The Digestive System

1. The Digestive System Gastrointestinal tract Physiology Dr. Suaad M. Ghazi MBChB, MSc, PhD

2. Objectives of lecture 7 1. Explain the digestion and absorption of carbohydrate, fat and protein in GIT.

3. Digestion & Absorption

4. Digestion of carbohydrates •  Three major sources of carbohydrates in diet • Sucrose (disaccharide) also known as cane sugar or table sugar • Lactose (disaccharide) in milk • Starches (polysaccharides) present in almost all non animal foods especially in the grains. •  Other carbohydrates ingested to a slight extent are glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, and others.

5. Digestion of carbohydrates In the mouth ♦ the food remains for short time , only 3 - 5% of all starches that have been eaten will have become digested. ♦ Enzyme α-amylase (ptyalin) hydrolyzes starches into the disaccharide maltose and other small polymers of glucose.

6. In the stomach: ♦ Salivary amylase can continue for an hour after the food has entered the stomach secretions. ♦ Activity of the salivary amylase is blocked by the acid of the gastric secretions. ♦ 30 - 40% of the starches will have been hydrolyzed mainly to maltose.

7. In the small intestine ♦ Pancreatic amylase that continue splitting starches into maltose and small polymers of glucose. ♦ Now, the starches are totally converted into maltose and other very small glucose polymers before they have passed beyond the jejunum.

8. ♦ The brush border of epithelial cells lining the small intestine contain enzymes lactase, sucrase, and maltase, which are capable of splitting the disaccharides lactose, sucrose, and maltose, into their constituent monosaccharides. • ♦ The monosaccharides, are then immediately absorbed into the portal blood. • Starches (polysaccharide)  Maltose (disaccharide) + Glucose. • Lactose (disaccharide)  Galactose + Glucose. • Sucrose (disaccharide)  Fructose + Glucose. • Maltose (disaccharide)  Glucose + Glucose.

10. Absorption of carbohydrates ♦ All carbohydrates are absorbed in the form of monosaccharides. ♦ Small fraction being absorbed as disaccharides. ♦ Almost none as larger carbohydrate compounds absorbed. (1) Glucose and galactose are transported from the intestinal lumen into the cells by Na+-glucose or Na+-galactose cotransport in the luminal brush border of the membrane. ♦ Sugar is transported uphill. ♦ Na+ is transported downhill. ♦ When glucose concentration increased to a higher level  diffuse by facilitated diffusion through the basolateral membrane of the epithelial cell  extracellular fluid  blood.

11. ♦ The Na+-K+ pump in the basolateral membrane keeps the intracellular Na+ concentration low, thus maintaining the Na+ gradient across the luminal membrane. (2) Fructose is transported to small intestinal enterocyte by facilitated diffusion. Within the enterocytes, it is mainly converted into glucose inside the epithelial cell before entering the portal blood.

13. Na+ K+

14. ♦ Deficiency of disaccharidase leads to diarrhoea, bloating, and flatulence after ingestion of sugar. ♦ The diarrhea is due to the increased number of osmotically active oligosaccharides molecules that remain in the intestinal lumen causing the volume of the intestinal contents to increase. ♦ The bloating and flatulence are due to the production of gas (CO2 and H2) from disaccharide residues which are metabolized by intestinal flora in the lower small intestine and colon.

15. ♦ Lactose intolerance is the most common cause of carbohydrate malabsorption. ♦ It results from the inability of the goblet cells to produce lactase. ♦ In infants, the diarrhea-produced dehydration can be life threatening. ♦ This disorder is hereditary, affecting 5-15% of Europians and 80-90% of Asians and Africans.

