Nutrient Assimilation
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- Created by: amazingemilyjones
- Created on: 23-04-19 13:42
Nutrient Assimilation
Nutrient Assimilation
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Assimilation = digestion + absorption
- Digestion
- physical reduction of food (chewing, grinding)
- chemical reduction of complex nutrients into constituent monomers, dimers or trimers by enzymatic hydrolysis
- luminal phase and membranous phase
- Absorption
- transport of products of digestion from gut lumen, across mucous membrane, into blood
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Carbohydrate Digestion
- Complex sugars - polysaccharides (starch)
- Simple sugars - mono and disaccharides
- Common dietary sugars (disaccharides)
- sucrose - glucose and fructose
- lactose - glucose and galactose
- maltose - glucose and glucose
- Cellulose
- plant material, indigestible, fibre - helps with propulsion through GI tract
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Glycosidic Bond Digestion
- alpha-amylases cleave alpha-1,4-glycosidic linkages in polysaccharides (starch)
- dextrinase cleaves 1,6-glycosidic bonds
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Carbohydrate Digestion
- Salivary amylase
- begins the 'luminal phase' digestion of starches action continues in stomach
- Pancreatic amylase
- continues 'luminal phase' digestion of starches in small intestine
- structurally different from salivary amylase but identical catalytic action
- Brush border enzymes
- membranous phase of digestion
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Carbohydrate Digestion
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Absorption of Monosaccharides
- Principal monosaccharide products of carbohydrate digestion are hexoses
- Hexose absorption occurs by three mechanisms:
- passive diffusion (slow and limited)
- Na+/glucose symport
- Na+-independent facilitative transpot
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Absorption of Monosaccharides
- Na+/glucose symport mechanism
- glucose and galactose are transported from the lumen into epithelial enterocytes by a Na+/glucose co-transporter SGLT-1 (sodium dependent glucose transporter)
- Facilitated transport of hexoses
- glucose transporter proteins expressed in all cell types
- GLUT-1, GLUT-2 and GLUT-5 expressed in enterocytes
- Fructose is absorbed from lumen by GLUT-5 on apical membrane
- GLUT-2 on basal membrane transports hexoses from enterocyte cytosol to extracellular space
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Absorption of Monosaccharides
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Protein Digestion
- 20 different amino acids may be joined by peptide linkaeges
- Forms many thousands of known proteins and polypeptides
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Protein Digestion
- Proteases cleave peptide linkages by hydrolysis
- Three origins for gut proteases:
- Gastric protease (pepsins)
- Pancreatic protease
- Brush border protease
- endopeptidases - hydrolyse peptide bonds in middle of polypeptide
- trypsin
- chymotrypsin
- exopeptidases - hydrolyse peptide bonds at end of polypeptide
- carboxypeptidases
- aminopeptidases
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Protein Digestion
- Gastric protease (pepsin)
- endopeptidases
- secreted by chief cells as pepsinogen (activated by H+ and pepsin autocatalysis)
- have acidic pH optimum
- initiates luminal phase of protein hydrolysis
- Pancreatic protease
- secreted from acinar cells in response to neural and hormonal stimuli
- secreted as inactive zymogens
- trypsinogen, chymotrypsinogen, procarboxypeptidase
- activated by enterokinase in small intestine
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Protein Digestion
- Intestinal brush border protease
- situated on apical surface of epithelial enterocytes in duodenum, jejunum and proximal ileum
- enterokinase, oligopeptidase, aminopeptidase
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Absorption and Transport of Amino Acids
- Products of protein digestion are:
- tripeptides, dipeptides and amino acids
- Further hydrolysis of tripeptides and dipeptides occurs by cytosolic peptidases within enterocytes
- Absorption from lumen across brush-border occurs by three main mechanisms:
- passive diffusion
- Na+/amino acid symport
- similar to SGLT-1 mechanism for glucose
- Na+-independent facilitated diffusion
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Absorption and Transport of Amino Acids
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Lipids
- Animal/dairy fats and vegetable oils
- Most dietary fat in the form of triacylglycerols (fatty acids condensed with glycerol)
- Fatty acids
- Saturated (C-C single bonds) - stearic acid
- Unsaturated (one or more C=C double bonds)
- monounsaturated - oleic acid
- polyunsaturated - arachidonic, linoleic, eicosapentaenoic (EPA)
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Acylglycerols
- Monoacylglycerols (monoglycerides)
- Diacylglycerols (diglycerides)
- Triacylglycerols (triglycerides) - majority of dietary fat
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Phospholipids
- Vital component of cell membranes
- Similar in structure to triglyceride but have one fatty acid chain replaced by a phosphate group
- Phosphate group has a substituted group attached (X), e.g. ethanolamine, choline, serine
