However, the method the glucose enters the yeast enters the cell is by facillitated diffusion and this requires carrier proteins. When glucose is being absorbed, there will be a point where the rate reaches it's maximum, i.e. all the carrier proteins are being used. When sucrose is the substrate, it is split into glucose and fructose. When all the carrier proteins are used transporting glucose, different proteins are used for fructose (due to the tertiary structure of the protein) so the composite monosaccharides of sucrose can enter the yeast at a higher rate. Hence the rate is faster.