Chemical Digestion and Absorption of Nutreints

Annotations:

• Continued on from the previous lecture, the enterocyte is a apical epithelium from the small intestine

Annotations:

• pancreatic amylase converts starch to disaccharides. Then brush border digestion at the enterocyte converts disaccharides to monosaccharides thru actions of disaccharidases i.e Sucrose to glucose and fructose via sucrase Lactose to glucose and galactose via lactase Maltose 

1. Absorption

   Annotations:

   •        Glucose, fructose, galactose are absorbed by the small intesOne in a two step process. Step 1. Across the apical cell membrane glucose and galactose  Na+/glucose co-transporter (SGLT1) fructose  facilitated sugar transporter (GLUT5) Step 2. Across the basal cell membrane glucose, galactose fructose  facilitated sugar transporter (GLUT2)    
2. Abnormalities

   Annotations:

   • lactose intolerance, enzyme lactase is deficient. Therefore lactose accumulates in the gut, it acts as an osmolytes (absorbs water in lumen), acts as nutrient for bacteria, accumulation of water and gas leads to irritation, bloating and diarrhoea. Coeliac disease:  Intolerance to gluten found in wheat, rye, barley and oats. Immune system damage esp the villi, interfere the absorption of nutrients and hence malnutrition.

Annotations:

• occur thru 4 pathways. 30%: Luminal Protrease from stomach, pancreas and small intestine to hydrolyse proteins to small peptides and amino acids 70% are broken down by brush border amino peptidase and cytoplasmic peptidase 1% lind of absorbed and cross directly into blood stream

1. Absorption

   Annotations:

   • 2 step similar to carbohydrates: 1: Apical transport of amino acids Using Na* dependent amino acid co-transporter, driven by inward directed Na+ gradient. H*-dependednt peptide co-transporter (pepT1) Active process driven by H+ gradient. 2: basal transport of amino acids  Na+ independent amino acid co-transporter... At least 5 amino acid transporters presented in basal membrane. 3 mediate transport out of the enterocyte and 2 absorb aa's into enterocytes. 90% of absorbed aa are transported across the basal membrane and into circ. 10% is used for intracellular protein synthesis

Annotations:

• Takes place in small intestine as a result on pancreatic lipase. Fats must be modified via: A. Emulsification by bile salts B. Hydrolysis by pancreatic lipase's

1. Absorption

Annotations:

• H2O is absorbed very well in the GI tract via a continual and bi-directional process.. Net movement occurs by osmotic diffusion. Predominantly via tight junctions and intercellular spaces between enterocytes.

1. Sodium

   Annotations:

   • Absorbed via SLGT1 and BAT1. Na/H exchanger Neutral co-transport with Cl Epithelial channel. Gradient is provided by na/k/atpase pump.
2. K

   Annotations:

   • Absorption is by passive secretion thru tight junctions and lateral intercellular spaces of the enterocytes. Driving force is due to the difference between the luminal K and blood k. Water absorption increases luminal K. Therefore increased K absorption.
3. Calcium

   Annotations:

   • Via active absorption. Using epithelial calcium channels Calcium binding proteins Ca/ATPase pump. In duodenum and jejunum Promoted by Vit D derivative. 1,25 dihydroxycholecalciferol.
4. Iron

   Annotations:

   • Actively absorbed in Ferrous form... Derric reductase turns ferric into ferrous Apical surface absorb it by divalent metal transporter (DMT-1) 2 fates: 1: Apoferritin to form ferriten to be stored 2. Ferroportin to be transported. It then bound and transported by transferrin in blood. Liver hormone hepcidin regulates iron homeostassis.
5. Vitamins
   1. Fat Soluble

      Annotations:

      • A, D, E K Absorbed along with lipids
   2. Water Soluble

      Annotations:

      • Vit B-complex and C Require transport proteins B12 esp need protection from IF secreted from gastric parietal cells.