Please!!!. explain why measuring carbon dioxide production is a good way to calculate the rate of respiration?

Question

....and what would cause a yeast cell to produce less CO2?

Answer 1

because CO2 output is directly proportional to oxygen intake.. you can get both factors by simply measuring one.  

Yeast cell would reduce its fermentation (ie, less C02) if it didnt have as much to feed on,  Ie. starches.