Lab 2 — Biomolecules
(adapted from A Laboratory Guide to the Natural World, by Richardson and Richardson)
- Experimental technique: 20 points
- Cleanup: 10 points
- Notebook: 20 points
- Writeup: 50 points
Today we're going to analyze a number of different samples for the presence of some of the biomolecules we've discussed in class. Work in groups of two or three. You may do the experiments in any order you like, but I recommend setting up the filter paper for experiment III first, as that can then dry while you do other experiments.
For this lab, bring some item of food or drink that you're interested in testing. Think about this before coming to lab and pick something interesting (you might as well pick something interesting, right?) and be sure to bring a pencil to lab. Because these tests are color-based, note that the results from dark-colored stuff like cola will be very hard to interpret, which will make your analysis that much more difficult.
Note also that some of the chemicals used in these experiments will form colored stains on clothing and/or skin!
Experiment I: Benedict's Test for Simple Sugars
"Simple sugars" is just another name for mono- and disaccharides: sweet substances we're familiar with such as glucose, fructose, and sucrose (table sugar). Benedict's solution will react with many monosaccharides, turning the solution from blue to green in the presence of small amounts of these molecules, and turning the solution yellow-orange or even red in the presence of larger amounts.
- Label 5 test tubes (plus one for each member of the group) so that you can tell which is which. Using a different dropper for each solution, add 1 mL of the solutions below to the appropriate tube:
Solution Expected Result Benedict's Test Result distilled water (negative control) 10% glucose (positive control) potato extract apple juice diet soda [food item #1] [food item #2]
You should set up a table like this in your notebook in order to record your results; it's much easier to quickly evaluate your data and discern patterns if the data are set out in a format like this, rather than as text.
- Add 1 mL of Benedict's solution to each tube, and mix well by gently tapping the side of the tubes with your fingers.
- Place the tubes in the heat block (which is Very Hot!).
- Check the tubes for color changes after 3 minutes and record your results in your table. Record the strength of the reaction (none, weak, moderate, strong) based on the color exhibited by each solution.
Experiment II: Iodine Test for Starch
Even though starch is made of exactly the same components as simple sugars, it reacts differently with various chemicals. Iodine will react with starch, turning blue or black in its presence, but will not react with simple sugars.
- As you did (or will do) for experiment I, label 5 test tubes (plus one for each member of the group); you will use exactly the same solutions as in that experiment except you'll use a 1% starch solution in place of the 10% glucose.
- Set up a table as in experiment I and add 1 mL of the appropriate solution to the corresponding test tube.
- Add 1 drop of iodine solution to each tube and mix each tube by tapping it with your fingers. Record the results in your table as none, weak, moderate, or strong, based on the intensity of the color produced.
Experiment III: Sudan Test for Lipids
Lipids, more commonly called "fats," are made up almost entirely of the same elements that make up carbohydrates: carbon, hydrogen, and oxygen. They don't behave the same way chemically, however. When it combines with lipids, Sudan III stain produces a bright orange color.
- Using a pencil, initial a piece of filter paper and draw a number of 1-cm circles on it (this is about the size of a penny) around the periphery. You will need a separate circle for each of the substances to be tested (listed below), as well as your own test substances. Draw the circles roughly equal distances apart and label each so that you'll know which is which.
Test substances: Distilled water, lard, butter, butter substitute, whole milk, 2% milk, non-fat milk, egg white, egg yolk, egg substitute
- Place 1 drop of each substance on the appropriate circle and allow the paper to dry. (Blot off any excess if necessary.)
- Place the paper in the dish containing the Sudan III stain for 5 minutes, immersing the paper completely.
- Remove the paper with forceps and gently wash it in a dish of water for about 1 minute.
- Examine each spot for coloration and record your results as before.
Experiment IV: Biuret Test for Proteins
Although proteins serve an incredibly diverse range of functions, they are chemically very similar in many ways. The Biuret test uses sodium hydroxide and copper sulfate to react with proteins, forming a purple color. The intensity of the color indicates the amount of protein present.
- Using droppers, add 1 mL of the substances below to labeled test tubes (don't forget to include an additional tube for each group member):
water, 1% protein, whole milk, 2% milk, egg white, egg yolk, egg substitute
- Add 1 mL of Biuret solution to each tube and mix thoroughly, tapping the tubes with your fingers.
- Record your results.
For this lab, briefly describe your results for each of the four experiments — a table sure would be a handy way to summarize your data, don't you think? Be sure to indicate whether your results made sense, based on what you know (or can find out) about the composition of the tested substances. (It's always a good idea to comment on whether your results agree with what you expected; it lets me (and you!) know that you know what's going on — for example, if you tested a rice cake and your Sudan test indicated there was a lot of lipid in it, you'd better be sure to discuss that!)
If any of your food items wasn't liquid, include a description of how you made it testable.
Please submit these as a group; in accordance with course policy, your write-ups are due by 5 pm one week from today.
For reasons that are not entirely clear to me, Moravian College considers this to be a personal page. Therefore it is incumbent on me to point out that "The views expressed on this page are the responsibility of the author, Christopher Jones (cjones-at-moravian-dot-edu) and do not necessarily reflect Moravian College or Moravian Theological Seminary policies or official positions."