Chemical Digestion in the Stomach: Enzymatic Breakdown of Food

Food breakdown by gastric enzymes is a classic example of a chemical change—the molecular structure of nutrients is altered, forming new substances.

Key Gastric Chemical Processes

• Pepsin (activated from pepsinogen by HCl): Hydrolyzes proteins into polypeptides and peptides (cleaves at aromatic amino acids). Optimal pH 1.5-2.5.
• Gastric Lipase: Initiates triglyceride digestion (10-30% of total fat hydrolysis); important in neonates.
• Hydrochloric Acid (HCl): Denatures proteins (unfolding), activates pepsinogen, kills microbes, provides low pH for enzyme activity.
• Intrinsic Factor: Binds vitamin B12 for ileal absorption (not digestion per se).

Why It's a Chemical Change

• Bonds are broken (peptide, ester) and new molecules formed (peptides, amino acids, fatty acids).
• Irreversible under physiological conditions.
• Enzymes act as catalysts, lowering activation energy.

Mechanical changes (peristalsis, churning) accompany chemical digestion but are distinct (physical size reduction without molecular alteration). Both are essential for nutrient absorption in the small intestine.