Aerobic respiration (One form of energy metabolism).
In this respiration, an organic molecule such asglucose is the electron donor and oxygen is the electron acceptor.Oxygen has a very high affinity for electrons, so a great deal ofenergy is released during aerobic respiration. Aerobic respirationis the primary mode of energy metabolism in animals.
Fermentation (common form of energy metabolism) Inthis, organic molecules serve as both electron donors and electronacceptors. Electrons are just moved from one carbon to another ofthe same molecule. Fermentation releases much less energy than respiration, since organic molecules have a lower affinityfor electrons than does oxygen.
Anaerobic conditions, glucose is oxidized to pyruvic acidvia glycolysis ,generating ATP and NADH. The pyruvic acid thenserves as the electron acceptor, producing CO2 andethanol as it re-oxidizes NADH back to NAD. Only two ATP are formedduring fermentation.
Under aerobic conditions, the pyruvic acid movesinto the mitochondria. In the mitochondria, the pyruvic acid isoxidized to acetyl CoA, which enters the tricarboxylic acid cycle(TCA cycle) where it is oxidized to CO2.
Three CO2 are produced for every pyruvate, or sixfor every glucose entering glycolysis. The electrons removed frompyruvic acid during its oxidation to CO2 aretransferred to NAD, reducing it to NADH. The NADH donates itselectrons to the electron transport system, which passes them on toO2 .
Many ATP are formed at the electron transport system.
Yeast obtains 36 moles of ATP per mole of glucose whenrespiring, compared with only two moles of ATP per mole of glucosein fermentation.
.Both processes usually startwith the same first step, glycolysis, but follow differentsubsequent pathways.
Duringfermentation,
1. Releases energy from sugars or other organic molecules,such as amino acids, organic acids, purines,pyrimidines.
Does not require oxygen (but sometimes can occur inits presence);
2. Does not require use of the Krebs cycle or anelectron transport chain .
3. Uses an organic molecule as the final electronacceptor; That produces only small amounts of ATP (only one or twoATP molecules for each molecule of starting material) because muchof the original energy in glucose remains in the chemical bonds ofthe organic end-products, such as lactic acid or ethanol. Duringfermentation, electrons are transferred (along with protons) fromreduced coenzymes (NADH, NADPH) to pyruvic acid or itsderivatives.
Those final electron acceptors are reduced to the end-products. In the process, NAD+ and NADP+ are regenerated andcan enter another round of glycolysis. An essential function of thesecond stage of fermentation is to ensure a steady supply of NAD +and NADP+ so that glycolysis can continue. In fermentation, ATP isgenerated only during glycolysis.
