12.1 Digestion
Key Takeaways
Salivary amylase in the mouth starts starch digestion, and peristalsis in the esophagus moves the bolus without digestion of note.
Stomach acid converts pepsinogen to pepsin, which begins protein digestion, while mucus protects the stomach wall.
The small intestine, lined with villi and microvilli, finishes most chemical digestion and absorbs almost all nutrients.
The pancreas supplies amylase, proteases such as trypsin, and lipase; the liver makes bile and the gallbladder stores it.
Bile emulsifies fats and is not an enzyme, and the large intestine absorbs water rather than serving as the main site of glucose absorption.
12.1 Digestion
Vertebrates must break food into molecules small enough to cross a cell membrane, then move those molecules into blood or lymph. Humans do that work along one continuous tube plus the pancreas, the liver, and the gallbladder. Digestion is the breakdown of food. Absorption is the uptake of the products. A region can start digestion and still absorb almost none of the meal. The usual question is which organ hydrolyzes a given food, and which organ actually takes up the products.
Mouth and esophagus
The mouth does both mechanical and chemical work. Teeth cut and grind the food, and the tongue mixes it with saliva to form a bolus. Salivary amylase, from the salivary glands, begins starch digestion. It hydrolyzes some bonds in starch and releases smaller polysaccharides plus the disaccharide maltose. Saliva also wets the bolus and adds mucus so swallowing can start. Protein and triglyceride are not chemically digested in any important way in the mouth, and glucose is not absorbed there.
The esophagus is the muscular tube from the throat to the stomach. Swallowing starts the movement, and waves of peristalsis push the bolus downward. Circular muscle contracts behind the bolus and relaxes ahead of it. Mucus lubricates the wall. There is no digestion of note in the esophagus, and there is no nutrient absorption.
Stomach
The stomach stores the meal, churns it, and begins protein digestion. Parietal cells secrete hydrochloric acid. Chief cells secrete pepsinogen, an inactive precursor. Acid converts pepsinogen to pepsin. Pepsin cuts proteins into smaller polypeptides. The acid denatures those proteins, unfolding them so the enzyme can reach more bonds, and it kills many swallowed microbes. Salivary amylase, which worked in the nearly neutral mouth, loses activity in this acid, so starch digestion pauses until the small intestine.
The stomach would digest its own wall without a shield. A thick coat of mucus, with bicarbonate held in that coat, keeps acid and pepsin off the lining. Churning turns the meal into chyme, an acidic slurry that leaves through the pyloric sphincter. The stomach can take up some alcohol and a few drugs. It is not the main site of nutrient absorption. Glucose, amino acids, and the products of fat digestion do not enter the body in useful amounts from the stomach.
Small intestine
The small intestine finishes most chemical digestion and performs almost all nutrient absorption. Its regions, in order, are the duodenum, the jejunum, and the ileum. The duodenum receives chyme plus secretions from the pancreas and from the liver and gallbladder. Segmentation mixes chyme with those secretions and presses it against the lining. Peristalsis then moves the residue toward the large intestine.
Villi, brush border, and accessory organs
The lining is folded into villi. Each epithelial cell on a villus bears microvilli, together called the brush border. Those folds create a huge surface for membrane enzymes and for transport proteins. Inside each villus, a capillary bed receives water-soluble nutrients, and a lacteal, a lymph vessel, receives the products of fat digestion.
Brush-border enzymes finish the last cuts. Maltase, sucrase, and lactase split disaccharides into monosaccharides. Peptidases split small peptides into amino acids. These enzymes sit on the intestinal cells. They are not poured in from the stomach.
The pancreas releases an alkaline juice into the duodenum. Bicarbonate neutralizes acid arriving from the stomach so intestinal enzymes can work. The juice supplies pancreatic amylase, which resumes starch digestion; proteases, including trypsin, chymotrypsin, and carboxypeptidase; and pancreatic lipase, which hydrolyzes triglycerides. The proteases leave the pancreas as inactive precursors so they do not digest the gland. Trypsinogen becomes trypsin when enteropeptidase, also called enterokinase, on the duodenal lining cuts it. Trypsin then activates the other protease precursors.
