🧪 Digestive System Lesson

ALPS: the key digestive enzymes

Each macromolecule in food — starch, fat, protein, sugar — needs its own dedicated enzyme to be broken down into an absorbable form.

Amylase

Lipase

Proteases

Sucrase

📖 Full Breakdown

Four enzyme categories and exactly what each one breaks down

Matching enzyme to macromolecule is one of the most testable relationships in digestive physiology.

Amylase

Breaks down starch

Salivary amylase starts the job in the mouth; pancreatic amylase continues it in the small intestine, breaking starch down into maltose.

Lipase

Breaks down fats

Produced by the pancreas, lipase breaks fats into fatty acids and monoglycerides — but only works efficiently after bile has emulsified the fat into smaller droplets first.

Proteases

Break down proteins

Pepsin works in the acidic stomach environment (activated by HCl); trypsin and chymotrypsin from the pancreas continue protein breakdown in the small intestine.

Brush border enzymes (incl. Sucrase)

Finish carbohydrate digestion

Maltase, sucrase, and lactase sit on the intestinal surface itself and complete the final breakdown of disaccharides into absorbable simple sugars.

🩺 Clinical / Exam Application

A patient reports bloating, gas, and diarrhea specifically after drinking milk. This points to lactose intolerance — a deficiency of lactase, one of the brush border enzymes that normally breaks lactose down into absorbable glucose and galactose at the intestinal surface. Without that enzyme, undigested lactose travels into the large intestine where bacteria ferment it, producing the gas and drawing in water that causes the classic symptoms.

⚠️ Exam Alert

Lipase requires bile pre-treatment to work efficiently — a commonly tested detail is that bile is NOT an enzyme itself but is required for lipase (an actual enzyme) to access enough surface area on fat droplets to function.

🚧 Common Trap

Don't confuse where an enzyme is PRODUCED with where it ACTS. Pancreatic amylase, lipase, and proteases are all made in the pancreas but do their actual digestive work in the duodenum after being secreted through the pancreatic duct.

✅ Quick Check

A patient lacks lactase. Which enzyme category does lactase belong to, and where in the digestive tract does that category act?

📝 Exam Prep

Common Exam Questions

❓ Which enzyme breaks down carbohydrates and where is it produced?

✅ Salivary amylase (mouth) and pancreatic amylase (duodenum) break down complex carbohydrates into disaccharides. Brush border enzymes (maltase, sucrase, lactase) complete digestion at the intestinal surface.

❓ Why does lipase need bile to work effectively, even though bile is not an enzyme?

✅ Bile emulsifies large fat globules into smaller droplets, dramatically increasing the surface area available for lipase to act on — without this emulsification, lipase alone would digest fat far too slowly.