The electron transport chain (ETC) transports electrons from electron donors like NADH to molecular oxygen. It consists of protein complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons out of the matrix, building up an electrochemical gradient used for ATP synthesis. Electrons flow from complex to complex via mobile carriers like coenzyme Q and cytochrome c. This transfers energy from electrons to protons, conserving energy as ATP. Mitochondria contain many copies of the ETC to generate sufficient ATP through oxidative phosphorylation.

1. Electron transport chain (ETC)
•ETC, also called electron-transporting system (ETS),
respiratory chain, hydrogen-carrying chain, or
oxidation reduction chain, is a complex multi-
enzyme chain which transports electrons or
energised hydrogen from a donor molecule to
molecular oxygen.
•In prokaryotes, it is located in the plasma
membrane, and in eukaryotes in the inner
mitochondrial membrane.
•There will be several thousand electron-transport
systems in each mitochondrion.

2. •Mitochondrial ETC is characterised by a series of
oxidation-reduction (redox) reactions when there is
a transfer of electrons from a donor (reductant) to
an acceptor (oxidant).
•The potentiality of a reductant to pass electrons to
an oxidant is called oxidation-reduction potential,
or redox potential.
•The oxidant and the reductant, operating in a
redox reaction, together form a redox couple, or
redox pair.

3. •The redox reactions of glyclolysis and Krebs cycle are
dehydrogenation reactions.
•So, each redox reaction releases a pair of hydrogen atoms.
•FAD and NAD accept these hydrogen atoms and get reduced
to FADH, and NADH + H+ respectively.
•Reduced FAD and NAD (i.e., FADH, and NADH + H+)
ultimately reduce molecular oxygen to form H₂O (molecular
oxygen serves as the terminal electron acceptor or oxidising
agent in aerobic respiration).
•This is accomplished by the step-by-step transfer of
hydrogen or electrons through a chain of intermediate
electron-carriers, known as electron transporting system
(ETS) or respiratory chain.

4. •In aerobic cells, the catabolic pathways of
carbohydrates, fats and amino acids ultimately
converge into the respiratory chain.
•Along this chain energised electrons flow from
organic molecules to molecular oxygen, yielding
energy.
•This energy is recovered and used for the
phosphorylation of ADP to synthesise ATP.
•Thus, respiratory chain functions as the final
common route for the flow of all electrons from
different fuels to molecular oxygen.

5. Components of the respiratory chain
•The major components of the respiratory chain
consist of several intermediate carriers of
hydrogen or electron.
•They include NAD, flavoproteins, non-haem
metalloproteins (Fe-S proteins), ubiquinone or co-
enzyme Q (UQ or CoQ) and cytochromes.
•Each member of the chain accepts electrons from
the preceding member and then passes them on to
the next member in a specific sequence.

6. •Flavoproteins are conjugate proteins whose prosthetic group
is a flavin molecule, either FMN or FAD.
•Flavoproteins and NAD serve as hydrogen - carriers in cellular
respiration.
•NAD can accept only one hydrogen atom and get reduced to
NADH+H+.
•Flavin nucleotides can accept either one hydrogen to yield
the semiquinone forms (FMNH or FADH), or two hydrogens to
yield FMNH₂ or FADH₂.
•Ubiquinone (UQ), or co-enzyme Q(CoQ), is a hydrophobic,
fat-soluble and electroncarrying benzoquinone.
•It can accept either one electron to become a semiquinone
radical (UQH) or two electrons to become ubiquinol (UQH₂).

7. •Cytochromes are red or brown iron-containing
haemoproteins (iron-porphyrin haemoproteins), which
serve as the electron-transporting enzymes in cellular
respiration and photosynthesis.
•Their characteristic strong colours are produced by the
heme prosthetic group.
•They are universally present in all aerobic cells and are
located in the plasma membrane of bacteria, thylacoid
membrane of chloroplasts and the inner membrane of
mitochondria.
•In cellular respiration, they receive electrons from
ubiquinone and transport them in a relay process to
molecular oxygen.

8. •All about 50 odd kinds of cytochromes have so far been
identified.
•They fall under three main classes, namely a, b and c.
•Five members of these usually exist in a chain, called
cytochrome series or respiratory assembly.
•They are arranged in the order cytochromes b-c₁-c-a-a3.
•Only the last member of the chain, namely a3 can donate
electrons directly to molecular oxygen. So, it alone is directly
oxidised by oxygen. Therefore, it is referred to as cytochrome
oxidase.
•Just as UQ, cyt.c also is a mobile carrier of electrons.

9. •Iron-sulphur proteins are iron-containing and
electron-transferring mitochondrial proteins.
•In them, iron is present not in the heme state (as in
cytochromes), but in association with inorganic
sulphur atom and also with the sulphur atom of the
cysteine residue of the protein.
•The Fe-S centre may be simple with a single Fe
atom, or complex with 2 or 4 Fe atoms.
•In the oxidised state, all iron atoms are in the ferric
(Fe³+) state.
•In the reduced state, one of them becomes ferrous
(Fe²+).

