Learning Objectives 1) Describe how complex foods that we consume are processed in the digestive system to prepare them for uptake and utilization. 2) Recognize the role of hydrolytic enzymes and bile acids in the digestive process. 3) Define how transporters function to move energy nutrients across cell membranes 4) Understand the distinct routes of absorption and transport that carbohydrates and aa's take from that of lipids. Digestion!- In order to absorb what we eat we have to break down (digest) these more complicated molecules into simpler forms that we can transport into the cells of the GI tract!- use digestive enzymes that hydrolyze specific bonds found in complex compounds of food!- process differs for each major type of energy nutrient!- Form ingested ----> Form absorbed!- Carbohydrates--> Monosaccharides!- Lipids --> Fatty acids!- Proteins --> Amino acids + Di/Tripeptides!- Different foods have different structures!- Digestive Enzymes!- Almost all are hydrolases= enzymes that add H20 to break a molecular bond!- Produced by: salivary gland (just amylase, no lipases or proteases here), stomach, small intestine, and exocrine pancreas!- Produced as inactive precursors= proenzyme/zymogen!- Precursor contains additional amino acids at amino term. end that inhibit the active site of the enzyme -->protect cell that synthesizes it!- After secreted--> cleavage of additional aa's --> activation!- Ex. Pepsinogen --> Pepsin (active) and Trypsinogen --> Trypsin (active)!- Gastrointestinal Hormones!- Digestion is coordinated by a large # of GI hormones that regulate secretion of digestive enzymes and fluids, intestinal motility, appetite, and satiety.!- Produced by endocrine cells through digestive tract!- Gastrin= stimulates acid and pepsinogen secretion - Cholecystokinin= triggers bile release - Glucagon-like peptide (GLP)= promotes insulin secretion - Polypeptide YY= reduces appetite!- Digestion of Carbohydrates!- Consume: starch (plant) and glycogen (animal)--complex glucose polymers linked in alpha 1-4 and alpha 1-6!- digestion initiated by alpha amylase!- produced in salivary glands-->mouth and exocrine pancreas-->duodenum!--> leads to short oligosaccs!- breaks 1-4 glycosidic bonds, can't break 1-6!- Digestion of Oligo and Disaccharides!- Final stage: in small intestine by hydrolases found on brush border of intestinal cells!- Lactose intolerance!- Lactase levels decrease in people w/age, when they consume foods w/lactose, they get undigested lactose that reaches the large intestine. Bacteria metabolize it there --> produce acid +gas --> bloating, diarrgea, etc.!- Individs from N Europe and some tribes of Saharan Africa have a recent mut that allows lactase levels to remain high in adults!- Lactaid= lactase in pill form or products that have already been treated w/lactase!- tastes sweeter because it continas 2x as much sugar w/ a higher sweetness value!- Absorption - Energy nutrients have to be transporter across the brush border membrane (via transport complexes) into the enterocyte (absorptive cell) and across the basolateral membrane into the interstitial space to enter capillaries/lymphatic vessels !- Transport!- Passive faciliated (carrier-mediated) diffusion - doesn't require E!- movement down electrochemical gradient!- bidirectional movement - Active (carried-mediated) transport - requires E!- can function against a [ ] gradient!- unidirectional movement !- Carbohydrate absorption - Na+ glucose cotransporter: active transporter, drives the uptake of glucose by coupliing it to moving Na+ inside the cell via concentration gradient, uses E from Na+K+ATPase. Pump also makes cell slightly electronegative--makes Na+ want to come in cell even more.!- Facilitated glucose transporter: don't use E, it's just going down the [ ] gradient!- Digestion of Proteins!- Stomach: HCl (made by parietal cells) --> acid pH that denatures proteins -->more accesible to digestive enzymes!- Main transporter to pump H+ against [ ] gradient is a K+/H+ exchanging ATPase that uses the K+ gradient and ATP hydrolysis to generate E req. = symporter!- Prilosec inhibits this --> use to treat acid reflux and ulcers!- Proteases hydrolyze the peptide bonds that hold the aa's together!- Many produced by exocrine pancrease and act as endopeptidases that clip proteins internally!- endopeptidases have distinct specificity for certain peptide bonds!- Final digestion thru exopeptidases that work from either end to get individ aa's!--> lead to amino acids, dipeptides, and tripeptides!- Exocrine pancrease also produces bicarbonate (HCO3-)-->increases the pH so certain enzymes can work at their optimal pH and to neutralize the acid chyme entering duodenum from stomach!- Pancreatic juice w/digestive enzymes + HCO3- enters duedenum thru common bile pancreatic duct!- Celiac Disease: gluten= protein in wheat, rye, and barley. Individs w/celiac lack an unidentified digestive protease that prevents them from fully digesting gluten!- you get short oligopeps --> inflammatory response that can damage and interfere w/normal digestive capacity of brush border!- Absorption of Amino Acids!- Brush border of intestinal enterocytes contains 6 aa transpoters w/overlapping specificities for taking up aa's and di/tripeptides!- they are active co-transporters dependent on Na+ ion [ ] gradient to provide the E!- aa's are delivered to the capillary bed and enter the portal circulation to be delivered to other tissues!- Digestion of Lipids (they're different!)!- Complicated because they're hydrophobic so they form large lipid droplets that are then shielded from digestive enzymes!- So the liver produces bile acids --> acts as detergents --> emulsify large lipid droplets into smaller micelles --> increase their surface area for lipases to act on --> ultimately allow for individ lipid molecules to be absorbed!- Bile acids are amphipathic: hydrophobic portion inserts into lipid droplets and polar portion interacts w/water!- Bile acids are synthesized from cholesterol in the liver by addition of hydroxyl and carboxly groups --> make it more polar!- Bile Acid Circulation!- Bile acids are synthesized in liver and stored in gallbladder w/cholesterol until hormone signals their release into duodenum!- also solubilize cholesterol, excess chol. --> gallstones!- after performing their function in aiding lipid digestion, 90% of bile acids are reabsorbed in distal ileum and returned to liver in portal circulation!- in
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