Nov.18 - Digestive System #3
Chemical Digestion and Absorption of the Major Nutrients
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Identify the enzyme that acts on each of the disaccharides listed below: Disaccharide Enzyme ___ Sucrose a. Lactase ___ Maltose b. Sucrase ___ Lactose c. Maltase
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Which salivary enzyme becomes more active in the stomach due to its lower pH?
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Absorption of Lipids Lipids are hydrophobic, and don't mix well with water Their digestion is supported by formation of micelles
Absorption of Monosaccharides #2
Absorption of Monosaccharides Galactose is also absorbed by the SGLT Fructose is absorbed by facilitated diffusion, and most is then converted to glucose All 3 monosaccharides are transported out of the base of the cell by facilitiated diffusion, enter the capillary of the villus, which leads to the portal system which supplies the liver
Absorption of Monosaccharides* Don't need details
Absorption of Monosaccharides* Don't need details ~80% of absorbed sugar is glucose, taken up by a sodium-glucose transporter (SGLT) in the enterocyte plasma membrane Glucose is then transported out of the base of the cell to the ECF, causing an increase in its osmolarity The ↑ osmolarity draws water (plus glucose & other nutrients) through leaky junctions between the epithelial cells (called the solvent drag)
Amino Acid Absorption
Amino Acid Absorption Enteroctes have a variety of sodium-dependent amino acid cotransporters
Amino acids leave the enterocytes by
Amino acids leave the enterocytes by facilitated diffusion, enter the capillaries of the villus and then the portal circulation
Starch Digestion - Begins in:
Begins in the mouth, with ~50% digested before reaching the small intestine
Starch Digestion
Continues in small intestine, when chyme mixes with pancreatic amylase Conversion to oligosaccharides and to maltose is completed within 10 minutes
Digestion is completed when
Digestion is completed when chyme contacts the brush border of enterocytes
Digestion of Disaccharides
Digestion of Disaccharides Two major dietary disaccharides: Sucrose (table sugar, digested by sucrase) Lactose (milk sugar, digested by lactase) NOTE: Maltose is present in some foods
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Digestion of Triglycerides 1 Triglyceride molecule ↓ lipase 1 monoglyceride + 2 free fatty acids
TRUE or FALSE? Leafy vegetables are the most important source of calcium in the human diet.
FALSE
TRUE or FALSE? The two major dietary disaccharides are lactose and maltose
FALSE
True or False? Digestion of starch begins in the mouth by the action of salivary amylase, an enzyme that requires an alkaline environment. This enzyme is destroyed immediately upon swallowing due to the low pH within the stomach.
FALSE
True or False? Protein digestion does not begin within the mouth, but occurs entirely within the stomach.
FALSE
True or False? The digestion of starch begins in the mouth and is completed in the stomach
FALSE
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Lipid Digestion Begins with lingual lipase from intrinsic salivary glands of the tongue, which becomes more active with lower pH of the stomach Most fat digestion occurs due to pancreatic lipase within the small intestine Exposure of fat molecules to these enzymes is increased via emulsification (breaking of large fat globules into many smaller ones by bile
Main Point
Main Point Digestion of protein begins in the stomach and continues within the small intestine
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Micelles Because their exterior surface is hydrophilic, micelles remain suspended in water better than lipids Micelles transport lipids to the surface of the enterocytes, where the lipids leave and are transported into the enterocytes for further processing The micelles get re-used Without these, only 40-50% of dietary fat gets digested and almost no cholesterol
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Micelles Made in the liver from 20-40 bile acid molecules Hydrophilic side groups facing outward, hydrophobic steroid rings facing inward Upon release into the duodenum, absorb fat-soluble vitamins, cholesterol, and FFA's and monoglycerides produced by fat digestion
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Minerals Absorbed along entire length of the small intestine by a variety of different mechanisms A few miscellaneous points: Iron and calcium are absorbed in proportion to the body's needs, most others at fairly constant rates Most absorbed calcium comes from milk or milk products (leafy vegetables are high in calcium, but contain a calcium-binding agent, oxalate, that makes it unavailable for absorption)
Digestible Dietary Carbohydrates
Mostly starch Negligible amounts of glycogen (digestion of which is identical to that of starch) NOTE: Cellulose is indigestible in humans, but serves as dietary fiber. It is readily digested in ruminants and horses.
