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Week 8
Aerobic cellular respiration continued
and Membrane structure
Review
• Glycolysis
– products include:
• 2 molecules of pyruvate
• 2 ATPs via substrate level phosphorylation
• 2 NADH (reduced NAD+)
– NAD+ is required to keep glycolysis going.
How is NAD+ regenerated from NADH?
– What is the fate of pyruvate?
Kreb’s cycle
• Syn. Tricarboxylic acid cycle (TCA) and
Citric acid cycle
• Occurs in matrix of mitochondria
• Requires:
– pyruvate
– oxaloacetate from Kreb’s cycle
– indirectly requires oxygen
Kreb’s cycle
• Pyruvate enters mitochondria
• Pyruvate + CoASH --> acetyl-CoA + NADH + CO2
– enzyme: pyruvate dehydrogenase complex
– hydrolysis of citrate-CoA intermediate releases
lots of energy that is used to drive the above
reaction to the right.
– See panel 4-2
Kreb’s cycle
• See panel 4-2
• Key points:
– you need to be able to account for the number
of carbons in each intermediate.
• Important intermediate compounds
– citrate
 a-ketoglutarate
– oxaloacetate
Kreb’s cycle
• Summary of products starting with acetyl
CoA
–
–
–
–
2 CO2 released - complete oxidation of acetate
1 ATP (GTP)
3 NADH
1 FADH2
• What is the fate of NADH and FADH2?
So, What is the fate of NADH
and FADH2?
• Like glycolysis, without NAD+ and FAD,
the Kreb’s cycle would shut down. They
are required cofactors for enzyme activity.
• NADH and FADH2 pass their high energy
electrons to the electron transport chain
embedded in the inner membrane of the
mitochondrion.
Electron transport chain
• The electrons move down the electron
transport chain (see figure 13-9) and
generate a proton motive force.
• Oxygen, indirectly required by the Kreb’s
cycle, is the terminal electron acceptor of
the electron transport chain.
• The proton motive force is the driving force
behind oxidative phosphorylation via ATP
synthase.
Yield of ATP
• ATP yields:
– Aerobic cellular respiration (glycolysis, TCA
cycle, and electron transport chain activities)
yields approximately 30 ATP from glucose
– glycolysis alone yields only 2 ATPs per glucose
Anaerobic conditions
• Exercise physiology
– strenuous exercise causes muscles to be
depleted of oxygen. What happens?
– Lactic acid builds up -burning sensation in
muscle. Why?
– Because the oxygen level has dropped, NADH
and FADH2 are not being oxidized back to
NAD+ and FAD.
Anaerobic cellular physiology
• Oxygen is depleted through normal cellular
respiration during strenuous exercise.
• To oxidize NADH to NAD+ needed for
glycolysis, muscles use pyruvate as a
terminal electron acceptor in what is called
lactic acid fermentation.
• Pyruvate + NADH --> lactic acid + NAD+
• NAD+ can return to function in glycolysis.
Alcohol fermentation
• Yeast can grow either aerobically using
aerobic respiration or anaerobically using
alcohol fermentation.
• Under anaerobic conditions, pyruvate is
oxidized to acetaldehyde + CO2
• Acetaldehyde + NADH --> ethanol + NAD+
• NAD+ can function in glycolysis again.
Comparison of aerobic
respiration and alcohol
fermentation
• Alcohol fermentation yields ATP via
substrate level phosphorylation only during
glycolysis. ONLY 2 ATPs per glucose
yielded
• Aerobic respiration yields ATP via both
substrate level phosphorylation and
oxidative phosphorylation. Up to 30+ ATPs
yielded per glucose!
Anabolic pathways
• A number of intermediates of both
glycolysis and Kreb’s cycle are used as
substrates for synthesis of nucleotides,
amino acids, lipids etc. See Figure 4-18.
Poisons of respiration
• Rotenone:
– blocks NADH dehydrogenase (complex I).
NADH cannot transfer its high energy electrons
to the electron transport chain in the
mitochondria. Therefore the cells cannot
generate a proton motive force and no ATP can
be made by oxidative phosphorylation. The
cells sense death!
Poisons cont
• Dinitrophenol - used as a diet supplement in the
1960s
– Dinitrophenol is called an uncoupler of oxidative
phosphorylation. It makes the inner membrane of
mitochondria permeable to protons and diffuses the
proton gradient. Electrons move through the electron
transport chain and try to make a proton gradient. But
the membrane is permeable to protons which disrupts
the proton motive force and very little ATP is made by
ATP synthase. Instead the energy of the electrons
moving down the electron transport chain is released as
heat.
Poisons cont
• Arsenate
– chemically similar to inorganic phosphate.
– Competes where inorganic phosphate is used to
make ATP via substrate level phosphorylation,
e.g., glycolysis and Kreb’s cycle
Poisons cont
• Cyanide
– interferes with complex III which is the complex that
reduces oxygen to water.
– If this is blocked, than the electron transport chain
would be saturated with electrons. Therefore NADH
and FADH2 would not be able to be oxidized to NAD+
and FAD respectively. Then glycolysis and Kreb’s
cycle would be negatively effected.
Hibernating bears
• Bears have lots of BAT- brown adipose
tissue (fat cells rich with mitochondria)
• During hibernation the fat of these cells is
metabolized to yield heat.
– How? Uncouple ATP synthesis from proton
motive force. Use the energy of the electrons to
generate heat instead of ATP
Membranes
Membranes
• Function:
– selective barrier between cytoplasm and its
contents and outside the cell.
– Composed of phospholipid, other lipid material
and proteins
Phospholipids
• Composed of a phosphate head, glycerol,
and two fatty acids. (see Fig. 11-6)
• Amphipathic molecule
– means that there is a hydrophilic and
hydrophobic region - the phosphate head is
hydrophilic and the fatty acid tails are
hydrophobic
Liposomes
• See Fig. 11-13
• Formed from phospholipids
• Lipid bilayer with an aqueous interior and
exterior environments.
• Energetically favorable structure due to the
amphipathic nature of the phospholipids.
membranes
• Plasma membrane
– circa 50 nm thick lipid bilayer
– separates inside from outside environment of a
cell
– Bacteria have a single plasma membrane
– Eucaryotes have a plasma membrane and
numerous internal membranes and membrane
bound organelles.
Lipid bilayer
• See Fig. 11-4
• Types of lipids
–
–
–
–
See Fig. 11-7
phospholipids
sterols
glycolipids
Lipid bilayer
• Fluid bilayer
• lipids float freely in their plane of the
bilayer
– rarely flip to otherside of bilayer
– enzyme called flipase carries out flips.
• Fluidity function of composition of fatty
acids in phospholipids.
– Short, unsaturated fatty acids increase fluidity
– long, saturated fatty acids decrease fluidity
Saturated vs unsaturated fatty
acids
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - COOH = saturated fatty acid
vs
CH3 - CH2 - CH2 - CH = CH - CH2 - COOH = unsaturated fatty acid
Adaptation of membrane
phospholipids
• As the temperature goes up, some yeast and
bacteria can reduce the number of double
bonds and increase the length of the fatty
acids of their phospholipids.
• Yeast can also include cholesterol in their
membranes to reduce the fluidity as the
temperature goes up.
Lipid bilayer
• See Fig. 11-17
• Asymetrical in composition of
phospholipids
– e.g., glycolipids found on non-cytosolic side of
membrane
– inositol phospholipids found on cytosolic side
of membrane