28.10
탄수화물 흡수
탄수화물은 신체의 주요 에너지원으로 작용하는 필수적인 거대 영양소입니다. 소화는 입에서 시작되며, 침샘 아밀라아제는 전분과 같은 복합 탄수화물을 작은 올리고당으로 부분적으로 분해합니다. 이러한 기계적 및 효소적 활동은 탄수화물을 위장관에서 추가 처리하기 위해 준비합니다…
식이 전분과 같은 탄수화물의 소화는 씹는 것과 타액 아밀라아제의 작용으로 입안에서 시작됩니다.
타액 아밀라아제에 의해 올리고당으로 부분적으로 소화된 후, 이러한 탄수화물은 chyme에 통합되어 십이지장으로 전달됩니다.
여기서 췌장 아밀라아제는 남은 전분을 더 작은 올리고당으로 분해하고, 이는 장 브러시 경계 효소에 의해 단당류로 전환됩니다.
주로 포도당, 과당, 갈락토오스와 같은 이러한 단당류는 이제 제주넘과 회장을 감싸고 있는 장세포에 의해 흡수될 준비가 되었습니다.
장세포의 정점 끝에서 포도당과 갈락토스는 나트륨 이온과 함께 2차 활성 수송으로 들어갑니다.
나트륨-포도당 수송체는 하나의 포도당 분자와 두 개의 나트륨 이온에 대한 결합 부위를 가지고 있으며, 이는 세포 내 수송을 위해 점유되어야 합니다.
Galactose는 또한 세포 진입을 위해 동일한 symporter를 사용하는 반면 과당은 촉진 확산을 사용합니다.
장세포(enterocyte) 내부로 들어가면 모든 단당류는 촉진 확산(facilitated diffusion)을 통해 기저외막(basolateral membrane)을 가로질러 융모의 모세혈관으로 운반됩니다.
결국, 모세혈관은 간문맥을 통해 단당류를 간으로 운반합니다.
View the full transcript and gain access to JoVE Core videos
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.