- Lab 3
note: You should probably read the background (Bio 113) information about Carbohydrates prior to this lab. You can find the link under course documents.

Carbohydrates - A carbohydrate is an organic compound that is composed of atoms of Carbon, Hydrogen and Oxygen in a ratio of 1 carbon atom, 2 hydrogen atoms, and 1 oxygen atom. Some carbohydrates are relatively small molecules, the most important to us is glucose which has 6 carbon atoms. These simple sugars are called Monosaccharides.

One primary Biological function of carbohydrates is as a source of energy (sugars are for Energy....duh). A secondary function is intermediate-term energy storage (as in starch for plants and glycogen for animals). Other carbohydrates are involved as structural components in cells, such as cellulose which is found in the cell walls of plants. Sugars are most often found in the form of a "RING". The sugar molecules in the image above and the sugars in the image below (Glc and Frc) are really the same molecules, just arranged differently. The Carbonyl (C=0) Carbon of the straight-chain form (above) has formed a bond with the next to last Carbon in the chain, forming a ring.

Hooking two monosaccharides together forms a more complex sugar, such as the union of glucose and fructose to give sucrose below (aka common table sugar). Compounds such as sucrose are called Disaccharides (two sugars). Both monosaccharides and disaccharides are soluble in water.

Here is a 3-D Disaccharide (chime)

Larger, more complex carbohydrates are formed by linking many sugars together to form long or very long sugar chains called Polysaccharides. Because of their size, these are often times not soluble in water. Many biologically important compounds such as starches and cellulose are Polysaccharides. In these molecules the many sugars are not "free" but are part of a large polymer. Starches are used by plants to store energy in their numerous carbon-hydrogen bonds, while cellulose is an important compound that adds strength and stiffness to a plant's cell wall. Animals synthesize a storage carbohydrate called Glycogen.

There are several chemical tests that indicate the presence of carbohydrates. The two we will use in Lab can also distinguish between Carbohydrates of different sizes.

Starch Test - Our starch test solution is a combination of iodine and potassium iodide (nicknamed IKI). IKI is a reddish-yellow solution that turns dark blue/black when put in contact with starch. Use this test by adding a couple drops of the IKI solution to a small quantity of the sample to be tested. In general, the more starch present, the darker blue the color. IKI does not react with small carbohydrates such as Glucose. Move your mouse over the image to see what the results of the IKI Starch test might look like.

Sugar Test - Benedict's reagent is used as a simple test for reducing sugars. Reducing sugars have either a free aldehyde functional group or a free ketone functional group as part of their molecular structure (polysaccharides lack these functional groups). The Benedict's test for sugar involves adding Benedict's solution to the solution to be tested and heating gently (often in a boiling water bath). Benedict's solution contains Copper, which can accept electrons from some sugars and consequently change color (read your lab manual for more on this). A positive Benedict's sugar test will produce an orange to brick-red color.

Below is an example of an experiment using Benedict's solution to test for sugar. Keep clicking the red button to walk thru the experiment. Pay attention to the experiment or you will have to start over.

Questions: Answer the questions below, then submit them along with your name and Lab instructor.

1. Glucose in the urine, glycosuria, can be an indicator of Diabetes. Which test would you use to test suspect urine for the presence of sugars?

Benedict's
IKI
Carbonyl

2. In the presence of monosaccharides IKI will turn what color?

Blue/Black
Orange/Red
Yellow
It won't change

3. In the Benedict's test experiment above, which tube was the negative control?

Tube 1
Tube 2
Tube 3
Tube 4
Tube 5

4. During the Benedict's test, why was there no color change in the solution containing Starch after heating?

5. Benedict's solution is a test for the presence of free sugars. Which of the following is NOT a free sugar?

Starch
Maltose
Glucose

6. Carbohydrates are particularly important in biological systems as...?

A. Enzymes
B. Sources of energy
C. Structural components of cells
D. A & B
E. B & C
F. A & C

Name:
Section:

Submit your answers to obtain a printable receipt. Be sure to select your section from the drop-down box.