2 - GI physiology two

principle dietary constituents

carbohydrates

proteins

fats

vitamins - fat or water soluble

minerals

water

main forms of carbohydrates

monosaccharides

disaccharides

oligosaccharides

polysaccharides

what are the main monosaccharides

glucose

galactose

fructose

aldose hexoses

glucose and galactose

ketose hexose

fructose

where are monosaccharides absorbed

small intestine then transported to the blood

how are disaccharides linked

glycosidic bond

main disaccharides

lactose

sucrose

maltose

what is lactose made of and what enzyme breaks it down

glucose and galactose

broken down by lactase

what is sucrose made of and what enzyme breaks it down

glucose and fructose

broken down by sucrase

what is maltose made of and what enzyme breaks it down

2 glucose

broken down by maltase

how are disaccharides broken down

hydrolysis into constituent monomers by brush border enzymes in small intestine

glycogen

storage form of glucose

how is glycogen formed

lots of glucose monomers linked by alpha 1-4 glycosidic bonds

how is glycogen broken down

by amylase from saliva and pancreas

alpha amylose structure

glucose straight chain

amylopectin structure

branched glucose

why can starch be digested in duodenum

there is amylase in pancreatic juices

transcellular transport

transport of material through cell without need of energy/help usually seen in hydrophobic molecules

paracellular transport

through cell junctions

what goes by paracellular transport

small molecules such as water

vectorial transport

movement of material across membrane via ion pump, directional transport

The transport of glucose from the intestinal lumen to the blood process

sodium glucose co transporter SGLT-1 transports glucose and sodium at the same time

transport of glucose from intestinal lumen to blood

active transporter ATPase pump present in basolateral membrane - 3x Na out and 2x K in

pump makes electrochemical gradients for Na so Na concentration inside cell decreases

Na get inside cell by SGLT1 which transports glucose and sodium into enterocytes - this is a co transporter

Na is passed to ATPase and glucose is passed to GLUT2

GLUT2 transports glucose secondary active to blood

where is GLUT1 present

red blood cell

blood brain barrier

what can GLUT1 transport

glucose AND galactose due to galactose similar structure to glucose

where is GLUT3 present

brain

where is GLUT4 present

adipose tissue

skeletal muscle

transport of fructose from lumen to blood

fructose crosses membrane by GLUT5 and is then passed onto GLUT2 where it is passively passed to blood

what is a special feature of GLUT4

insulin dependent

how are polymers of amino acids linked

peptide bond

glycoprotein

protein with one or more carbohydrates attached

lipoprotein

fat molecule and a protein

what are small proteins called

peptides

how are proteins digested

hydrolysis via proteases or peptidase

how are peptidases classified

endopeptidases - work on middle of chain

exopeptidase - work on end of chain or externally

how are exopeptidases classified

aminopeptidases - work on N terminal to release amino acids

carboxypeptidases - work on C terminal to hydrolyse amino acids

how are free amino acids absorbed

3 steps

1. ATPase pump

2. sodium outside cell

3. potassium inside cell

process of free amino acid absorption

similar to glucose transport - transporter called sodium amino acid transporter 1 (SAAT1)- works to co transport sodium and amino acids from lumen to inside cells

makes electrochemical gradient for sodium inside cell and absorption of Na will help with absorption of amino acids

sodium out of cell and potassium inside cell

amino acids are transported by a different transporter passively from basolateral membrane to blood

why does movement of sodium help with movement of water

movement of sodium = movement of water from lumen to blood or absorption of water due to osmotic gradient formed due to movement of sodium from lumen to blood - sodium increases water absorption and movement

transport of dipeptides or tripeptides - from lumen of small intestine

absorbed from lumen to blood by PepT1

co transports dipeptides or tripeptides with a proton

lumen pH less than 6 - acidic = microclimate

what's responsible for making the acid microclimate

sodium proton exchanger 3

can pump sodium from outside cell to inside and pump out portion to epithelial cells outside of cell

makes pH closer to microvilli acidic or will increase proton concentration to help activate PepT1

this is known as proton motive force

Describe Cellulose

B-1,4 glycosidic bond, constituent of plant cell walls

Animals/humans cannot enzymatically digest cellulose due to the lack of cellulase enzyme. Humans mechanically break down cellulose when chewing fibrous foods. It passes through the gut mostly intact.

Main “benefit” is as a dietary fibre for bowel movement (reducing constipation).

Cellulose could be fermented by normal flora in the large intestine, producing short-chain fatty acids (important for gut and human health)