The primary function of the digestive system is to transfer nutrients, water, and electrolytes from the food consume into the body’s internal environment. The ingested food is essential as an energy source, or fuel, from which the cells can generate ATP to carry out their particular energy-dependent activities such as contraction, transport, synthesis, secretion and even renewal of body tissues. Three primary categories of food ingested by humans which are carbohydrates, proteins and fats emerge as large molecules. These large molecules cannot cross plasma membranes intact to be absorbed from the lumen of the digestive tract into the blood or lymph; hence, it must undergo degradation in size (Sherwood, 2013). This degradation process is catalyzed by hydrolytic enzymes, which split large molecules into smaller, absorbable units by combining with water. (Samsulrizal, 2013)
The hydrolysis of molecules becomes more effective by the release of specific enzymes that works at optimum pH in different regions of digestive tract. The three major GI hormones are gastrin from the stomach mucosa, secretin and cholecystokinin (CCK) from the duodenal mucosa. Gastrin is released primarily in response to protein in the stomach, and its effects promote digestion of protein. Secretin is released in response to acid in the duodenum, and its effects will neutralize the acid. CCK is released in response to fat in the duodenum, and its effects optimize conditions for fat digestion reactions. (Sherwood, 2013)
Digestion of carbohydrates begins in the mouth (salivary digestion), where the salivary glands (submandular, sublingual or parotid) will secrete an amylase called ptyalin that begins the hydrolysis of complex polysaccharides: Plant starchesOrAnimal glycogen
| Disaccharides(maltose, sucrose, lactose)
| Ptyalin has an optimum pH of around 6.8, which is roughly the pH found in the mouth.
Protein digestion begins in the stomach (gastric digestion) where the enzyme pepsin splits proteins to shorter polypeptide chains containing amino acids. Secretion and activation of pepsin occurs as follows: Pepsinogen(chief cells)
| HCl (parietal cells)―――――――――→
| Besides activating pepsinogen, HCl provides the stomach a pH of 2, so the pepsin activity works at optimum level.
For digestion of fat, pancreatic lipase reaction must be aided with the presence of bile salts as an emulsifier. Lipase is a water-soluble enzyme, and it is not effective alone to act on the large lipid droplets which are water insoluble. Bile salts emulsify by breaking the fat into smaller droplets so that lipase has a larger surface area for the hydrolysis of fats.
The pancreas also aids digestion by secreting sodium bicarbonate. This compound provides a pH of around 7.8 in the small intestine, which is optimal for the action of the pancreatic enzymes (Samsulrizal, 2013).
* To examine the action of some of the key digestive enzymes and the factors that can alter their activities.
3.0. MATERIALS AND METHODS
Test tubes (15 – 20 ml), test tube clamps, measuring cylinders (10 ml), watch glass, pH paper, water baths with test tube racks, hard-boiled egg whites, 1% acetic acid, 1% pancreatin solution in 0.2% Na₂CO₃, 0.5% starch paste, Lugol’s solution, Benedict’s solution, 5% pepsin solution, 0.5% HCl, concentrated HCl, 0.5% NaOH, ice, powdered litmus, litmus paper. 3.2. Methods
3.2.1. Activity 1
1. About 10 ml of human saliva (fresh) is collected in a graduated cylinder. 2. A small amount of saliva is placed in a watch glass and a few drops of 1% acetic acid are added. A precipitate indicated that the present of mucin (a glycoprotein) is indicated by precipitate occurrences. 3. Four test tubes are prepared and labeled as follows:
| Tube 2
| Tube 3
| Tube 4
3 ml starch+3 ml water↓in37°C water bath
| 3 ml starch+3 ml saliva↓in37°C water bath
| 3 ml starch...
References: Campbell, M. K., & Ferrel, S. O. (2012). Biochemistry (7th ed.). Canada: Brooks/Cole, Cengage Learning.
Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2008). Biology (8th ed.). San Francisco: Pearson Benjamin Cummings.
Samsulrizal, N. (2013). Lab Report Rubric BIO560 Animal Physiology. UITM, Faculty of Applied Science. Shah Alam: UITM.
Sherwood, L. (2013). Introduction to Human Physiology (8th ed.). Canada: Brooks/Cole, Cengage Learning.
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