Effect of pH on Protein Digestion

Essay by theshortkidHigh School, 10th grade February 2007

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Objective: To investigate how pH affects the rate of protein digestion

Hypothesis: I predict that since pepsin breaks down the material the quickest at a pH of 2 in the stomach, test tubes #3 will reduce the greatest because it has pepsin to further break down the egg. Whereas the other test tubes will have a smaller reduction. I think test tube #2 will have the second largest reduction rate and then #6 because they all will have a greater rate at breaking down the material, Then, I predict the test tubes that did not have pepsin added will come in last in the order of test tubes #4,#1, and #5.

Manipulated variable:

*Using different solutions to the test tubes (pepsin, distilled water, HCI, and NaOH)

Controlled variable:

*the same amount of solution poured into each test tube

*ensuring that each test tube is clean

Responding variable:

*The pH levels and the size of the egg white


Hydronium pH paper

6 large test tubes (22*175mm)

10 mL graduated cylinder

10% pepsin solution


Tweezers or forceps

Wax pen

1.0 mol NaOH solution

1.0 mol HCI solution

Hard-boiled egg

Metric ruler

Plastic gloves


1)Using a wax pen label the test tubes 1 to 6

2)Measure and cut 2cm cubes of boiled egg white. Place two cubes in each of the test tubes.

3)Using the 10 mL graduated cylinder, measure 5mL of distilled water and pour it into test tube #2. Repeat the procedure, but measure and pour 5 mL of HCI into test tube #3 and #4. Rinse the graduated cylinder. Repeat again, but this time measure and pour 5ml of NaOH into test tubes #5 and #6.

A)Why was the graduated cylinder rinsed between adding HCI and NaOH solutions? The graduated cylinder was rinsed between adding HCI...