IPHY

Enzymatic Digestion in Small Intestine 1. Proteins degraded to small polypeptides 2. Carbohydrates degraded to disaccharides 3. Lipids must receive special treatment before they can be attacked by enzymes.

Fat digestion: 1. Bile salts and lecithin emulsify lipid particles (micelle) 2. Lipase breaks down to monoglycerides and free fatty acids.

Epithelial cell Mucous cell Central lacteal Capillaries Crypt of Lieberkühn Arteriole Venule Lymphatic vessels Microvilli (brush border) Fig , p. 614 Villus Epithelial cell

Absorption of nutrients 1. Small peptides. A. Peptides broken down to amino acids by aminopeptidases in the epithelial wall or by intracellular peptidases. B. Most amino acids are absorbed into epithelial cell by active transport (with expenditure of ATP and co- transport of Na+ C. Cl- and water follow Na+ into epithelial cell. D. Amino acids exit cell via passive carrier molecules and into blood. Na+ exits by active transport, Cl- and water follow.

2.Absorption of Disaccharides A. In epithelial cell, disaccharides broken down to single sugars. Lactose --> galactose and glucose by lactase Maltose--> glucose and glucose by maltase Sucrose--> glucose and fructose by sucrase B. Glucose and galactose actively (with ATP) co- transported into epithelial cell with Na+. Cl- follows with water. Fructose moved by passive facilitated diffuion. C. Glucose, galactose and fructose exit the cell by passive facilitated diffusion, and enter blood by diffusion

3. Absorption of Fats A. micelles containing monoglycerides and free fatty acids deliver them to epithelial lining of small intestine B. monoglycerides and FFA passively diffuse through cell membranes C. monoglycerides and FFA resynthesized into triacylglycerides in cell D. triacyglycerides surrounded with protein coat, forming a chylomicron.

Fats, continued: E. Chylomicron moved by exocytosis out of epithelial cell into lymph circulation F. Chylomicron transported by lymph to circulatory system, and then to fat depot where protein coat shed, and fat stored. G. ATP used for bile and triacylglyceride synthesis.

Fig , p. 618

Absorption of other nutrients: A. vitamins 1. Fat soluble (A,D,E, K)--absorbed passively via micelle 2. Water soluble absorbed passively or with facilitated diffusion 3. Vitamin B12 unique--needs gastric intrinsic factor and receptor-mediated uptake

B. Ions 1. Active transport (Na+, K+, Ca++, Mg++, Mn++, Fe++, PO4-, etc) 2. Cl-, HCO3- passive diffusion C. Water Moves by osmosis with all nutrients transported [~10 L/day absorbed (most from digestive process); less than 500 ml still in feces as it goes to large intestine]

Accomplishments in the small intestine 1. Complete breakdown of proteins, fats, and carbohydrates and absorption of their constituents 2. Almost complete absorption of ions, water, vitamins. 3. Addition of bilirubin to feces for elimination.

Large intestine 1. Store feces and move towards anus 2. Continue absorption of ions (mostly Na + and Cl-) and water (less than 100 ml/day lost). 3. Bacterial breakdown of non- digestible material and vitamin synthesis.

Bacterial action in large intestine There are more bacteria in the large intestine than cells in an adult body. Bacteria use  -amylase to digest cellulose (some glucose may be used by large intestinal cells) Gases produced (CO2, methane, H2S) as waste products Vitamins K, B12, riboflavin, and thiamine produced in small amounts

Elimination of feces Feces contain bilirubin, non-digestible organic matter from food (roughage), ferritin, any non-absorbed nutrients (fats), bacterial breakdown products, sloughed cells, water, and a few ions. Control of anal sphincters: Internal: smooth muscle controlled by PNS External: skeletal muscle controlled by motor