a movement in which stationary ringlike constrictions appear at several places along the intestine and then relax as new constrictions form elsewhere ● Mixing/contact digestion ● When most nutrients have been absorbed and little remains but undigested residue, segmentation declines and peristalsis begins ■ Peristalsis- The duodenum secretes a hormone called motilin that triggers a peristaltic wave beginning in the duodenum. ● The wave travels 10 to 70 cm and dies out ● only to be followed by another wave that begins a little farther down the tract than the first one ○ Migrating motor complex- overlapping waves of contraction ● Movement through Small Intestine ○ Carbohydrate Digestion - Pancreatic amylase digests starch into maltose and small oligosaccharides. Brush border enzymes (maltase, dextrinase, and glucoamylase) digest these to glucose, which is absorbed by the epithelial cells. ■ Starch is digested first to oligosaccharides up to eight glucose residues long ■ then into the disaccharide maltose ■ finally to glucose, which is absorbed by the small intestine
■ The Process ● Mouth- starts here ○ Salivary amylase- breaks starch down into shorter segments (oligosaccharides) ● Stomach- the acid in the stomach ○ Churning, mixing ● Small Intestine- 50% of the starch is digested before it gets here, ○ Pancreatic Amylase ○ Dextrinase and Glucoamylase ○ Maltase, Sucrase, Lactase ■ The end product of all of these is glucose, which is then absorbed ○ Carbohydrate Absorption ■ Glucose transported into absorptive cells ● Sodium-Dependent Glucose Transporter (SGLT) ○ Secondary active transport coupled to sodium ■ Galactose- monosaccharide sugar ● Uses SGLT as well ■ Fructose ● Via facilitated diffusion using a separate carrier that doesn’t depend on Na + ● Inside the enterocyte fructose is converted into glucose ■ All transported to liver by the hepatic portal system ○ Protein Absorption ■ Amino Acid absorption similar to monosaccharides ● Enterocytes have several sodium-dependent amino acid cotransporters for different classes of amino acids ● they leave the cell by facilitated diffusion, enter the blood capillaries of the villus, and are carried away in the hepatic portal circulation ■ Dipeptides and tripeptides also absorbed ● they are hydrolyzed within the enterocytes before their amino acids are released to the bloodstream ○ Lipid Digestion ■ Small Intestine ● Emulsification droplets are promptly passed on to the duodenum and coated by certain components of the bile—lecithin and bile acids ○ Lecithin and bile salts coat fat droplets ■ Pancreatic Lipase- TRG into FFA and Monoglyceride ● when lipase acts on a triglyceride, it removes the first and third fatty acids from the glycerol backbone and usually leaves the middle one.
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