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TChemicalandPhysicalProcessesofDigestionhe digestive system, also called the gastrointestinal system, consists of the digestive tract (also gastrointestinal tract or GI tract) and accessory glands that secrete enzymes and fluids needed for digestion. The digestivetract includes the mouth, pharynx, esophagus, stomach, small intestine, colon, rectum, and anus. The major functions of the digestive system are to ingest food, to break food down to its simplest components, to extract nutrients from these components for absorption into the body, and to expel wastes.Most of the food we consume cannot be absorbed into our bloodstream with- out first being broken down into smaller subunits. Digestion is the process of breaking down food molecules into smaller molecules with the aid of enzymes in the digestive tract. See Figure 8.1b for an overview of chemical digestion sites in the body. Digestive enzymes are hydrolases: They catalyze (accelerate) the ad- dition of water to food molecules in order to break them down into smaller sub- units. For example, when two amino acids join together to form a protein, an—OH–group is removed from the carboxyl end of one amino acid, and an —H+isremovedfromtheaminogroupofthesecondaminoacidtoformadipeptidebondbetweenthetwoaminoacidspluswater.Tobreakdownsuchaprotein,adigestive enzyme catalyzes the addition of water (—OH– plus —H+)to the dipeptidebond,cleavingthebondtorestorethecarboxylgroupofthefirstaminoacidandtheaminogroupofthesecondaminoacid,andeffectivelybreakstheproteindownintotwoaminoacidsubunits.Onceafoodmoleculeisbrokendownintoitssimplestcomponents,thecomponentsareabsorbedthroughtheep-ithelialcellsthatlinetheintestinaltractandthenenterthebloodstream.In addition to being hydrolases, digestive enzymes are substrate specific— they work on some substances but not others. For example, salivary amylase is an enzyme in the saliva that breaks down starch (found in foods like corn, pota- toes, bread, and pasta) and glycogen (animal starch), but not cellulose (found in the cell walls of plants), even though cellulose is made up of glucose, just like starch and glycogen.Two factors that play key roles in the efficacy of digestive enzymes are tem- perature and pH level. An increase in temperature may cause a reaction to speed up, as it causes molecules to move faster and thus increases contact with an en- zyme; however, too high a temperature will disrupt molecular bonding that sta- bilizes enzyme configuration, causing the enzyme to denature (i.e., undergo a103EXERCISE8OBJECTIVES1.To define digestive tract, accessory glands, digestion, hydrolases, sali- vary amylase, carbohydrates, proteins, lipids, bile salts, pepsin, and li- pase2.To understand the main functions and processes of the digestive system3.To understand the specificity of enzyme action4.To explain the impact of temperature and pH levels on enzyme activity5.To identify the three main categories of food molecules6.To explain how enzyme activity can be assessed with enzyme assays7.To identify the main enzymes, substrates, and products of carbohydrate, protein, and fat digestionTChemicalandPhysicalProcessesofDigestionhe digestive system, also called the gastrointestinal system, consists of the digestive tract (also gastrointestinal tract or GI tract) and accessory glands that secrete enzymes and fluids needed for digestion. The digestivetract includes the mouth, pharynx, esophagus, stomach, small intestine, colon, rectum, and anus. The major functions of the digestive system are to ingest food, to break food down to its simplest components, to extract nutrients from these components for absorption into the body, and to expel wastes.Most of the food we consume cannot be absorbed into our bloodstream with- out first being broken down into smaller subunits. Digestion is the process of breaking down food molecules into smaller molecules with the aid of enzymes in the digestive tract. See Figure 8.1b for an overview of chemical digestion sites in the body. Digestive enzymes are hydrolases: They catalyze (accelerate) the ad- dition of water to food molecules in order to break them down into smaller sub- units. For example, when two amino acids join together to form a protein, an—OH–group is removed from the carboxyl end of one amino acid, and an —H+isremovedfromtheaminogroupofthesecondaminoacidtoformadipeptidebondbetweenthetwoaminoacidspluswater.Tobreakdownsuchaprotein,adigestive enzyme catalyzes the addition of water (—OH– plus —H+)to the dipeptidebond,cleavingthebondtorestorethecarboxylgroupofthefirstaminoacidandtheaminogroupofthesecondaminoacid,andeffectivelybreakstheproteindownintotwoaminoacidsubunits.Onceafoodmoleculeisbrokendownintoitssimplestcomponents,thecomponentsareabsorbedthroughtheep-ithelialcellsthatlinetheintestinaltractandthenenterthebloodstream.In addition to being hydrolases, digestive enzymes are substrate specific— they work on some substances but not others. For example, salivary amylase is an enzyme