Carbohydrate Digestion flashcards

All 20 cards

1. Card 1

   How does salivary alpha-amylase contribute to carbohydrate digestion in the mouth, and what are its limitations?

   Salivary alpha-amylase, secreted by salivary glands, initiates carbohydrate digestion by cleaving internal alpha-1,4 glycosidic bonds in starch and glycogen. This produces maltose, maltotriose, and limit dextrins. However, it cannot cleave alpha-1,6 branch points and is activated by chloride ions.

2. Card 2

   Why is there no specific enzyme for carbohydrate digestion in the stomach?

   There are no specific enzymes for carbohydrate digestion in the stomach because salivary alpha-amylase, which begins the process in the mouth, is inactivated by the stomach's low pH, which is around 2. This acidic environment denatures the enzyme, halting its activity.

3. Card 3

   How are glucose and galactose absorbed from the intestinal lumen into intestinal cells, and what type of transport mechanism is involved?

   Glucose and galactose are absorbed from the intestinal lumen into intestinal cells via SGLT1 (sodium-glucose linked transporter 1) on the apical side and then transported out by GLUT2 on the basolateral side. This process is secondary active transport, as it co-transports with Na+ and relies on the Na+ gradient maintained by the Na+/K+ ATPase.

4. Card 4

   Explain the enzymatic basis of lactose intolerance and its physiological consequences.

   Lactose intolerance results from a deficiency or reduced activity of the brush border enzyme lactase (lactase-phlorizin hydrolase). Without sufficient lactase, undigested lactose passes into the large intestine, where colonic bacteria ferment it. This fermentation produces gas (CO2, H2, methane) and short-chain fatty acids, leading to symptoms like bloating, flatulence, diarrhea, and abdominal cramping due to gas production and osmotic water retention.

5. Card 5

   Define a glycoside and provide examples of both O-glycosides and N-glycosides.

   A glycoside is a compound where a monosaccharide is linked via a glycosidic bond to another molecule, known as an aglycone. O-glycosides feature a glycosidic bond through an oxygen atom, with examples including lactose, sucrose, maltose, and cardiac glycosides. N-glycosides have a glycosidic bond through a nitrogen atom, such as nucleosides (in DNA/RNA) and coenzymes like NAD+ and FAD.

6. Card 6

   How do starch (amylose and amylopectin) and glycogen differ in terms of their structural characteristics and branching?

   Amylose is a linear helical coil with only alpha-1,4 glycosidic bonds, while amylopectin and glycogen both have alpha-1,4 and alpha-1,6 bonds, forming branched structures. Glycogen is more highly branched than amylopectin, with branches every 8-12 glucose residues compared to every 25-30 residues in amylopectin. This higher branching makes glycogen more compact and provides more non-reducing ends for faster mobilization.

7. Card 7

   Why are humans unable to digest cellulose, despite it being composed of glucose units?

   Humans cannot digest cellulose because it consists of glucose units linked by beta-1,4 glycosidic bonds. Humans lack the specific enzyme, beta-glucosidase (cellulase), required to cleave these bonds. Consequently, cellulose passes through the human digestive system as dietary fiber, undigested.

8. Card 8

   What is the role of pancreatic alpha-amylase in carbohydrate digestion, and how does its specificity compare to salivary alpha-amylase?

   Pancreatic alpha-amylase, secreted by the pancreas into the small intestine, continues the digestion of starch oligosaccharides. It cleaves internal alpha-1,4 glycosidic bonds, producing maltose, isomaltose, and limit dextrins. Its specificity is the same as salivary alpha-amylase, meaning it also cannot cleave alpha-1,6 branch points.

9. Card 9

   Describe the function of maltase, sucrase, and lactase enzymes at the brush border of the jejunum.

   Maltase hydrolyzes maltose into two glucose molecules by cleaving an alpha-1,4 bond. Sucrase, part of the sucrase-isomaltase complex, digests sucrose into glucose and fructose. Lactase (lactase-phlorizin hydrolase) breaks down lactose into glucose and galactose.

10. Card 10

    How is fructose absorbed by intestinal cells, and what type of transport mechanism is employed?

    Fructose is absorbed by intestinal cells via GLUT5 on the apical (luminal) side and then transported out by GLUT2 on the basolateral side. This process is facilitated diffusion, which is a passive mechanism that does not require direct energy input.

