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13 Cards in this Set
- Front
- Back
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Key principles of absorption of macromolecules
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Absorption of macronutrients (molecules that provide energy and substrate for growth, cellular repair, and reproduction) is a matter of life or death
The small intestine is where this happens. The enterocytes lining the small intestine are a barrier between the inside and outside worlds Digestion & absorption enable macronutrient molecules to pass selectively through the barrier Macronutrients are large bio-polymers: - Carbohydrates (chains of sugars) - Proteins (chains of amino acids) - Fats (fatty acids esterified to glycerol) These molecules are too large to be transported (absorption) through enterocyte cell membranes Before they can be absorbed, they must be broken down into smaller constituent molecules (digestion) |
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Sites of digestion: glucose, protein, lactose, peptides, TG
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Glucose: none
Protein: intraluminal Lactose: brush border Peptides: intracellular Triglycerides: combined |
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Processes involved in absorption of macromolecules
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Mechanical (increases surface area)
- Mouth and teeth - mastication - Stomach - trituration into particles and emulsions Chemical (enzymatic depolymerization) - Mouth & stomach: amylase, gastric lipase, pepsin - Pancreatic enzymes: amylase, lipase, proteases - Enterocyte enzymes: disaccharidases, peptidases Physical-chemical (increases solubility) - Intraluminal formation of lipid-bile salt micelles Biophysical (transmembrane transport) - Highly specific membrane transport proteins |
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Carbohydrate digestion and absorption: source, type and structures
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Source: plants, fungi, animals (glycogen)
Types and structures: - Simple Sugars: glucose, fructose, galactose - Disaccharides: maltose (glucose + glucoses), sucrose (glucose + fructose), lactose (glucose + galactose) - Polysaccharides: --Dextrins & Starches: simple linear glucose polymers --Glycogen: highly branched alpha-linked glucose polymers --Cellulose: linear beta-linked glucose polymers |
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Carbohydrate digestion and absorption: luminal digestion, brush border digestion, sugar transporters
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Luminal Digestion
- Salivary alpha-amylase: breaks starch 1:4 links Brush Border Digestion - Lactase: lactose to glucose + galactose - Sucrase: sucrose to glucose + fructose - Glucoamylase: breaks starch 1:4 links - alpha-dextrinase (isomaltase): breaks starch 1:6 links Sugar Transporters - SGLT1: Apical glu/gal transporter --Cotransporter with Na+ - GLUT2: Basolateral glu/gal transporter - GLUT5: Apical/basolateral fructose transporter --Facilitated diffusion |
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Carbohydrate digestion and absorption: intraluminal digestion
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Maltase
Alpha-dextrinase Break 1:6 links |
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Protein digestion and absorption: source, structure, AA classes
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Source: animals, fish, plants
Structure: - Linear polymers of 20 amino acids Amino acid classes: - Polar: Gly, Ala, Ser, Thr, Asn, Asp - Aromatic: Phe, Tyr, Trp - Charged: Glu-, Asp-, Lys+, Arg+, His+ - Branched chain: Val, Leu, Ile - Cyclic: Pro - Sulfur containing: Met, Cys |
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Protein digestion and absorption: luminal digestion, brush border digestion, transporters
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Luminal Digestion - stomach
- Gastric pepsin (the only acidic peptidase) --Don't really need Luminal Digestion – small intestine - Pancreatic peptidases - Enterocyte peptidases Brush Border Digestion - Amino-oligopeptidase: oligopeptides-> 2,3 AA - Aminopeptidase: di- and tri-peptides -> single AA - Dipeptidyl-aminopeptidase -> removes Ala, Pro Transporters: amino acid specific |
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Lipid digestion and absorption: source, energy, types and structure, solubility
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Source: animals, fish, plants, fungi
Most concentrated storage form of energy Lipid types and structures: - Triglycerides – Fatty acids (3) esterified to glycerol - Phospholipids – Fatty acids (2) esterified to PC - Cholesterol – sterol Very Low solubility - Absorption varies - TG, PL: 95% - Cholesterol: 50% |
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Lipid digestion and absorption: luminal digestion, brush border, intracellular events
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Luminal Digestion - stomach
- Lingual & gastric lipases release some FA’s - Stomach mixing forms emulsion particles Luminal Digestion - small intestine - Pancreatic lipase releases large amounts of FA - Hydrolysis products form mixed micelles Brush Border - passive diffusion Intracellular Events: - Re-esterification to TG - Chylomicron assembly and secretion |
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Triglyceride digestion and absorption: gastric and small intestine
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Stomach:
-Gastric mixing -Lingual and gastric lipase hydrolyze dietary fats and liberate FA’s, mono- and di-glycerides -These serve as lipid emulsifying agents Small bowel lumen -Pancreatic lipase-colipase forms 2-monoglycerides and fatty acids -Lipid solubilization in mixed micelles |
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Absorption of micellar lipids at the brush border
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Mixed micelles diffuse through unstirred layer at brush border.
Through simple diffusion (no transporters), lipids dissociate from micelle and pass through membrane into cell. -Slow process, requires a lot of small bowel Within enterocyte, monoglycerides and FA are reassembled into TG. Then packaged and coated with protein products to form chylomicrons. Chylomicrons are so large they cannot reenter bloodstream but are instead absorbed into lymphatics |
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Regional nutrient absorption
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Carbohydrates
-Most digestion is complete by the upper third of small bowel Protein, fat, H20, Na+ -Take entire small bowel Bile salts -Absorbed in ileum (only location of transporter) -Can't be compensated Vitamin B12 -Absorbed in ileum Iron -Duodenum |
