Chemical and Physical Processes of Digestion
he digestive system is a physiological marvel, composed of finely orchestrated chemical and physical activities. The food we ingest must be broken down to its molecular form for us to get the nutrients we need, and digestion involves a complex sequence of mechanical and chemical processes designed to achieve this goal as efficiently as possible. As food passes through the gastrointestinal tract, it is progressively broken down by the mechanical action of smooth muscle and the chemical action of enzymes until most nutrients have been extracted and absorbed into the blood.
1. To list the digestive system enzymes involved in the digestion of proteins, fats, and carbohydrates; to state their site of origin; and to summarize the environmental conditions promoting their optimal functioning. 2. To recognize the variation between different types of enzyme assays. 3. To name the end products of digestion of proteins, fats, and carbohydrates. 4. To perform the appropriate chemical tests to determine if digestion of a particular food has occurred. 5. To cite the function(s) of bile in the digestive process. 6. To discuss the possible role of temperature and pH in the regulation of enzyme activity. 7. To define enzyme, catalyst, control, substrate, and hydrolase. 8. To explain why swallowing is both a voluntary and a reflex activity. 9. To discuss the role of the tongue, larynx, and gastroesophageal sphincter in swallowing. 10. To compare and contrast segmentation and peristalsis as mechanisms of propulsion.
Chemical Digestion of Foodstuffs: Enzymatic Action
Nutrients can only be absorbed when broken down into their monomer form, so food digestion is a prerequisite to food absorption. Enzymes are large protein molecules produced by body cells. They are biological catalysts that increase the rate of a chemical reaction without becoming part of the product. The digestive enzymes are hydrolytic enzymes, or hydrolases, which break down organic food molecules, or substrates, by adding water to the molecular bonds, thus cleaving the bonds between the subunits or monomers. A hydrolytic enzyme is highly specific in its action. Each enzyme hydrolyzes one or, at most, a small group of substrate molecules, and specific environmental conditions are necessary for an enzyme to function optimally. For example, temperature and pH have a large effect on the degree of enzymatic hydrolysis, and each enzyme has its preferred environment. Because digestive enzymes actually function outside the body cells in the digestive tract lumen, their hydrolytic activity can also be studied in a test tube. Such in vitro studies provide a convenient laboratory environment for investigating the effect of various factors on enzymatic activity.
Starch Digestion by Salivary Amylase
In this experiment you will investigate the hydrolysis of starch to maltose by salivary amylase, the enzyme produced by the salivary glands and secreted into the mouth. For you to be able to detect whether or not enzymatic action has occurred, you need to be able to identify the presence of these substances to determine to what extent hydrolysis has occurred. Thus, controls must be prepared to provide a known standard against which comparisons can be made. The controls will vary for each experiment and will be discussed in each enzyme section in this exercise. Starch decreases and sugar increases as digestion proceeds according to the following equation: Starch amylase water → X maltose
Because the chemical changes that occur as starch is digested to maltose cannot be seen by the naked eye, you need to conduct an enzyme assay, the chemical method of detecting the presence of digested substances. You will perform two en-
zyme assays on each sample. The IKI assay detects the presence of starch and the Benedict’s assay tests for the presence of reducing sugars, such as...
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