50. Absorption in the gastrointestinal system: general characteristic . Absorption of the carbohydrates, proteins and lipids. General Characteristics

Absorption in the gastrointestinal system: general characteristic . Absorption of the carbohydrates, proteins and lipids. General Characteristics

Absorption in the gastrointestinal system refers to the movement of nutrients, water, and electrolytes from the intestinal lumen into the bloodstream. The small intestine, with its extensive surface area enhanced by villi and microvilli, is the primary site for absorption. This intricate structure, with longitudinal folds called Kerckring, maximizes the exposure of nutrients to digestive enzymes and facilitates efficient absorption. However, the high turnover rate of intestinal epithelial cells (replaced every 3-6 days) makes them susceptible to damage from radiation and chemotherapy.

There are two primary pathways for absorption:

• Cellular path: Substances cross the apical (luminal) membrane into the intestinal epithelial cell, then exit across the basolateral membrane into the blood. This process involves specific transporters and channels.
• Paracellular path: Substances move through tight junctions between intestinal epithelial cells, bypassing the intracellular space, to enter the blood. The permeability of these tight junctions varies along the GI tract; for example, tight junctions are "leakier" in the small intestine compared to the colon.

Absorption of Carbohydrates

• Monosaccharides (glucose, galactose, and fructose) are the absorbable forms of carbohydrates.
• Glucose and galactose are absorbed through a Na+-dependent cotransport mechanism:

1. 1. SGLT1 (sodium-glucose cotransporter 1) transports glucose and galactose from the lumen into the cell against their concentration gradient, using the energy from the Na+ gradient.
2. 2. GLUT2 (glucose transporter 2) facilitates the movement of glucose and galactose from the cell into the blood via facilitated diffusion.

• Fructose is absorbed solely by facilitated diffusion:

1. 1. GLUT5 (fructose transporter) facilitates the movement of fructose from the lumen into the cell.
2. 2. GLUT2 facilitates the movement of fructose from the cell into the blood.

Absorption of Proteins

• Proteins are absorbed as amino acids, dipeptides, and tripeptides.
• L-amino acids are absorbed via a mechanism similar to glucose and galactose:

1. 1. Na+-amino acid cotransporters transport L-amino acids from the lumen into the cell against their concentration gradient.
2. 2. Facilitated diffusion transports L-amino acids across the basolateral membrane into the blood.

• Dipeptides and tripeptides are absorbed via a different mechanism:

1. 1. H+-dependent cotransporters transport dipeptides and tripeptides from the lumen into the cell.
2. 2. Cytosolic peptidases hydrolyze most dipeptides and tripeptides into amino acids within the cell.
3. 3. Facilitated diffusion transports amino acids across the basolateral membrane into the blood.
4. 4. Some dipeptides and tripeptides are absorbed intact.

Absorption of Lipids.

• Lipids are absorbed as monoglycerides, fatty acids, cholesterol, and lysolecithin.