10. Organization of the respiratory chain
•The mitochondrial respiratory chain is organized as
four electron-carrier complexes, all embedded in
the inner mitochondrial membrane.
•Complex I catalyzes the transfer of electrons from
NADH to UQ, complex II catalyzes the transfer from
succinate to UQ, complex III catalyzes the transfer
from UQ to cytochrome c, and complex IV
completes the sequence by transferring electrons
from cytochrome c to O₂.

11. Complex I
•(NADH to UQ)Also known as NADH dehydrogenase complex.
•It is composed of as many as 40 polypeptide sub-units, and its
prosthetic groups include FMN and Fe-S.
•The whole complex remains embedded in the inner
mitochondrial membrane in such a way that its NADH-binding
site faces the mitochondrial matrix.
•This arrangement enables the complex to interact with the
NADH produced by the dehydrogenases in the mitochondrial
matrix.
•The overall reaction catalyzed by the complex is as follows:
NADH + H+ + UQ ↔ NAD+ + UQH₂ (ubiquinol)

12. •In this reaction, oxidized UQ accepts hydride ions (two
electrons and one proton) from NADH and a proton from the
aqueous medium of the matrix.
•During this, the enzyme complex transfers a pair of reducing
equivalents from NADH to its prosthetic group FMN.
•Through the Fe-S of the complex, electrons pass from FMN to
UQ.
•The ubiquinol (UQH₂), formed in the reaction, soon diffuses
from complex I to complex III and then gets oxidised to UQ.
•The electron flow from UQ through complex I is always
accompanied by the outward flow of protons from the
mitochondrial matrix to the intermembrane space.

14. Complex II (Succinate to UQ)
•Also called succinate dehydrogenase (the only membrane-
bound enzyme of the TCA cycle).
•It is formed of at least 4 different protein units, and its
prosthetic groups include FAD and Fe-S with 4 Fe atoms.
•In this complex, electrons are believed to pass from succinate
to UQ through FAD and Fe-S.

15. •[Not all substrates for mitochondrial dehydrogenases
pass electrons into the respiratory chain through
complex II; some bypass complex II.
•An example is the first step in the betaoxidation of fatty
acyl-CoA, catalysed by the flavoprotein acyl-CoA
dehydrogenase.
•In this case, electrons are first transferred from the
substrate to the FAD of the dehydrogenase, then from
FAD to the electron-transferring flavoprotein (ETFP),
then from ETFP to the enzyme ETFP uniquinone
dehydrogenase, and finally from this enzyme to UQ].

16. Complex III (UQ to cytochrome c)
•Also called UQ-cytochrome c oxidoreductase or
cytochrome b-c1 complex.
•It consists of Cyt.b (b562 and b566), Cyt.c, and up
to 10 protein units.
•Its prosthetic groups include hemes and Fe-S.
•The flow of electrons through complex III
ultimately results in the oxidation of UQ and the
reduction of cytochrome c.

17. •Complex III serves as a "proton pump".
•The protons, produced during the oxidation of
UQH2 to UQ, are released to the
intermembrane space of the mitochondrion.
•This produces a difference in proton
concentration across the mitochondrial
membrane, called proton gradient.
•This is very significant in the synthesis of ATP.

18. Complex IV (reduction of O₂)
•Also called cytochrome oxidase.
•It consists of cytochromes a and a3, and its prosthetic groups
include hemes and the copper ions CuA and CuB.
•These copper ions are very vital in the transfer of electrons to
O2, without generating incompletely reduced, highly reactive
and seriously harmful intermediates, such as H₂O₂ or hydroxyl
free radicals.
•Electron flow from Cyt c to O₂ along complex IV causes a net
movement of protons from the intermembrane space.
•Thus, complex IV functions as a proton pump that generates
a proton-motive force.

19. •On the whole, it becomes clear that the
combined functioning of the electron carrier
complexes I, III and IV results in the transfer of
electrons from NADH to O₂ and the combined
action of complexes II, III & IV brings about
the transfer of electrons from succinate to O₂.
•Under normal cellular conditions, the
mitochondrial oxidation of NADH or succinate
yields more free energy than what is actually
required for the synthesis of one ATP (51.8 kJ/
mol).

21. Functioning of the respiratory chain
•In the respiratory chain, the constituent molecules are
arranged in the order of increasing redox potential,
decreasing electron pressure and diminishing free
energy level (redox potential or oxidation - reduction
potential is the tendency or potential to donate or
accept electrons).
•So, along the chain there would be a step-by-step flow of
electrons from a most negative initial donor (e.g., NAD)
to the most positive terminal acceptor (molecular
oxygen) through a series of increasingly more positive
intermediate carriers.

