Final answer:
Protein digestion is initiated in the stomach by pepsin, carbohydrate digestion that starts in the mouth is halted in the stomach because amylase is denatured by stomach acid, and fat digestion has not yet begun as it occurs in the small intestine.
Step-by-step explanation:
For a food bolus in the stomach, protein digestion is initiated by pepsin; carbohydrate digestion, which began in the oral cavity, continues until the enzyme responsible is denatured by acid; and the digestion of fats has not begun.
Protein digestion starts in the stomach with the action of the enzyme pepsin, which breaks down proteins into smaller peptides and eventually into amino acids. Carbohydrate digestion begins in the oral cavity where the enzyme amylase begins breaking down starches into simpler sugars like maltose. This process is halted in the acidic environment of the stomach because the amylase is denatured. The digestion of fats does not commence in the mouth or stomach; it begins in the small intestine with the help of enzymes such as lipase.