Normally you would think that increasing the concentration of glucose
should increase repiration. At low concentrations this is true. However,
the problem with glucose is that it capable of catabolic repression.
Glucose is the preferred carbon source for many strains of yeast, including
baker's yeast, Saccharomyces cerevisiae. At high concentrations, the
glucose causes catabolic repression. In high concentration it has the
ability to repress and regulate a number of metabolic pathways, including
those involved in respiration. Many of the enzymes that act in the
respiration cycles undergo feedback inhibition, by other intermediaries in
these pathways, but also can be regulated by high concentrations of
glucose. So in this case, at high concentrations, glucose actually causes
a reduction in respiration, because it is downregulating either the
production of enzymes involved in respiration or decreasing their activity.