TO THE STUDENT

You may be part of a group for the first part of this exercise. Each group should carry out the experiment and collect the data together, but each student must record the data in his or her own booklet. Be sure to record the data exactly as you see it. After the data has been collected, each student should answer the questions independently.

After you have finished your experiment and have recorded all of the data, you will be asked to answer some questions about the experiment and the data you recorded. Your answers must be written in this test booklet in the space provided. Make sure that you understand each question before you begin to write. At any time while you are writing your answers, you may look back to the directions for the experiment and the data you collected. Be sure that your answers are written as clearly and neatly as possible.

Before you turn the page, read the list of materials given below and check to make sure that your group has everything listed.

Materials 1 25-ml graduated cylinder 1 Celsius thermometer 1 permanent marker 6 90-ml disposable plastic cups 6 disposable coffee stirrers 2 Styrofoam cups 1 plastic pipette plastic bottle with rennin 1 container of crushed ice 10 ml 2% milk at 35oC 10 ml 2% milk at room temperature (22-24oC) 10 ml 2% milk at 15oC 1 clock with second hand or digital watch paper towels pen or pencil

Effect of Temperature on Enzymes

Ancient shepherds often used pouches made from the stomachs of sheep or goats to carry milk into the fields where they worked. The shepherds found that the milk, which had been placed in the stomach in the morning, had curdled and formed cheese by noon. (Curdling occurs when the milk becomes thicker and somewhat lumpy.) Today, it is known that the curdling is due to the presence of an enzyme, rennin, in the animal's stomach.

In this experiment you will add rennin to milk at different temperatures and measure the time required for the milk to curdle. Read the directions before you begin.

Before you begin the experiment, write down a prediction about how the temperature of the milk may affect the time it takes rennin to cause curdling.

Write your prediction here.

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1. Follow laboratory safety rules as directed by the teacher.

2. You will need two 5 ml samples of milk at each of the following temperatures: 15^o C, room temperature (22^oC-24^oC), and 35^oC.

3. At each temperature, one sample will contain rennin and the other will not. For example, one sample at 35^oC will have rennin while the other sample at 35^oC will not. The samples without rennin are called the control samples. Refer to Table 1 for the six different cup labels. Use the marker to label the cups appropriately.

4. To keep a set of samples at 15^oC, half fill each of the two Styrofoam cups with crushed ice. The milk samples in each of the smaller plastic cups can be set in the crushed ice.

5. When using the thermometer, hold it by the top. Hold on to the thermometer when you measure the temperature so that the cups do not tip. Rinse the thermometer and dry it with a paper towel before using it in different samples.

6. Use the plastic pipette to add 15 drops of rennin to one of the cups for each temperature.

7. In Table 1, you should record the time when you add the milk to each cup (the start time) and the time that the sample begins to curdle (the end time).

8. Use the graduated cylinder to measure 5 ml samples of milk. Add milk at the appropriate temperature to each pair of cups. Record the time as directed. Use a different stirrer to stir the milk in each cup. Do NOT use the same stirrer to stir more than one sample.

   Sample	Temperature (oC)	Number of drops of rennin	Start time (hr:min:sec)	End time (hr:min:sec)	Elapsed time (sec)	Did the milk curdle? (yes/no)
   Control	15o
   Rennin	15o
   Control	Room (22o-24o)
   Rennin	Room (22o-24o)
   Control	35o
   Rennin	35o

   For the samples that curdled, calculated the elapsed time (i.e., the total time required for the sample to curdle) by subtracting the start time from the end time. Express your answer in seconds and record it in Table 1. Show your work in the space below.

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   Questions
   
   Please answer the following questions by yourself.
   
   1. Why are controls used at each temperature?
   
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   2. Compare the time it took the milk in each of the cups to curdle? What relationship, if any, exists between the temperature of the milk and the time it takes to curdle? Compare this result with your earlier prediction. Explain any agreements/differences.
   
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