The science of making bread

• When supplied with water and sugar, but no oxygen, yeast reproduce and carry out anaerobic respiration (fermentation) to produce carbon dioxide, ethanol and a little energy.
• Yeast obtains the nutrients by extra-cellular digestion. Enzymes released from yeast into the food source digest the sugar releasing glucose. The smaller glucose is absorbed into the yeast.
• An experiment was done to determine the best (optimum) temperature for anaerobic respiration using yeast mixed with sugar and water in a bottle. Foam is produced due to the carbon dioxide and the level was highest a 40 degrees showing the most carbon dioxide produced. That is the optimum temperature.
• When bread is being made the yeast, sugar, water mixture is warmed so the rate of anaerobic respiration increases quickly reducing the time taken to make bread. 
• When yeast is stored, it should be at 20 degrees so yeast respire slowly. The container yeast is stored should be sealed to keep out moisture. With no water the glucose and enzymes can't move and collide so no fermentation occurs.
• If respiration is high when stored, the yeast can die from a lack of food. The low temperature of  a fridge can also slow respiration so no energy is available for reproduction which increases the time yeast remains alive in storage.