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Carbohydrates, Lipids, Proteins

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    You may wish to read about Carbohydrates before you begin this part of the lab. Read up to the "Lipids" section, then push the "back" button on your browser to return here.

    Cleaning Up

    It is important to keep the lab in the same clean condition that is in when you arrive. After the experiments, rinse all of the equipment and glassware and wipe down the lab bench.

    The contents of test tubes can be disposed down the drain. The tubes should be rinsed with tap water and put upside down in a test tube rack. Leave used glassware near the sink area.


    Benedict's test is a test for the presence of monosaccharides or certain disaccharides in a solution. When a solution containing these sugars is mixed with Benedicts reagent and heated, a reduction reaction causes the Benedicts reagent to change color. The color varies from yellow to green to dark red, depending on the amount of and type of sugar.

    • Fill four of the test tubes to the 1 cm mark with the following solutions:
      Test tube #1: water (deionized)
      Test tube #2: glucose solution
      Test tube #3: sucrose solution
      Test tube #4: starch solution

    Below: The test solutions and Benedict's reagent are boiled in a water bath for five minutes.


    Click on the image to view an enlargement. Press the "Back" button to return.

    Below: Results of several solutions tested with the Benedict's test



    Iodine solution (IKI) reacts with starch to produce a dark purple or black color.

    • Use a wax marker to mark two test tubes 1 cm from the bottom.
    • Fill one of the tubes to the 1 cm mark with water and fill the other to the 1 cm mark with starch solution. Be sure to stir the starch before filling your tube.
    • Add two drops of IKI solution to each tube and note the color change.

    Record your results in your notebook.

    Below left: starch solution and IKI - Iodine turns dark in the presence of starch.

    Below right: distilled water and IKI


    • Place a small slice of potato on a piece of paper towel in your work area. Put a drop of IKI solution on the potato. Note the color after several minutes and record your observation in your notebook.
    • Place a small slice of onion on a paper towel and then add a drop of IKI to the onion. Record your observation in your notebook.
    • Put a thin slice of potato on a slide and stain it with IKI. The potato should be sliced as thin as possible; thinner than paper is best. If you cannot get it thin enough, press down on the cover glass to crush the specimen.

    Draw a potato cell from the slide that you prepared in the previous step. Label the cell wall and starch granules.

    Which macromolecule are the dark granules within the potato cells composed of? [Hint ? What caused the iodine to turn dark?]

    potato_stained_with_IKI_100x_small.jpg potato_stained_with_IKI_200x_small.jpg  
    Potato cells stained
    with IKI X 100
    Potato cells stained
    with IKI X 200

    Draw an onion cell.

    Does onion store food as starch?


    Left: Onion stained with IKI X 100 - The nuclei of these cells are light brown in this photograph. The numerous starch granules seen in potatoes are absent.


    Read about lipids in the class notes before you begin this part of the lab. Read up to the "Proteins" section, then push the "back" button to return here.


    Lipids are nonpolar and therefore do not dissolve in water. Emulsifiers are molecules have both polar and nonpolar parts and thus are capable of dissolving in or interacting with both lipids and water. When emulsifiers are mixed with lipids and water, they may act to suspend small droplets of the lipid in water. The lipid is not dissolved in water, but is broken into smaller fragments that may remain suspended for long periods of time.

    Bile salts are emulsifiers that are produced by the liver and assist in the digestion of lipids by enabling lipids to be broken up into small particles so that enzymes can break them down quicker.

    Detergents are emulsifiers that aid in cleaning by enhancing the ability of water to wash away nonpolar substances.

    • Sudan IV is a stain used to stain lipids. Add six drops of Sudan IV to each tube and mix the contents by swirling the test tubes.

      Describe what happens to the oil and water mixture.

    Below: The tube on the right contains oil and water. The one on the left contains oil, water, and a detergent. Both tubes were shaken to mix the oil and water. The oil can be seen floating on the water in the tube on the right. The tube on the left shows that the oil droplets remain mixed with the water longer before separating.


    Below left: oil and water X 40 - Note the large fat droplet on the upper, right half of the photograph. The smaller bubbles scattered throughout the photograph are air bubbles due to vigorous shaking.

    Below right: oil, water and detergent (emulsifier) X 40 - The large oil droplets have been broken up into smaller droplets after shaking.

    oil_and_water_40X_small.jpg oil_water_and_detergent_small.jpg


    Read about Proteins before you begin this part of the lab. Read up to the "Nucleic Acids" section, then push the "back" button to return here.

    Biuret Test

    The copper atoms of Biuret solution (CuSO4 and KOH) will react with peptide bonds, producing a color change. A deep violet color indicates the presence of proteins and a light pink color indicates the presence of peptides.

    Color Indication
    Light blue No protein or peptides
    Violet Protein
    Pink Peptides

    We will perform the biuret test on egg albumin, a protein found in chicken eggs.

    • Mark three test tubes at 2 cm.
    • Fill one of the tubes to the 2 cm mark with water, the second one to the 2 cm mark with albumin solution (a protein), and the third one to the 2 cm mark with starch solution.
    • Add 5 drops of 3% copper sulfate solution (CuSO4) to each tube.
    • Add 10 drops of 10% potassium hydroxide solution (KOH) to each tube.

    Record the final color of each test tube in your lab notebook.


    Tube 1: Water (control)

    Tube 2: Albumin (protein)

    Tube 3: Starch


    In the experiment below, we will study how pepsin, an enzyme found in the stomach, is capable of breaking protein down into smaller fragments called peptides.


    Pepsin is normally found in the warm (37 degrees C) acidic environment of the stomach. To simulate these conditions, HCl will be added and the test tube will be incubated at 37 degrees C.

    A violet color indicates the presence of protein. A lighter, pinkish color results in the presence of peptides.

    Record the final color of each test tube in your lab notebook.


    Tube 1 (left): water
    Tube 2 (center): albumin, pepsin, water
    Tube 3 (right): albumin, pepsin, HCL


    1) Explain why tube 3 was incubated at 37 degrees C (this is body temperature).

    2) What is the function of pepsin in the stomach?

    3) Explain why HCl was added to tube 3? (Hint: What is the pH of the stomach?)

    4) What is the name of the enzyme involved in this experiment?

    5) What is the optimal pH range of this enzyme (acid, neutral, or base)? What happens to enzymes when the pH is not appropriate for the enzyme?

    6) Trypsin is an enzyme found in the small intestine. It cleaves larger peptide fragments into smaller peptides. The pH of the small intestine is slightly alkaline. Knowing this, approximately what pH range (acid, neutral, or base) do you predict trypsin to function best?

    7) Based on your answer to the two previous questions, what can you conclude about the optimal pH of enzymes. Does it depend on the enzyme?

    8) Explain why you expect tube 2 to contain protein and tube 3 to contain peptides. [Hints: Is the pH the same in each test tube? How does changing pH effect the function of an enzyme?]

    Carbohydrates, Lipids, Proteins is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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