Gut Absorption - Lecture 145

Gut Absorption - Lecture 145 

Learning objectives

1. Explain the mechanisms used to transport the products of carbohydrate and protein digestion across the intestinal membrane and into the blood

Carnivores/omnivores - most absorption (~90%) occurs in small intestine.
Non-ruminant herbivores - large intestine important for absorption
Ruminant herbivores - considerable absorption in rumen

Water soluble compounds and small lipids pass directly into blood (small fatty acids and glycerol)
Lipids (longer fatty acids) and fat-soluble vitamins are secreted into lymph and pass to blood via thoracic duct.

Transport mechanisms:

• Passive diffusion:
• molecules pass down concentration gradient
• small polar molecules like water
• non-polar molecules
• Channel-mediated diffusion: 
• facilitated diffusion
• protein spans membrane
• has aqueous pore
• specific solutes (usually ions) able to pass through
• Carrier-mediated 
• Large polar molecules or ions
• Uniport
• only solute passes down concentration gradient
• Symport
• 2 solutes transported in same direction
• one down concentration gradient
• one up concentration gradient
• no net energy used
• Antiport
• 2 solutes move in opposite directions
• one up/one down concentration gradient
• no net energy used 
• Active transport
• uses energy/ATP
• against concentration gradient
• large polar molecules

Carbohydrate absorption:

• Products mainly glucose - some fructose and galactose
• Glucose/galactose require energy for transport
• Fructose absorbed by carrier-mediated diffusion down concentration gradient out of cell and into blood

Glucose (and galactose) transport: 

1) Saturable - finite number of binding sites on binding protein

2) competitively inhibited by galactose

3) inhibited by glucose analogues (phlorizin)

4) sodium dependant

5) inhibited by Na⁺/K⁺ ATPase inhibitor ouabin

• glucose and Na⁺ transported into cell together 
• symport system (SGLT-1)
• Glucose travels up concentration gradient
• Na⁺ travels down concentration gradient
• glucose passes from cell into blood via carrier mediated diffusion using GLUT5 transporter
• no energy required
• concentration higher inside cell
• Na⁺ concentration inside cell kept low by Na⁺/K⁺ ATPase
• pumps Na⁺ out and K⁺ in
• ATP used
• Very efficient system

Protein Absorption

• Saturable
• Sodium dependent
• Energy-requiring
• Competitively inhibited by some aas, but not others

4 carrier proteins: 

1) neutral amino acids: eg. alanine, valine, leucine (15 total)

2) dibasic amino acids: lysine, arginine, ornithine  

3) proline, hydroxyproline, glycine (collagen synthesis)

4) acidic amino acids: glutamine, Aspartic acid

• all amino acids compete for transport with others in same group, but not with those in different groups
• some amino acids are in more than one group

Amino acid transport: 

• amino acids transported into cell with Na⁺ 
• via carrier protein in symport system
• amino acids travel up concentration gradient
• Na⁺ travels down concentration gradient
• Na⁺ concentration inside cell kept low by Na⁺/K⁺ ATPase
• pumps Na⁺ out and K⁺ in
• ATP used

Backup amino acid transport system: Gamma-glutamyl cycle

• gamma-glytamyl transferase spans membrane
• combines with glutathione (tripeptide) inside cell with aa from outside cell and brings aa inside cell
• forms gamma-glu-aa
• amino acid is released
• glutathione reformed using 3 ATPs
• brings in any aa
• needs a lot of energy

Di, tri, oligopeptide transport:

• specific carrier protein for di, tri, oligopeptides
• same system as amino acid carriers
• Once inside a cell, peptides broken down to amino acids by aminopeptidases

2. Explain the processes involved in the uptake of the products of fat digestion by intestinal cells and their subsequent secretion into lymph as chylomicrons

Products of lipid absorption:

• 2- monoacylglycerols ~ 70%
• 1- monoacylglycerols ~ 6%
• glycerol ~ 20%
• free fatty acids

• Micells: Large fat droplets in duodenum dispersed into smaller particles by bile salts and phospholipids from bile
• Products of lipid digestion absorbed by intestine and triacylglycerols re-synthesised inside cells - before going into blood

Lipid absorption

Fatty acids and 2-MAG:

1. Fatty acids and 2-monoacylglycerol (MAG) enter cell by passive or carrier mediated diffusion
2. fatty acids converted to fatty acylCoA by addition of CoA
3. Acyl group added to 1 and 3 positions of 2-MAG.
4. Forms 1,2 DAG then TAG

1-MAG:

1. enters cell by passive diffusion
2. fatty acyl group removed from 1 position
3. forms glycerol
4. glycerol converted to 1,2 DAG by addition of fatty acyl groups
5. Another fatty acyl group added to form TAG

TAG:

1. Once reassembled in cell, packaged with protein and lipoproteins into chylomicrons
2. Secreted into lymph by exocytosis
3. packaging allows insoluble TAG to be transported in aqueous blood
4. chylomicrons 90% TAG
5. outside is protection layer of protein and phospholipids
6. TAGS inside the cell are different in composition than the TAGS in lumen

Glycerol:

1. passes through cell and goes directly into blood

Absorption of other compounds: 

Water: 

• large amounts of water are secreted into GIT in digestive fluids
• reabsorbed in small and large intestine

Vitamins/minerals: 

• obtained from diet
• do not require digestion
• enter via carrier-mediated or active transport