16. Bad things happen when solutes are not absorbed Colon Jejunum Ileum Na+ Lactose glucose + galactose H2O H2O Normal Diarrhea SCFA CO2 Lactose Lactose H2O Lactose H2O Lactase deficiency GI-LI-24 H2O

17. Digestion of fat ♦ Fat in the diet are: The neutral fats (triglycerides), Small quantities of phospholipids, cholesterol, and cholesterol ester. ♦ Very little digestion of fat occur by Lingual and gastric lipase. Essentially all fat digestion occurs in the small intestine as follows:

18. [1] Emulsification of fat by bile acids [2] Digestion of fats by pancreatic lipases [3] Transportation of fats to the brush border of the epithelial cells

19. [1] Emulsification of fat by bile acids ♦ The first step in fat digestion is to break the fat globules into small sizes so that the water soluble digestive enzymes can act on the globule surfaces. This process is called emulsification of the fat and it is achieved under the influence of bile salts and aided by lecithin in the bile.

20. ♦ Both decrease the interfacial tension of the fat globules which result in breaking up these globules into many minute particles when they are agitated. This result to an increase in surface area of fat globules as much as 1000 fold.

21. [2] Digestion of fats by pancreatic lipases ♦ Lipases, cholesterol estrase, and phospholipase A2. ♦ Small intestine contain a minute quantity of lipase known as enteric lipase. ♦ TG are spilt into FFAs and MG. ♦ Small portions are not digested and remain in the diglyceride state.

22. [3] Transportation of fats to the brush border of the epithelial cells Bile salts play an important role in removing the MG and FFAs from the vicinity of the digesting fat globule, by the following way : ♦ TG → MG & FFA dissolved in the fatty portion of the micelles. ♦ Also the bile salt micelles act as a transport medium to carry MG & FFA.

23. ♦ Both relatively insoluble to the brush border of the epithelial cells. There, MG & FFA are absorbed. ♦ Bile salts are again released back into the chyme to be used again and again for this ferrying process. ♦ Few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell, lipase.

24. ♦ Cholesterol esters and phospholipids are hydrolyzed by lipases in the pancreatic secretion → FFA. ♦ The bile salt micelles also play role in ferrying free cholesterol and phospholipids same with MG & FFA. role of the bile salt micelles is absolutely essential to absorption of cholesterol and no cholesterol absorption can take place without bile salts.

25. ♦ 60% of the TG can be digested and absorbed even in the absence of bile salts. ♦ Because short- and medium chain TG are more water soluble (not digested by lipases and are not incorporated into micelles) ♦ They diffuse through water layer adjacent to the brush border of the enterocytes and are absorbed intact. ♦ Inside the cell, they are hydrolyzed by esterase.

26. Absorption of fats ♦ As micelles come in contact with the brush border microvilli→ MG & FAs diffuse through the epithelial membrane, because they are equally soluble in this membranes as in the micelles. ♦ These fat products leave the micelles and are absorbed across the epithelial cell membranes, the micelles can pick up more MG & and FFAs which have been produced from digestion of other TG molecules in the fat emulsion.

27. ♦ Bile salts continuously repeat their fat-solubilizing function down the length of the small intestine until all fat is absorbed. ♦ Then the bile salts themselves are reabsorbed in the terminal ileum by a Na+ - dependent active transport process.

28. Absorption of fats

30. ♦ After entering the epithelial cell, FAs & MG are taken up by the smooth endoplasmic reticulum (SER)→ recombined to form new TG → TG aggregate within the ER into globules → newly synthesized cholesterol and phospholipids. ♦ The phospholipids arrange in globules with fatty portions toward the center and the polar portions toward the surface.

31. ♦ This provides an electrically charged surface makes globules miscible (water soluble) with the fluids of the cell. ♦ Small amounts of β-lipoprotein synthesized by ER, coat part of the surface of each globule.

32. ♦ In this form the globule diffuse to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells, from these it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons. ♦ Chylomicrons propelled along with the lymph upward through the thoracic duct to be emptied into the great veins of the neck.

33. Lipid malabsorption ♦ Lipid malabsorption is much more common than carbohydrate or protein malabsorption. [1] The pancreas does not secrete sufficient quantities of lipase (as in pancreatitis). [2] The liver does not secrete sufficient quantities of bile (as in liver parenchymal diseases).

35. Absorption of Fats Figure 14.14