Pathways that reduce NAD+ to NADH
• How do cells transfer electrons from food to
NAD+ to make NADH?
– From sugars
– From proteins
– From fatty acids
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Oxidation of glucose to CO2
Glycolysis
Pyruvate
oxidation
Citric acid cycle
http://www.biologie.uni-hamburg.de/b-online/e19/19d.htm
Glycolysis: partial oxidation of glucose to
pyruvate
Glycolysis – in cytoplasm of all cells
1. Uses 2 ATPs to “activate” glucose
2. Splits glucose into two 3-carbon sugars
3. Partially oxidizes the sugars, yielding 2
NADH per glucose
4. Substrate-level phosphorylation of ADP to
make 4 molecules of ATP
5. Nets 2 pyruvates, 2 ATP, 2 NADH
In respiring cells, pyruvate is oxidized to
CO2 + acetyl CoA
http://www.micro.siu.edu/micr201/chapter8N.html
Two molecules of pyruvate are oxidized to 2 acetyl CoA +
2CO2, generating 2 NADH per molecule of glucose.
Citric acid cycle generates ATP, reduced
electron carriers and oxidizes acetate to CO2
Campbell & Reece Biology 8th ed.
Fats and proteins
http://www.ruf.rice.edu/~bioslabs/studies/
mitochondria/mitokrebs.html
http://www.life.umd.edu/classroom/bsci424/BS
CI223WebSiteFiles/GeneralCatabolism.gif
Locations of eukaryotic energy pathways
Campbell & Reece Biology 8th ed.
Respiring cells make most of their ATP via
oxidative phosphorylation. What happens in the
absence of respiration (no electron transport
chain function)?
Fermentation reduces
pyruvate to lactic acid or
ethanol, to regenerate
NAD+ from NADH
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
In the absence of respiration
• Cells can make ATP through glycolysis and
fermentation of glucose
• Cells cannot run pyruvate oxidation or citric
acid cycle (no mitochondrial function in
eukaryotes)