The initial part of the respiration pathway, Glycolysisconverts Glucose to Pyruvate and then allows the rest of the pathway to occur(either aerobic or anaerobic). This means that whether the yeast respires ornot is determined by the initial step of the pathway (Glycolysis). Simplecarbohydrates, the mono- and disaccharides, generally enter respiration at thebeginning of glycolysis. Disaccharides are first hydrolysed into monomers, andthen each monosaccharide enters the pathway as a reactant for one of thereactions during glycolysis. 7Fructose is “primed” in many cells by phosphorylation andcan enter glycolysis as a reactant, this is similar to glucose as they bothhave 6 carbons (hexose) and are there for able to undergo Lysis and furtherPhosphorylation. In other cells, fructose is split by special enzymes andenters as two three-carbon molecules in a later glycolysis reaction.

7Galactose again a monosaccharide which is able to berespired however differs slightly to the pathway of glucose. Galactosemetabolism is the process where galactose is converted into glucose is done bythree enzymes (Galactose Mutarotase,Galactokinase and Galactose-1-phosphate Uridylyltransferase) in theLeloir pathway and can then be respired normally. 8 As maltose is a s disaccharide made up of two glucose monosaccharide’s,this means that it simply be respired normally however it does initiallyrequire an enzyme called maltose to break it into the two monosaccharide’s. 9To break down sucrose the yeast use an enzyme calledinvertase, which breaks it down outside the yeast cell. 9 Once the glucoseand fructose monosaccharide’s have been obtained, they take their respective routesto be respired (as discussed previously)Lactose (disaccharide) is made up of glucose and Galactosewhich yeast would be able to use for respiration, however yeast does not havean enzyme to break the lactose down. The enzymes that process galactose andglucose are unable to process lactose directly.

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10 Thus meaning it cannot beused.Glucose is the simplest to respire as it does not have to bebroken down to be used during the reaction, however surcose and maltose must bebroken down. Yeast must activate a gene for the production of the enzyme(s) tobreak them down. 11

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