Digestive system lecture 18 B

Mechanical digestion

mastication… masturbation..

Chemical digestion

duh

Saliva

• amylase, lipase, lysozume hydrolysis with enzymes and acid

Chemical digestion of proteins

• HCl attempts to break covalent bonds

  • disulfide broken, which makes the protein linear. this leaves enough space for enzymes to do their job and break peptide bonds

• HCl also activates pepsin (pepsinogen → pepsin)

  • its in the form of a zymogen bc we don’t want active enzymes that break down proteins to be active all the time because then they’ll just break down tissues

    • highly regulated type shift by HCl

• pepsin is an endoprotease

  • meaning, it’ll attack peptide bonds somewhere in the middle of the linear protein

• Trypsin is an enoprotease

  • does same as above but to the smaller fragments that were broken down by pepsin

• Carboxypeptidases are exoproteases

  • cuts the very outsides of the smaller peptide fragments

  • this creates monomers!!!!! eventually

This is all occuring in the duodenum!

Amino peptidases

• on the brush border!

• membrane bound enzymes IN JEJUNUM

Absorption of animo acids (monomer)

1. AA transported into enterocyte with sodium dependent AA transporter

   • sodium and AA get transported into the enterocyte together from concentration of the sodium out in the lumen

     • diffusion! theres low conc of salt inside the cell, so naturally it wants to go from high to low duh

   • the AA transport can transport all AA no matter their charge.

2. AA is transported across the basolateral membrane by the sodium antiporter

   • essencially trading an AA for a sodium!

   • So an amino acid goes out, and a sodium goes in

   • Ok so now we have sodium in the cell that we don’t want, so we’re gonna kick some out using ATPase

     • Na+ gradient maintained by NKA

Absorption of di- and tri- peptides

1. Di-/Tri- peptides cotransported with H+ (hydrogen/proton)

2. H+ exchanged for Na+ with NHE (sodium-hydrogen exchanger)

3. There are now two possibilities:

   • it can be hydrolyzed to an amino acid by aminopeptidase, so now it just follows the AA pathway we learned previously

   • OR

   • It could be exchanged for H+

     • this will occur on the basalateral side

     • this causes a buildup of H+ in the enterocyte and increases acidity

       • CELL DEGRADES NOOOOO

     • to prevent this we’re just gonna use NHE to balance everything out again!