Macromolecule Lab with Potatoes

In Biology, a few days ago, we conducted a lab about Macromolecules. In this lab we were given a food, either egg white, egg yolk, potato, apple, onion, strawberry or lemon.  My group was assigned potato. We had to conduct tests on the potato to determine what kinds of macromolecules it contained. Before testing our substance, we tested substances that we knew contained protein, starch, glucose and lipids. In the protein test we measured 5 mL of gelatin, and added ten drops of Biuret solution to it. The result was positive, and the substance turned a dark brown and tan color. When we later performed this test on our potato, the color of the potato did not change, thus we determined that the potato did not contain protein. The next test was the glucose test. In the glucose test we took five mL of pure glucose, and added three mL of Benedict's solution to it. We then heated the test tube that contained the mixture in boiling water for five minutes. Because the  substance was positive, the mixture turns green, then yellow, and then a dark reddish-orange. We put our potato and Bendict's solution mix in the water, and within a minute our test tube had turned a yellowy-orange color. The potato was clearly glucose positive. Our third test was the starch test. In the starch test we took five mL of pure starch and added five drops of Lugol's iodine solution to it. The color of the starch changed from pure white to black immediately. When we measured our potato and added the Lugol's iodine solution, the substance turned a very dark black. This was no surprise, as potatoes are commonly known to contain starch. The fourth and final test that we performed was the lipid, or fat, test. In this test we rubbed butter of on a paper towel, then let the towel dry. When the towel was dry, we held it up to the light. Where the butter had been rubbed on the towel, it was shiny and greasy. There was no shininess on the paper towel when we rubbed the potato on it, and we determined that the potato did not contain lipids.