Q.Pick out the proenzyme present in gastric juice from the following :
(Trypsinogen, Procarboxypeptidase, Pepsinogen, Amylase)

Gastric Digestion: Breaking Down Proteins in the Stomach

Think of the stomach as a chemical reactor. Food arrives from the mouth as a chewed-up, but chemically unchanged, mass. The real breakdown of large molecules — especially proteins — begins here. Your body needs to turn the chicken or dal you ate into tiny building blocks (amino acids) that can be absorbed later in the small intestine. Gastric digestion is the first serious step in that process.

The Acidic Environment: Why HCl Matters

The stomach lining secretes hydrochloric acid (HCl) , which drops the pH to around 1.5–3.5. This is brutally acidic — strong enough to kill most bacteria and to denature proteins. Denaturation means the protein's carefully folded 3D structure unravels, like a tangled necklace becoming a straight chain. Once unfolded, the protein's peptide bonds are exposed and vulnerable to attack by enzymes.

Pepsin: The Protein-Cutter

The stomach wall secretes an inactive precursor called pepsinogen. When pepsinogen meets the acidic pH, it gets cleaved into active pepsin. Pepsin is a protease — it chops long protein chains into shorter fragments called peptides.

Pepsin works best at pH 1.5–2.5. It prefers to cut next to the amino acids phenylalanine, tyrosine, and tryptophan. The result is a mixture of polypeptides, not free amino acids. Pepsin cannot complete the job alone; it just makes the pieces smaller.

Rennin: The Milk-Clotting Enzyme (in Infants)

Rennin (also called chymosin) is present in large amounts in infants. Its job is specific: it coagulates milk protein (casein). Milk is liquid; if it passed through the stomach too quickly, the enzymes wouldn't have time to act. Rennin converts soluble casein into paracasein, which then reacts with calcium ions to form a semi-solid curd. This slows gastric emptying and gives pepsin time to digest the trapped proteins.

In adults, rennin is largely absent — pepsin and HCl handle milk proteins directly, though less efficiently. …