Chapter 3 Reading Guide Be sure to use the many figures and tables provided by the book to help answer these questions. 1. What is digestion? What is the difference between mechanical and enzymatic digestion? Why must food be digested? 2. What is absorption? 3. Trace the path of an undigested food particle (for example, fiber) from the mouth to the rectum. 4. What is food called when it is in the esophagus? In the stomach and small intestine? In the large intestine? 5. List the 3 sections of the small intestine, starting with the one that is adjacent to the stomach and ending with the one that is adjacent to the large intestine. 6. Describe peristalsis and segmentation. 7. What is a sphincter (describe its function and anatomy)? 8. What sphincter separates a) esophagus from stomach, b) stomach from duodenum, c) ileum from cecum? 9. Which organs produce secretions that contain digestive enzymes? 10. What’s the name of the reaction carried out by digestive enzymes? 11.What acid is in the stomach? Is it a strong or a weak acid? 12.How are stomach cells protected from acid? 13.What happens to the enzyme that was digesting starch in the mouth and esophagus, once the bolus gets to the stomach? (Reminder: enzymes are proteins) 14.There are digestive enzymes that digest carbohydrates, fats and proteins in the small intestine. Where did these enzymes come from, and how did they get to the small intestine? 15.What substance neutralizes chyme as it moves from the stomach to the small intestine? Where does it come from? 16. Explain the “7 obstacles” food must face in order to be efficiently digested and absorbed. 17.What is the general term for an enzyme that digests starch? Proteins? Triglycerides? 18.What’s the difference in action between bile and lipase? 19.Why are normal intestinal bacteria good? How can you feed your flora? 20.Which nutrients need to be enzymatically digested before they can be absorbed? Which simply need to be separated away from other particles? 21.What nutrients are absorbed in the colon (don’t forget, water is a nutrient)? 22.Follow the fate of nutrients and fiber from whole wheat crackers with cheese and apples. (Crackers- fiber, starch; Cheese- fat, protein; Apples- sugar, fiber. All three provide ample vitamins and minerals.) 23.Compare and contrast villi and microvilli: a) which is larger? B) which is composed of many cells, and which is simply extensions of individual cells? 24.Describe the structure of a villus. Don’t forget the capillaries and lacteal! Draw a villus with microvilli. 25. How does a water-soluble nutrient, such as glucose, get from the lumen (open space where food passes) of the small intestine to the capillary within the villus? 26. In which organ of the digestive tract are most nutrients absorbed? 27. Between water-soluble and fat-soluble nutrients, which enter the blood as they are absorbed? They lymphatic system? 28. To what organ do water soluble nutrients go directly from the small intestine? How do they get there (name the vein)? 29. What is a feedback mechanism, and how does that relate to hormones and the nervous system? 30. Explain the roles of the hormones gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP). 31. Describe the relationship or arteries, capillaries, and veins. 32. Describe the lymphatic system. 33. Why is it important that the pyloric sphincter only lets out small amounts of chyme at a time, and how is that careful release controlled? 34. How are pancreatic cells protected from their own enzymes? 35.Where is bile manufactured? Where is it stored? 36. Describe why “balance, moderation, variety and adequacy” of diet are important in maintaining GI health. 37. Explain the roles of the liver, gall bladder, and pancreas. Does chyme ever actually pass through any of these accessory organs? 38. What is the only place in the digestive tract where all three energy nutrients (carbs, proteins, fats) are being enzymatically digested at the same time? 39. Where in the body does the enzymatic digestion of starch begin? Triglycerides? Protein? I. Supplemental information Digestion and Absorption a. The point of the digestive system is to get nutrients into the body (via the blood). To get into the blood, nutrients need to pass through cells that line the digestive tract. These cells have little tunnels in them that allow specific nutrients to pass through. The nutrient passes from the lumen of the small intestine, through a tunnel into the cell, then through a tunnel on the other side of the cell and into a blood vessel. In order to get through these tunnels, nutrients must be very small. b. The process of digestion breaks food into smaller and smaller pieces, until the food is separated into its component nutrient molecules: amino acids, monosaccharides (ex glucose), fatty acids, vitamins, minerals (which are mostly just single atoms, like Na+, K+, and Ca++), and a few other types of molecules like phytochemicals. These molecules are now small enough to pass through cells and enter the bloodstream. Anything that doesn't get broken into small enough pieces passes right through and goes out the other end without giving us any nutrition. For example, we can't digest fiber, so it goes right out the other end and into the toilet (of course, fiber does offer some nice health benefits, it just doesn't offer us any energy. For example, it drags cholesterol out into the toilet with it). c. The process of absorption describes the component molecules of food moving from the GI tract, across cells, and into the blood. That is, getting nutrients into the body. d. Digestion includes both mechanical (chewing, etc) and chemical (acid and enzymes) processes. Mechanical digestion just breaks food chunks down into smaller chunks. Enzymatic activity is what gives us the component food molecules. II. Enzymes a. General- enzymes are substances that drive virtually all the chemical reactions that occur in your body. There are a few things you need to know about enzymes in general: i. They are all proteins ii. Each is specific; each drives