Yeast is a microorganism and doesn\'t itself rise. Yeast eats carbohydrate and makes Carbon Dioxide (CO2) - the gas in soda water - as a waste product and it is this gas that makes the bread or cake \"rise\". Solution Yeast is a microorganism and doesn\'t itself rise. Yeast eats carbohydrate and makes Carbon Dioxide (CO2) - the gas in soda water - as a waste product and it is this gas that makes the bread or cake \"rise\"..