28.10
Absorção de Carboidratos
Carboidratos são macronutrientes essenciais que servem como principal fonte de energia do corpo. Sua digestão começa na boca, onde a amilase salivar q…
A digestão de carboidratos, como o amido dietético, começa na boca com a mastigação e a ação da amilase salivar.
Depois de serem parcialmente digeridos pela amilase salivar em oligossacarídeos, esses carboidratos são incorporados ao quimo e passados para o duodeno.
Aqui, a amilase pancreática decompõe o amido restante em oligossacarídeos menores, que são convertidos em monossacarídeos pelas enzimas da borda da escova intestinal.
Esses monossacarídeos - principalmente glicose, frutose e galactose - estão agora prontos para absorção pelos enterócitos que revestem o jejuno e o íleo.
Na extremidade apical dos enterócitos, a glicose e a galactose entram por transporte ativo secundário com íons de sódio.
O transportador de sódio-glicose possui sítios de ligação para uma molécula de glicose e dois íons de sódio, que devem ser ocupados para o transporte intracelular.
A galactose também usa o mesmo simportador para entrada celular, enquanto a frutose usa difusão facilitada.
Uma vez dentro do enterócito, todos os monossacarídeos são transportados através da membrana basolateral para os capilares das vilosidades por meio de difusão facilitada.
Eventualmente, os capilares transportam os monossacarídeos para o fígado através da veia porta hepática.
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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.