3. link and krebs cycle
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The Link Reaction
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If oxygen is available energy can be released from pyruvate via
the Krebs Cycle and oxidative phosphorylation
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Pyruvate from glycolysis can move into the matrix of the mitochondrion
Pyruvate must cross the outer and inner membranesThis is done by active transport which requires energy
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Pyruvate must be converted to Acetyl Coenzyme A (Acetyl CoA)
This used Coenzyme A
Pyruvate + NAD + CoA Acetyl CoA + NADreduced + CO 2
NAD is reduced and CO2 is made
Remember this happens twice for each glucose molecule
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3Carbon pyruvate
Pyruvate + NAD + CoA Acetyl CoA + NADreduced + CO2
Coenzyme A NAD
CO2 Acetyl CoAIn effect this is a 2Carbon acetyl moleculeThese 2 carbons will enter the Krebs Cycle
H
NADreduced
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Remember this process happens twice per glucose
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The Krebs Cycle
Also calledCitric Acid Cycle and the Tricarboxylic Acid Cycle
Described by Hans Krebs in 1937
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The cycle is a series of enzyme controlled reactions:
In Step 1 - Citrate (6C) is formed from acetyl CoA + oxaloacetate(4C)
In Steps 2-8 Citrate is decarboxylated to yield CO2 gas and dehydrogenated to release Hydrogen ions
these will reduce NAD and FAD
Oxaloacetate is regeneratedOne ATP is formed by phosphorylation
The Krebs Cycle will go around twice for each molecule of glucose
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This is a cycle but for convenience we will start with 4-carbon oxaloacetate This combines with the 2C acetyl group to form 6-carbon Citrate
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This is what oxaloacetate looks like There are 4 carbons
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This is the structure of citric acid (citrate)
Note it has 6 carbons
There are 3 carboxylic groups so it used to be called tricarboxyilic acid TCA
This is the first step in the Krebs cycle so that is why it is also called the citric acid or tricarboxylic acid cycle
Citric acid is what makes citrus fruit acidicIt is also used as a food additive
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In a series of steps citric acid is dehydrogenated (hydrogen is removed and transferred to NAD)And decarboxylated(A carbon and 2 oxygens are removed)
This leaves a 5C cpd
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This happens again to reduce another NAD and release another CO2
Leaving a 4C cpd
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In a further series of reactions more hydrogens are removed and used to reduce NAD
ATP is made by substrate level phosphorylation
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(6C)
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Products of the Krebs cycle per glucose molecule
4CO2
2ATP2FAD red6NAD red
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NAD reduced is a hydrogen carrier moleculeIt will carry hydrogen to the electron transfer chain where ATP will be madeThe more hydrogens available the more energy in the form of ATP can be made
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FAD is also simply a hydrogen carrier moleculeEach molecule of glucose will only make 2 FAD reduced
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ATP NAD red
FAD CO2
Glycolysis
2 2 - -
Link - 2(1) - 2(1)
Krebs cycle
2(1) 2(3) 2(1) 2(2)
Total 4 10 2 6
Where it goes next
used To ETC To ETC Diffuses out
Summary so far…(per glucose molecule)