The liver makes bile continuously. The gallbladder stores and concentrates that bile and empties it into the small intestine when fat and protein arrive. Bile salts emulsify fats. They break one large globule into many tiny droplets and increase the surface pancreatic lipase can attack. Emulsification is physical. Bile is not an enzyme, and it does not hydrolyze triglycerides. Two duodenal hormones time this help. Secretin, released when acidic chyme arrives, calls for bicarbonate-rich pancreatic juice. Cholecystokinin, released when fats and amino acids arrive, makes the gallbladder contract and the pancreas release its enzyme-rich juice.
Large intestine and one breakfast
The large intestine absorbs water and some salts, and it houses a dense community of bacteria. Those bacteria ferment material human enzymes cannot break, including much of the cellulose in plant fiber, and they produce gases plus vitamins such as vitamin K. The remaining material is compacted into feces. This region has no villi. It is not the main site of glucose absorption. Glucose from digested starch crossed the small-intestinal lining farther upstream.
Follow toast, eggs, and butter. Starch in the toast meets salivary amylase in the mouth, pauses in the acid stomach, and is continued by pancreatic amylase. Brush-border maltase frees glucose. Glucose enters villus capillaries and travels in the hepatic portal vein to the liver. Protein in the eggs is only mechanically broken in the mouth. Acid and pepsin cut it in the stomach. Trypsin and the other pancreatic proteases continue in the small intestine, and brush-border peptidases free amino acids, which enter the same villus capillaries. Fat in the butter waits for the small intestine. Bile emulsifies the droplets, and pancreatic lipase hydrolyzes triglycerides into fatty acids and monoglycerides. Intestinal cells rebuild those pieces as chylomicrons. Chylomicrons leave in the lacteal. Lymph later delivers them to the blood. Pepsin never does that fat job.
| Food | Enzyme that hydrolyzes it | Where that enzyme acts | What is absorbed, and where |
|---|---|---|---|
| Starch | Salivary amylase, then pancreatic amylase | Mouth, then small intestine | Glucose into villus capillaries |
| Protein | Pepsin, then trypsin and other proteases | Stomach, then small intestine | Amino acids into villus capillaries |
| Triglyceride | Pancreatic lipase, after bile emulsifies the fat | Small intestine | Fatty acids and monoglycerides into intestinal cells; chylomicrons then enter lacteals |
Warning
Bile emulsifies fat, and bile is not an enzyme. The stomach begins protein digestion with acid and pepsin, but almost all nutrient absorption happens in the small intestine. The large intestine absorbs water and is not the main site of glucose absorption.
Starch is an amylase substrate, protein is a pepsin and trypsin substrate, and triglyceride is a lipase substrate after bile has emulsified it.
A student traces glucose produced from a meal of bread. Where does almost all of that glucose enter the blood?
Across the esophageal wall during peristalsis
Across the villi of the small intestine
Across the large-intestine lining, treated as the main site of sugar uptake
Across the stomach lining, where pepsin is active
What does bile contribute when a person digests a fatty meal?
Bile emulsifies large fat droplets so pancreatic lipase can reach more surface
Bile acts as a lipase and hydrolyzes triglycerides into fatty acids
Bile absorbs glucose from the colon after lipase has finished
Bile converts pepsinogen to pepsin inside the stomach
Lunch is pasta, fish, and olive oil. Which pairing of food molecule, enzyme, and organ is right?
Salivary amylase hydrolyzes the fish protein in the mouth
Enzymes of the large intestine are the main route that absorbs glucose from the pasta
Pepsin hydrolyzes the olive oil in the stomach
Pancreatic lipase hydrolyzes the olive oil in the small intestine after bile emulsifies it
Sections you finish are checked off in the contents.