22. •The electrons, entering the respiratory chain, are
energy-rich.
•However, as they flow down, they lose free energy.
•Much of this energy gets conserved in ATP through the
coupled synthesis of ATP from ADP and Pi.
•The rest is lost as heat.
•In the respiratory chain, enzyme-bound hydrogen is
used as the fuel for energy generation.
•These hydrogen atoms are available from the
dehydrogenation reactions of substrate oxidation
(glycolysis, conversion of pyruvic acid to acetyl CoA,
oxidation of fatty acids and amino acids, Krebs cycle).

23. •From the hydrogen-donating substrates, hydrogen
atoms are collected by dehydrogenases and then
supplied to the respiratory chain through two
coenzyme channels, namely NAD and FAD (in some
synthetic pathways a third channel also operates,
namely NADP).
•These co-enzymes serve as the acceptors, carriers and
donors of hydrogen, or as a hydrogen-carrying system.
•The hydrogen atoms they carry soon donate their
electrons to the electron-transport chain and become
H+ ions.
•These H+ ions soon escape to the aqueous medium.

24. •NAD-linked (or NADP-linked) dehydrogenase removes
two hydrogen atoms from the substrate.
•One of them is transferred to NAD as a hydride ion (:H),
and the other one appears in the medium as H+.
•Reduced substrate + NAD+ ↔ Oxidised substrate +
NADH+H+
•FAD-linked dehydrogenase also removes two hydrogen
atoms from the substrate.
•In most cases, both of them will be accepted by FAD,
yielding FADH₂.
•Reduced substrate + FAD ↔ Oxidised substrate +
FADH₂

25. •From the co-enzyme channels, electrons are funnelled to
molecular oxygen through a chain of electron-carriers.
•They include flavoproteins, non-heme iron-sulphur (Fe-S)
proteins, ubiquinone or coenzyme Q (UQ or CoQ) and
cytochromes.
•In the respiratory chain, electrons are transferred in
three ways: (i) as electrons (ii) as hydrogen atom (H+ +
e-) and (iii) as hydride ion (:H-) which bears two
electrons.
•During the flow of electrons, every member of the
electron-transporting chain gets alternately reduced and
oxidised in a cyclic manner.

26. •In this process, their iron atom oscillates between
ferric (Fe³+) and ferrous (Fe2+) states.
•At the initial end, this redox reaction chain is
linkded to dehydrogenation reactions, and at its
terminal end to molecular oxygen.
•At the same time, some of its intermediate
reactions are coupled with the phosphorylation of
ADP.
•Dehydrogenation reactions maintain a steady flow
of hydrogen, and phosphorylation reactions
ensure a constant synthesis of ATP.

27. •The coupling of the redox reactions of the respiratory chain
with the phosphorylation of ADP constitutes a reaction
complex, known as oxidative phosphorylation or respiratory
chain phosphorylation (the coupling enzymes and the
phosphorylation enzymes are located in the mitochondrial
oxysomes).
•The biochemical reactions in which electrons are transferred
from one molecule to the next in the electron-transport chain
are called oxidation-reduction reactions or redox reactions.
•In them, the electron-donating molecule is called the reducing
agent or the reductant and the electron accepting molecule is
called the oxidising agent or oxidant.

28. •A reducing and an oxidising agent together
function as a conjugate redox pair, just as an
acid and its base together form a conjugate
acid-base pair.
•Each member of the cytochrome series
alternately serves as an oxidising agent and a
reducing agent.
•The oxidation of hydrogen in the respiratory
chain is completed in several sequential steps.

29. •The major steps are the following:
•(I) Dehydrogenation reactions remove hydrogen
atoms from the substrate. In carbohydrate
catabolism, there are six such reactions, one in
glycolysis, one in the oxidation of pyruvic acid to
acetyl CoA and four in each turn of the TCA cycle.
•(ii) The hydrogen acceptors NAD and FAD collect the
hydrogen atoms in pairs and get reduced to NADH +
H+ and FADH₂ respectively. In carbohydrate
catabolism, FAD receives hydrogen from one reaction,
and NAD from the remaining five reactions.

30. •(iii) In the hydrogen acceptors, hydrogen atoms
undergo ionisation and get split up to protons and
electrons. Soon, each hydrogen acceptor gets re-
oxidised by transferring electrons to the first
member of the cytochrome chain through certain
intermediate carriers. The intermediate carriers
include FMN-containing proteins (flavoproteins),
Fe-S-proteins and CoQ for the transfer of electrons
from NADH; Fe-S proteins and CoQ for the transfer
from FADH₂. Protons are simultaneously released
from the hydrogen acceptors to the medium.

31. •(iv) Every member of the cytochrome chain
first accepts electrons from the preceding
member and then passes them on to the
next member. Thus, a series of redox
reactions take place in the cytochrome chain.
The last member of the chain passes
electrons directly to molecular oxygen. This
reduces and excites oxygen, and results in the
formation of hydroxyl (OH-) ions. Hydroxyl
ions combine with the protons (H+) in the
medium and form H₂O.

32. Biogenesis of mitochondria (mitochondriogenesis)

33. •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.

34. (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.

35. • (b) Transformation of non-mitochondrial systems to mitochondriaAccording
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.