11. Card 11

    Explain why boiling saliva affects the results of the Trommer and Lugol tests when starch is present.

    Boiling saliva denatures salivary alpha-amylase, causing it to lose its 3D structure and become inactive. As a result, the boiled saliva cannot digest starch, leaving it intact. Therefore, a Trommer test would be negative (no reducing sugars produced), while a Lugol test would be positive (starch is still present and turns blue-black with iodine).

12. Card 12

    Distinguish between primary and secondary lactase deficiency.

    Primary lactase deficiency is the most common form, characterized by a genetic reduction in lactase activity after weaning, typically in adulthood. Secondary lactase deficiency, on the other hand, results from intestinal damage caused by conditions such as celiac disease or Crohn's disease, which impairs the production or activity of lactase.

13. Card 13

    How does the structural difference in branching between glycogen and amylopectin relate to their roles as energy stores?

    Glycogen is more highly branched than amylopectin, with branches occurring every 8-12 glucose residues compared to every 25-30 residues. This increased branching in glycogen provides more non-reducing ends, allowing for faster mobilization of glucose units. This makes glycogen a superior rapid-response energy store compared to starch.

14. Card 14

    What are glycoconjugates? List the major classes and provide an example for each.

    Glycoconjugates are molecules composed of carbohydrates covalently linked to proteins or lipids. The major classes include glycoproteins (proteins with N-linked or O-linked oligosaccharide chains, e.g., immunoglobulins), proteoglycans (proteins with long glycosaminoglycan chains, e.g., heparan sulfate), and glycolipids (lipids with covalently attached sugar chains, e.g., cerebrosides).

15. Card 15

    How does isomaltase (alpha-dextrinase) contribute to the complete digestion of carbohydrates in the small intestine?

    Isomaltase, which is part of the sucrase-isomaltase complex at the brush border, is crucial for completely digesting carbohydrates by cleaving alpha-1,6 glycosidic bonds. These bonds are found in isomaltose and limit dextrins, which are products of alpha-amylase activity. By breaking these branch points, isomaltase ensures the release of glucose for absorption.

16. Card 16

    Explain the mechanism by which SGLT1 facilitates glucose and galactose absorption, highlighting the role of the Na+ gradient.

    SGLT1 facilitates glucose and galactose absorption through secondary active transport. It co-transports one glucose or galactose molecule with two sodium ions from the intestinal lumen into the enterocyte. This process is driven by the electrochemical gradient of Na+, which is maintained by the Na+/K+ ATPase pump on the basolateral membrane, actively pumping Na+ out of the cell.

17. Card 17

    What are the most common dietary carbohydrates (polysaccharides and disaccharides) that undergo digestion in the human body?

    The most common dietary carbohydrates that undergo digestion are polysaccharides like starch and glycogen, and disaccharides such as maltose, sucrose, and lactose. These complex carbohydrates are broken down into monosaccharides for absorption.

18. Card 18

    Describe the structural features of amylose and amylopectin components of starch.

    Amylose is a linear polymer of glucose units linked exclusively by alpha-1,4 glycosidic bonds, forming a helical coil. Amylopectin, on the other hand, is a branched polymer, also containing alpha-1,4 glycosidic bonds but with alpha-1,6 glycosidic bonds forming branches approximately every 25-30 glucose residues, giving it a tree-like structure.

19. Card 19

    How do the enzymes involved in carbohydrate digestion change in location and specificity from the mouth to the small intestine brush border?

    Digestion begins in the mouth with salivary alpha-amylase, which cleaves internal alpha-1,4 bonds in starch and glycogen. In the small intestine lumen, pancreatic alpha-amylase continues this, with the same specificity. Finally, at the brush border, enzymes like maltase, sucrase, lactase, and isomaltase (alpha-dextrinase) complete digestion by breaking down disaccharides and alpha-1,6 bonds, respectively, into monosaccharides.

20. Card 20

    Why is glycogen considered a better rapid-response energy store than starch?

    Glycogen is a better rapid-response energy store than starch because it is more highly branched. With branches occurring every 8-12 glucose residues, compared to every 25-30 in amylopectin, glycogen presents more non-reducing ends. These numerous ends allow for faster enzymatic cleavage and glucose mobilization when energy is quickly needed.

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