Digestion and Absorption

Amplification of GI Mucosal Surface area

• Circular folds: increase surface area 3x

  • Villi on folds: increases by 30x

    • On cells of villi: microvilli: 600x increase: largest increase!

Polysaccharides

• Starch: from plants, complex

  • most abundant carb in diet

• Glycogen: from animals (meat)

  • muscle glycogen

• Cellulose: Plant cell walls

  • NOT digestible

  • Serves as fibers (bulk in GI to aid in motility)

sucrose

table sugar: glucose + fructose (fruit sugar)

Lactose

milk sugar: glucose + galactose

Maltose

• glucose + glucose

Disacchardies

Simple sugars:

• sucrose

• Maltose

• maltose

Monosaccharides

• Glucose (most abundant)

• Fructose

• Galactose

Digestion of Carbs

• Mouth

  • Salivary amylase

  • Initiates digestion (small amt)

• Stomach: NONE

  • Pepsin inactivates amylase

• Small intestine

  • Pancreatic amylase

    • Majority of carb digestion

  • Epithelial Disaccharides: In cell membranes

    • break down disaccharides

Absorption of Carbs

• secondary active transport using sodium gradient

• From lumen, carrier mediates transport across membrane

Types of lipids

• Triglycerides (90%)

  • 2-monoglyceride + fatty acids

• Cholesterol

• Phosphipids

Digestion of Lipids

• Mouth: Lingual lipase

• Small Intestine: fat is emulsfied, increasing SA for lipases to break down fats

• Pancreatic Lipase

  • Breaks fats into monoglycerides that are able to cross membrane

  • Resynthesized into chylomicrons and removed via exocytosis

    • Low density: high fat, low protein

    • Tryglyceride core with phospholipid membrane

    • Picked up by lymphatics

Absorption of fats

• Fats and cholesterol are packaged into chylomicrons and released via exocytosis into villia

• Lacteals: lymphatic capillary that are highly permeable

  • Enter circulation via lymphatic system

Proteins Types

• proteins

• Peptides

• Amino acid

Digestion of Proteins

• Stomach: pepsin

• Small intestine:

  • pancreatic enzymes

    • Trypsinogen:

    • Endopeptidases

    • Exopeptidases

  • Epithelial enzymes: in wall of duodenum

    • Cleaves small peptides (dipeptides, tripeptides) into amino acids

Pancreatic Enzymes: Trypsinogen

activated by enteropeptidase to trypsin, which activates other proeznymes (digestion of proteins)

Pancreatic enzyme: Endopeptidase (3)

cleave internal peptide bones: trypsin, chymotrypsin, elastase

Pancreatic enzyme: Exopeptidases:

cleave terminal peptide bonds: procarboxypeptidases A + B

Absorption of Proteins

• has to be broken down into amino acids, if absorbed: immune rxn

  • take up via secondary active transport and sodium gradient

• Di and tripeptides:

  • more efficiently absorbed than amino acids

Vitamin absorption: fat soluble

• A, D, E, K

• passively absorbed in small intestine

• Can cross membrane

• Not well regulated: can lead to toxicity

Water soluble vitamin absorption

• Passive and active processes in small intestine

• Requires transport mechanisms

vitamin B12 absooprtion and consequence of deficiency

• Via intrinsic factor (parietal cells in stomach)

  • binds with ingested B12

• Receptor-mediated endocytosis in small intestine of B12 intrisinc factor complex that releases into blood

• Consequence

  • Shiny red tongue

  • Pernicious anemia: inability to form RBCs, lack of O2, fatigued

B12 is needed for

RBC synthesis

Calcium absorption

• does not cross membrane

• high conc in gut

• moves down concentration gradient thru calcium channels

• CaBP: calcium binding protein

• Transported out/in by primary active transport thru ATPase

• Vitamin D increases CaBP and Ca ATPase

  • steroid that changes gene expression, takes a few hours

Why is iron absoprtion ususal?

• Toxic molecule: oxidizes membranes and produces problems

• Keep it bound to protein: heme

Iron Absorption

• iron and heme absorbed by DMT-1: facilitated diffusion

• heme iron absorbed by heme transporter

• Equilibrates with ferritin (holds thousands of Iron molecules)

  • In gut epithelial cells: storage of iron

Elimination of iron

• keeping iron in ferritin is a way of eliminating:

  • lost with enterocytes (they turnover rapidly and ferritin is removed with them)

  • Eliminated in feces

• Free iron is transported out into circulation where it beings with transferrin in plasma

low plasma iron/iron deficiency

• increase DMT-1: take up more iron from gut lumen

• Increase ferraportin: more transfer into plasma

• Decrease ferritin: decrease storage/elimination (more goes into plasma)

iron excess

• decrease DMT-1: less iron take up from gut lumen

• Decrease ferraportin:

  • from liver produced hepcidin

• Increase ferritin: iron is trapped in ferritin and lost as enterocytes are sloughed off from gut lining