The initial part of the respiration pathway, Glycolysis
converts Glucose to Pyruvate and then allows the rest of the pathway to occur
(either aerobic or anaerobic). This means that whether the yeast respires or
not is determined by the initial step of the pathway (Glycolysis). Simple
carbohydrates, the mono- and disaccharides, generally enter respiration at the
beginning of glycolysis. Disaccharides are first hydrolysed into monomers, and
then each monosaccharide enters the pathway as a reactant for one of the
reactions during glycolysis. 7

Fructose is “primed” in many cells by phosphorylation and
can enter glycolysis as a reactant, this is similar to glucose as they both
have 6 carbons (hexose) and are there for able to undergo Lysis and further
Phosphorylation. In other cells, fructose is split by special enzymes and
enters as two three-carbon molecules in a later glycolysis reaction. 7

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Galactose again a monosaccharide which is able to be
respired however differs slightly to the pathway of glucose. Galactose
metabolism is the process where galactose is converted into glucose is done by
three enzymes (Galactose Mutarotase,
Galactokinase and Galactose-1-phosphate Uridylyltransferase) in the
Leloir 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 initially
require an enzyme called maltose to break it into the two monosaccharide’s. 9

To break down sucrose the yeast use an enzyme called
invertase, which breaks it down outside the yeast cell. 9 Once the glucose
and fructose monosaccharide’s have been obtained, they take their respective routes
to be respired (as discussed previously)

Lactose (disaccharide) is made up of glucose and Galactose
which yeast would be able to use for respiration, however yeast does not have
an enzyme to break the lactose down. The enzymes that process galactose and
glucose are unable to process lactose directly. 10 Thus meaning it cannot be

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

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