DIGESTION and ABSORPTION Mark Louie D. Lopez Department of Biology College of Science Polytechnic University of the Philippines
SUMMARY OF DIGESTION
ABSORPTIVE SURFACES
ABSORPTIVE SURFACES Plicae circulares In some books: spiral or circular concentric folds; valvulae conniventes (or folds of Kerckring) Large folds which increases the surface area of the absorptive mucosa about threefold Well-developed in the duodenum and jejunum
ABSORPTIVE SURFACES Villi Fingerlike projections of the mucosal surface that enhance absorptive area another 10-fold. Involved in actual absorption Within the villi: arrangement of the vascular system for absorption of fluid and dissolved material into the portal blood
ABSORPTIVE SURFACES Brush borders Found at the apex of enterocytes Layer of densely packed microvilli that is a cylindrical protrusion of the cytoplasm that contains actin filaments. It also increase the area between intestinal surface and nutrients by 20-fold.
ABSORPTIVE SURFACES Thus, the combination of plicae circulares, villi, and microvilli increases the total absorptive area of intestinal mucosa by 600x
CARBOHYDRATE DIGESTION AND ABSORPTION 50% of digestible carbohydrate derived from plants and cereals as starch Glycogen from animal meat small amounts in normal diet Widely available, low in cost, stored easily Raffinose oligosaccharide in legumes, not digested by humans, digested by bacteria in colon = gas
DIETARY FIBERS Dietary Fiber Class Functions Cellulose: - Holds water, laxative, reduces main cell wall constituent colonic pressure, binds minerals Noncellulose polysaccharides: Hemicellulose, Pectin, Gums, Mucilages, Algal substances (sand in pears) - Holds water, increases stool bulk, slows gastric emptying time, binds cholesterol and bile acids, fermentable material for colonic bacteria (volatile acids and gas) Noncarbohydrate: - antioxidant, binds bile acids and Lignins woody part of plants metals
FUNCTION OF CARBOHYDRATES Energy Glycogen reserves Protein-sparing Antiketogenic Reserve fuel of heart CNS function fat-sparing
CARBOHYDRATE DIGESTION Mouth Ptyalin (alpha-amylase) Stomach stops once food is mixed with HCl Small intestine lumen: Pancreatic amylases digestion products = maltose α-limit dextrin lactose sucrose
CARBOHYDRATE DIGESTION
CARBOHYDRATE DIGESTION Small intestine - brush border enzymes Maltase (glucoamylase) maltose = glucose monomers Lactase lactose = glucose + galactose Sucrase sucrose = glucose + fructose Isomaltase α-limit dextrins = glucose Trehalase trehalose (from mushrooms) = glucose
CARBOHYDRATE DIGESTION Small intestine brush border enzymes: further digest the molecules into 3 sugar monomers 1. glucose (80%) Na + -glucose 2. galactose (5%) transporter 3. fructose (15%) - Fructose carrier
FAILURE TO ABSORB CARBOHYDRATES Diarrhea unabsorbed CHO will osmotically draw fluids from the epithelium into the lumen ex: lactose intolerance (absence of lactase) * As we grow older, enzymes at the brush border are insufficient. Hence, adults are more prone to have lactose intolerance. Intestinal gas H2, methane, CO2, metabolic products of intestinal flora
PROTEIN DIGESTION AND ABSORPTION
PROTEIN DIGESTION AND ABSORPTION The pancreatic juice contains zymogens like trypsinogen, chymotrypsinogen, and procarboxypeptidase. The enterokinase present in the brush border is capable of activating trypsinogen into trypsin. Trypsin then activates the other zymogens into their active forms (carboxypeptidase and chymotrypsin) so that long peptides can be broken down into simpler forms (di-/tripeptides or amino acids). Peptidases inside the enterocyte (cytoplasmic peptidases) can transform di-/tri-peptides into amino acids.
PROTEIN DIGESTION AND ABSORPTION
PROTEIN DIGESTION AND ABSORPTION Amino acid transportes are symporters that carry their substrate aa in conjuction with obligatory uptake of Na+ The small intestine is also notable for its ability to take up short peptides (PepT1). Peptides taken up into enterocytes are hydrolyzed by a series of cytosolic peptidases. AA not required by the enterocyte are exported across the basolateral membrane -> blood capillaries -> liver via portal vein.
LIPID DIGESTION Lipids: supply 40% of adult energy needs Majority of fatty acids (FAs) in dietary triglyceride: oleate and palmitate Essential Fats linoleic 2% linolenic 0.5% Arachidonic Triglycerides animals = long-chain saturated FAs plants = polyunsturated FAs Phospholipids majority from bile, commonest ingested = lecithin (arachidonic and linoleic)
mitochondria in aging = use of fat as fuel (since mitochondria is the site of B-oxidation).
LIPID DIGESTION
LIPID DIGESTION
LIPID DIGESTION The more saturated fat (more H + ), less safe Polyunsaturated fats are good for the health (like fish oils and linseed oils)
LIPID ABSORPTION Most dietary lipid absorbed in upper 2/3 of the jejunum Absorption influenced by the presence of other foods and reduced by dietary fiber Goes through 3 phases: water in the lumen, lipid in the epithelial membrane and water in the lymphatics & bloodstream >95% ingested fat absorbed by adults
LIPID ABSORPTION The products of enzymatic digestion (free fatty acids, glycerol, lysophospholipids, cholesterol) form micelles with bile acids in the intestinal lumen. The micelles interact with the enterocyte membrane and allow diffusion of the lipid soluble components across the enterocyte membrane into the cell. The bile acids, however, do not enter the enterocyte at this time. They remain in the intestine, travel further down, and are then reabsorbed and sent back to the liver by the enterohepatic circulation.
