(a) : Yeast species are used in alcoholic fermentation due to the presence of zymase enzyme. It was known that the yeast extract contained an enzyme zymase, which is nondialyzable and a coenzyme which is dialyzable. It is now well known that the zymase is a complex mixture of many enzymes and that several coezymes are necessary for their function. The activity of this enzyme was lost because the main enzyme was separated from its coezyme during dialyzation. Thus, it was established later that extracellular enzyme zymase, secreted by yeast cells, carry out the process of fermentation.