Hello there,

Well you are correct about glucose being a monosaccharide and both sucrose and lactose being disaccharides (requiring sucrose phosphorylase and beta-galactosidase to respectively release monosaccharide sugars). You have in fact already hit on the answer.

If you were measuring the initial rate of CO₂ evolution, you might expect glucose to evolve more given that there will be a metabolic lag in the processing of the other two sugars - though you'd need some pretty sensitive equipment. However, assuming you have the same number of sugar molecules dissolved in each solution, you would eventually end up with roughly the same volume of CO₂ in each case.

You need to think of your chemistry a little more. Think about the scenario of dissolving say 4 g of each sugar in a equal volume of water. The molecular weight of glucose is 180, which means if 180 g were dissolved in 1 litre of water, you would have a 1 molar solution. Now, in that 180 g we have approximately 6 x 10²³ molecules (Avogadro's Number).

Now sucrose is quite a bit heavier than glucose, with a molecular weight of 342. So we would need 342 g to achieve the same number of 6 x 10²³ molecules. Likewise for lactose (MW 360).

So bringing this back to your work, and those 4 g you have just theoretically dissolved in water. Can you now see why 4 g of glucose would give you more CO₂ than the other two?

Hope this helps,