- Key for cell apoptosis signalling and blood clotting response
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Digestive Lipases
- Hydrolyse triacylglycerols to fatty acids, monoacylglycerols, diacyglycerols and glycerol
- Acidic lipases
- gastric and lingual
- Pancreatic lipases
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Digestive Lipases (Acidic Lipases)
- Gastric lipase produced by peptic (chief) cells of fundus
- Lingual lipase from serous glands of tongue (not secreted by salivary glands, but present in saliva)
- Potent and rapidly acting lipases, pH optima 4.0-5.5
- Initiate lipid hydrolysis in stomach (independent of bile) forming fatty acids and diacylglycerols)
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Digestive Lipases (Pancreatic Lipase)
- Acts on diacyl- and triacylglycerols in duodenum; neutral pH optimum
- Major hydrolysis products are fatty acids and monoacylglycerols
- Responsible for the majority of lipid hydrolysis in adults; completes hydrolysis initiated by the acidic lipases
- Optimal activity requires bile acids
- Bile acids emulsify fat into small droplets maximising exposure of the glycerol 'head' structure to the enzyme
- Colipase, a peptide co-factor, produced by pancreas
- essential for optimum pancreatic lipase activity
- prevents bile acids from inhibiting lipase activity
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Pancreatic Lipase
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Lipid Absorption
- Formation of mixed micelle (mainly monoacylglycerol, free fatty acids, bile acids and cholesterol)
- Diffusion across apical membrane of monoacylglycerol, free fatty acids and glycerol
- Binding to fatty acid binding protein in the cytosol for transport to smooth endoplasmic reticulum
- Re-synthesis of triacylglycerol
- Packaging of triacylglycerol with cholesterol, apoproteins and phospholipids to form chylomicrons
- Transport of chylomicrons across basal membrane into lacteals
- Lipids are re-processed within enterocyte cell, unlike carbohydrate and amino acids
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Lipid Absorption
Electrolyte and Fluid Balance
- Major regulation of fluid balance is via kidneys, GI tract, breathing and skin
- Electrolytes have chemical bonds that allow dissociation into ions, which carry an electrical charge
- Electrolytes are of critical importance in fluid balance, i.e. fluid balance and electrolyte balance are interdependent
- Fluid balance can be maintained only if intake equals output
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Bidirectional Fluid Flux in the GI Tract
- Water movement into lumen (secretion)
- Water recovery from lumen (absorption)
- Net absorption of water by passive movement
- response to osmotic and hydrostatic pressures
- Luminal fluid generally remains isotonic with plasma
- Most solutes and water are re-absorbed in the ileum, some in the colon
- Major mechanism is Na+-glucose co-transporter (SGLT)
- some transcellular or paracellular passive transport
- Malabsorption of solutes and water by ileum may overwhelm the colon's absorptove capacity
- leads to diarrhoea and electrolyte loss
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Bidirectional Fluid Flux in the GI Tract
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Passive Electrolyte Transport
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Mechanisms of Na+ Reabsorption in Ileum
- Most efficient absorption in the distal ileum coupled with water
- Co-transporter mechanisms
- glucose/galactose
- amino acids
- Coupling to Na+/H+ exhange
- Passive transcellular/paracellular diffusion
- Cl- absorption is coupled to Na+ absorption
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Absorption of Minerals
- Divalent cations (e.g. Ca2+ and Mg2+) are poorly absorbed
- osmotic laxactive effect - milk of magnesia
- Ca2+ absorption by enterocytes is tightly regulated to maintain total body Ca2+ stores
- 1,25-dihydroxy-vitamin D3 stimulates synthesis in enterocytes of Ca2+ binding protein which transports Ca2+ across the apical membrane
- Inorganic Fe2+ is complexed with transferrin in the lumen and absorbed into enterocytes by a receptor mechanism
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Absorption of Vitamins
- Water-soluble vitamins
- most are absorbed by simple passive diffusion
- vitamin C (ascorbic acid)
- some are absorbed by specialised mechanisms
- vitamin B12 (cobalamins) absorbed by ileal enterocyte receptors for vitamin B12-intrinsic factor complex
- most are absorbed by simple passive diffusion
- Fat-soluble vitamins
- vitamins A, D, E depend on solubilisation by bile salts for intestinal absorption
- dietary vitamin K (phytomenadione, vitamin K1) absorbed by active transport in ileum)
- bacterially-synthesised vitamin K2 absorbed passively in colon
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Absorption of Water
- Approximately 9L of fluid enter the small intestine each day
- Approximately 2.3L are ingested in foods and beverages
- rest is from GI secretions
- 90% of this water is absorbed in the small intestine
- Water absorption is driven by the concentration gradient of the water
- concentration of water is higher in chyme than in epithelial cells
- water moves down its concentration gradient from the chyme into cells
- remaining water is then absorbed in the colon
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