1. 22.5 Pathways for Pyruvate
Lactate is transported to the liver, where it is converted back into pyruvate. After vigorous exercise, rapid breathing helps to repay the oxygen debt incurred during exercise.
Learning Goal Give the conditions for the conversion of pyruvate to lactate, ethanol, and acetyl coenzyme A.

2. Pathways for Pyruvate
The pyruvate produced from glucose can now enter pathways that continue to extract energy.
The available pathway depends on whether there is sufficient oxygen in the cell.
Under aerobic conditions, oxygen is available to convert pyruvate to acetyl coenzyme A (acetyl CoA) and CO2.
When oxygen levels are low, pyruvate is reduced to lactate.

3. Pyruvate: Aerobic Conditions
Under aerobic conditions (oxygen present), pyruvate
moves from the cytosol into the mitochondria to be oxidized further.
is oxidized when a carbon atom is removed as CO2 as the coenzyme NAD+ is reduced.
The resulting two-carbon acetyl group is attached to CoA, producing acetyl CoA, an important intermediate in many metabolic pathways.

4. Pyruvate: Anaerobic Conditions
When we engage in exercise, oxygen in our muscle cells is depleted. A person breathes rapidly to repay the oxygen debt incurred.
Under anaerobic conditions (without oxygen),
pyruvate is reduced to lactate and NAD+ by lactate dehydrogenase.
NAD+ is used to oxidize glyceraldehyde-3-phosphate in the glycolysis pathway, producing a small amount of ATP.

5. Pyruvate: Anaerobic Conditions
The accumulation of lactate causes the muscles to tire and become sore. Lactate is subsequently transported to the liver and converted back to pyruvate. Under anaerobic conditions, the only ATP production in glycolysis occurs during the steps that phosphorylate ADP directly, giving a net gain of only two ATP.

6. Anaerobic versus Aerobic Conditions

7. Fermentation
Some microorganisms, particularly yeast, convert sugars to ethanol by fermentation. Fermentation
occurs in anaerobic microorganisms, such as yeast.
decarboxylates pyruvate to acetaldehyde, which is then reduced to ethanol, regenerating NAD+.

8. Fermentation, Ethanol Production
During fermentation, enzymes in the yeast convert sugars to glucose and then to ethanol and CO2 gas.
The fermentation of
barley malt produces beer.
sugars in grapes produces
wine and champagne.
potatoes or grain produces vodka.
rice produces sake.
corn or rye produces whiskeys.
Beer is produced by the fermentation of pyruvate from barley malt, which gives carbon dioxide and ethanol.

9. Study Check
Match the following terms with their descriptions. 1) catabolic reactions 2) coenzymes 3) glycolysis 4) lactate A. produced during anaerobic conditions B. reaction series that converts glucose to pyruvate C. metabolic reactions that break down large molecules to smaller molecules + energy D. substances that remove or add H atoms in oxidation and reduction reactions

10. Solution
Match the following terms with their descriptions. 1) catabolic reactions 2) coenzymes 3) glycolysis 4) lactate A. produced during anaerobic conditions 4) lactate B. reaction series that converts glucose to 3) glycolysis pyruvate C. metabolic reactions that break down large 1) catabolic molecules to smaller molecules + energy reactions D. substances that remove or add H atoms in 2) coenzymes oxidation and reduction reactions