All cells аre mаde оf fоur mаjоr classes of biological macromolecules. These macromolecules build cellular structures, carry out cellular functions, and allow intra- and intercellular communications. The four classes of macromolecules found in living systems are carbohydrates, lipids, proteins, and nucleic acids. Although these molecules are large and complex, they are formed by connecting a few types of repeating units or building blocks. The order and number of these building blocks linked together determines the size, shape, code, and function of the molecules. Common to all these macromolecules and their building blocks, is the fact that they all contain the atom carbon (C). Any molecule that contains two or more carbons is considered an organic molecule. All molecules absent of carbon are, therefore, inorganic molecules. Usually, organic molecules can be found to be directly derived from living things, whereas inorganic molecules are not. Scientists often use chemical assays (tests) to quickly determine the presence of certain classes of molecules. In this lab, we will learn some simple chemical assays to determine the presence of some macromolecules that we learned about in lecture. The chemical assays that we will conduct do not give us quantitative results (amount or concentration). Instead, all these tests produce colors that are qualitative forms of data. We are able to determine whether the class of molecule we are looking for is present or not detectable from the color. When the molecules are present, we say the result is positive. When the molecule is not detected, we say the result is negative. To interpret the assays, positive and negative controls are often set up. These controls set the standards for assay comparisons. A positive control will demonstrate the expected change(s) when the molecule is detected. A negative control will demonstrate the result when the molecule is not detected. Furthermore, when positive and negative controls produce the expected results, the investigator is assured that the assay reagents are working, and all the results obtained from the assay for any substance can be trusted. Because these assays produce qualitative tests, remember that they are not always absolute answers. If the concentration of the molecules to be identified is too low, the assay may not be sensitive enough to produce a positive result. Therefore, a negative result does not necessarily mean the molecule is absent, it could simply mean the concentration of the molecule is too low to be detected.