chapt07 lecture 8
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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CHAPTER 7
LECTURE
SLIDES
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Cellular respiration
Cellular respiration is a series of reactions
Oxidationsloss of electrons
Dehydrogenationslost electrons areaccompanied by protons
A hydrogen atom is lost (1 electron, 1 proton)
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Redox
During redox reactions, electrons carry
energy from one molecule to another
Nicotinamide adenosine dinucleotide
(NAD+)
An electron carrier
NAD+accepts 2 electrons and 1 proton to
become NADH
Reaction is reversible
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Aerobic respiration
Final electron receptor is oxygen (O2)
Anaerobic respiration
Final electron acceptor is an inorganicmolecule (not O2)
Fermentation
Final electron acceptor is an organic molecule
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Aerobic respiration
C6H12O6+ 6O2 6CO2 + 6H2O
DG = -686kcal/mol of glucose
DG can be even higher than this in a cell
This large amount of energy must bereleased in small steps rather than all at
once.
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Electron carriers
Many types of carriers used
Soluble, membrane-bound, move within
membrane
All carriers can be easily oxidized and
reduced
Some carry just electrons, some electrons
and protons
NAD+ acquires 2 electrons and a proton to
become NADH10
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ATP
Cells use ATP to drive endergonic
reactions
G = -7.3 kcal/mol
2 mechanisms for synthesis
1. Substrate-level phosphorylation
Transfer phosphate group directly to ADP
During glycolysis
2. Oxidative phosphorylation
ATP synthase uses energy from a proton gradient
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Oxidation of Glucose
The complete oxidation of glucose proceeds
in stages:
1. Glycolysis
2. Pyruvate oxidation
3. Krebs cycle
4. Electron transport chain & chemiosmosis
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Glycolysis
Converts 1 glucose (6 carbons) to 2
pyruvate (3 carbons)
10-step biochemical pathway
Occurs in the cytoplasm
Net production of 2 ATP molecules by
substrate-level phosphorylation 2 NADH produced by the reduction of
NAD+
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NADH must be recycled
For glycolysis to continue, NADH must berecycled to NAD+by either:
1.Aerobic respiration
Oxygen is available as the final electronacceptor
Produces significant amount of ATP
2.Fermentation Occurs when oxygen is not available Organic molecule is the final electron
acceptor
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Fate of pyruvate
Depends on oxygen availability.
When oxygen is present, pyruvate is oxidized
to acetyl-CoA which enters the Krebs cycle
Aerobic respiration
Without oxygen, pyruvate is reduced in order
to oxidize NADH back to NAD+
Fermentation
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Pyruvate Oxidation
In the presence of oxygen, pyruvate is
oxidized
Occurs in the mitochondria in eukaryotes
multienzyme complex called pyruvate
dehydrogenase catalyzes the reaction
Occurs at the plasma membrane in
prokaryotes
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For each 3 carbon pyruvate molecule:
1 CO2 Decarboxylation by pyruvate dehydrogenase
1 NADH 1 acetyl-CoA which consists of 2 carbons from
pyruvate attached to coenzyme A Acetyl-CoA proceeds to the Krebs cycle
Products of pyruvate oxidation
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Krebs Cycle
Oxidizes the acetyl group from pyruvate
Occurs in the matrix of the mitochondria
Biochemical pathway of 9 steps in threesegments
1. Acetyl-CoA + oxaloacetate citrate
2. Citrate rearrangement and decarboxylation
3. Regeneration of oxaloacetate
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Krebs Cycle
For each Acetyl-CoA entering:
Release 2 molecules of CO2
Reduce 3 NAD+to 3 NADH
Reduce 1 FAD (electron carrier) to FADH2
Produce 1 ATP
Regenerate oxaloacetate
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At this point
Glucose has been oxidized to:
6 CO2
4 ATP
10 NADH
2 FADH2
Electron transfer has released 53kcal/mol
of energy by gradual energy extraction
Energy will be put to use to manufacture
ATP
These electron carriers proceedto the electron transport chain
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Electron Transport Chain
ETC is a series of membrane-boundelectron carriers
Embedded in the inner mitochondrial
membrane Electrons from NADH and FADH2are
transferred to complexes of the ETC
Each complexA proton pump creating proton gradient
Transfers electrons to next carrier
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Chemiosmosis
Accumulation of protons in the
intermembrane space drives protons into
the matrix via diffusion
Membrane relatively impermeable to ions
Most protons can only reenter matrix
through ATP synthase
Uses energy of gradient to make ATP from
ADP + Pi
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Energy Yield of Respiration
Theoretical energy yield
38 ATP per glucose for bacteria
36 ATP per glucose for eukaryotes
Actual energy yield
30 ATP per glucose for eukaryotes
Reduced yield is due to
Leaky inner membrane
Use of the proton gradient for purposes other than
ATP synthesis
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Regulation of Respiration
Example of feedback inhibition
2 key control points
1. In glycolysis
Phosphofructokinase is allosterically inhibited by
ATP and/or citrate
2. In pyruvate oxidation
Pyruvate dehydrogenase inhibited by high levels ofNADH
Citrate synthetase inhibited by high levels of ATP
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Oxidation Without O2
1. Anaerobic respiration
Use of inorganic molecules (other than O2) as
final electron acceptor
Many prokaryotes use sulfur, nitrate, carbondioxide or even inorganic metals
2. Fermentation
Use of organic molecules as final electronacceptor
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Anaerobic respiration
Methanogens
CO2is reduced to CH4(methane)
Found in diverse organisms including cows
Sulfur bacteria
Inorganic sulphate (SO4) is reduced to
hydrogen sulfide (H2S)
Early sulfate reducers set the stage forevolution of photosynthesis
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Fermentation
Reduces organic molecules in order to
regenerate NAD+
1.Ethanol fermentation occurs in yeast
CO2, ethanol, and NAD+are produced
2.Lactic acid fermentation
Occurs in animal cells (especially muscles)
Electrons are transferred from NADH to
pyruvate to produce lactic acid
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Catabolism of Protein
Amino acids undergo deamination toremove the amino group
Remainder of the amino acid is converted
to a molecule that enters glycolysis or theKrebs cycle
Alanine is converted to pyruvate
Aspartate is converted to oxaloacetate
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Catabolism of Fat
Fats are broken down to fatty acids and
glycerol
Fatty acids are converted to acetyl groups by
b-oxidation
Oxygen-dependent process
The respiration of a 6-carbon fatty acid
yields 20% more energy than 6-carbonglucose.
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Evolution of Metabolism
Hypothetical timeline1. Ability to store chemical energy in ATP
2. Evolution of glycolysis
Pathway found in all living organisms3. Anaerobic photosynthesis (using H2S)
4. Use of H2O in photosynthesis (not H2S) Begins permanent change in Earths atmosphere
5. Evolution of nitrogen fixation
6. Aerobic respiration evolved most recently
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