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The Krebs Cycle
9-2
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At the end of glycolysis, about 90% of the chemical energy available in glucose is still unused
To extract the rest, cells need oxygen Powerful electron acceptor Required for final steps of cellular
respiration b/c they require oxygen - aerobic
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Cellular respiration should not be confused with respiration
Cellular respiration is the releasing of energy stored in food
Respiration gives our cells oxygen The oxygen needed to power cellular
respiration
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The Krebs Cycle
In the presence of oxygen, pyruvic acid from glycolysis is passed to the Krebs cycle
Second stage of cellular respiration
Named after Hans Krebs
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During the Krebs cycle - pyruvic acid is broken down into carbon dioxide
Also known as citric acid cycle Citric acid is the first compound formed
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Step A
Begins with pyruvic acid produced by glycolysis entering the mitochondria
1 carbon atom from pyruvic acid becomes part of a CO2 molecule Eventually released into the air
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Remaining 2 carbon atoms from pyruvic acid are joined to a compound called coenzyme A Forms acetyl-CoA Acetyl is made of 2 C atoms, 1 O atom,
and 3 H atoms
Acetyl-CoA adds the 2 carbon acetyl group to a 4-carbon molecule Produces 6-carbon citric acid
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Step B
Citric acid is broken down into a 4-carbon molecule
More CO2 is released
Electrons are transferred to electron carriers NAD+ and FAD
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At each turn of the cycle, a molecule similar to ADP is converted to a molecule similar to ATP
At 4 places in the cycle, a pair of high-energy electrons is accepted by electron carries Changes NAD+ to NADH and FAD to
FADH2
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What happens to the products? CO2 released to atmosphere
ATP produced can be used for cellular activities
High-energy electrons can be used to generate large amounts of ATP in electron transport chain