28.10
碳水化合物的吸收
碳水化合物是人体的主要能量来源，是必需的常量营养素。它们的消化始于口腔，唾液淀粉酶将复杂的碳水化合物（如淀粉）部分分解成较小的寡糖。这种机械和酶的活性为碳水化合物在胃肠道中进一步加工做好准备。
吞咽后，部分消化的碳水化合物与胃分泌物在胃中混合。然而，酸性环境会使唾液淀粉酶失活，暂时停止碳水化合物的消…
碳水化合物（如膳食淀粉）的消化从口腔开始，通过咀嚼以及唾液淀粉酶的作用进行。
这些碳水化合物经唾液淀粉酶部分消化为寡糖后，进入食糜并被推入十二指肠。
在此，胰淀粉酶将剩余的淀粉分解为较小的寡糖，后者由小肠刷状缘酶进一步转化为单糖。
这些单糖——主要是葡萄糖、果糖和半乳糖——现已可被衬于空肠和回肠的肠上皮细胞吸收。
在肠上皮细胞的顶端，葡萄糖和半乳糖通过与钠离子的继发性主动运输进入细胞。
钠-葡萄糖转运蛋白具有一个葡萄糖分子和两个钠离子的结合位点，这些位点必须被占据才能进行细胞内转运。
半乳糖也利用相同的同向转运蛋白进入细胞，而果糖则通过易化扩散进入。
进入肠上皮细胞后，所有单糖均通过易化扩散穿过基底侧膜，进入绒毛的毛细血管中。
最终，毛细血管通过门静脉将单糖运送至肝脏。
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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.