The most common organic compounds found in living organisms are lipids, carbohydrates, proteins, and nucleic acids. Common foods, which often consist of plant materials or substances derived from animals, are also combinations of these organic compounds. Substance called indicators can be used to test for the presence of organic compounds. An indicator is a substance that changes color in the presence of a particular compound. In this investigation, you will use several indicators to test for the presence of lipids, carbohydrates, and proteins in various foods.
Problem: What are the major types of organic compounds in some common foods?
Make a data table with the following information: food samples (IV); prediction of biomolcs present in food; results from benedicts; results from biurets; results from lugols; results from grease spot test
Substances (food samples) being tested:
Part A: Testing for Carbohydrates: Monosaccharides
Glucose, commonly called “blood sugar,” and fructose, “fruit sugar,” are two of the most common monosaccharides. Ribose, an important component of nucleic acids, is also a monosaccharide and will be discussed during our study of DNA and RNA. Monosaccharides can be identified in a substance using an indicator called Benedict’s solution. When Benedict’s is heated in the presence of a monosaccharide, the color changes from blue to green to yellow to reddish-orange, depending on the amount of monosaccharide present.
Procedure: You can start step 1 and then proceed to another test while the water heats up.
Part B: Testing for Carbohydrates: Polysaccharides
Monosaccharides may join together to form long chains called polysaccharides that may be either straight or branched. Starch is an example of a polysaccharide formed entirely of glucose monosaccharides. Polysaccharides can be tested for using the indicator Lugol’s reagent (iodine/potassium iodine). Lugol’s will change color from yellow-brown to blue-black-purple in the presence of starch.
Part C: Testing for Lipids
Lipids can be generally categorized into two subgroups: saturated fats and unsaturated fats. In saturated fats, the carbon chains are bonded together with only single covalent bonds and the remaining electrons are bonded to hydrogens. This structure keeps the hydrocarbon tails relatively straight, so they readily align with each other, pack closely together, and thus tend to be solid at room temperature. Saturated fats are most common in animals. Unsaturated fats has double bonds between the carbons and therefore do not have the maximum number of links to hydrogens. These regions of double bonding allow kinks to form in the hydrocarbon tails, keeping them from packing closely together. For this reason, unsaturated fats are usually liquid at room temperature. Unsaturated fats are common in plants. Because fats are nonpolar, we will test for their presence using Sudan IV, a nonpolar dye that will readily mix with lipids but form small bubbles or separate with non-fats. Lipids can also be tested for using the “sack lunch” method: the lipid is rubbed into a brown bag. If a grease spot appears (the brown bag appears translucent), then a lipid is present in the food.
Procedure: Sudan IV
Procedure: Grease spot test (sack lunch):
Part D: Testing for Proteins
Proteins are made up of one or more polypeptides, which are linear polymers of monomers called amino acids. Amino acids derive their name from the amino group and the carboxyl group (which is acidic). Polypeptides are formed when amino acids are joined together by peptide bonds between the amino group of one amino acid and the carboxyl group of another amino acid. Proteins can be tested for using Biuret’s reagent, which reacts with the peptide bond between amino acids in the polypeptide. Biuret’s reagent turns from blue to purple-violet if peptides bonds are present.
Lab Write up requirements (20pts total):
Title: Must be descriptive (includes the IV and DV of the experiment) 1 point
Results: Data Table of class results: (9 points total)
a. includes a descriptive title (scientific...not cutsey <sorry all you English buffs!>). (2 points)
b. column headings include the name of the indicator and what the indicator detects in parenthesis... example: Benedict's Solution (monosaccharides)... (5 points)
c. Key for explaining the symbols you might use... example "+" = positive for biomolc (2 points)
Post Lab Questions: make sure you pick one version of questions to answer and provide any citations requested for full credit. Also remember incomplete sentences, sentences starting with "it" or "because", or sentences using pronouns will not receive full credit.
Answer questions 1-5 to earn a C on this section: (up to 5 points), answer questions 6-9 to earn a B (up to 8 points), to earn an A (up to 11 points answer 6-10).
Helpful articles to answer #10 article #1 http://tea.armadaproject.org/atwood/12.2.1998.html (Especially see section F)