28.10
L’absorption des glucides
Les glucides sont des macronutriments essentiels qui constituent la principale source d'énergie du corps. Leur digestion commence dans la bouche, où l…
La digestion des glucides, tels que l’amidon alimentaire, commence dans la bouche avec la mastication et l’action de l’amylase salivaire.
Après avoir été partiellement digérés par l’amylase salivaire en oligosaccharides, ces glucides sont incorporés dans le chyme et passés dans le duodénum.
Ici, l’amylase pancréatique décompose l’amidon restant en oligosaccharides plus petits, qui sont convertis en monosaccharides par les enzymes de bordure du pinceau intestinal.
Ces monosaccharides – principalement le glucose, le fructose et le galactose – sont maintenant prêts à être absorbés par les entérocytes qui tapissent le jéjunum et l’iléon.
À l’extrémité apicale des entérocytes, le glucose et le galactose entrent par transport actif secondaire avec des ions sodium.
Le transporteur sodium-glucose a des sites de liaison pour une molécule de glucose et deux ions sodium, qui doivent être occupés pour le transport intracellulaire.
Le galactose utilise également le même symporteur pour l’entrée cellulaire, tandis que le fructose utilise une diffusion facilitée.
Une fois à l’intérieur de l’entérocyte, tous les monosaccharides sont transportés à travers la membrane basolatérale dans les capillaires des villosités via une diffusion facilitée.
Finalement, les capillaires transportent les monosaccharides vers le foie via la veine porte hépatique.
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Q1: Where does carbohydrate digestion begin and what enzyme starts the process?
Carbohydrate digestion begins in the mouth with chewing and the action of salivary amylase, which partially breaks down dietary starch into oligosaccharides. This mechanical and enzymatic activity prepares carbohydrates for further processing. The partially digested carbohydrates are then incorporated into chyme and passed into the duodenum for continued breakdown.
Q2: What happens to carbohydrates when they enter the stomach?
When chyme enters the stomach, the acidic environment inactivates salivary amylase, halting carbohydrate digestion temporarily. This pause in enzymatic activity occurs until the partially digested carbohydrates move into the duodenum, where pancreatic amylase resumes the breakdown process in the alkaline environment.
Q3: How do brush border enzymes complete carbohydrate digestion?
Brush border enzymes—maltase, sucrase, and lactase—are located on the microvilli of enterocytes and catalyze the final breakdown of disaccharides into monosaccharides at the site of absorption. Maltase splits maltose into glucose, sucrase hydrolyzes sucrose into glucose and fructose, and lactase breaks down lactose into glucose and galactose. By the time carbohydrates reach the jejunum, they are almost entirely reduced to monosaccharides.
Q4: How do glucose and galactose enter enterocytes differently from fructose?
Glucose and galactose are absorbed via secondary active transport using sodium-glucose symporters at the apical surface of enterocytes, requiring two sodium ions and one glucose or galactose molecule to bind for cellular entry. Fructose, however, enters through facilitated diffusion mediated by GLUT5 transporters, which does not require energy and relies on a concentration gradient between the intestinal lumen and enterocyte cytoplasm.
Q5: What role does the sodium-potassium pump play in monosaccharide absorption?
The sodium-potassium pump at the basolateral membrane maintains the sodium gradient essential for secondary active transport of glucose and galactose. It expels sodium ions from the cell in exchange for potassium ions, creating the concentration difference that drives sodium-glucose symporters to function. This indirect support is critical for efficient glucose and galactose uptake at the apical membrane.
Q6: How are absorbed monosaccharides transported from enterocytes into the bloodstream?
Once inside enterocytes, all monosaccharides are transported across the basolateral membrane into capillaries of the villi via GLUT2 transporters using facilitated diffusion. This process does not require additional energy and ensures rapid transfer into the bloodstream. The capillaries then carry monosaccharides via the hepatic portal vein to the liver for metabolism, storage, or distribution to peripheral tissues.
Q7: How is the small intestine structurally adapted for efficient carbohydrate absorption?
The small intestine maximizes carbohydrate absorption through villi and microvilli that significantly increase surface area for nutrient uptake. A dense capillary network within villi enables swift transport of absorbed nutrients into the bloodstream. Brush border enzymes are strategically positioned on microvilli to immediately hydrolyze disaccharides into monosaccharides at the site of absorption, minimizing nutrient loss and supporting metabolic states of the body the absorptive state.