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Week 4
Mitochondrion
• See Figure 13-7
• Double membrane organelle
• Outer membrane
– large pores (channel-forming proteins permeable to molecules
<MW=5000)
– lipid metabolism
• Inner membrane
– convoluted to increase surface area
– functions: electron transport chain; maintain
proton gradient; transport proteins
Mitochondrion
• Intermembrane space
– enzymes to phosphorylate nucleotides using
ATP
• Matrix
– contains Kreb’s cycle (TCA or citric acid cycle)
enzymes; fatty acid oxidation enzymes
Endosymbiont theory
• Theorizes about the origin of mitochondria
and chloroplasts
• Mitochondrion origins - aerobic,
heterotrophic bacteria
• Evidence:
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RNA
Ribosomes
antibiotic sensitivity
DNA
ATP formation
• Driving force- electrochemical proton
gradient = “proton-motive force”
• Electrochemical gradient is made up of
membrane potential + proton gradient (syn.
pH gradient)
• see Figure 13-12
Membrane potential
• See Figure 13-2
• = matrix side negative and intermembrane
space positive due to protons
Proton gradient
• See Figure 13-12
• = difference in proton concentration across
inner membrane
– pH of intermembrane space = 7
– pH of matrix = 8
• Remember - membranes are impermeable to
protons
ATP synthase
• See Figures 13-14 & 15
• Two functions
– ATP synthesis - synthesizes ATP using
oxidative phosphorylation
– ATP hydrolysis - pumps protons from matrix
into intermembrane space to maintain
electrochemical proton gradient
Chemiosmotic coupling theory
• Peter Mitchell, 1968
• theory: couple electron transport and proton
pumping with ATP synthesis
• electron transport generates membrane
potential plus proton gradient (=proton
motive force)
• proton motive force drive ATP synthesis
Other Roles
• See Figure 13-16
• Proton gradient
– pyruvate and phosphate import into matrix
• Membrane potential
– ADP-ATP exchange
High energy electrons
• Source
– oxidation of organic compounds all
heterotrophic organisms
– light reactions in photosynthesis in
photosynthetic eukaryotes and prokaryotes
– oxidation of reduced inorganic compounds
(e.g., NH4+ or H2S) in some prokaryotes
• Carrier: NAD+ & FAD (see Figure 13-8)
– NAD+ + 2e- + H+ ---> NADH
Electron transport chain
• See Figure 13-10
• Syn. Respiratory chain
• embedded in inner membrane of
mitochondrion
• composed of 3 large complexes
– NADH dehydrogenase
– cytochrome b-c1
– cytochrome oxidase
Electron transport chain
• Begins with reduced NADH donating
electron pair to NADH dehydrogenase
complex
• Electrons move from NADH
dehydrogenase to cytochrome b-c1 to
cytochrome oxidase to finally O2 to form
water.
– Why do electrons move in this fashion?
– What drives protons out of matrix into
intermembrane space?
Why do electrons move in this
fashion?
• Depends on the relative affinity of the
complexes for electrons.
• Relative affinity is measured by the redox
potential (E’o; mVolts) of the complex
– negative potential = good electron donor
– positive potential = good electron acceptor
– this is all relative, e.g., -320 mV (good donor)
vs -250 mV (good acceptor)
• See panel 13-1
Redox potentials
• NADH; - 320 mVolts
• O2; +820 mVolts
• electrons move from molecules with more
negative redox potentials to more positive
redox potentials
What drives protons out of matrix
into intermembrane space?
• Energy released as electron moves down the
electron transport chain.
• Quantify the energy
 DGo = - n (0.023) (DE’o); n = number of
electrons used; DE’o = difference in redox
potential between oxidizing agent and reducing
agent (see below)
Calculating DGo
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NADH + O2 <----> NAD+ + H2O
NAD+ + 2 e- + H+ ---> NADH (-320mV)
O2 + 2e- + 2H+ ---> H2O (+820mV)
Combine above half reactions:
– NADH + O2 + 2e- + 2H+ ---> NAD+ + 2 e- + H+ + H2O
– Cancel to get : NADH + O2 + H+ --> NAD+ + H2O
 DGo = - n (0.023) (DE’o)
Calculating DGo
 DGo = - n (0.023) (DE’o)
 DE’o = E’o oxidizing agent - E’o reducing
agent
 DE’o = 820 - (-) 320 mV = 1140 mV
 DGo = - 2 (0.023) (1140mV)
 DGo = - 52.4 kcal / mole
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