Final answer:
Despite the fact that lipids and proteins are present in plant and animal cell membranes, they may not always be detected in food tests due to challenges in extraction, food processing alterations, test sensitivity, and the presence of interfering substances.
Step-by-step explanation:
In an activity where you are recording the presence of various organic compounds in different foods, it might be surprising not to get a positive test result for lipids and proteins in all food samples. Although all plant and animal cells have membranes comprised of lipids and proteins, not all food tests will indicate their presence. This discrepancy can be due to several factors.
Firstly, the detection of lipids and proteins in foods requires that these molecules be in a detectable state, which might not always be the case. The extraction and solubility of lipids are sometimes challenging, and the way foods are processed can make lipids difficult to detect. For instance, lipids may have been transformed or removed during food processing. Similarly, proteins in foods may be denatured or their structures altered, making them less reactive to the specific reagents used for detection.
Moreover, the sensitivity of the testing methods might not pick up small amounts of lipids and proteins, or the specific tests might be designed to detect particular types of these molecules that are not present in all food samples. For instance, different foods have different phospholipid-derived fatty acid (PLFA) profiles, which can result in differences in detectability when using biochemical tests.
Lastly, other factors such as the presence of interfering substances, improper sample preparation, or the test's specific procedures could contribute to a negative result, even if lipids and proteins are present in small quantities.