Digestive System - Intestinal Enzymes Absorption

Question 1

What is digestion ?

Answer 1

Digestion is the chemical degradation of nutrient molecules into simpler component molecules by enzymes secreted by cells in the mouth, stomach, pancreas and small intestine

Question 2

What is absorption ?

Answer 2

Involves the transfer of the products of digestion from the lumen of the gut into the blood

Question 3

How does the structure of the small intestine aid in digestion and absorption ?

Answer 3

Mucosal folds, called folds of Kerckring (valvulae conniventes) which increase the absorptive mucosal area about 3 fold
Microvilli - brush border, increase SA
Enterocyte membrane - capacity to control the flux of solutes and fluid between the lumen and the blood

Question 4

Explain how proteins are digested and absorbed.

Answer 4

Protein digestion begins in the stomach with the action of pepsin
Peptidases are also secreted from brush border for splitting small peptides into amino acids.
Absorbed through the intestinal epithelia by a secondary active transport system - Na+ co-transport system
Local and enteric nervous reflexes in the submucosal plexus initiated by stimuli from the chyme in the intestines control this mechanism

Question 5

Explain how lipids are digested and absorbed.

Answer 5

Fats in the diet are absorbed from the intestinal epithelium into the lymph.
Digested by pancreatic enzymes and bile salts to form micelles and free fatty acids.
These products diffuse into the intestinal epithelium, where they are reconstituted into triglycerides and packaged into chylomicrons.
These chylomicrons enter the submocasal interstitium, intravillous lymphatics, and return to the blood via the lymphatic system.
After a fatty meal, the plasma concentration may rise to 1-2%, but is quickly removed.

Question 6

Explain how water and the major electrolytes are absorbed.

Answer 6

• Around 9L of water enters the small intestine daily, with 0.5L absorbed before reaching the large intestine.
• Water is transported through the intestinal membrane through osmotic movement, reducing gut osmolarity.
• Na+ is transported out of epithelial cells, followed by Cl- and water, causing hydrostatic pressure in the paracellular space.
• Fluid is forced into the interstitium, absorbed by blood capillaries.
• Na+ absorption enhanced by Na+ co-transport system.

Question 7

Explain how vitamins are absorbed.

Answer 7

Vitamins A, D, E, and K are fat-soluble and absorbed with lipids, while B12 requires intrinsic factor and carrier molecule in the ileum. Mineral absorption is proportional to dietary intake, but can be regulated.

Question 8

Explain how calcium is absorbed

Answer 8

Actively absorbed into the blood. Absorption is dependent on Ca2+ binding protein in the mucosal epithelia, parathyroid hormone and vitamin D. Parthyroid hormone activates vitamin D and the activated vitamin D in turn greatly enhances Ca2+ absorption

Question 9

Explain how iron is absorbed

Answer 9

Occurs in the duodenum and jejenum
Ferrous (Fe2+) rather than ferric (Fe3+) ions are absorbed
Iron absorption relies on transferrin (transport protein) which is then taken into the cell by endocytosis

Question 10

Explain how carbohydrates are digested and absorbed.

Answer 10

• Amylase breaks down long oligosaccharides, which are digested in the mouth and small intestine to form monosaccharides.
• These monosaccharides are then absorbed as glucose, the final digestion product of starches.
• Absorption occurs through secondary active transport or facilitated diffusion.
• Secondary active transport uses Na+ diffusion into the cell as an energy source
• Primary active transport pumps Na+ out again
• High intracellular concentrations of glucose/galactose allow them to diffuse into the blood through mucosal capillaries.

Question 11

Explain symport mechanisms

Answer 11

Symport mechanisms are membrane transport mechanisms that allow two or more molecules or ions to move across a cell membrane in the same direction
Use the energy from the electrochemical gradient of one of the transported species.

Question 12

Explain how symport mechanisms are used in oral dehydration therapy.

Answer 12

• In oral dehydration therapy (ORT), these mechanisms are crucial for the absorption of electrolytes and water from the gastrointestinal tract, particularly in treating dehydration caused by conditions like diarrhea and vomiting.
• Involves the co-transport of sodium ions and glucose across the intestinal epithelium, typically in the small intestine.
• Creates an osmotic gradient, allowing water to be absorbed from the intestinal lumen.
• Once inside the enterocyte, sodium ions and glucose are transported across the basolateral membrane into the bloodstream, maintaining the sodium gradient and promoting passive reabsorption of water.