Test and analysis: (1) protein is an important raw material for the construction and repair of human body, and the growth and development of human body and the repair and renewal of damaged cells are inseparable from protein. In addition, protein can be decomposed to provide energy for human life activities. Teenagers are in a critical period of growth and development, and there is a great demand for protein. Milk is rich in protein, which is indispensable for the growth and development of teenagers and the repair and renewal of damaged cells.
(2) Bread is rich in starch; As can be seen from the curve A shown on the right, the digestion of starch starts from the mouth of A. Under the action of salivary amylase, starch is decomposed into maltose, and then reaches the small intestine, where it is completely decomposed into glucose by intestinal juice and pancreatic juice.
(3) The digestive tract represented by D in the figure is the small intestine, which is the main place for human body to digest food and absorb nutrition. The structural characteristics of small intestine's adaptation to digestion and absorption are as follows: ① small intestine is long; ② There are annular folds on the inner surface, and there are small intestinal villi on the folds, which greatly increases the inner surface area; ③ There are many kinds of digestive juices, including bile, pancreatic juice and intestinal juice; ④ The villi are thin and consist of only one layer of epithelial cells. Villi are rich in capillaries and lymphatic capillaries.