Pathways that reduce NAD+ to NADH How do cells transfer electrons from food to NAD+ to make NADH?...

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

http://www.biologie.uni-hamburg.de/b-online/e19/19d.htm

Glycolysis Pyruvate oxidation

Citric acid cycle

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

Two molecules of pyruvate are oxidized to 2 acetyl CoA + 2CO2, generating 2 NADH per molecule of glucose.

http://www.micro.siu.edu/micr201/chapter8N.html

Citric acid cycle generates ATP, reduced electron carriers and oxidizes acetate to CO2

Campbell & Reece Biology 8th ed.

Fats and proteins

http://www.life.umd.edu/classroom/bsci424/BSCI223WebSiteFiles/GeneralCatabolism.gif

http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/mitokrebs.html

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 2 ATP

Glucose Glycolysis

2 NAD+ 2 NADH

2 Pyruvate

+ 2 H+

2 Acetaldehyde2 Ethanol

(a) Alcohol fermentation

2 ADP + 2 Pi2 ATP

Glucose Glycolysis

2 NAD+ 2 NADH+ 2 H+

2 Pyruvate

2 Lactate

(b) Lactic acid fermentation

2 CO2

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)