23.7 Chemical Digestion and Absorption: A Closer Look

Chemical Digestion and Absorption: A Closer Look

Chemical digestion breaks down complex nutrients into simpler forms your body can absorb. Enzymes in the mouth, stomach, and small intestine work together to break apart carbohydrates, proteins, lipids, and nucleic acids into their basic building blocks. Understanding where each enzyme acts and what it produces is the key to mastering this topic.

Most nutrient absorption happens in the small intestine. Water-soluble nutrients enter the bloodstream directly, while lipid-soluble nutrients are packaged into chylomicrons and enter the lymphatic system. These two pathways determine how quickly nutrients reach the liver and the rest of the body.

Chemical Digestion

Chemical Digestion of Macromolecules

Each macromolecule is broken down in stages as it moves through the GI tract. Notice the pattern: digestion often starts in the mouth or stomach, but the small intestine does the heavy lifting.

• Carbohydrates
  • Mouth: Salivary amylase begins starch digestion, breaking polysaccharides into maltose, maltotriose, and α-dextrins.
  • Small intestine: Pancreatic amylase continues breaking remaining polysaccharides into disaccharides (maltose, maltotriose, and α-dextrins). Note that sucrose and lactose come from the diet directly; they aren't products of amylase.
  • Brush border: Enzymes on the microvilli (maltase, sucrase, lactase, and α-dextrinase) split disaccharides into absorbable monosaccharides: glucose, fructose, and galactose.
• Proteins
  • Stomach: Pepsin, activated by the acidic environment (pH ~1.5–3.5), cleaves proteins into smaller polypeptides. Pepsin is secreted as inactive pepsinogen by chief cells and activated by HCl from parietal cells.
  • Small intestine: Pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidase) further digest polypeptides into smaller peptides and some individual amino acids. Trypsin is especially important because it activates the other pancreatic proteases.
  • Brush border: Aminopeptidases and dipeptidases break remaining peptides into individual amino acids (and some di/tripeptides, which are absorbed and then cleaved intracellularly).
• Lipids
  • Mouth and stomach: Lingual lipase and gastric lipase begin triglyceride digestion, but they handle only a small fraction of total fat breakdown.
  • Small intestine (emulsification): Bile salts, produced by the liver and stored in the gallbladder, emulsify large fat globules into smaller droplets. This dramatically increases the surface area available for enzymatic action. Bile salts are not enzymes; they're detergent-like molecules.
  • Small intestine (enzymatic): Pancreatic lipase (with its cofactor colipase) does the majority of triglyceride digestion, producing fatty acids and monoglycerides.
• Nucleic Acids
  • Small intestine: Pancreatic nucleases (ribonuclease and deoxyribonuclease) break RNA and DNA into nucleotides.
  • Brush border: Nucleotidases and phosphatases further digest nucleotides into their components: nitrogenous bases (purines and pyrimidines), pentose sugars (ribose and deoxyribose), and phosphate groups.

Enzymatic Processes in Chemical Digestion

• Hydrolysis is the primary mechanism. Enzymes use water to break covalent bonds in macromolecules, splitting them into smaller subunits. Every digestive enzyme described above catalyzes a hydrolysis reaction.
• Emulsification is not a chemical reaction. It's a physical process where bile salts break large fat globules into smaller droplets (micelles), increasing the surface area so pancreatic lipase can work efficiently.
• Enzymes are biological catalysts. They speed up reactions without being consumed, and each one is specific to a particular substrate. Most digestive enzymes are secreted in inactive forms (zymogens) to prevent self-digestion of the organs that produce them.

Nutrient Absorption

Nutrient Absorption into Circulatory Systems

Absorption occurs across the mucosa of the small intestine. The epithelial cells here have microvilli (the brush border), which massively increase surface area for nutrient uptake.

Water-soluble nutrients (monosaccharides, amino acids, short-chain fatty acids, minerals, water-soluble vitamins) cross the intestinal wall into blood capillaries using three transport mechanisms:

1. Passive diffusion: Nutrients move down their concentration gradient with no energy required. Water follows osmotic gradients, aided by aquaporins.
2. Facilitated diffusion: Nutrients move down their concentration gradient but require carrier proteins to cross the membrane (e.g., GLUT transporters move fructose into epithelial cells).
3. Active transport: Nutrients move against their concentration gradient, requiring ATP. The $\text{Na}^+$-glucose cotransporter (SGLT1) is a classic example, using the sodium gradient to pull glucose and galactose into cells.

Lipid-soluble nutrients (long-chain fatty acids, monoglycerides, fat-soluble vitamins) take a different route into the lymphatic system:

1. Fatty acids and monoglycerides diffuse into intestinal epithelial cells (often from micelles that deliver them to the cell surface).
2. Inside the cell, the smooth endoplasmic reticulum reassembles them into triglycerides.
3. Triglycerides are packaged with cholesterol, phospholipids, and fat-soluble vitamins (A, D, E, K) into protein-coated droplets called chylomicrons.
4. Chylomicrons are too large to enter blood capillaries, so they enter lacteals (lymphatic capillaries in each villus) instead.

Water-Soluble vs. Lipid-Soluble Nutrient Absorption

This distinction matters because it determines how nutrients reach the rest of the body.

• Water-soluble nutrients
  • Absorbed into blood capillaries in the villi via passive diffusion, facilitated diffusion, or active transport
  • Enter the hepatic portal vein, which carries them directly to the liver
  • The liver processes, stores, or distributes these nutrients before they enter systemic circulation
  • Water-soluble vitamins (B-complex and C) follow this same portal route
• Lipid-soluble nutrients
  • Absorbed into epithelial cells and reassembled into triglycerides
  • Packaged into chylomicrons with cholesterol and fat-soluble vitamins (A, D, E, K)
  • Enter the lymphatic system through lacteals, bypassing the liver initially
  • Drain into the venous blood via the thoracic duct (which empties into the left subclavian vein), reaching peripheral tissues before the liver