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Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways
All organisms produce ATP by releasing energy stored in glucose and other sugars.
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Plants make ATP during photosynthesis.
All other organisms, including plants, must produce ATP by breaking down
molecules such as glucose
Aerobic respiration - the process by which a cell uses O2 to "burn" molecules and
release energy
This reaction takes place over the course of three major reaction pathways
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Glycolysis
The Kreb's Cycle (TCA cycle)
Electron Transport Chain
Glycolysis (glyco = sugar; lysis = breaking)
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Goal: break glucose down to form two pyruvates
Where: the cytoplasm
Glycolysis produces 4 ATP's and 2 NADH, but uses 2 ATP's in the process for
a net of 2 ATP and 2 NADH
NOTE: this process does not require O2 and does not yield much energy
The First Stage of Glycolysis
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Glucose (6C) is broken down into 2 (3Carbon molecules) (glyceraldehyde -3phosphate) This requires two ATP's
The Second Stage of Glycolysis
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2 PGAL's (3C) are converted to 2 pyruvates
This creates 4 ATP's and 2 NADH's
The net ATP production of Glycolysis is 2 ATP's
Krebs's Cycle (citric acid cycle, TCA cycle)
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Goal: take pyruvate and put it into the Krebs's cycle, producing NADH and
FADH2
Where: the mitochondria
There are two steps
o The Conversion of Pyruvate to Acetyl CoA
o The Kreb's Cycle proper
In the Krebs's cycle, all of Carbons, Hydrogens, and Oxygen in pyruvate end
up as CO2 and H2O
The Krebs's cycle produces 2 ATP's, 8 NADH's, and 2FADH2's per glucose
molecule
The Conversion of Pyruvate to Acetyl CoA for Entry Into the Kreb's Cycle
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2 NADH's are generated
2 CO2 are released
The Kreb's Cycle
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6 NADH's are generated
2 FADH2 is generated
2 ATP are generated
4 CO2's are released
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Therfore, for each glucose molecule that enters into the Kreb's cycle (including
the prepatory conversion to Acetyl CoA), the net production of products are:
o 8 NADH
o 2 FADH2
o 2 ATP
o 6 CO2
Electron Transport Chain
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Goal: to break down NADH and FADH2, pumping H+ into the outer
compartment of the mitochondria
Where: the mitochondria
In this reaction, the ETS creates a gradient which is used to produce ATP
Electron Transport Chain typically produces 32 ATP's
Net Energy Production from Aerobic Respiration
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Glycolysis: 2 ATP
Kreb's Cycle: 2 ATP
Electron Transport Phosphorylation: 32 ATP
Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) - the
NADH is worth 3 ATP, but it costs an ATP to transport the NADH into
the mitochondria, so there is a net gain of 2 ATP for each NADH
produced in gylcolysis
o Each NADH produced in the conversion of pyruvate to acetyl COA and
Kreb's Cycle is worth 3 ATP (8 x 3 = 24)
o Each FADH2 is worth 2 ATP (2 x 2 = 4)
o 4 + 24 + 4 = 32
Net Energy Production: 36 ATP!
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Anaerobic Respiration
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Goal: to reduce pyruvate, thus generating NAD+
Where: the cytoplasm
Why: in the absence of oxygen, it is the only way to generate NAD+ and ADP
Lactic Acid Fermentation o The product of Lactic Acid fermentation, lactic acid
o This is the "burn" felt when undergoing strenuous activity
The only goal of fermentation reactions is to convert NADH to NAD+ (to use in
glycolysis).
No energy is gained
Note differences - fermentation - 2 ATP's produced, aerobic respiration - 36
ATP's produced
Related Catabolic Processes - Beta Oxydation
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Fats consist of a glycerol backbone three fatty acids connected to it
The body absorbs fats and then breaks off the fatty acids from the glycerol
Glycerol is converted to glyceraldehyde phosphate, an intermediate of
glycolysis
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The fatty acids are broken down into two-carbon units which are then
converted to acetyl CoA.
o An eight-carbon fatty acid can produce 4 acetyl CoA's
o Each acetyl CoA is worth 12 ATP's (3 NADP, 1 FADH2, 1 ATP)
o Therefore, this short fatty acid is worth 48 ATP's, a fat with three chains
of this length would be worth 144 ATP's!
o This is why fats are such a good source of energy, and are bad if you
want to lose weight