in the saliva that breaks down starch (found in foods like corn, pota- toes, bread, and pasta) and glycogen (animal starch), but not cellulose (found in the cell walls of plants), even though cellulose is made up of glucose, just like starch and glycogen.Two factors that play key roles in the efficacy of digestive enzymes are tem- perature and pH level. An increase in temperature may cause a reaction to speed up, as it causes molecules to move faster and thus increases contact with an en- zyme; however, too high a temperature will disrupt molecular bonding that sta- bilizes enzyme configuration, causing the enzyme to denature (i.e., undergo a103EXERCISE8OBJECTIVES1.To define digestive tract, accessory glands, digestion, hydrolases, sali- vary amylase, carbohydrates, proteins, lipids, bile salts, pepsin, and li- pase2.To understand the main functions and processes of the digestive system3.To understand the specificity of enzyme action4.To explain the impact of temperature and pH levels on enzyme activity5.To identify the three main categories of food molecules6.To explain how enzyme activity can be assessed with enzyme assays7.To identify the main enzymes, substrates, and products of carbohydrate, protein, and fat digestion

104Exercise 8(a)F I G U R E 8 . 1Chemical digestion. (a) Opening screen of the Amylase experiment. Part (b) follows.structural change that renders it functionless). In addition, each enzyme has an optimal pH at which it is most active. Within range of this optimal pH, the enzyme will work as ex- pected; beyond the optimal pH, the enzyme may have no effect.Most food molecules fall into one of the following cate- gories: carbohydrates, proteins, or lipids. Carbohydrates are the principal source of calories for most people and include glucose, sugars, and starch. Larger carbohydrates are broken down into monosaccharides (simple sugars, such as glucose) before being absorbed into the blood. Proteins are very im- portant for growth, especially among young people. Proteins are broken down into amino acids before being absorbed into the body to build new proteins. Lipids, most of which are triglycerides (the major constituents of fats and oils), are not water soluble and thus pose special problems for digestion. Lipase, the enzyme that acts on lipids, is hydrolytic (like all digestive enzymes) and can only work on the surfaces of lipid droplets because the lipids are water insoluble. To increase the rate of digestion by lipase, lipids are first emulsified (bro- ken down into smaller droplets) with the aid of bile salts, acholesterol derivative. Emulsification results in smaller droplets with larger surface areas, making it easier for lipase to bind to substrates and digest lipids. Bile salts also form micelles, which aid in the absorption of products of lipid di- gestion: fatty acids and monoglycerides.In the following experiments you will be examining the effects of different digestive enzymes on carbohydates, pro- teins, and lipids. To begin, follow the instructions for starting up PhysioEx 8.0 in the Getting Started section at the begin- ning of this lab manual. From the Main Menu, choose Exer- cise 8: Chemical and Physical Processes of Digestion from the drop-down menu and click GO. Then click Amylase. The opening screen will appear (see Figure 8.1). This screen will be used for the first two activities, in which you will be test- ing the effects of salivary amylase on starch and cellulose.AmylaseNotice the 11 dropper bottles in the top right quadrant of the screen. You will be preparing test tubes containing various

Chemical and Physical Processes of Digestion105Salivary glands: produce salivary amylase for the digestion of starch.Small intestine: produces enzymes for digestion of starch and proteins.Pancreas:produces pancreatic lipase for the digestion of fats, and pancreatic amylase for the digestion of starch (enzymes are ducted to the small intestine).Liver:produces bile for the emulsification of fats (ducted to the small intestine).Stomach glands: produce pepsin (in the presence of HCl) forthe digestion of proteins.(b)F I G U R E 8 . 1(continued) Chemical digestion. (b) A few sites of chemical digestion and the organs that produce the enzymes of chemical digestion.combinations of the dropper bottle contents. Below the drop- per bottles is an incubation unit that will enable you to boil, freeze, and incubate the test tubes. The closed cabinet in the upper left quadrant of the screen is an assay cabinet, contain- ing chemicals that you will add to your experimental test tubes in order to interpret your test results. Below the assay cabinet is a test tube washer where you will deposit used test tubes. Next to this is a test tube dispenser from which you will click test tubes and drag them to the holders in the incu- bation unit, where you will prepare them for experimenta- tion. Along the bottom of the screen is the data recording box, where you will be recording your experimental data.ACTIVITY1Salivary Amylase and Starch