Lab #12: Digestive Physiology p.1Lab #12: Digestive Physiology Background In order for the nutrients in food to be absorbed, they must first be broken down into particles that are small enough to be transported through carrier proteins into the epithelial cells that form the mucosal lining of the digestive tract. This process of breaking down food is called digestion, and occurs primarily within three particular segments of the digestive tract: the mouth, the stomach, and the small intestine. Digestion occurs through two different processes: physical digestion, where large chunks of food are ground into tiny particles, and chemical digestion, whereby through the use of enzymes released into the digestive tract large polymeric biomolecules are broken into individual monomers or oligomers (e.g. dimers or trimers). Chemical digestion is essential for breaking food into particles that can be absorbed by the epithelium of the small and large intestine, and will be the focus of this lab exercise. The Importance of Enzymes in Chemical Digestion There are many different substances that are secreted into the different segments of the digestive tract. Mucus, bile salts, bilirubin, hydrochloric acid (HCl), and solium bicarbonate (NaHCO3) are just some of the substances mixed with the food as it passes through the digestive tract, and many of these substances facilitate the breakdown of food. However, the most important substances secreted for the purpose of digestion are the digestive enzymes. Digestive enzymes greatly enhance the rate at which the covalent bonds that link subunits together to form polymeric biomolecules are broken. Indeed, without the presence of these enzymes, chemical digestion would essentially not occur. Although substances such as HCl and NaHCO3 can alter noncovalent bonding patterns within and among biomolecules, they typically cannot break down covalent bonds. We have previously discussed many aspects of enzyme structure, catalytic activity, and factors that influence this catalytic activity (e.g., enzyme, substrate, and cofactor concentrations, see Lab #4). Two factors that influence enzyme activity deserve further consideration here in the context of digestion: temperature and pH. Recall that temperature can have considerable influence on the rate at which enzyme-catalyzed reactions proceed. Low temperatures result in slow reaction rates because overall kinetic energy is reduced—since particles are moving more slowly, substrate binds to the enzyme less frequently, the reaction converting substrate to product lasts longer, and the random collisions that dislodge the product from the substrate occur less often and with less force. Very high temperatures can also slow the rate of an enzyme catalyzed reaction because high temperatures destabilize the noncovalent interactions that give enzymes the specific tertiary and quaternary structures that enable them to function as catalysts. This highlights the importance of the stable core body temperatures of humans in their digestive processes. The relatively high (but not extremely high) temperature of the human body enables digestive enzymes to break down food at near maximal rates. Organisms with more variable and lower core body temperatures (e.g. reptiles) Temperature37 °CEnzymatic Activity Fig 11.1. Influence of temperature on the rates of enzyme-catalyzed reactions. Not that both low and very high temperatures lead to the reduction of reaction rates.Lab #12: Digestive Physiology p.2will often bask after feeding to elevate body temperature and facilitate enzymatic digestion. Enzyme activity is also influenced by the pH of the surrounding fluid. Recall that [H+] can influence whether acidic and basic amino acid side chains are bound to a hydrogen ion or not, thus influencing the charge on those side chains and, in turn, alter ionic and hydrogen bonding patterns in those proteins, thus inducing changes in the tertiary and quaternary structure of proteins. Thus enzymes have a particular pH where they have the proper conformation to have maximal catalytic activity, and significant deviation from that pH will typically result in a decrease in catalytic activity. Interestingly, pH varies widely among different segments of the digestive tract, and different enzymes have maximum catalytic activity at these various pH’s, effectively restricting the function of that enzyme to a particular region of the digestive tract (Fig 11.2). For example, gastric enzymes such as pepsin have maximum catalytic activity at the very low pH of the stomach, and no longer function once moved into the alkaline conditions of the small intestine. In contrast, intestinal and pancreatic enzymes, such as trypsin, function optimally at moderately alkaline pH. Altering pH among different regions of the digestive tract effectively enables a stepwise process of chemical digestion, where enzymes are activated for digestion at one point and then deactivated at the next. Chemical Digestion of Carbohydrates. Carbohydrates are the first type of biomolecule to be chemically digested in the digestive tract, as chemical digestion begins in the oral cavity through a salivary enzyme called salivary amylase (or ptyalin). Salivary amylase begins the breakdown of the polysaccharide amylose (starch, the principle storage carbohydrate in plants) into the disaccharide maltose. However, when food is swallowed and transferred to the small intestine, chemical digestion of carbohydrates effectively stops. Only when the chyme passes into the small intestine will carbohydrate digestion resume. In the small intestine the chyme is exposed to pancreatic amylase (which continues the process of breaking down starch and glycogen into disaccharides and trisaccharides) and certain brush-border enzymes (e.g., lactase, sucrase, and maltase) that break down specific oligosaccharides into the monosaccharides that are absorbed by the intestinal epithelium. Chemical Digestion of Protein. Chemical digestion of protein begins in the stomach. The lining of the stomach produces a mixture of fluids called gastric juice in response to neural stimulation (induced by smell, site and taste of food), by distension of the stomach as food enters, and by pH changes induced as the more neutral pH food enters the acidic stomach. Gastric juice contains a number of substances, but the two most important for initiating protein digestion are hydrochoric acid