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Carbohydrate absorption

  • Carbohydrates are large macromolecules consisting of carbon, hydrogen and oxygen atoms. They have the empirical formula Cm(H2O)n, where m is different from n. Exceptions do exist though.
    • Four different forms exist, depending on their size: Polysaccharides, oligosaccharides, disaccharides, and monosaccharides.
  • Most of our dietary carbohydrate is in the form of starch, with a small amount of sucrose and lactose.
  • Polysaccharide hydrolysis begins in the oral cavity with salivary alpha-amylase, however most of the polysaccharide breakdown occurs in the upper intestines by pancreatic amylase. This converts polysaccharides into maltose, a disaccharide.
  • The dietary carbohydrates are then presented to the small intestines in the form of disaccharides: Lactose (Galactase + Glucose), Maltose (Glucose + Glucose) & Sucrose (Glucose + Fructose)
  • The brush-border membrane of the small intestine contains disaccharidases (Lactase, Maltase and Sucrase), which further reduce the disaccharides into monosaccharides which can be absorbed.
  • Glucose and galactose are absorbed by secondary active transport, in which co-transport carriers (Sodium-glucose transporter proteins) on the luminal border transports both the monosaccharide and Na+ from the lumen into the interior of the intestinal cell. This therefore does not use energy, but instead depends on the sodium concentration gradient.
  • When the glucose/galactose is in the cell, it is then able to move down its concentration gradient by means of a passive carrier (GLUT2 receptors) in the basolateral border to enter the blood within the villus.
  • Fructose however is absorbed into the blood by facilitated diffusion via the GLUT5 transporter.

Protein absorption

  • Proteins are macromolecules involving the polymerisation of amino acids (empirical formula: H2N-HCR-COOH) linked together by peptide bonds. Note: R is a variable region.
  • Protein absorption involves both exogenous (food sources) and endogenous proteins that have entered the digestive tract lumen, e.g. digestive enzymes, proteins within cells and small amounts of plasma protein that leak from the capillaries.
  • Dietary protein digestion typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid. This reduces the polypeptides into smaller peptide chains or amino acids.
  • Digestion then continues via pancreatic proteolytic enzymes. The pancreas release pro-enzymes, trypsinogen and chymotrypsinogen.
  • These become active within the lumen as trypsinogen is converted into trypsin by enterokinases, an enzyme present on the brush border of the small intestine. Trypsin then activates chymotrypsinogen; together these digest proteins into amino acids or into smaller oligopeptide fragments.
  • The amino acids are then absorbed across the intestinal cells by secondary active transport, i.e. via the sodium concentration gradient. Small peptides however gain entry through a different carrier, and may be broken down by aminopeptidases within the brush border or intracellular peptidases.
  • Once broken down within the cell, they are able to diffuse passively into the blood.

Fat absorption

  • Since fat is not soluble in water, it must undergo a series of transformations in order to be digested and absorbed.
  • Dietary fat in the form of large fat globules composed of triglycerides is first emulsified by the detergent action of bile salts (secreted in the liver; stored in the gallbladder) into a suspension of smaller fat droplets. This lipid emulsion prevents the fat droplets from coaelescing and thereby increases the surface area available for attack by pancreatic lipases.
  • Lipase then hydrolyzes triglicerides into monoglycerides and free fatty acids.
  • These water-insoluble products are then carrier into the interior of water-soluble micelles, which are formed by bile salts and other bile constituents, to the luminal surface of the small intestine epithelial cells
  • When a micelle approaches the absorptive epithelial surface, the monoglycerides and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes
  • The monoglycerides and free fatty acids are then resynthesized into triglycerides inside the epithelial cells
  • These triglycerides aggregate are are coated with a layer of lipoprotein to form water-soluble chylomicrons, which are extended through the basal membrane of the cells by exocytosis
  • Chylomicrons however are unable to cross the basement membrane of blood capillaries, so instead enter the lymphatic vessels, the central lacteals.

Vitamin absorption

  • There are two groups of vitamins
    • Water-soluble vitamins (primarily absorbed with water)
    • Fat-soluble vitamins (A, D, E, K)
      • These are vitamins carried in the micelles and absorbed passively with the end products of fat digestion.

References

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