Electron Transport Chain (ETC)
• series of compounds, mainly proteins, which
are associated with the inner mitochondrial
• arranged in order of increasing
★ NADH dehydrogenase < ubiquinone
< cytochrome b-c1 complex < cytochrome c
< cytochrome oxidase complex
Electron Transport Chain (ETC)
Textbook p. 105
Electron Transport Chain (ETC)
• Each component is alternately
a. reduced (gaining two electrons from the
component before it in the chain)
b. oxidized (losing two electrons to the
component after it in the chain)
Electron Transport Chain (ETC)
• Most of the energy produced during the
Kreb’s cycle is held in energy carriers such
as NADH and FADH2 that stay within the
mitochondrial matrix.
• They transfer their high energy electrons to
proteins on the inner membrane of the
Electron Transport Chain (ETC)
• Physical Characteristics:
- inner mitochondrial membrane
separates the matrix from the
outer compartment
- large proteins embedded
in inner membrane
- these proteins are the electron
carriers in the electron
transport chain (ETC)
- electron carriers arrayed side by
side like the links in the chain
- 3 of the proteins are called ion pumps
Electron Transport Chain (ETC)
• There are 9 quicks steps that occur in
the electron transport chain:
Electron Transport Chain (ETC)
(1) NADH transfers two high
energy electrons to the first
electron carrier: an ion pump.
(2) NADH changes to NAD+.
(3) The carrier protein uses
energy from the electrons to
pump two H+ ions from the
matrix into the outer
Electron Transport Chain (ETC)
(4) The electrons are then passed
to the next protein in the chain
and then on to the third electron
(5) This third protein is also
an ion pump. It uses a bit
more of the electron’s energy to
pump two H+ ions through the
(6) The electrons shift again to
the fourth protein then to the
fifth. This protein is a third ion pump.
Electron Transport Chain (ETC)
(7) Using energy from the
electrons, the protein pumps two
more H+ ions into the outer
(8) The two electrons are now
much lower in energy. They
combine with two H+ ions and
an oxygen atom. Oxygen,
absorbed from air during
inhalation is the final electron
acceptor at the end of the electron transport chain (ETC)
(9) A molecule of water (H2O) is formed.
Electron Transport Chain (ETC)
- NADH gives up 2 electrons to
the first ETC complex: NADH
- Q and cytochrome c (mobile
electron carriers) shuttle the
electrons from one protein to
the next until they reach the
final protein complex in the
chain: cytochrome oxidase complex
- the enzyme cytochrome oxidase complex catalyzes (speeds up) the
reaction between the electrons, protons and molecular oxygen to
form water
Electron Transport Process
highly exergonic (energy of the products is less than
the energy of the reactants)
free energy lost by the electron pair during electron
transport is used to pump 3 protons into the
intermembrane space
converts one form of energy into another:
chemical potential energy of electron position 
electrochemical potential energy of a proton gradient
that forms across the inner mitochondrial membrane
Three main points
•Electrochemical potential energy
•Important to distinguish between NADH and
FADH2 in terms of their relationship with the
electron transport system
•A distinction must be made between the NADH
molecules produced in glycolysis and those
produced in the pyruvate oxidation& Krebs cycle.
Electrochemical Potential Energy
- Stored energy possessed by a charged battery
- NADH -> oxygen, free energy (-222kJ/mol)
- Stored and will be used to power ATP
synthesis (chemiosmosis)
NADH and FADH2 in the electron
transport system
- NADH passes its electrons on to the 1st protein
complex (NADH dehydrogenase)
- FADH2 transfers its electrons to Q, the second
component of the chain
- The free energy released by the oxidation of
FADH2 is used to pump 2 protons into the
intermembrane space, while NADH pumps 3
A distinction between the NADH
- The NADH molecules produced in
glycolysis and those produced in the
pyruvate oxidation& Krebs cycle are
- NADH(glycolysis) in the cytoplasm may
diffuse through the outer mitochondrial
membrane into the intermembrane
2 Shuttle Systems
- Glycerol-phosphate shuttle
-> transfers the electrons from cystolic
NADH to FAD to produce FADH2 and 2
ATP molecules(chemiosmosis)
- Aspartate shuttle
-> transfers electrons to NAD+ instead of
FAD, forming NADH, and then three
ATP molecules
- The many folds of the inner membrane
increase surface area & allow multiple copies
of the ETC
- A limited number of NAD+ and FAD molecules
-> recycled
- The resulting oxidized compounds pick up
more hydrogen atoms in glycolysis, pyruvate
oxidation, or the Krebs cycle
Chemiosmosis and Oxidative
ATP Synthesis
• Chemiosmosis: a process for
synthesizing ATP using the energy of an
electrochemical gradient and the ATP
synthase enzyme
• The protons that accumulate in the
intermembrane space of the mitochondion
during electron transport create an
electrochemical gradient that stores free energy
• Electrochemical gradient: a concentration
gradient created by pumping ions into a space
surrounded by a membrane that is
impermeable to the ions.
• The free energy stored in the electrochemical
gradient produces a proton-motive force(PMF)
that moves protons through an ATPase
• The energy of the gradient is reduced as the
protons pass through the ATPase complex
back into the mitochondrial matrix
• Proton-motive force(PMF): a force that moves
protons through an ATPase complex on
account of the free energy stored in the form of
an electrochemical gradient of protons across a
biological membrane
• ATPase: a catalyst for the synthesis of ATP
from ADP and inorganic phosphate in the
• Electron transport followed by chemiosmosis is
the last stage of the oxidative phosphorylation
What Happens When it stops?
• No food(glucose) = no electrons
• Heterotrophs must keep eating
• Autotrophs must keep photosynthesizing
• No O2 = no flow of electrons in ETC since O2
is final electron acceptor, freeing up the
• There’s no chemical in body Electronegative
enough to oxidize last protein, O2 used to free
• H+ not pumped and the concentration becomes
even between matrix and intermembrane space
• Chemiosomosis and ATP manufacturing stops
As for NADH and FADH2…
• ETC clogged since O2 isn’t there to take
away the final electron, holding up the
entire production line
• NADH and FADH2 can’t give up their
electrons, and remain in their reduced
form unable to oxidize H from other
• Pyruvate Oxidation, Kreb’s Cycle, ETC and
chemiosmosis are linked and all depend on
glycolysis for pyruvate
• ETC and chemiosmosis BOTH depend on O2
and electrons to function, in this way they are
Electron Transport Chain (ETC)

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