52 - Digestion and absorption of proteins, fats and carbohydrates in the gastrointestinal tract. Absorption of salts, water and vitamins.

proteins + carbs have brush border thingy but fatsdont

proteins

Digestion

• Begins in the stomach with pepsin (active in pH 2–3) → breaks proteins into peptides.

• In the small intestine, pancreatic proteases complete most of the breakdown:

  • Trypsin, chymotrypsin, carboxypeptidase, elastase

  • These enzymes break proteins into smaller peptides (mainly di- and tripeptides)

• Final digestion occurs at the brush border of enterocytes:

  • Enzymes: aminopeptidase, dipeptidase

  • Convert peptides → amino acids, di-/tripeptides

prot ab

Absorption

• Amino acids absorbed via Na⁺-dependent co-transporters

• Di-/tripeptides absorbed via H⁺-dependent transporters

• Inside enterocytes: all peptides are further broken into single amino acids

• Amino acids enter blood via facilitated diffusion into capillaries

fats

Fats (Lipids) Digestion

• Begins in mouth (lingual lipase) and stomach (gastric lipase) – mainly targets short-chain fatty acids

• Main digestion occurs in the small intestine:

  1. Bile salts emulsify fats → increase surface area

  2. Pancreatic lipase breaks triglycerides (TAGs) → free fatty acids (FFA) + monoglycerides

fats

Absorption

• Lipid products form micelles with bile salts → diffuse into enterocytes

• Inside enterocytes:

  • FFA + monoglycerides → re-esterified into TAGs

  • Packaged into chylomicrons

  • Released into lacteals (lymphatics) → then enter bloodstream

Carbohydrates Digestion

• Starts in mouth: salivary α-amylase begins breaking down starch

• Main digestion in small intestine via pancreatic amylase:

  • Breaks polysaccharides → disaccharides

• Brush border enzymes:

  • Lactase, maltase, sucrase → break disaccharides → monosaccharides

Absorption

• Glucose & galactose: Na⁺-dependent symport (SGLT1) → then GLUT2 into blood

• Fructose: via GLUT5, then GLUT2 into blood

Water, Salts, and Vitamins

Water, Salts, and Vitamins

Water

• Absorbed mostly in the large intestine, also in small intestine

• Via osmosis, depending on solute (esp. Na⁺) concentration

Sodium (Na⁺)

• Actively transported via Na⁺/K⁺ ATPase + na dependent symport on basolateral membrane

• Drives secondary transport of glucose, amino acids, etc.

Vitamins

• Fat-soluble (A, D, E, K):

  • Absorbed with fats via micelles, then in chylomicrons

• Water-soluble:

  • Absorbed via diffusion or carrier-mediated transport

  • Vitamin B12: requires intrinsic factor (from stomach), absorbed in ileum