Unfortunately, for Investigations 3.1, 4.1, and 5.1, we have run out of the chemicals necessary to conduct the investigations – even after getting a second delivery of supplies from the district! This has everything to do with the final food project that I designed for the unit. Any missed investigations will have to be discussed during lunch and data used from other students' experiments.
Investigation 5.1 Worksheet
For Investigation 5.1, we gathered evidence of digestion in the stomach. We were introduced to a new enzyme: pepsin. This enzyme breaks down proteins into amino acids. The only problem is, we are made from proteins! We have to protect ourselves from the pepsin, so there is a very clever way our body creates the enzyme so that it doesn't break down our cells. It doesn't build the pepsin all the way, so that it is fully functioning. Instead, specialized cells create pepsinogen, which is released into the stomach. Other specialized cells release hydrochloric acid into the stomach. When the two meet, the acid converts the pepsinogen into pepsin, and the enzyme is active!
The combination of pepsin and hydrochloric acid is called gastric juice. Our claim was that gastric juice breaks down proteins into amino acids. Pepsin alone (in the form of pepsinogen) will not break down proteins. Acid alone will not break down proteins. But when mixed together, the acid activates the pepsin, and the proteins are broken down.
We did not have an indicator for proteins or amino acids in the middle school lab for this investigation, so we had to find another way to obtain valid evidence to support our claim. We used egg white from a hard boiled egg to represent our protein, since it is a solid protein source.
We measured a small cube of egg white and calculated its volume. We then tied a string around it and submerged it in water in a test tube. We created several sample tubes like this. To one, we added gastric juice. To another, we added hydrochloric acid. To a third, we added pepsin. We left one tube alone with just distilled water. We then submerged all four tubes into a hot water bath set for 37ºC, which is approximate body temperature, and let the tubes sit for at least one hour.
The next day, we removed the egg whites, re-measured them, and calculated the volumes again. We looked for any change in volume to use as evidence that the proteins were broken down into amino acids. We used all four results to generate the reasoning to support the focus question: does gastric juice break down proteins into amino acids?
