Digestive Enzymes

Amylase, protease and lipase

Match each enzyme to its own substrate rather than to food in general: amylase acts on starch specifically, not on carbohydrates as a whole and not on sugars; protease acts on protein; lipase acts on fats and oils. “Amylase digests food” earns nothing on its own — the substrate has to be named. For lipase, both products are required: fats and oils become fatty acids andglycerol, and naming only one is an incomplete answer. At Core level, amylase’s product is described as simple reducing sugars; at Supplement level it is named specifically as maltose, with a second enzyme needed to reach glucose — covered below.

Where each enzyme is secreted, and where it acts

“Secreted by” and “acts in” are not the same fact, and a question can ask for either one separately. Being secreted by the pancreas does not mean an enzyme digests anything inside the pancreas — the pancreas is not part of the alimentary canal, and food never passes through it. Pancreatic enzymes travel into the duodenum and act there. Amylase and protease each have two named sources — amylase from the salivary glands and the pancreas, protease from the stomach and the pancreas — so naming only one source when a question asks for both is an incomplete answer. The full causal sequence, tracing one meal through the mouth, stomach and small intestine, is set out on the chemical digestion page.

Starch digestion in two steps

At Supplement level, the breakdown of starch is resolved into two separate enzyme-controlled steps rather than one: starch is converted to maltose by amylase, and maltose is then converted to glucose by a second enzyme, maltase. Missing out the maltose step — writing starch straight to glucose — is the most common way this pathway loses a mark. Maltase itself is not free-floating in the gut contents; it is held on the membranes of the epithelial cells that line the small intestine, so the final step of starch digestion happens at exactly the same surface across which glucose is then absorbed. Glucose, being small and soluble, can cross that surface once maltase has produced it. Keep maltose and maltase apart, too — maltose is the sugar, produced by amylase; maltase is the enzyme that then acts on it.

Pepsin and trypsin — same reaction, different conditions

Both pepsin and trypsin are proteases, and both digest the same substrate into the same products — protein into amino acids. What differs between them is not the reaction but the site and the pH each one is suited to. Assigning pepsin to alkaline conditions, or placing trypsin in the stomach, is the standard way this point is lost, so it is worth anchoring the pairing firmly: pepsin is the acidic, stomach one; trypsin is the alkaline, small-intestine one. The pH itself is a property of the region, not of the enzyme — hydrochloric acid makes the stomach acidic, and pepsin is simply suited to the result. Moving either enzyme away from its own optimum pH reduces its activity rather than destroying it instantly; only a sufficiently extreme pH denatures an enzyme, but the difference between the stomach and the small intestine is large enough that each protease is genuinely ineffective in the other’s region. Why bile creates that alkaline environment in the first place is covered on the bile page, and hydrochloric acid’s own two functions are covered in chemical digestion.