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) transformation of non-mitochondrial systems to
mitochondria.
•Of these, the self-duplication hypothesis is most widely
accepted now.

3. (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.

4. (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.

5. (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.

6. 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.

7. •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.

8. •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.

9. •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.

10. 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:

11. (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.

12. •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.

13. •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.

14. •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.

15. •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.

16. •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.

17. (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.

18. •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.

19. 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.

20. •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.

21. •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.