The free energy for the oxidation of glucose to CO2 and water is -686 kcal/mol and the free energy for the reduction of NAD+ to NADH is +53 kcal/mol. Why are only two molecules of NADH formed during glycolysis when it appears that as many as a dozen could be formed?

Cellular respiration is a process that breaks down glucose to produce energy. Fermentation is a metabolic process that converts carbohydrates into acids or alcohols.  Here are some differences between cellular respiration and fermentation: Inputs: Cellular respiration uses glucose and oxygen as inputs. Fermentation uses carbohydrates as inputs. Outputs: Cellular respiration produces carbon dioxide, water, and energy (ATP) as outputs. Fermentation produces small, reduced organic molecules as waste. Complete oxidation: Cellular respiration completely oxidizes glucose into CO2 and water.... Show more

Cellular respiration is a process that breaks down glucose to produce energy. Fermentation is a metabolic process that converts carbohydrates into acids or alcohols. 

Here are some differences between cellular respiration and fermentation:
Inputs: Cellular respiration uses glucose and oxygen as inputs. Fermentation uses carbohydrates as inputs.
Outputs: Cellular respiration produces carbon dioxide, water, and energy (ATP) as outputs. Fermentation produces small, reduced organic molecules as waste.
Complete oxidation: Cellular respiration completely oxidizes glucose into CO2 and water. Fermentation does not fully oxidize glucose.
Energy release: Cellular respiration releases more energy from glucose than fermentation.
Oxygen: Cellular respiration uses oxygen. Fermentation can occur without oxygen. 

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