroplasts, might have evolved from prokaryotes by the accidental internalisation of prokaryotes by eukaryotic cells. It has been argued some time before that mitochondria and chloroplasts represent bacteria-like symbiotic prokaryotes, living within eukaryotic cells. This argument is on the ground that they are similar to bacteria in form, size, genetic and metabolic machinery, etc.

6. Accordingly, it was postulated that mitochondria and chloroplasts might have evolved from bacteria-like prokaryotes which were accidentally ingested by primitive eukaryotic cells at an early stage of evolution, more than one billion years ago. In course of time, these internalised organisms established a symbiotic association with their host cells and ultimately became mitochondria and chloroplasts. During this, they lost most of their genome, became dependent on the proteins and enzymes encoded by the nuclear genome of the host cell, and ultimately evolved into the energy-transducing organelles of the host cell. At the same time, they conserved their DNA which contains some genes coding for some proteins.

7. The host cell, in turn, became dependent on mitochondria or chloroplast for its energetic requirement, to meet its demand for ATP. Most of the mitochondrial and chloroplast enzymes and proteins are encoded by chromosomal genes. This probably indicates that during the evolution of eukaryotes there occurred extensive transfer of genes from the endosymbiotic prokaryotes to the nuclear genome of the host cell. This view is supported by the fact that some eukaryotic nuclear genes which code for mitochondrial proteins are very much similar to bacterial genes.

8. During duplication, mitochondria actively engage in DNA replication, RNA synthesis, formation of ribosomes and the synthesis of mitochondrial proteins . This is followed by the growth, elongation and internal compartmentalisation of mitochondria. Finally, an inward furrowing or constriction appears in the inner membrane, followed by the furrowing of the outer membrane. The furrowing deepens further, ultimately dividing the mitochondrion into two. Replication of mt DNA and duplication of mitochondria take place independently , almost out of phase with cell cycle and cell division.

9. Functions of mitochondria Mitochondria are primarily the "power plants" or the energy transducing centres of the cell. They can transform the chemical energy contained in the "low-grade" food stuffs and fuel molecules to the biologically available energy of the energy bonds of the "high-grade" fuel ATP by oxidative phosphorylation. By the oxidation of fuel molecules, they release energy for the synthesis of ATP. This brings about the transformation and conservation of energy. The major functions of mitochondria are the following:

10. ( i ) Oxidative phosphorylation and ATP synthesis Mitochondria are the centres of oxidative phosphorylation and ATP synthesis. In them, fuel molecules are completely and finally oxidised , releasing their chemical energy. This oxidation process is always coupled or linked with the phosphorylation of ADP. In this process, ADP molecules capture the released energy, undergo phosphorylation with its help and produce ATP . In the energy-bonds of ATP, energy is conserved in a biologically available form to be used for various cellular functions.

11. Thus, mitochondria bring about the extraction, trapping and conservation of energy. This involves the transformation of the potential chemical energy of fuel molecules to the potential biological energy of ATP molecules. The whole process requires an input of O₂, ADP and inorganic phosphate, and it results in an output of CO₂, ATP and H₂O.

12. Mitochondrial energy transduction is completed in three major steps, namely ( i ) the enzymatic oxidation of fuel molecules in the TCA cycle (ii) repeated redox reactions in the respiratory or electron transport chain and (iii) coupling of the energy-releasing redox reactions with phosphorylation reactions forming a reaction complex, called oxidative phosphorylation.

13. Oxidation of fuel molecules in the TCA cycle occurs in the mitochondrial matrix (except the conversion of succinic acid to fumaric acid, which occurs in the inner membrane), redox reactions occur in the inner mitochondrial membrane, and oxidative phosphorylation occurs mostly in the F1 particles.

14. The dehydrogenation reactions of glycolysis, TCA cycle, etc. release hydrogen atoms. The hydrogen acceptors FAD and NAD capture them in pairs and get reduced to FADH₂ and NADH+H+ respectively. In them, hydrogen atoms undergo ionization and get split up to protons and electrons. The electrons are soon transferred to a chain of electron-transporting respiratory enzymes, called cytochromes. Every member of the cytochrome ( electrontransporting ) chain first accepts electrons and get reduced.

15. Then, it passes the electrons to the next member and gets oxidised . Thus, a series of redox reactions takes place in the respiratory chain. The last member of the chain passes electrons to molecular oxygen to form H₂O. Three of the redox reactions release energy. ADP molecules capture this energy, undergo phosphorylation utilizing it and form ATP.

16. (ii) Extra-chromosomal inheritance Mitochondrial DNA ( mtDNA ) contains extra- chromosomol genes, known as plasma genes. Their role is somewhat similar to that of chromosomal genes. They can store biological information, transmit them through duplication, and express them through transcription and translation. In fact, mitochondrial DNA duplication, transcription and translation are dependent on the nuclear genetic system.

17. This is because all the necessary enzymes and protein factors are synthesised on cytoribosomes under the direction of chromosomal genes. Thus, chromosomal genes direct the synthesis of enzymes and proteins, which in turn, mediate the action and expression of mitochondrial genes.

18. iii) Synthesis of mitochondrial DNA, RNAs and proteins Synthetically, mitochondria are semi-autonomous organelles. With some degree of autonomy, they can synthesise their own DNA, RNAs and proteins. In other words, they can bring about gene expression through gene duplication and genetic transcription and translation. mtDNA can undergo duplication and form multiple copies of it. It can also serve as a template and guide the synthesis (transcription) of mRNA, rRNA and tRNA. The rRNA gets complexed with ribosomal proteins and forms mitoribosomes .

19. These proteins, in turn, are synthesised in cytoribosomes under the control of chromosomal genes. Now, mitochondria contain the complete protein synthetic machinery, namely DNA, mRNA, tRNA, ribosomes and the necessary enzymes and protein factors. Making use of this machinery, they can synthesise nearly 12 different kinds of mitochondrial proteins. These proteins are mostly hydrophobic and they get incorporated with the inner membrane.

20. Even though mitochondria can independently synthesise DNA, RNAs and proteins, the enzymes and protein factors necessary for the process are specified by chromosomal genes, and are synthesised on cytoribosomes . The major enzymes include DNA polymerase, RNA polymerase, aminoacyl transfer RNA synthetase, peptidyl transferase, etc. Thus, mitochondrial genetic system and synthetic mechanisms are dependent on nuclear genetic system.