 The contraction of circular and longitudinal muscle layers of the small intestine mixes the food with enzymes and moves it along the gut  The pancreas secretes enzymes into the lumen of the small intestine  Enzymes digest most macromolecules in food into monomers in the small intestine  Villi increase the surface area of epithelium over which absorption is carried out  Villi absorb monomers formed by degestion as well as mineral ions and vitamins  Different methods of membrane transport are required to absorb different nutrients.

 Mouth – Mechanical digestion of food (chewing). Saliva has amylase which digests starch  Esophagus – Food moves by peristalsis to stomach  Stomach – Water and acid kills bacteria. Churning breaks up food. Pepsin begins protein digestion  Small Intestine – Digestion of all macromolecules. Absorption of monomers  Pancreas – Secretion of lipase, amylase and protease  Liver – Production of bile  Gallbladder – Storage and secretion of bile  Large Intestine – Reabsorbs water. Forms and stores feces

 Peristalsis is the coordinated contraction of muscles in the esophagus and intestines  It forces food to move through the digestive tract  Can even move food against gravity

 The pancreas is probably best known for its role in controlling blood sugar by secreting insulin and glucagon into the bloodstream but it also plays a crucial role in digestion  The pancreas secretes pancreatic juice into the small intestine that contains:  Amylase to digest starch  Lipases to digest triglycerides & phospholipids  Proteases to digest proteins

 The small intestine is about 6 meters long and can be divided into three parts: the duodenum, the jejunum and the ileum  The processes of digestion and absorption take place in it  Digestion: Pancreatic juices digest  Starch to maltose  Triglycerides and phospholipids to glycerol, phosphates and fatty acids  Proteins into smaller peptides  Epithelial cells on the intestinal wall break molecules down even further. These enzymes include lactase, sucrase and peptidases. They break the remaining food into monomers.

 The intestinal walls contain small fingerlike projections called villi  Villi increase the surface area of the intestine which allows more food to be digested and absorbed  Surface area is increased even further by the presence of microvilli on the villi  Food absorbed across the intestinal wall include the monomers of carbs, lipids, protein and nucleic acids. It also includes vitamins and minerals.  The intestinal wall is selectively permeable and prevents some harmful substances from entering the bloodstream

 To get into the bloodstream, broken down food has to travel through the epithelial cells in the villi and into the capillaries or lacteals (lymph vessels)

 Different particles are transported in different ways. For example, fatty acids and monoglycerides (fatty acid with glycerol) travel into the epithelial cells via simple diffusion or facilitated diffusion. They then reassemble and combine with cholesterol and protein to form a lipoprotein or chylomicron before passing by exocytosis into the lacteal  Look at Figure 8 on page 284 and read pages to see how other food molecules are absorbed