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Delivery of Nutrients to Cells As discussed previously, digestive enzymes break down large complex food molecules into smaller useable molecules. The components of these small molecules are required for the bodies growth, development, metabolism and production of enzymes and hormones. These small molecules are soluble and are absorbed through the small intestine directly into the blood stream. The Small Intestine The Small Intestine (cont.) The inner lining of the small intestine Is folded and covered in finger like projections called villi. These are further covered with more folds called microvilli. This folding increases the surface area for absorption of nutrients through the epithelial cell which line the villi. This structure is so efficient because the epithelial cells are thin being only one cell thick and found directly within each villi is a blood capillary and central lacteal (lymphatic vessel) which are ready to absorb nutrient molecules. Absorption of Nutrients Carbohydrate Proteins Fats/Lipids Glucose Amino Acids Fatty Acids Glucose and Amino acids are absorbed into the epithelial cells and then pass directly into the blood capillary. Fat products pass into the epithelial cells and then pass into the central lacteal. Calcium, iron and vitamin B12 are also absorbed through the epithelium of the small intestine. B12 requires the presence of and additional factor produced in the stomach to be absorbed. Intrinsic factor is a polypeptide produced in the stomach. It encourages endocytosis and therefore the vitamin is transported into the epithelium of the small intestine. Emulsifiers An emulsion is when two normally separate products or liquids become mixed and form droplets e.g. oil in water. An emulsifier is something which maintains the emulsion state. In the body, bile salts act as emulsifiers. Bile Salts Bile salts are a steroid compound produced in the liver and stored in the gall bladder. They consist of a water soluble (hydrophilic) head and a lipid soluble (hydrophobic) tail. If bile salts and lipid droplets are shaken together the bile salts arrange themselves around the outside of the lipid droplet with their tails on the inside of the lipid droplet. As the hydrophilic heads are negatively charged the droplets repel each other and stay apart. This process is called emulsification. Lipase Lipase is a digestive enzyme produced in the pancreas which in the small intestine digests lipids into monoglycerides and fatty acids. As shown in our experiment, bile salts increase the action of lipase on fatty acids. This occurs because the bile salts have maintained an emulsion and therefore increase the total surface area and the total number of substrate molecules for the lipase to act on the lipids. Micelles After lipase has digested lipids into monoglycerides and fatty acids and bile salts have attached around creating and maintaining an emulsion, minute water soluble molecules called micelles are formed which travel to the surface of the intestinal epithelium microvilli. (fig 22.6) Absorption of Monoglycerides and fatty Acids (fig 22.7) Monoglyceride and Fatty Acid Diffuse through cell membranes into Epithelium Pass into Smooth Endoplasmic Reticulum and rebuilt into lipid molecules Coated with Lipoprotein and leave cell by exocytosis Absorbed by lacteal Enter lymphatic system Enter blood stream The Liver The Livers Blood Supply (fig 22.8) Hepatic Artery: Carries oxygenated blood to the liver ( branched from aorta) Hepatic Vein: Carries deoxygenated blood away from the liver ( leads to vena cava) Hepatic Portal Vein: Carries blood from another source ( e.g. from the intestine) to the liver which then leads to the general circulation. The Liver and Metabolism Carbohydrate: Regulation of Blood Glucose excess glucose converted to glycogen by insulin and stored in the liver. When glucose is required glucagon produced in the pancreas converts glycogen back to glucose. Lipid: Cholesterol required for cell membrane structure and synthesis of steroids. Liver synthesises cholesterol if not enough obtained through dietary intake. Excess cholesterol excreted by liver and passed out of body as bile. Protein: The liver synthesises three types of plasma proteins. Amino acids required for this are absorbed from the blood stream by liver cells (table 22.1) Albumins Globulins Fibrinogen Fate of Absorbed Materials Absorbed material Immediate use Fate of excess of substance Carbohydrates (glucose) Aerobic Stored as glycogen in respiration muscle/liver or as fat in (energy) adipose Lipids (fatty acids, Energy source Stored in adipose glycerol) tissue Proteins (Amino acids) Protein synthesis Energy source or urea excreted in urine Vitamins and minerals Enzyme action RBC’s etc Some stored, some excreted