Aerobic respiration occurs within cells.
It starts with glycolysis in the cytosol of the cell.
It then goes through the link reaction moving into the matrix of the mitochondria.
It then goes through the citric acid cycle within the mitochondria matrix.
Then it goes through the electron transport chain and chemiosmosis in the inner mitochondrial
membrane producing energy in the form of ATP.
In this video we are going to look at the fourth process involved in Aerobic Respiration,
The Electron Transport Chain.
At the end of the combined steps of Glycolysis, The Link Reaction and The Citric Acid Cycle,
we were left with 10 NADH and 2 FADH2’s.
During the electron transport chain these NADH’s and FADH2’s are used to create
a hydrogen concentration gradient between the mitochondria matrix and the intermembrane
space.
The electron transport chain occurs in the inner mitochondrial membrane.
Within the membrane there are four protein complexes involved; complexes I, II, III and
IV.
There are also several mobile electron carriers within the membrane; Ubiquinone (Q for short),
Ubiquinol (QH2 for short) and cytochrome c.
The Electron Transport Chain starts with Complex I.
Complex I, also known as NADH-Coenzyme Q Reductase, consists of a flavin mononucleotide (FMN)
and a chain of 9 Iron-Sulfur (Fe-S) redox centres.
The chain starts with the NADH produced in Glycolysis, Pyruvate Dehydrogenase and the
Citric Acid Cycle each being oxidised at Complex I to produce NAD+ + H+ (proton) by the reduction
of the Flavin Mononucleotide (FMN) into FMNH2.
In this process 2 electrons (2e-) are transferred from NADH to the reduced flavin mononucleotidae
FMNH2.
These 2 electrons (2e-) are then transferred to the chain of Iron-Sulfur (Fe-S) redox centres
through the oxidation of FMNH2 back into FMN.
These 2 electrons (2e-) then move through Complex I via the chain of Iron-Sulfur (Fe-S)
Redox Centres, from a higher energy state to a lower one.
Complex I is also a proton pump.
Energy is released by the electrons as they move to lower energy states along the chain
of Iron-Sulfur (Fe-S) redox centres.
This energy is used to pump four hydrogen (H+) protons across the inner mitochondrial
membrane from the matrix into the intermembrane space.
This establishes the hydrogen concentration gradient between the intermembrane space and
matrix of the mitochondria.
There is a pool of carrier proteins; Ubiquinone (Q) and the reduced form Ubiquinol (QH2).
At the end of the chain of Iron-Sulfur (Fe-S) centres the 2 electrons (2e-) are transferred
to the mobile electron carrier Ubiquinone (Q) one at a time.
The first electron (e-) is transferred along with a hydrogen (H+) creating the semiquinone
intermediate form (Q.-) The second electron (e-) is transferred along
with another hydrogen (H+) creating the reduced form Ubiquinol (QH2).
The next part of the electron transport chain occurs at Complex II and occurs parallel to
the processes in Complex I. Complex II also known as Succinate-Coenzyme
Q Reductase or just as Succinate Dehydrogenase consists of a flavoprotein, a chain on 3 Iron-Sulfur
(Fe-S) redox centres and a heme b containing subunit.
When succinate is oxidised into fumarate in the Citric Acid Cycle, the oxidisation produces
2 hydrogens (2H+) but it also produces 2 electrons (2e-).
These 2 electrons (2e-) are accepted by FAD which is bound in the flavoprotein of Complex
II, reducing it to form FADH2.
The FADH2 transfers these 2 electrons (2e-) to the Iron-Sulfur (Fe-S) Redox Centres, in
turn converting FADH2 back into FAD.
The 2 electrons (2e-) move along the Iron-Sulfur (Fe-S) Redox Centres, until they reach the
heme b containing subunit.
This heme b containing subunit is the binding site for Ubiquinone (Q).
It is here that the electrons (e-) are transferred to the mobile electron carrier Ubiquinone
(Q), reducing it to Ubiquinol (QH2).
Complex II is not a proton pump.
