6.1 Digestion & Absorption D.2Digestion The Digestive System 6.1 Digestion & Absorption D.2Digestion

Accessory Digestive Organs These are organs located outside of the GI tract These organs create, or store and secrete enzymes, hormones and chemicals that are required for digestion.

Pancreas

Pancreas Long flat gland, nestled between the stomach and duodenum. When chyme from the stomach enters the duodenum, the hormone CCK (cholecystokinin) is secreted (by the walls of the duodenum) and causes the pancreas to release its pancreatic juices and the liver to release bile The pancreas does 3 things to aide in digestion

1. Releases pancreatic juice Travels from the pancreas to the duodenum via the pancreatic duct Contains 28 different enzymes including: Amylases: convert big, complex carbs into simple sugars Lipases: split lipids into glycerol and fatty acids Trypsin/peptidase – breaks proteins into amino acids

2. Neutralizes the acidic chyme The pancreatic juices are alkaline (basic) because of its high concentration of bicarbonate ions (HCO3-) Changes the chyme from pH 2 (which is the pH in the stomach) to pH 8 If the acidic chyme was not neutralized, it could cause ulcers in the intestinal walls

Secretin Secretin is a hormone produced by the duodenal walls. The optimal pH of the duodenum is pH 8 When the pH in the duodenum is too low, secretin is released into the blood stream, and will stimulates the production of bicarbonate ions from the pancreas Inhibit gastric juice production in the stomach

3. Releases Hormones INSULIN – controls blood sugar levels by allowing glucose to enter the cells allowing the glucose to be stored as glycogen. GLUCAGON – when more glucose is needed in the blood, glucagon will allow glycogen to be broken down into glucose again

Exocrine vs Endocrine Glands A gland is an organ that secretes a substance such as a hormone or an enzyme. Endocrine Glands: secrete hormones and release them directly into the blood stream. Ex: the pituitary glands, adrenal glands, the ovaries, the testes……

Exocrine vs Endocrine Glands Exocrine Glands: secrete their products into a duct. Ex: sweat glands, digestive glands

EXOCRINE GLANDS - Secrete products into a duct ENDOCRINE GLAND - Secretes products (hormones) directly into the blood stream Acinar cell Makes products Ex: pancreatic lipase Acinus (plr acini) - Where product accumulates Duct

Exocrine Gland - Hollow space surrounded by secretory cells. - Collects products and sends it to the duct Produce the products and secrete them into the acinus Can either lead to a lumen (space between cells, ex: the lumen of the small intestine) or out of the body

Identification of exocrine glands from EMs See data base questions on page 674

Examples of Exocrine Glands Place Released pH Secretion Enzyme Substrate Product Salivary glands Mouth 7 Saliva Salivary Amylase Gastric glands Stomach 2 Gastric juice Pepsin/ pepsinogen pancreas Small intestine (duodenum) 8 Pancreatic Juice Bicarbonate ions Pancreatic amylase Trypsin Lipase Phospholipase Not an enzyme Starch Protein Lipid Phosopholipids Maltose Smaller Polypeptides Glcerol & Fatty acids Phosphate, glycerol & fatty acids Examples of Exocrine Glands Neutralizes stomach acid

Trypsin Trypsin is released as the inactive precursor trypsinogen (made by the pancreas) because in it’s active form it would destroy the proteins and cells that make it. The duodenum secretes the enzyme enternokinase which will catalyse the conversion of trypsinogen to trypsin in the duodenum. enterokinase Trypsinogen Trypsin

HCl Pepsinogen pepsin enterokinase Trypsinogen trypsin

Lipid Digestion Not easy because they are not water soluble Lipid molecules group together to from droplets of fat Lipase acts on the surface of lipid spheres

Lipid Digestion - BILE BILE – produced by the liver Aides in lipid digestion by emulsifying fats (dividing fat droplets into smaller droplets) Creates a larger surface area for lipase to work

The emulsification of fat by bile

Bile is continuously produced in the liver but is stored in the gall bladder When food enters the duodenum, the gall bladder contracts and bile is squeezed out of the gall bladder and travels to the duodenum via the bile duct.

Digestion in Small Intestine As mentioned before, the majority of digestion occurs in the small intestine. Macromolecules are broken down into monomers. The pancreas add pancreatic juice and the liver/gallbladder add bile to aide in digestion. The walls of the small intestine also produce a variety of enzymes to digest ingested materials.