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Nucleic Acids Present only in small quantities Hydrolyzed by ribonuclease and deoxyribonuclease to → nucleotides Nucleosidases and phosphatases decompose to phosphate ions, ribose or deoxyribose and nitrogenous bases Transported across the intestinal epithelium and enter the capillary blood of the villus
Pepsin
Pepsin Has an optimal pH of 1.5 - 3.5 Becomes inactive upon entering the duodenum
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Processing Within the Enterocytes FFA's and monoglycerides are transported to smooth ER and resynthesized into triglycerides Golgi complex combines with small amounts of cholesterol and coats with a film of phospholipids and protein to form chylomicrons, packages these in secretory vesicles for release into the core of the villus From there, chylomicrons enter the lacteal and then pass to larger lymphatic vesicles and then to the thoracic duct, entering the bloodstream at the left subclavian vein
Protein Digestion in the Small Intestine Final digestion by:
Protein Digestion in the Small Intestine The pancreatic enzymes trypsin and chymotrypsin hydrolyze polypeptides into shorter oligopeptides Final digestion by: Carboxypeptidase Aminopeptidase Dipeptidase
Protein Digestion* Three sources of amino acids:
Protein Digestion* Three sources of amino acids: 44-60g/day of dietary proteins ~30g/day from - Digestive enzymes Sloughed epithelial cells
The two digestive enzymes contributed by the saliva are: ____________ ____________
SALIVARY AMYLASE
Salivary amylase
Salivary amylase ● Catalyzes hydrolysis of starch to → oligosaccharides
Some dipeptides and tripeptides are absorbed, but are
Some dipeptides and tripeptides are absorbed, but are hydrolyzed before release as amino acids
Starch Digestion - Steps
Starch ↓ Oligosaccharides (up to 8 glucose monomers long) ↓ Maltose (a disaccharide) ↓ Glucose ↓ Absorbed by sm. intestine
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TRUE
TRUE or FALSE? Lipids are hydrophobic and tend to form globules. Within the small intestine, these are broken up into smaller droplets by bile acids, thereby increasing their exposure to lingual lipase and pancreatic lipase. The break-up of fat globules by the bile acids is called emulsification.
TRUE
TRUE or FALSE? The two enzymes that catalyze the digestion of starch are salivary amylase and pancreatic amylase
TRUE
TRUE or FALSE? Most glucose is absorbed by a sodium-glucose transporter in the enterocyte plasma membrane. The subsequent release of this glucose into the ECF at the base of these cells raises the osmolarity of the ECF. This elevated osmolarity pulls water, glucose and other nutrients through leaky tight junctions between the adjacent epithelial cells. The latter is called the solvent drag.
TRUE
True or False? The enzyme involved with the initial phases of protein digestion within the stomach is pepsin. This enzyme is deactivated upon entering the small intestine.
TRUE
True or False? The product of starch digestion is glucose
TRUE
True or False? Within the small intestine, the pancreatic enzymes trypsin and chymotrypsin hydrolyze polypeptides into shorter oligopeptides. Final digestion is then catalyzed by carboxypeptidase, aminopeptidase and dipeptidase. The end product of this process is free amino acids.
TRUE
True or False? Most lipid digestion occurs within the small intestine due to pancreatic lipase
TRUE
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Vitamins Absorbed unchanged Fat-soluble vitamins A,D,E and K are absorbed with lipids (they are not absorbed if they are consumed without fat-containing foods) Water-soluble vitamins (B complex vitamins and vitamin C) are absorbed by simple diffusion, with exception of vitamin B12 Vitamin B12 binds to intrinsic factor from the stomach, forming a complex that's absorbed in the distal ileum by receptor-mediated endocytosis
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Water Absorption Diarrhea occurs when the large intestine absorbs too little water Constipation results when the large intestine absorbs too much water
Salivary amylase optimal pH
● Optimal pH for salivary amylase is 6.8 - 7.0
Salivary amylase is denatured by
● Salivary amylase is denatured by stomach acid and digested by pepsin, but can function within the stomach for 1-2 hours IF in the middle of the food mass, avoiding contact with stomach acid