The electrons from FADH2 bypass Complex I and do not add to the net flow of Hydrogen
(H+) protons leaving the matrix, meaning FADH2 yields less ATP molecules than NADH.
From here the electrons from Complex I and Complex II are moved with the mobile electron
carrier Ubiquinole (QH2) to Complex III.
Complex III also known as Coenzyme Q- Cytochrome c Reductase, consists of iron protoporphyrin
containing cytochrome b, a heme b containing cytochrome c and one 2iron-2sulfur cluster
centre called the Rieske protein.
Within Complex III, a process known as the Ubiquinone or (“Q”) Cycle, occurs.
This cycle starts with one Ubiquinole (QH2) binding with a Quinone binding site on cytochrome
b called Qp..
This Ubiquinole (QH2) is oxidised in two steps.
The first electron is transferred to the Rieske protein, which also releases 2 hydrogens (2H+)
into the intermembrane space and converts Ubiquinole (QH2) into the semiquinone anion
form (Q.-).
The second electron is transferred to the cytochrome bL heme.
This converts the semiquinone (Q.-) into Ubiquinone (Q) which leaves the complex to return to
the pool of Ubiquinone (Q) and Ubiquinole (QH2) in the inner mitochondrial membrane.
The electron that was transferred to the cytochrome bL heme is then transferred to the cytochrome
bH heme.
From the cytochrome bH heme, the electron is transfered to a second Quinone-binding
site called Qn.
At the Qn site the electron converts a Ubiquinone (Q) into the semiquinone (Q.-).
This semiquinone (Q.-), in this form, is strongly bound to the Qn site until another electron
is available to release it in the Ubiquinole (QH2) form.
This commences the first half of the (“Q”) Cycle.
The second half requires a second molecule of Ubiquinole (QH2) to be oxidised at the
Qp Quinone binding site.
One electron again goes to the Rieske Protein, again releasing another 2 H+ into the intermembrane
space.
The second goes to the cytochrome bL heme, then onto the cytochrome bH heme.
The electron this time binds to the bound semiquinone (Q.-) at the Qn Quinone binding
site, in addition to 2 Hydrogen (2H+) from the matrix to form Ubiquinole (QH2) that is
released back into the inner mitochondrial Complex III is a proton pump, it is through
this Ubiquinone (“Q”) Cycle that 4 hydrogens (4H+) are pumped into the intermembrane space.
That concludes the (“Q”) Cycle portion of what goes on in Complex III, now we need
to look at what happens to the electrons passed onto the Iron-Sulfur (Fe-S) Cluster Centre
of the Reiske Protein.
From the Reiske Protein, the electrons are transferred to cytochrome c1.
From cytochrome c1 the electrons are transferred to the mobile electron carrier cytochrome
c.
This mobile electron carrier cytochrome c carries the electrons to Complex IV.
Complex IV, also known as Cytochrome c Oxidase, consists of a double Copper (CUA) centre,
a heme a site and a copper (CUB) plus heme a3 site.
Cytochrome c transfers a single electron to the CUA centre in Complex IV.
Four cytochrome c’s sequentially pass on four electrons into Complex IV.
Each electron is then passed on to the heme a site.
From here the electron is transferred to the Copper (CUB) and heme a3 site.
At this site the four electrons split an O2 molecule into two oxygen atoms which can be
denoted as 1/2 oxygen.
This Oxygen molecule (O2) is the final acceptor of electrons in the electron transport chain.
This oxygen is essential in the electron transport chain as it takes the electrons that would
otherwise block the chain up, so it can continue running over and over again.
Each half oxygen (1/2 O2) accepts 2 hydrogen (2 H+) protons from the matrix, each forming
a water molecule (H2O).
Complex IV is a proton pump, as the electrons move to lower energy states as they move through
complex IV, 2 H+ are pumped through.
This ends the Electron Transport Chain, leaving a high concentration of hydrogen (H+) protons
in the intermembrane space, and a low concentration in the matrix.
This concentration gradient is vital for the production of ATP in the next step called
Chemiosmosis.
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