only one reaction iii. Each can only work under certain pH and temperature conditions; for example, the enzyme in your saliva that breaks down starch is unraveled by stomach acid. b. The nutrients that must be enzymatically digested- These are nutrients that are made of chains of subunits: starches and disaccharides, proteins, and triglycerides (a type of lipid). These three specific nutrients each are pretty large, too large to get through the tunnels in cells. They are made of smaller subunits, though: starch is made of hundreds of small glucose molecules linked together (like a beaded necklace, where each bead represents a glucose), proteins are made of hundreds of small amino acids (here, the beads are amino acids), and triglycerides are made of just 3 linked fatty acids (the beads are the fatty acids). So, in order to get any nutrition from these particular nutrients, the individual “beads” must be clipped off of the necklace. Glucose, amino acids, and fatty acids are all small enough to be absorbed. This is what digestive enzymes do: they release glucose, amino acids, and fatty acids from starch, proteins, and triglycerides. Now, vitamins and minerals do NOT need to be cut up; they are already small enough. They just need to be separated from the rest of the food molecules, and that does not require enzymes. c. The major digestive enzymes- all use hydrolysis to split up starches, proteins and triglycerides. (I’ll talk about disaccharides in chapter 4) I will introduce the 3 major classes of digestive enzymes now; as we get into more detail on carbs, proteins, and fats, we will see more enzymes being introduced. i. Amylase- hydrolyzes (splits up) starch, released in saliva by salivary gland cells and into the small intestine by the pancreas ii. Proteases- hydrolyze proteins- released in the stomach by stomach cells and into the small intestine by the pancreas iii. Lipases- hydrolyze triglycerides- released in small amounts in saliva (by salivary gland cells) and the stomach (by stomach cells), and in large amounts into the small intestine by the pancreas III. Comparing the roles of enzymes, acid, and bile in digestion a. Enzymes- you already know the role of enzymes: they split carbs, protein, and triglycerides into their component subunits (glucose, amino acids and fatty acids), which are small enough to be absorbed. b. Acid- hydrochloric acid is produced by cells of the stomach. This acid serves a couple of important functions: first, it kills most undesirable critters (bacteria, etc) that you ingested with your food. Second, and this is the digestive role, it unravels most proteins. It simply unravels them; proteins are like beaded necklaces that are all folded and tangled up. HCl does not clip off amino acids (beads); instead, all it does is untangle the necklace a bit; unfold the protein chain. This makes it easier for proteases to get in there and actually digest the protein. c. Bile- bile is produced by cells of the liver and is sent to the gallbladder for storage. When you eat a meal with fat, CCK from the small intestine encourages the gall bladder to squeeze and send bile through a little tube into the small intestine. What does bile do? First, you must know that the fluid inside the small intestine is watery. Fats that you eat would tend to form big blobs (made of thousands of molecules of triglycerides) and separate away from the water because fat and water don’t mix. Lipases are water soluble, so they cannot get into those big blobs of fat to digest triglycerides. Bile helps in fat digestion because of bile salts. These molecules have a lipid-soluble (cholesterol based) end, and a water-soluble end. Bile salts grab small blobs of fat away from the big blobs with their cholesterol ends, leaving their water soluble ends sticking out. In this way, they suspend small blobs of lipids in the watery environment of the small intestine. In surrounding lipids, bile salts prevent lipid droplets from reblobbing together, which lipids tend to do in water (think of salad dressing...no matter how much you shake it, the oil still ends up separating from the vinegar). So, bile salts keep lipid droplets small. Why is this important? It allows the maximum number of lipases to work on lipids at once, by increasing the surface area available to those enzymes. Think about lipids as a loaf of bread and lipases as ants. Lets say there are 1000 ants. If they were to work on an unsliced loaf of bread, maybe only 500 ants would be able to fit around the loaf. But, if the bread were sliced and spread out, all 1000 ants would be able to work on it at the same time, making sure that all the bread was accessed. This process is called emulsification. Again, bile salts are not hydrolyzing individual triglycerides (they are not chopping off fatty acids from each triglyceride); instead, they are limiting how many triglycerides can blob up together. IV. The many roles of the liver- The book made reference to how important the liver is, but didn’t give you a full view. While we will see more roles of the liver as we go through the book, I think it’s important you have an overview now. So, here is a short list of the things the liver does for you: a. Detoxification of substances (ex, alcohol) and storage of heavy metals b. Receives all water-soluble nutrients from the small intestine and determines what to do with them; for example, if there is a lot of glucose in circulation, liver cells will store some of it. If there is excess protein, liver cells will convert some amino acids to fatty acids. c. Monitors and adjusts blood lipids; for example, the cholesterol carriers LDLs and HDLs are both made by liver cells d. Makes bile e. Stores nutrients: fat-soluble vitamins, vitamin B-12, iron, glucose f. Liver cells make most of the proteins that are in your blood; for example, clotting proteins. g. Rids the body of waste products and excess cholesterol by putting them into bile; so, bile is not only an emulsifier, it is also a carrier of waste. That waste will simply end up becoming part of feces.