Enzymes are proteins which catalyse, or speed up, biological reactions. Several enzyme catalysed reactions occur during breadmaking. First, starch has to be broken down into sugar. The sugar then has to be broken down into simple sugars to allow yeast to react with these sugars during the process called fermentation (rising).
Starch belongs to a group of chemical compounds called carbohydrates. Pure dry starch is a white granular powder. Wheat flour contains 70-73% starch and most commonly anywhere between 8 -14.5% protein.
Yeast belongs to the fungi family. It is a very small single cell micro-organism. Like all other fungi it doesn't have the power to produce food by photosynthesis. Instead it ferments carbohydrates (sugars) to produce carbon dioxide and alcohol which gives bread it's texture, colour and aroma.