- Carbohydrates, fats, and proteins are the major nutrients the body needs for growth, repair, movement, and maintaining tissue and organ function.
- These macromolecules are broken down and absorbed into the body at different rates and into specific forms as they travel through the organs in the digestive system.
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Digestion of carbohydrates
Among carbohydrates, only the monosaccharide forms are absorbed. Hence, all carbohydrates must be digested to glucose, galactose, and fructose for absorption to proceed.
Enzymes Involved
- a-Amylases (salivary and pancreatic) hydrolyze 1,4-glycosidic bonds in starch, yielding maltose, maltotriose, and α-limit dextrins.
- Maltase, a-dextrinase, and sucrase in the intestinal brush border then hydrolyze the oligosaccharides to glucose.
- Lactase, trehalase, and sucrase degrade their respective disaccharides lactose, trehalose, and sucrose to monosaccharides.
- Lactase degrades lactose to glucose and galactose.
- Trehalase degrades trehalose to glucose.
- Sucrase degrades sucrose to glucose and fructose.
Absorption of carbohydrates
1. Glucose and Galactose
- They are transported from the intestinal lumen into the cells by a Na+-dependent co-transport (SGLT 1) in the luminal membrane.
- The sugar is transported “uphill” and Na+ is transported “downhill.”
- They are then transported from cell to blood by facilitated diffusion (GLUT 2).
- The Na+–K+ pump in the basolateral membrane keeps the intracellular [Na+] low, thus maintaining the Na+ gradient across the luminal membrane.
2. Fructose
- Fructose is transported exclusively by facilitated diffusion; therefore, it cannot be absorbed against a concentration gradient.
Digestion of proteins
- Dietary proteins are a source of amino acids that are utilized for the formation of various cellular substances.
- Mostly, proteins must be broken down into amino acids for absorption. Digestive products of protein can be absorbed as amino acids, dipeptides, and tripeptides
- Both endopeptidases enzymes which degrade proteins by hydrolyzing interior peptide bonds and exopeptidases enzyme that hydrolyzes one amino acid at a time from the C-terminus of proteins and peptides are involved in the digestion of proteins.
- Digestion takes place in the stomach and the small intestine.
Enzymes Involved
- Pepsin
- Pepsin is secreted in its zymogen form as pepsinogen by the chief cells of the stomach.
- Pepsinogen is activated to pepsin by gastric H+. The optimum pH for pepsin is between 1 and 3.
- Pepsin hydrolyzes proteins into peptones and proteoses.
- When the pH is >5, pepsin is denatured. Thus, in the intestine, as HCO3− is secreted in pancreatic fluids, duodenal pH increases, and pepsin is inactivated.
- Pancreatic proteases
- The digestion is completed in the small intestine by the action of pancreatic and intestinal juice.
- The proteases include trypsin, chymotrypsin, elastase, carboxypeptidase A, and carboxypeptidase B.
- They are secreted in inactive forms that are activated in the small intestine as follows:
- Trypsinogen is activated to trypsin by a brush border enzyme, enterokinase.
- Trypsin then converts chymotrypsinogen, proelastase, and procarboxypeptidase A and B to their active forms.
Absorption of Proteins
1. Free amino acids
- Na+-dependent amino acid cotransport occurs in the luminal membrane. It is analogous
- to the cotransporter for glucose and galactose.
- The amino acids are then transported from cell to blood by facilitated diffusion.
- There are four separate carriers for neutral, acidic, basic, and imino amino acids, respectively.
2. Dipeptides and tripeptides
- They are absorbed faster than free amino acids.
- H+-dependent cotransport of dipeptides and tripeptides also occurs in the luminal membrane.
- After the dipeptides and tripeptides are transported into the intestinal cells, cytoplasmic peptidases hydrolyze them to amino acids.
- The amino acids are then transported from cell to blood by facilitated diffusion.
Digestion of Fats
- Fats not being soluble in water by their nature are both difficult to digest and absorb. They do not mix with the stomach or intestinal contents.
- Lipids include triglycerides, phospholipids, cholesterol, steroids, and fat-soluble vitamins.
- The first step in lipid digestion is emulsification, which is the transformation of large lipid droplets into much smaller droplets.
- The emulsification process increases the surface area of the lipid-exposed to the digestive enzymes by decreasing the droplet size.
Enzymes Involved
1. In the mouth
- Lingual lipases digest some of the ingested triglycerides to monoglycerides and fatty acids.
- However, most of the ingested lipids are digested in the intestine by pancreatic lipases.
2. Stomach
- In the stomach, mixing breaks lipids into droplets to increase the surface area for digestion by pancreatic enzymes.
3. Small intestine
- Bile acids emulsify lipids in the small intestine, increasing the surface area for digestion. The hydrophobic products of lipid digestion are solubilized in micelles by bile acids.
- Pancreatic lipases hydrolyze lipids to fatty acids, monoglycerides, cholesterol, and lysolecithin. The enzymes are pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2.
Absorption of Fats
- Micelles bring the products of lipid digestion into contact with the absorptive surface of the intestinal cells.
- Then, fatty acids, monoglycerides, and cholesterol diffuse across the luminal membrane into the cells. Glycerol is hydrophilic and is not contained in the micelles.
- In the intestinal cells, the products of lipid digestion are re-esterified to triglycerides, cholesterol ester, and phospholipids and, with apoproteins, form chylomicrons.
- Chylomicrons are transported out of the intestinal cells by exocytosis.
- Because chylomicrons are too large to enter the capillaries, they are transferred to lymph vessels and are added to the bloodstream via the thoracic duct.
References
- Chung, K. W., Chung, H. M., & Halliday, N. L. (2015). BRS Gross anatomy (Eighth edition.). Philadelphia: Wolters Kluwer Health.
- Waugh, A., & Grant, A. (2009). Ross and Wilson: Anatomy and Physiology in Health and Illness. (11th edition). Churchill Livingstone
- Hall, J. E. 1. (2016). Guyton and Hall textbook of medical physiology (13th edition.). Philadelphia, PA: Elsevier.
- http://www.onlinebiologynotes.com/physiology-of-digestion/