8 F. Z a m o r a
consumed previously during the phosphorylation of the hexoses, the net energy gain
for the cell is two ATPs per hexose.
Pyruvate produced by glycolysis can be used by yeasts for several metabolic path-
ways. However, yeasts must regenerate NAD+from the NADH to re-establish the
oxydoreduction potential of the cell. This can be done by fermentation or respiration.
1.4 Fermentation and Respiration
Yeasts are facultative anaerobic microorganisms because they possess the genetic
equipment for metabolizing sugars aerobically or anaerobically (Boulton et al.
1996). Therefore, yeasts can consume sugars using two different metabolic path-
ways: respiration and fermentation (Racker 1974). Figure 1.3 illustrates these bio-
chemical pathways.
P l a s m a M e m b r a n eDihydoxyacetone
phosphate Glyceraldehyde -3-phosphateFermentationGlycerol
PyruvateHexose
transporterD-GlucoseD- FructoseEthanolCO 2EthanalPyruvate
decarboxylasePyruvate dehydrogenaseAcetyl -coANAD+NADH + H+HScoACO 2COOxidized
coenzymesReduced
coenzymesKrebs
CycleRespiratory chainsO 2 H 2 OEnergyEnergyRespirationP l a s m a M e m b r a n eGrape juiceDihydoxyacetone
phosphate Glyceraldehyde-3-phosphateFermentationGlycerol Glycerol
PyruvateD-GlucoseD- FructoseHexose
transporterHexose
transporterD-GlucoseD- FructoseD-GlucoseD- FructoseEthanolCO 2Ethanol EthanalPyruvate
decarboxylasePyruvate dehydrogenaseAcetyl-coANAD+NADH + H+HScoACO 2CO 2 CO 2CO 2Oxidized
coenzymes
Reduced
coenzymesKrebs
CycleKrebs
CycleRespiratory chainsRespiratory chainsO 2 O 2 H 2 OEnergyEnergyRespirationCO 2Fig. 1.3Fermentation and respiration