Chemical Digestion and Absorption: A Closer Look

Core Concepts

Hydrolysis: the master reaction

Digestion is a long chain of hydrolysis reactions: water is added to a bond, and the bond breaks. Starch → maltose → glucose; proteins → peptides → amino acids; triglycerides → fatty acids + monoglycerides; nucleic acids → nucleotides. The reverse — building large molecules by removing water — is condensation, which the body uses in anabolism (Chapter 24). Every digestive enzyme is a hydrolase: it speeds up a specific hydrolysis reaction, and each is selective about its substrate.

Where the enzymes work: three locations

Digestive enzymes act in three places. Luminal digestion happens in the chyme itself, using enzymes secreted into the tract (salivary and pancreatic amylase, pepsin, pancreatic proteases, pancreatic lipase). Brush-border digestion happens on the membrane of the absorptive cells, using enzymes anchored to the microvilli (disaccharidases such as sucrase, maltase, and lactase, plus peptidases). Intracellular digestion finishes the job inside the enterocyte, where small peptides are split into amino acids. The pattern is progressive: big polymers are trimmed in the lumen, disaccharides and small peptides are finished at the brush border, and the final pieces cross into the cell.

Carbohydrate digestion and absorption

Salivary amylase begins starch breakdown in the mouth; stomach acid inactivates it, and pancreatic amylase takes over in the duodenum. The products — disaccharides like maltose, sucrose, and lactose — cannot be absorbed yet. Brush-border enzymes split them: sucrase, maltase, and lactase produce the monosaccharides glucose, galactose, and fructose. Glucose and galactose enter the enterocyte by secondary active transport, riding a sodium gradient through the SGLT1 carrier; fructose enters by facilitated diffusion through GLUT5. All three exit into the blood through GLUT2. When lactase is low — the common pattern in lactose intolerance — undigested lactose stays in the lumen, osmotically draws water (diarrhea), and is fermented by gut bacteria (gas and bloating). The missing enzyme is at the brush border, not in the stomach.

Protein digestion and absorption

Protein digestion begins in the stomach, where HCl denatures proteins and activates pepsinogen into pepsin. In the duodenum, pancreatic proteases continue: trypsin and chymotrypsin cut chains into peptides, and carboxypeptidase trims amino acids off the ends. Brush-border peptidases (aminopeptidases, dipeptidases) finish, releasing free amino acids plus di- and tripeptides. Amino acids cross the membrane by sodium-dependent cotransporters; di- and tripeptides enter through a hydrogen-coupled carrier (PepT1) and are split inside the enterocyte. The absorbed amino acids travel in the portal blood to the liver, which decides their fate — building proteins or breaking them down for energy (Chapter 24).

Fat digestion and absorption: a different route

Fat is hydrophobic, so it needs special handling at every step. Bile salts emulsify large fat droplets (Topic 6); pancreatic lipase then digests triglycerides into free fatty acids and monoglycerides. These products are hydrophobic too, so bile salts wrap them into micelles — tiny ferries that carry them through the watery chyme to the enterocyte membrane, where they diffuse in. Inside the enterocyte, the pieces are reassembled into triglycerides, packaged with cholesterol and phospholipids into chylomicrons, and released by exocytosis into lacteals — the lymphatic capillaries of the villi. Chylomicrons travel through lymph and the thoracic duct into the bloodstream; fat, unlike glucose, does not enter the portal blood directly. Medium-chain fatty acids are commonly taught to be absorbed more directly into portal blood, and the fat-soluble vitamins (A, D, E, K) ride along with the fat — which is why fat malabsorption can deplete them.

Nucleic acids, water, and electrolytes

Pancreatic nucleases split nucleic acids into nucleotides, which are further broken down and absorbed. Water crosses the intestinal wall mostly by osmosis, following the gradients created by active solute absorption — sodium is pumped out of the cell and into the blood, dragging water along. Iron and calcium are absorbed mainly in the duodenum through regulated, saturable mechanisms (commonly taught: calcium needs vitamin D; iron absorption tracks the body's iron status). Vitamin B12 must first bind intrinsic factor, a protein made by the stomach, and the complex is absorbed in the ileum — so problems with either the stomach or the ileum can cause B12 deficiency.

Where and when

The duodenum and jejunum perform most chemical digestion and absorption; the ileum mops up remaining bile salts and vitamin B12. Segmentation contractions constantly mix chyme against the brush border, maximizing contact, and chyme spends hours in the small intestine. The large intestine's main job is reclaiming water and electrolytes, and its resident bacteria produce some vitamins (commonly taught, e.g., some B vitamins and vitamin K).

Common Confusions