Final answer:
Protein digestion begins in the stomach with the enzyme pepsin, unlike carbohydrates, which start breaking down in the mouth, and lipids, which are mainly digested in the small intestine.
Step-by-step explanation:
The enzymatic breakdown that doesn't begin until it reaches the stomach is that of proteins. The enzyme pepsin is responsible for starting the digestion of proteins in the stomach. Unlike carbohydrates, which begin to break down in the mouth through the action of the enzyme amylase, and lipids, whose digestion predominantly occurs in the small intestine with the help of bile and lipase, protein breakdown begins in the stomach. The acidic environment of the stomach, with a pH that activates pepsin, is ideal for breaking down proteins into smaller peptides. This process, known as proteolysis, is essential for the absorption of amino acids later in the digestive tract, specifically in the small intestine.