Yeast cells respire aerobically if there is enough oxygen supply. If there is not enough oxygen present then fermentation will take place as the yeast cells have to respire anaerobically. Fermentation makes energy that is used for cell growth. When yeast cells ferment they also produce ethanol (alcohol) which is important to brewers. The yeast cells need sugar to ferment as without sugar, the yeast cells would not be able to respire at all. The sugar supply provided is called sucrose, Enzymes in the yeast cells break down the sucrose into glucose. Yeast reproduces by budding in other words when the yeast cell grows too big. It splits to make two metical copies of itself. This form of reproduction is called asexual as it only involves one parent cell. Yeast needs amino acids to build into proteins so it cans growth. Anion acids are supplied by malt extract.