Q.Complete the analogy: Ptyalin : Amylase : : Pepsin : .................

Protein digestion is a two-stage process: it starts in the stomach with pepsin, but the major, completing phase happens in the small intestine (mainly the duodenum), using pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) and intestinal brush-border enzymes.

In the stomach, gastric juice contains pepsinogen, which HCl activates into pepsin. Pepsin cleaves large protein molecules into smaller peptones and proteoses — an important first step, but not a complete digestion, since pepsin cannot fully break proteins down to single amino acids.

The food then passes into the duodenum, the first part of the small intestine, where pancreatic juice delivers trypsin and chymotrypsin (which continue breaking peptide bonds) and carboxypeptidase (which clips amino acids off the ends of peptide chains). Finally, enzymes embedded in the intestinal wall itself (aminopeptidases, dipeptidases) finish the job, releasing free amino acids that can be absorbed into the bloodstream across the intestinal lining.

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Enterokinase, secreted by the intestinal mucosa, activates the inactive pancreatic enzyme trypsinogen into its active form, trypsin.

The pancreas secretes several digestive enzymes in inactive (zymogen) forms to prevent them from digesting the pancreas's own tissue. Trypsinogen is one such inactive form, released into the duodenum via the pancreatic duct.

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Lipase digests lipids, pepsin digests protein in general, rennin specifically curdles the milk protein casein, and ptyalin (salivary amylase) digests starch — giving the pairing (1)-(iv), (2)-(iii), (3)-(ii), (4)-(i).

Each enzyme listed acts on a specific substrate: Lipase, secreted by the pancreas (and present in gastric and intestinal juice too), hydrolyses lipids (fats) into fatty acids and glycerol/monoglycerides — so Lipase pairs with (iv) Lipid. Pepsin, secreted as pepsinogen by gastric chief cells and activated by HCl, is a proteolytic enzyme that breaks down dietary proteins into smaller peptones and proteoses in the acidic environment of the stomach — so Pepsin pairs with (iii) Protein. Rennin (chymosin), present in the gastric juice of infants, acts specifically on the milk protein casein, converting it to a curdled/coagulated form (calcium paracaseinate) that can be acted upon further by pepsin — so Rennin pairs with (ii) Casein. Ptyalin, the saliva …