LIPID ABSORPTION The intestinal epithelial cells will resynthesize triacylglycerol from free fatty acids and 2-monacylglycerol and will package them with a protein, apolipoprotein B-48, phospholipids, and cholesterol esters into a soluble lipoprotein particle known as a chylomicron. The chylomicrons are secreted into the lymph and eventually end up in the circulation, where they can distribute dietary lipids to all tissues of the body.
LIPID ABSORPTION The fat will be resynthesized in the SER and golgi apparatus inserting some protein making it a chylomicrons for transport into the circulation through the lymph. Some of the fat (monoglycerides and MCFA) will be extruded in the lacteals and go directly into the blood streams. Some of the absorbed fat will got the lymph while the other will go to the portal circulation
NUCLEIC ACID DIGESTION
NUCLEIC ACID DIGESTION Pancreas release pancreatic nucleases that converts nucleic acids into nucleotides Small intestines has intestinal brush border enzymes that converts nucleotides into N- containing bases, ribose, deoxyribose and phosphate
NUCLEIC ACID DIGESTION
ABSORPTION IN GASTROINTESTINAL TRACT
SUMMARY Carbohydrate and Protein digestion - depends on: combined effects of secreted enzymes in lumen, brush border and within the mucosa close proximity at brush border of enzymes and sites of absorption of digestive products Lipid digestion depends on the presence of bile salts, pancreatic lipase and colipase at nearly neutral ph Efficient digestion and absorption salt and water secretion by jejunum motor response of the gut intestinal blood flow (may limit absorption)
INTESTINAL ABSORPTION Water - most absorbed in jejunum & ileum - absorbed against an osmotic gradient in the colon Na + - co-transport with glucose, amino acids & peptides Cl - - flows passively down electrochemical gradient established by Na + K + - secreted in the colon HCO3 - - secreted partly in exchange for Cl -
BLOW FLOW IN GASTROINTESTINAL TRACT
BLOOD FLOW IN GASTROINTESTINAL TRACT Abdominal Aorta supplies oxygenated blood to the organs of the GIT Most of the blood that goes to the liver is venous in nature and is supplied via the portal vein from the intestines This limits the entry of harmful molecules(drugs/bacteria) into the blood stream while receiving nutrients absorbed in the GIT
PANCREAS
PANCREAS Physiological Characteristics Functions both as an exocrine (releases pancreatic juice w/c contains enzymes, lipases, amylases, proteases, & bicarbonate) and an endocrine (releases insulin & glucagon) gland. By bulk however, mostly exocrine in function.
PANCREATIC JUICE COMPONENTS
PANCREATIC ENZYMES Proteases: digests proteins Chymotrypsin, Trypsin: Proteins to Peptides Carboxypolypeptidase: Peptides to amino acids Amylase: digests carbohydrates Starch to Maltose Lipase: digests fat; needs to combine with bile salts (secreted by liver) Dietary fat triglycerides to monoglycerides + fatty acids Pancreatic amylase: digest carbohydrates (except cellulose)
PANCREATIC BICARBONATE SECRETION Secreted by pancreatic duct cells CO2 and H2O diffuses into the cell Carbonic anhydrase facilitates H2CO3 formation Carbonic acid (H2CO3) splits into H+ and HCO3- Active transport of HCO3- to the lumen, associated with Na+ diffusion into the lumen. H+ is exchanged with Na+ from the blood by active transport (this supplies process #4)
PANCREATIC BICARBONATE SECRETION
PANCREATIC BICARBONATE SECRETION How bicarbonate perform its neutralizing action: HCl* + NaHCO3 NaCl** + H2CO3 H2CO3 H2O + CO2*** *from duodenal contents ** NaCl solution neutralizes the acid contents emptied into the duodenum *** goes to the bloodstream
STIMULATION OF PANCREATIC SECRETION
STIMULATION OF PANCREATIC SECRETION
STIMULATION OF PANCREATIC SECRETION
LIVER A large, multi-lobed organ located in the abdominal cavity 75% of the blood that passes it are deoxygenated and are for filtration. 25% comes from hepatic vein (100% oxygenated).
LIVER Functions of the Liver: Bile secretion main digestive function Storage of vitamins and minerals Degrade and excrete hormones and other wastes from the blood (e.g. bilirubin and excess cholesterol) Transform and excrete drugs and toxins Disposal of ammonia from protein metabolism by converting it to urea
LIVER
GALL BLADDER Standard color: green Store bile in between meals (25-50cc) Bile flows into the gallbladder when the sphincter of Oddi is closed. Also concentrates bile by active transport of sodium and the secondary absorption of H2O, Cl-, and other constituents.
BILIARY SYSTEM
BILE FLOW
REGULATORY MECHANISM OF BILIARY PHYSIOLOGY
GALL BLADDER EMPTYING Stimuli: Cholecystokinin (major stimulus): initiated by fatty food in the duodenum Acetylcholine secreted by both vagus and ENS nerve fibers (minor stimulus) Stimuli results to rhythmical contractions of the gallbladder wall and simultaneous relaxation of the sphincter of Oddi.
RECYCLING OF BILE Four ways that bile salts are recycled out of the small intestines: carrier-mediated active absorption in the terminal ileum (most important route) passive diffusion along the small intestine (minor role) deconjugation to primary bile acids before being absorbed passively or actively conversion of primary bile acids to secondary bile acids, then subsequent absorption of deoxycholic acid