7.1 Mechanical and Chemical Digestion

Mechanical and Chemical Digestion

Mechanical and chemical digestion work together to break down food into absorbable nutrients. Mechanical digestion physically breaks food into smaller pieces, while chemical digestion uses enzymes to cleave molecular bonds through hydrolysis. Understanding how these two processes coordinate across the GI tract is central to everything else in this unit.

Mechanical Digestion of Food

Physical Breakdown of Food

Mechanical digestion breaks food into smaller pieces without changing its chemical composition. The whole point is to increase surface area so that digestive enzymes can access more molecules at once. Think of it this way: a whole cracker sitting in a pool of amylase would take forever to digest, but cracker crumbs in that same pool break down quickly because there's so much more exposed surface.

Mechanical digestion occurs in three main locations:

• Mouth (mastication): Teeth grind and tear food while the tongue repositions it. Chewing mixes food with saliva to form a soft mass called a bolus.
• Esophagus (peristalsis): Rhythmic waves of smooth muscle contraction push the bolus toward the stomach. This isn't just gravity; peristalsis works even if you're upside down.
• Stomach (churning): The stomach's three layers of smooth muscle (longitudinal, circular, and oblique) contract in coordinated waves, mixing food with gastric secretions to produce a semi-liquid called chyme.

By the time food leaves the stomach, mechanical digestion has done most of its job. The small intestine relies primarily on chemical digestion and segmentation (back-and-forth contractions that mix chyme with enzymes) rather than further physical breakdown.

Chemical Digestion in the GI Tract

Overview of Chemical Digestion

Chemical digestion breaks the covalent bonds in food molecules through hydrolysis, a reaction where water is added across a bond to split a larger molecule into smaller subunits. Each hydrolysis reaction is catalyzed by a specific digestive enzyme, and those enzymes are substrate-specific: carbohydrate enzymes won't touch proteins, and protein enzymes won't touch fats.

Chemical digestion takes place in three regions: the mouth, the stomach, and the small intestine.

Chemical Digestion in the Mouth and Stomach

• Mouth: Salivary amylase begins breaking starch into the disaccharide maltose. This enzyme works best at a near-neutral pH (~6.8), so it becomes inactivated once it reaches the acidic stomach.
• Stomach: The chief cells of the stomach secrete pepsinogen, an inactive zymogen. Hydrochloric acid (HCl) from parietal cells converts pepsinogen into its active form, pepsin, which cleaves proteins into shorter peptide fragments. The low pH of the stomach (around 1.5–3.5) also denatures proteins, unfolding them so pepsin can reach more peptide bonds.
• Stomach (lipids): Gastric lipase begins a small amount of fat digestion, but this enzyme accounts for only a minor fraction of total lipid breakdown. The bulk of fat digestion happens in the small intestine.

Chemical Digestion in the Small Intestine

The small intestine is where the majority of chemical digestion occurs. Two sources of enzymes converge here:

1. Pancreatic juice enters the duodenum through the hepatopancreatic ampulla. It contains pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, pancreatic lipase, and nucleases. Pancreatic juice is also rich in bicarbonate ($HCO_3^-$), which neutralizes acidic chyme and raises the pH to ~7–8, the optimal range for these enzymes.
2. Brush border enzymes are anchored to the plasma membranes of intestinal epithelial cells (enterocytes). They perform the final step of digestion, breaking dimers and small oligomers into absorbable monomers.

The liver and gallbladder also contribute: bile salts emulsify large fat globules into smaller droplets (micelles), dramatically increasing the surface area available to pancreatic lipase. Bile salts are not enzymes; they don't catalyze a chemical reaction, they just physically disperse fat.

Digestive Enzymes and Their Functions

Carbohydrate-Digesting Enzymes

• Salivary amylase: Secreted by the salivary glands; breaks starch into maltose in the mouth.
• Pancreatic amylase: Secreted by the pancreas into the duodenum; continues breaking starch and glycogen into maltose.
• Brush border enzymes (maltase, sucrase, lactase): Located on the surface of enterocytes. Each one targets a specific disaccharide:
  • Maltase: maltose → glucose + glucose
  • Sucrase: sucrose → glucose + fructose
  • Lactase: lactose → glucose + galactose

The final products of carbohydrate digestion are monosaccharides (glucose, fructose, galactose), which are small enough to be absorbed.

Protein-Digesting Enzymes

Protein digestion requires multiple enzymes working in sequence:

1. Pepsin (stomach): Cleaves large proteins into shorter polypeptides. Active only in acidic conditions.
2. Trypsin and chymotrypsin (pancreatic): Continue breaking polypeptides into smaller peptide fragments in the small intestine. Both are secreted as inactive zymogens (trypsinogen and chymotrypsinogen) and activated in the duodenum. Trypsinogen is activated by enterokinase (a brush border enzyme), and active trypsin then activates chymotrypsinogen.
3. Carboxypeptidase (pancreatic): Removes amino acids one at a time from the carboxyl end of peptide chains.
4. Peptidases (brush border): Dipeptidases and aminopeptidases complete digestion by splitting dipeptides and tripeptides into individual amino acids.

Fat-Digesting Enzymes

Fat digestion has an extra step because lipids are hydrophobic and don't mix with the watery environment of the GI lumen:

1. Bile salts emulsify large fat globules into tiny droplets, increasing the surface area available to enzymes.
2. Pancreatic lipase (with its cofactor colipase) then hydrolyzes triglycerides into two fatty acids and one monoglyceride.

Gastric lipase contributes a small amount of fat digestion in the stomach, but pancreatic lipase does the heavy lifting.

The Small Intestine in Chemical Digestion

Anatomy of the Small Intestine

The small intestine has three segments, each with a primary role:

• Duodenum (~25 cm): The main site of chemical digestion. Receives pancreatic juice and bile. Most enzymatic breakdown is completed here.
• Jejunum (~2.5 m): The primary site of nutrient absorption. Has the tallest, most densely packed villi.
• Ileum (~3.5 m): Absorbs remaining nutrients, especially vitamin $B_{12}$ and bile salts, which are recycled back to the liver.

Final Stages of Chemical Digestion

Brush border enzymes on the apical surface of enterocytes handle the last step. By this point, carbohydrates have been reduced to disaccharides and proteins to dipeptides or tripeptides. The brush border enzymes convert these into their final absorbable forms:

• Disaccharides → monosaccharides
• Dipeptides/tripeptides → amino acids
• Nucleotides → bases, sugars, and phosphate groups

Absorption of Nutrients

The small intestine is extraordinarily efficient at absorption because of its massive surface area, estimated at roughly 200 square meters (about the size of a tennis court). Three structural features create this area:

• Circular folds (plicae circulares): Large, permanent folds in the intestinal wall that force chyme to spiral as it moves through, increasing contact time.
• Villi: Finger-like projections (~0.5–1 mm tall) covering the circular folds. Each villus contains a capillary network and a central lacteal (lymphatic vessel) for absorbing fats.
• Microvilli: Microscopic projections on the apical surface of each enterocyte, forming the brush border. These increase absorptive surface area by roughly 20-fold.

Whatever remains undigested and unabsorbed passes from the ileum through the ileocecal valve into the large intestine, where water and electrolytes are reclaimed before the residue is eliminated as feces.