I wasn't drinking it while in class. I can think of no better way to relax after an exam, than with a home brew.
Also, as has been mentioned above, dextrin (glucose) is a monosaccharide. Sucrose, is a disaccharide of glucose and fructose. Yeast convert the glucose to ethanol (fermentation), going through fructose (an intermediate molecule in the pathway). So, it seems that using table sugar should increase carbonation. That makes sense why you guys said to cut back on the amount used.
The Lemon juice (citric acid) lowers the pH, causing the sucrose to break up into glucose and fructose, speeding up the whole process.
Here is a map of the biochemical pathways of yeast. The fermentation process is on the right hand side.