Stage 4: Electron Transport
and Chemiosmosis
Angel Gu, Samantha Wong, Linda Yang, Angel Zhang
What is the electron
transport chain?
A chain of protein complexes embedded in the inner
mitochondrial membrane. Transports electrons and pumps
hydrogen ions into the intermembrane space to create a
gradient.
Components of the electron
transport chain (ETC)
•
•
•
•
•
•
NADH dehydrogenase
Ubiquinone (Q)
Succinate dehydrogenase
Cytochrome b-c1
Cytochrome c
Cytochrome oxidase
• Arranged in order of increasing electronegativity
(weakest to strongest)
NADH produced
from glycolysis
NADH produced
from Krebs cycle
VS
VS
• NADH produced in the cytoplasm by glycolysis:
CAN diffuse from outer mitochondrial membrane to
intermembrane space.
CANNOT diffuse from inner membrane to matrix.
• NADH produced from Krebs cycle:
Already in the matrix.
What to do?
• Cytosolic NADH (NADH that are produced by glycolysis)
• Pass electrons to shuttles
• Glycerol-phosphate shuttle:
Transfers electrons from cytosolic NADH to FAD to produce FADH2
• Aspartate shuttle:
Transfers electrons from cytosolic NADH to NAD+ to produce NADH
Complex I: NADH
dehydrogenase
• NADH dehydrogenase oxides NADH back to NAD+
NADH + H+  NAD+ + 2H+ + 2e• The electrons are transferred to flavin mononucleotide (FMN)
which reduces to FMNH2
FMN + 2H+ + 2e-  FMNH2
• The electrons are then passed to iron-sulphur proteins (FeS)
located in NADH dehydrogenase
Complex I Continued…
• Electrons are accepted by Fe3+ which is reduced to Fe2+
2Fe3+ + 2e-  2Fe2+
• These two electrons are then given to ubiquinone (Q)
• The two hydrogen ions are pumped into the intermembrane
space
*One hydrogen ion is
pumped per electron
transferred
Ubiquinone (Q)
• A mobile electron carrier
2Fe2+ gives 2e- to Q
Q is reduced to QH2 (ubiquinol) and
2Fe2+ is oxidized 2Fe3+
• Carries electrons to complex III, cytochrome b-c1
Interactive
Animation:
http://www.brookscole.com/chemistry_d/templates/student_re
sources/shared_resources/animations/oxidative/oxidativephosp
horylation.html
Complex II: Succinate
Dehydrogenase
• oxidation of succinate from Krebs cycle to fumarate
Succinate + FAD  Fumarate + FADH2
• FADH2 then tries to oxidize back into FAD by passing its
electrons to 2Fe3+, which is reduced to 2Fe2+ (Like in Complex 1)
2Fe3+ + 2e-  2Fe2+
Complex II Continued...
• These electrons from 2Fe2+ are then stolen by ubiquinone (Q),
which carries them to complex III.
Q + 2Fe2+  2Fe3+ + QH2
• *Unlike complex I, complex II doesn’t have enough free energy
for active transport of the hydrogen ions (protons) from
FADH2 across the intermembrane . This is why oxidation of
FADH2 only yields 2 ATP molecules instead of 3 ATP molecules
like NADH.
Complex III: cytochrome b-c1
• Contains cytochrome b, cytochrome c1, and FeS proteins.
• QH2 passes two electrons to cytochrome b causing Fe3+ to
reduce to Fe2+
• Same way from cyt b to FeS protein and then to cyt c1
Cytochrome C (c)
• I’m a protein and like Q...
• I am also a water soluble mobile electron
carrier. I transport electrons one at a time
from complex III to complex IV.
Complex IV: Cytochrome Oxidase
(The End of the Line)
• This is the end of the line for electrons from NADH or succinate.
• Oxygen (breathed in) is reduced and when combined with these
electrons, they form...
WATER
½ O2 (g) + 2H+ + 2e-  H2O (l)
THIS REACTION IS EXPLOSIVE! It’s highly exergonic, releasing large
amounts of energy.
Electrochemical gradient
• A concentration gradient created by pumping ions into a
space surrounded by a membrane that is impermeable
to the ions.
• Two components: electrical and chemical
• Proton-motive force (PMF) 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.
Chemiosmosis
• Peter Mitchell – Nobel Prize in Chemistry in 1978
• A process for synthesizing ATP using the energy of an
electrochemical gradient and the ATP synthase enzyme.
• “osmosis”
• After chemiosmosis, ATP molecules are transported
through both mitochondrial membranes.
Connections!
• Electrons from NADH and FADH2 were passed down the
ETC
• As the electrons move down, energy released moves
protons to create electrochemical gradient
• Protons move through proton channels, and release
energy to synthesize ATP from ADP and Pi
• The many processes of ATP synthesis are all continuous
Respiration
• C6H12O6 + 6O2  6H2O + 6CO2
• The oxygen is necessary to take away the electrons from
the ETC so the new electrons can keep flowing. Only
oxygen is electronegative enough
• The ETC cannot get clogged up by electrons
• Oxygen takes two electrons and two protons from the
matrix, and forms water
• Glucose provides electrons and energy in its bonds for
the body.
Oxidative Phosphorylation
• Each step of ATP synthesis is completely dependent on its
previous step
• Glycolysis  Pyruvate Oxidation  The Krebs Cycle 
Electron Transport Chain and Chemiosmosis
• Each step depends on the step before for the correct
molecules and energy to work
Bibliography
• Highered Mcgraw Hill. Electron Transport System and ATP Synthesis.
October 26, 2011. <http://highered.mcgrawhill.com/olcweb
/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/1200
71/bio11.swf::Electron%20Transport%20System%20and%20ATP%20S
ynthesis>
• Interactive Animations. Oxidative Phosphorylation. October 26, 2011.
<http://www.wiley.com/college/boyer/0470003790/animations/elec
tron_transport/electron_ transport.htm>
• Leeds Lecture. Electron Transport Chain. October 26, 2011.
<http://www.dentistry.leeds.ac.uk/biochem/lecture/etran
/etran.htm>
• Nelson Biology 12. Chapter 2 Cellular Respiration. Pages 103-108.
• Thomson Brooks/Cole. The Mitochondrion. October 26, 2011.
<http://www.brookscole.com/chemistry_d/templates/student_resou
rces/shared_resources/animations/oxidative/oxidativephosphorylatio
n.html>
Bibliography continued…
• Brooks, Dr. S. J.. "Electron Transport Chain." University of Leeds. N.p., n.d.
Web. 19 Oct. 2011. <www.dentistry.leeds.ac.uk/biochem/lecture
/etran/etran.htm>.
• Carlson, Rebecca. "Hey Soul Sister/Electron Transport Chain." YouTube. N.p.,
n.d. Web. 18 Oct. 2011. <http://www.youtube.com/watch?v
=DNReloT3QYU>.
• Cengage. "Oxidative Phosphorylation." Thomson Books. N.p., n.d.
Web. 18 Oct. 2011. <www.brookscole.com/chemistry_d/templates
/student_resources/shared_resources/animations/oxidative/oxidativ
ephosphorylation.html>.
• Khan Academy. "Electron Transport Chain." YouTube. N.p., n.d. Web. 18 Oct.
2011. <http://www.youtube.com/watch?v=mfgCcFXUZRk>.
• Wiley, John. "Concepts in Biochemistry - Interactive Animations." Wiley
College . N.p., n.d. Web. 19 Oct. 2011.
<http://www.wiley.com/college/boyer/0470003790/animations/elec
tron_transport/electron_transport.htm>.