MEDI211 Lecture Notes - Lecture 3: Golgi Apparatus, Chylomicron, Acetyl-Coa
Week 1 Lecture 3 – Mechanisms of macronutrient digestion
Lecture objectives:
1. To introduce students
to the physiological
basis of macronutrient absorption and storage
2. Demonstrate the processes regulation by insulin.
•Triglycerides are broken down into glycerides and fatty acids, which are able to pass through
the lumen, and these components are then joined back together to form triglycerides again.
Carbohydrate digestion: two major groups:
•- monosaccharides: small molecules - molecules - directly absorbed into intestine-di, oligo
and poly saccharides - these are non-digestible so cannot be directly absorbed by the
intestines.
•Oligo and polysaccharide digestion starts with salivary amylase in the mouth and pancreatic
amylase in the small intestine.
•Products of amylase digestion are still too large to be absorbed, so oligosaccharides are
broken down by enzymes on the brush border of the small intestine lumen —> they are
broken down into monomers such as glucose and fructose.
CHO absorption:
•Transporter proteins (Glucose transporter 5 and 2 - GLUT5 & GLUT2) and Na+/Glucose
Transporter 1 (SGLT1) transport monosaccharides across the intestinal cell into the
interstitial space. Sodium/potassium pumps create a sodium gradient in the epithelium, which
causes of a flow of Na+ down the concentration gradient, allowing glucose and galactose to
flow through the epithelium at the same time.
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Week 1 Lecture 3 – Mechanisms of macronutrient digestion
•Fructose is transported by transporter GLUT5.
•CHO’s enter the epithelium by SGLT1 or GLUT5, and exit the epithelium by GLUT2.
•These microvilli increase the surface area thus the rate of absorption.
•The CHOs then move into the bloodstream - insulin then directs the uptake of glucose by the
cells.
Lipid digestion:
•Triglycerides are composed of 3 free fatty acid chains + glycerol.
•Lipids are hydrophobic so tend to accumulate together in emulsion droplets. Emulsion
droplets are multi-lumelar membranes - they have multiple lumen layers of triglycerides,
monoglycerides, cholesterol etc —> hard for the pancreatic enzymes to digest.
•Bile salts are used to breakdown these emulsion droplets into micelles, which are then
diffused and able to pass across the lumen.
•The environment close to the brush border is acidic due to the sodium gradient. Sodium ions
are being pumped through an ion channel into the enterocyte, allowing hydrogen ions to be
moved in the opposite direction. This results in there being a high level H+ ions outside the
epithelium which cause an acidic environment. This causes protonation of the fatty acids so
they become neutral —> thus are more easily able to be taken up in the bloodstream.
•Most absorbed lipids are converted back into the larger molecular lipids in the Smooth
Endoplasmic Reticulum (SER).
•Lipids are packaged into lipoproteins (chylomicrons, HDL, LDL, VLDL) in the Golgi
apparatus. Chylomicrons are made from dietary lipids, and VLDLs are made from
endogenous lipids in the enterocytes during times of fasting to provide more energy to the
body.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
To introduce students to the physiological basis of macronutrient absorption and storage. Triglycerides are broken down into glycerides and fatty acids, which are able to pass through the lumen, and these components are then joined back together to form triglycerides again. Monosaccharides: small molecules - molecules - directly absorbed into intestine-di, oligo and poly saccharides - these are non-digestible so cannot be directly absorbed by the intestines. Oligo and polysaccharide digestion starts with salivary amylase in the mouth and pancreatic amylase in the small intestine. Products of amylase digestion are still too large to be absorbed, so oligosaccharides are broken down by enzymes on the brush border of the small intestine lumen > they are broken down into monomers such as glucose and fructose. Transporter proteins (glucose transporter 5 and 2 - glut5 & glut2) and na+/glucose. Transporter 1 (sglt1) transport monosaccharides across the intestinal cell into the interstitial space.
