L. digestion and absorption Use figure 16-18 on page 541 of the textbook to follow along as we look at what happens to the materials being processed in the small intestine. BIOL 2404 Chapter 16

1. carbohydrates salivary amylase works during chewing to break starch and glycogen down into disaccharides (mostly maltose) salivary amylase is inactivated once the food gets to the stomach when food arrives in the duodenum, pancreatic amylase continues the breakdown of starch and glycogen into disaccharides when the disaccharides are pushed up against the intestinal mucosa epithelial cells, the intestinal epithelial enzymes break all disaccharides (maltose, sucrose and lactose) down into monosaccharides (glucose, fructose and galactose) the monosaccharides are absorbed through intestinal epithelial cells into vascular capillaries BIOL 2404 Chapter 16

2. proteins when food reaches the stomach, the stomach secretes the enzyme pepsin pepsin breaks proteins and peptides in the chyme down into smaller peptides (chains of amino acids) when chyme reaches the duodenum, pancreatic proteases continue to break the peptides down into smaller and smaller sections (mostly dipeptides) when the dipeptides are pushed up against the intestinal mucosa epithelial cells, the intestinal epithelial enzymes break all dipeptides down into amino acids the amino acids are absorbed through intestinal epithelial cells into vascular capillaries BIOL 2404 Chapter 16

3. lipids the major type of lipid in ingested food is triglycerides before any enzymes can begin to break down these triglycerides, they have to be emulsified by bile (this is not chemical digestion) bile makes smaller drops of fat so that more enzymes can work on digesting the lipids the only enzyme that digests lipids is pancreatic lipase it breaks triglycerides down into fatty acids and monoglycerides these are absorbed into the intestinal epithelial cells cholesterol does not need to be digested before it is absorbed along with fatty acids and monoglycerides then they are converted back into triglycerides and combined with proteins to make chylomicrons chylomicrons are leave the intestinal epithelial cells and are absorbed into lacteals (lymphatic capillaries inside the villi) BIOL 2404 Chapter 16

4. water and electrolytes are absorbed along with food (there are no diagrams showing this process) a. sodium and chloride are transported from the lumen into the intestinal epithelial cells and then go into the vascular capillaries b. water follows by osmosis c. iron absorption is carefully controlled to prevent toxicity; the intestinal epithelial cells have a “locking” mechanism that keeps too much iron from entering the blood BIOL 2404 Chapter 16

5. vitamins are absorbed along with food a. most water soluble vitamins (B, C) diffuse through the intestinal epithelial cells and then go into the vascular capillaries b. the fat soluble vitamins (A, D, E, K) are absorbed along with lipids from food c. vitamin B12 is a water soluble vitamin that can only be absorbed if it has first been bound to intrinsic factor, a protein made in the stomach BIOL 2404 Chapter 16