Starch or amylum is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as energy storage. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, wheat, maize (corn), rice, and cassava. When starch is heated with water, granules absorb the water, and swell. Eventually they burst and the inner part of the granule spills out to form a thick gel. This is what occurs when you make a gravy or sauce. This process is called gelatinisation. In bread making not as much water is added as when making a sauce or gravy, and gelatinisation isn't completed - the starch granules swell, and many don't burst to form a gel. This forms a network of bloated starch granules all touching at the edges. Starch also interacts with gluten during baking. The gluten breaks down and gives up water which is quickly taken up by the starch. This makes the gluten set and become rigid, which is why our loaves of bread don't collapse when they come out of the oven.