Fig. 6.1 How Enzymes Work
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Lesson Six: The Role of Enzymes in Digestion
Teaching suggestions: Lecture, experiment, group work, diagram or chart illustrating the effect of temperature and PH on the action of enzymes.
Engage students by asking the following questions.
Let the students tell what they know about enzymes. They might give definition of enzymes in different ways. But the key concept to be noted is that enzymes are biological catalysts.
A catalyst enhances the speed of a chemical reaction. Enzymes speed up the rate of biological reactions by lowering the energy of activation. Here the students should be introduced to the concept of activation energy. Also, the term substrate is important as enzymes act upon substrates. The teacher is required to outline the reaction between enzymes and substrates as follows.
Suggested Teaching Methodology: - lecture method and small group work
Properties of Enzymes
As enzymes are protein in their chemical nature, they become denatured by heat, which means that when heated above 40oC, they change shape and do not work anymore. When the temperature is lower than 00C, enzymes become inactive. Likewise, a shift in pH highly affects enzymatic actions.
When an enzyme acts upon its substrate, an enzyme substrate complex is formed. Finally,
a product is formed and the enzyme becomes free as shown. The students should be informed that enzymes have active sites on which the substrate can be attached.
Enzyme + substrate <==> Substrate Complex <==> nzyme + Product
Fig. 6.1 How Enzymes Work
Fig. 6.2 Effect of Temperature on the action of an Enzyme
Activity: Effect of temperature on the action of an enzyme
Let the students in a small group explain the effect of temperature on enzymatic action by looking at the above graph. The graph shows the rate of the enzymatic action in relation to temperature. The rate increases slowly when the temperature rises between 10oC to 40oC, but when the temperature rises further, the activity of an enzyme decreases drastically, as the enzyme is being denatured.
Activity: Effect of PH on the action of an enzyme
Let the students in a small group explain the effect of pH on enzymatic action by looking at the graph below.
The graph below shows the sensitivity of enzymes to pH. It is a bell-shaped graph, showing that the enzyme in the stomach work best at optimum pH, which in this case is pH2. (Example: pepsin).
Ask students to mention the type or name of the enzyme that works at a lower pH (pH2).
Fig. 6.3 Effect of pH on the action Pepsin
Enzymes work best at a particular pH known as the optimum pH. In laboratory test, the speed at which amylase breaks down starch is measured at three different pH values, 3, 7, and 9. Amylase works best at pH 7 or neutral. When pepsin is similarly tested, it worked best at pH 2, which is why hydrochloric acid in the stomach maintains a pH of 2-3.
Lipase breaks down fat but it is assisted by bile salts secreted from the gall bladder. Bile is involved in mechanical digestion of fats. In the laboratory tests, the action of lipase and bile salts is investigated using whole milk. When fat is digested fatty acids are formed. The fall in pH towards acidity then causes an indicator to change color from pink to colorless. This change occurs most quickly when lipase and bile work together. Bile causes larger fat droplets to break up into small pieces giving lipase a larger surface area to work on.
Materials
Cornstarch, Distilled water, Saliva, Vinegar, Benedict's qualitative solution three graduated cylinders (10mL), 250-ml beaker, Stirring rod, three test tubes (16 x 125mm), Test tube rack, pencil, and Water Bath.
Pre-lab activity for the teacher- The teacher is required to make a preparation by adding 1g of cornstarch to a beaker containing 100ml of cold distilled water. While stirring frequently, heat the mixture just until it begins to boil. Finally, allow it to cool.
Procedure
OBSERVATIONS:
Tube A: Starch + saliva treated with vinegar (acid)
Tube B: Starch + saliva and water, treated in a boiling water bath
Was the test positive or negative? Negative
What does this indicate? High temperature denatures proteins. Amylase works at normal body temperature of humans (370C).
Tube C: Starch + saliva
Was the test positive or negative? Positive
What does this indicate? Amylase acted upon starch because there was an optimal temperature and pH. In this case starch was converted into maltose. This was confirmed by the formation of brownish-red with Benedict's reagent.
From the above experiment, the students are expected to explain the action of the enzyme amylase on starch. Amylase changes starch (polysaccharides) into a simpler form called the sugar maltose (double sugar) which is soluble in water. Amylase is present in our saliva, and begins to act on the starch in our food while still in the mouth. Exposure to heat or extreme pH (acid or base) will denature proteins. Enzymes, including amylase, are proteins. If denatured, an enzyme can no longer act as a catalyst for the reaction. Benedict's solution is a test reagent that reacts positively with simple reducing sugars like maltose, but will not react with starch. A positive for maltose test is observed as the formation of a brownish-red cuprous oxide precipitate. A weaker positive test will be yellow to orange.
Evaluation/ Assessment Questions