All food undergoes both mechanical and chemical digestion. The type of digestion that occurs is not dependent on food-type. Instead, the type of digestion that occurs is dependent on the location within the digestive system at which the digestion is occurring. Mechanical digestion involves the physical breaking down of food. This occurs in the mouth with the grinding of the teeth. Chemical digestion involves the chemical breaking down of food into smaller particles that can be absorbed and used by cells. Chemical digestion begins in the mouth as food mixes with amylase of saliva. Amylase is an enzyme that breaks down starches (such as bread) into sugars. Throughout the digestive system there are other enzymes that help to break down specific biomolecules. For example, peptidase helps to break down proteins (such as the turkey) into amino acids. Lipase helps to break down lipids (fats, such as the butter).
The digestive system is unable to digest fiber (such as the roughage of the lettuce). However, lettuce also contains carbohydrates, which would have been broken down via chemical digestion.