Organic Lab Rubric
___/2-Title
___/18-Introduction
Function, examples and indicators for:
Carbohydrates
a. Monosaccharides
b. Disaccharides
c. Polysaccharides
Proteins
Lipids
a. Triglycerides
b. Phospholipids
c. Steroids
Nucleic Acids
a. DNA
b. RNA
___/20-Materials for each test, procedure for each test, include controls for each and why they are
important
___/20-Results
___/25-Conclusion
For each section summarize results and state sources of error.
___Points Earned + ___/15 (lab clean-up, name and rubric attached)=___/100
Biologically Important Molecules
Carbohydrates, Proteins, Lipids and Nucleic Acids
Objectives:
By the end of this exercise you should be able to:
1. Understand how to test for the presence of carbohydrates, lipids, proteins and nucleic
acids.
2. Understand the importance of a control in biochemical tests.
3. Use biochemical tests to identify an unknown compound.
Most organic compounds in living organisms can be classified as carbohydrates, proteins, lipids,
or nucleic acids. Each of these macromolecules are made of smaller subunits (Figs 2-1, 2-2, 2-3 and 2-4).
These subunits of macromolecules are held together with covalent bonds, and have different structures
and properties. For example, lipids made of fatty acids have many C-H bonds and relatively little
oxygen, while proteins made of amino acids have amino groups (-NH2) and carboxyl (-COOH) groups.
These characteristic groups impart different chemical properties to macromolecules-for example,
monosaccharides such as glucose are polar and soluble in water, while lipids are nonpolar and insoluble
in water.
Controlled Experiments to Identify Organic Compounds
Scientists have devised several biochemical tests to identify the major types of organic
compounds in living organisms. Each of these tests involves two or more treatments; an unknown
solution to be identified and controls. As its name implies, an unknown solution may or may not contain
the substance that the investigator is trying to detect. Only a carefully conducted experiment will reveal
its contents. In contrast, controls are known solutions. We use controls to validate that our procedure
is indeed detecting what we expect it to detect and nothing more. During the experiment we compare
the unknown solution’s response to the experimental procedure with the control’s response to that
procedure.
A positive control contains the variable for which you are testing. It reacts positively and
demonstrates the test’s ability to detect what you expect. For example, if you are testing for protein in
unknown solutions, then an appropriate positive control is a known solution of protein. A positive
reaction proves that your test reacts correctly, and shows you exactly what a positive test looks like.
A negative control does not contain the variable for which you are searching. It contains only
the solvent (often distilled water with no solute) and does not react in the test. A negative control
shows you what a negative result looks like.
Controls are important because they allow us to determine the specificity of a particular test.
For example, if water and glucose solution react similarly in a particular test, then the test cannot
distinguish water from glucose. However, if the aqueous solution of glucose reacts differently from
distilled water, then the test can distinguish water from glucose. In this instance, the distilled water
would be a negative control for the test and a known glucose solution would be a positive control.
Procedures
CARBOHYDRATES
Benedict’s Test for Reducing Sugars
Carbohydrates are molecules made of C, H and O in a ratio of 1:2:1 (ex, the chemical formula for
glucose is C6H12O6). Carbohydrates are made of monosaccharides, or simple sugars (Fig 2-1). Paired
monosaccharides form disaccharides-for example, sucrose is a disaccharide of glucose linked to
fructose. Similarly, linking three or more monosaccharides form a polysaccharide such as starch,
glycogen, or cellulose.
Many monosaccharides such as glucose and fructose are reducing sugars, meaning that they
possess free aldehyde (-CHO) or ketone (-C=O) groups that reduce oxidizing agents such as the copper in
Benedict’s reagent. Benedict’s reagent contains cupric (copper) ion complexed with citrate in alkaline
solution. Benedict’s test identifies reducing sugar based on their ability to reduce the cupric (Cu2+) ions
to cuprous oxide at basic (high) pH. Cuprous oxide is green to reddish orange.
Oxidized Benedict’s reagent (Cu2+) + Reducing sugar (R-COH)
(blue)
Heat
High pH
Reduced Benedict’s reagent (Cu+) + Oxidized sugar (R-COOH)
(green to reddish orange)
Green color indicates a small amount of reducing sugars, and reddish orange color indicates an
abundance of reducing sugars. Non-reducing sugars such as sucrose produce no change in color (i.e.,
the solution remains blue).