Digestion in Small Intestine While the gland cells produce some intestinal juice that will mix with the chyme, most of the enzymes remain immobilized in the plasma membrane of the epithelium cells that line the lumen of the small intestine

Digestion in Small Intestine This includes: Nucleases: digest DNA and RNA into nucleotides Maltase: digests maltose into glucose Lactase: digests lactose into glucose and galactose Sucrase: digests sucrose into glucose and fructose Exopeptidases: digest proteins by removing one amino acid at a time Dipeptidases: digests dipeptides into 2 amino acids Note: these are all hydrolysis reactions

Absorption of Digested Foods

Cross Section of ileum

Identify the tissue layers in a transverse section of the small intestine (EM) (page 281) mucosa serosa

Epithelial cell features (of small intestine) Epithelial Cells are the cells that line cavities and surfaces They are the cells that create the villi that line the small intestine lumen

Epithelial Cell Features Form villi – increase surface area Contain microvilli – increase surface area Tight Junctions – connections between epithelial cells which fuse adjacent cells together at their membranes to form a seal Rich in mitochondria to produce ATP for active transport Pinocytotic vesicles –to aide in endocytosis

Ileum - Cross section Villus (plr villi) lumen Circular muscle Longitudinal muscle submucosa Folds in small intestine

Absorption by Villi The epithelium that covers the villi forms a barrier to harmful substances but also must be permeable to allow for useful nutrients to pass through into villi. Villi will absorb: Monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, nitrogeneous bases, mineral ions (Ca2+, K+, Na+), vitamins (ex: vitamin C, ascorbic acid) Some harmful and harmless but unwanted substances will be absorbed as well (ex: ethanol, food dyes)

How are nutrients absorbed into epithelial cells? Passive Transport Active Transport

Passive Transport Transport across a membrane that does not require energy Driven by the concentration gradient SIMPLE DIFFUSION – the molecules move through the membrane moving from an area of high concentration to low concentration. Ex: lipids

Simple Diffusion

Passive Transport Facilitated Diffusion – the molecules are too large to pass through membrane on their own They enter the membrane via a pore or a carrier protein/protein channel Ex: amino acids, glucose

Facilitated Diffusion

Active Transport Requires ATP energy Often moves molecules against their concentration gradient (from low concentration to high concentration)

Active Transport ACTIVE TRANSPORT via Transport Proteins/ Protein Pumps These proteins embedded in the cell membrane require ATP to activate them

Endocytosis The plasma membrane forms a vesicle around the nutrient molecule(s) to bring them into the cell Pinocytosis: for liquids or small molecules Phagocytosis: for larger molecules

Transport Animations http://programs.northlandcollege.edu/biology/biology1111/animations/passive1.swf

Animations http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Triglycerides Absorption Triglycerides are digested into monoglycerides and fatty acids Monoglycerides are absorbed via simple diffusion Fatty acids are absorbed by facilitated diffusion (by proteins on the membrane of microvilli) Once inside the epithelium cells, the fatty acids combine with the monoglycerides to reform triglycerides

Triglyceride Absorption Triglycerides then form small lipoprotein droplets by combining with cholesterol and being surrounded by phospholipids The lipoproteins then exit the epithelium cells on the inner side of the villus by exocytosis. Lipoproteins will either enter a lacteal and be carried away by lymph or a capillary and enter the blood stream.

Glucose Absorption Glucose cannot pass through the plasma membrane by simple diffusion. Sodium-potassium pumps will pump Na+ out of the villus epithelium cells (and K+ in) => low [Na+]

Glucose Absorption A sodium-glucose co-transporter protein will move both Na+ and glucose together from the intestine lumen into the epithelium cells. This is facilitated diffusion (moves with the Na+ concentration gradient) Glucose protein channels will then move glucose into interstitial spaces inside the villi and into the capillaries

Unabsorbed Materials The following molecules are not absorbed by the body (because there is no system of membrane proteins or receptors to absorb them or enzymes to break them down): Cellulose Lignin (large polysaccharide that strengthens wood) Bile pigments Bacterial and intestinal cells These materials are egested in feces

Why isn’t Cellulose Digested? Cellulose is a large polysaccharide (made of glucose molecules) that provides structure in plant cell walls. The main difference between starch and cellulose are the linkages Starch can be made of amylose which has α-1,4 glycosidic bonds and/or amylopectin has α-1,4 glycosidic bonds and α-1,6 glycosidic bonds Cellulose has β-1,4 glycosidic bonds

Starch Digestion Amylase can work on the 1,4 alpha glycosidic bonds to make maltose or maltotriose Amylase cannot work on the 1,6 alpha bonds of amylopectin The fragments with the 1,6 bonds are known as dextrins. 3 enzymes on the membranes of microvilli will complete the digestion of these three small sugars into glucose Maltase: digests maltose to glucose Gluodidase: digests maltotriose to glucose Dextrinase: digests dextrin to glucose

Cellulose indigestion Amylase only works on the 1,4 alpha linkages and cannot work on the 1, 4 beta linkages of cellulose Cellulose does not dissolve in water The enzyme cellulase can break down cellulose however mammals do not poses the enzyme

Ruminant Animals Some forms of bacteria can produce cellulase and they can live symbiotically in the guts of herbivores (such as cattle, llamas, sheep, goat, giraffes, deer….)

Role of Dietary Fibre The rate of movement of materials (feces)through the large intestine is correlated to their fibre content. A healthy diet contains fibre. Fibre increases the bulk of material passing through the intestines It prevents constipation because it draws water into the intestines The higher the water content, the faster the movement of feces.

Role of Dietary Fibre The presence of bulky fibre also increase feelings of satiety, making you feel satisfied and preventing overeating. Fiber slows down the absorption of sugars (preventing type II diabetes) Fibre rich foods: whole grain breads and cereals, cabbage, celery