The digestion of proteins starts in the______

Step 1: Analyze the question The question asks where the chemical digestion of proteins begins in the alimentary canal.

Step 2: Explain the concept Different food classes start being digested in different places:

• Carbohydrates start in the mouth, where salivary amylase (ptyalin) breaks starch into maltose.
• Proteins start in the stomach. The gastric glands release gastric juice containing hydrochloric acid and the enzyme pepsin (secreted as inactive pepsinogen). The acid creates the right pH (about 2) to activate pepsin, which breaks proteins into shorter chains called peptides (peptones/polypeptides).
• Fats start mainly in the small intestine, where lipase and bile act on them.

Step 3: Solution

• A. Small intestine: Proteins are digested further here (by trypsin, erepsin/peptidases) into amino acids, but digestion does not start here.
• B. Stomach: Pepsin and HCl begin protein digestion here. ✔
• C. Large intestine: Mainly absorbs water and forms feces. No major enzymatic digestion of proteins.
• D. Mouth: Only mechanical breakdown (chewing) and starch digestion occur here.

Step 4: Answer Protein digestion starts in the stomach, so the correct option is B.

Tip: Remember "Pepsin = Proteins in the Pit of the stomach." Also remember the order of starting points: Carbohydrates → mouth, Proteins → stomach, Fats → small intestine.