Absorbed Nutrients & Transport Mechanisms
Absorption
- Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
- Major site of nutrient absorption.
Three folded
mucosal structures maximize surface area for absorption*
1 - Plicae circulares: wavy, folds on the inner walls of small intestine ? form circular folds ? increase surface area 3-fold.
2 - Villi: finger-like projections that protrude from the plicae circulares ? surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.
3 - Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) ? increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase
Nutrients are absorbed in the small intestine
- Monosaccharides (digested carbohydrate products)
- Amino acids, di-peptides and tri-peptides (digested protein products)
- Intact proteins
- Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
- Vitamins
- Water and electrolytes
Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.
- Apical surface: interfaces the intestinal lumen and epithelium.
- Basolateral surface: opposite to the apical surface, lines the inside of the villi.
- Capillaries and lacteals inside villi
– Most nutrients cross the basolateral surface and pass directly into circulation.
- Fats, however, pass directly into lacteals (lymphatic system).
Key Transport Mechanisms
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Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
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Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
– Simple
diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
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Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.
– Facilitated diffusion.
– Simple diffusion.
–
Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.