Materials:
7 test tubes
Apple juice
Test tube rack
Lemon juice
Labeling tape
Yogurt
Water
Benedict’s solution
Glucose solution
Hot plate
Regular soda
Hot water bath
Diet soda
Test tube holder
Procedure:
1. Obtain 7 test tubes and number them 1-7.
2. Add to each tube the materials to be tested
Test tube 1-Water
Test tube 2-Glucose solution
Test tube 3-Diet soda
Test tube 4-Regular soda
Test tube 5-Apple juice
Test tube 6-Lemon juice
Test tube 7-yogurt
3. Add 2 ml of Benedict’s solution to each test tube.
4. Place all of the test tubes in a hot water bath for 3 minutes and observe color changes during
this time.
5. After 3 minutes use a test tube holder to remove tubes and let them cool to room temperature.
Record the color of their contents.
Iodine Test for Starch
Iodine (iodine-potassium iodide, I2KI) staining distinguishes starch from monosaccharides,
disaccharides, and other polysaccharides. The basis for this test is that starch is a coiled polymer
of glucose. Iodine interacts with these coiled molecules and becomes bluish black. Iodine does
not react with other carbohydrates that are not coiled, and remains yellowish brown.
Therefore, a bluish black color is a positive test for starch and a yellowish brown color (i.e. no
color change) is a negative test for starch. Notably, glycogen, a common polysaccharide in
animals has a slightly different structure than does starch and produces only an intermediate
color reaction.
Materials:
7Test tubes
Pasta
Test tube rack
Glucose solution
Water
Soda
Corn starch colution
Yogurt
Potato
Iodine
Procedure:
1. Obtain 7 test tubes and number them 1-7.
2. Add to each test tube the materials to be tested:
Test tube 1-Water
Test tube 2-Corn starch solution
Test tube 3-Potato
Test tube 4-Pasta
Test tube 5-Glucose solution
Test tube 6-Regular Soda
Test tube 7-yogurt
3. Add 3-5 drops of iodine to each test tube.
4. Record the color of the tube’s contents in data table.
PROTEIN
Proteins are made of amino acids, each of which has an amino group (-NH3) and a
carboxyl, or acid group (-COOH). The bond between these two groups found on adjacent amino
acids in a protein is a peptide bond (Fig 2-2) and is identified with a Biuret test. Specifically,
peptide bonds (C-N bonds) in proteins complex with Cu2+ in biuret reagent and produce a violet
color. A Cu2+ion must complex with four to six peptide bonds to produce a color; therefore, free
amino acids do not react positively. Long-chain polypeptide (proteins) have many peptide
bonds and produce a positive reaction. Biuret reagent is a 1% solution of Cu SO4, copper sulfate.
A violet color is a positive test for the presence of a protein, and the intensity of color is
proportional to the number of peptide bonds reacting.
Materials:
5 test tubes
Test tube rack
Water
Protein solution
Procedure:
1. Obtain 5 test tubes and number 1-5.
2. Add the materials listed to be tested:
Test tube 1-Water
Test tube 2-Protein solution
Test tube 3-Tuna
Test tube 4-Egg white
Test tube 5-Yogurt
3. Add 10 drops of Biuret reagent to each tube.
4. Record the color of the tubes.
Chicken
Egg white
Yogurt
Biuret
LIPIDS
Lipids react with paper to make it translucent. A translucent spot after drying is a positive
result.
Materials:
Brown paper bag
Water
Oil
Butter
Egg white
Yogurt
Procedure:
1. Obtain a sample of paper bag and label it with the 5 substances to be tested.
2. Place a small sample of each substance to be tested on the paper bag above the labeled
spot and allow to try.
Substance 1: water
Substance 2: oil
Substance 3: butter
Substance 4: Egg white
Substance 5: Yogurt
3. Record results.
UNKNOWN:
Test your unknown with all of the above tests and attempt to identify which of the 3 substances tested
in the previous labs are contained in your unknown.
Figure 2-1
Figure 2-2
Figure 2-3
Figure 2-4