In this lab, we will test for various biomolecules in your everyday world, using common household products as reacts to extract and/or detect sugars (carbohydrates), fats (lipids), DNA (nucleic acids), and proteins (amino acids).
Part 1: Testing for Simple Sugars
- Identify the location of sugar production in a plant.
- Identify the presence of the carbohydrate, glucose, in the form of starch.
Hypothesis: Because plants store the sugar they produce as starch, a chemical indicator specific for starch will detect its presence in a plant leaf.
Overview of Testing for Starch in a Plant Leaf:
Safety: This lab activity uses ethanol or isopropyl alcohol. Wear eye protection.
Ethanol is flammable.
Background: Some organisms can produce their own food by accomplishing photosynthesis. We call these organisms, producers. Plants are producers, because they produce glucose (a simple carbohydrate, or sugar) during photosynthesis.
A glucose molecule looks like this:
It is a simple carbohydrate. It can be linked to more simple sugars to make complex carbohydrates, like starch.
The storage form of sugars in a plant is starch. It is a special chain of glucose molecules that looks like this:
Some of the sugar produced by photosynthesis is converted into starch, which can be stored in the roots and leaves of the plant. This lab activity involves taking a leaf from a plant and testing it for the presence of starch.
Our research question is: Do plants produce carbohydrates (sugars) during photosynthesis? Based on what we know about plants and photosynthesis, we develop the following Hypotheses:
- Because plants need light for photosynthesis, presence of carbohydrates in the plant leaf will be identified by iodine solution in the parts of the leaf exposed to light.
- Because plants store the sugar they produce as starch, a chemical indicator specific for starch will detect its presence in a plant leaf.
Plants require light, carbon dioxide, and water to accomplish photosynthesis. To set up this lab, you will cover part of the leaf to block the light and therefore halt photosynthesis in that section of the leaf. This is your Experimental Control. You know you do not have the presence of starch there, because no photosynthesis occurred in that part of the leaf:
No light = no photosynthesis = no carbohydrates (sugar production) = no starch
You will use this “known” result to compare the results from the test variables in your experiment. Your test variable in this experiment is the leaf left in the light:
Light = photosynthesis = carbohydrate (sugar) production = starch
Prep: Using a potted plant, cover a broad leaf partially with aluminum to prevent exposure to light all day (a).
Pot with water
Test tube (tall shot glass or craft tubes from a craft store, like Michaels or Walmart)
Petri dish (small shallow dish)
2 leaves taken from the potted plant
Ethanol (found in some hand sanitizers) or nail polish remover
- Fill pot half way with water.
- Heat until boiling
- Turn off heat to stop active boiling
- Add the leaf to this beaker while the water is still hot. Leave the leaf in there for a few seconds. This will stop any further chemical reactions taking place in the leaf.
- Half fill a test tube with ethanol (ensure you do this away from any naked flames or heat sources).
- Remove the leaf from the water using tongs and put it into the ethanol in the test tube.
- Stand the test tube in the hot water until the ethanol turns green. This removes the chlorophyll in the leaf which makes the later staining clearer to see.
- Remove the leaf with tongs (remembering the water is still very hot) and dip back into the hot water to remove any ethanol and to soften the leaf.
- Place the leaf into the petri dish and flatten out. Add some of the iodine solution to the surface of the leaf. If there is starch present, the leaf will turn blue/black in color.
Record your results here. Include a description of your observations and images of your set up and results of the iodine test.
- What is glucose?
- Where did we find glucose in our experiment? Why was it found there?
- Did your leaf show a color change?
- Do we have evidence to support the hypothesis that the presence of carbohydrates would be detected in the plant leaf in the parts of the leaf exposed to light? If, yes, what is the evidence?
- Try testing other parts of a plant for the presence of starch. For example, does the iodine detect the presence of starch in a potato, or carrot? Why would it make sense for starch to be present in these vegetables? Include pictures of your results from a potato or carrot, or other plant part in your answer.
Part 2: NUCLEIC ACIDS: HOMEMADE EXTRACTION OF YOUR OWN DNA
Background: Cells are the smallest unit of life. They contain DNA, the instructions for the cell that keep it organized and operating as a living unit. DNA is composed of biomolecules called, nucleic acids. Each nucleic acid includes a glucose and phosphate molecule and a nitrogenous base. There are four different nitrogenous bases, each pairs with another.
Nucleic acid DNA Structure (Composed of nucleic acids)
Objective: In this laboratory exercise, you will break the cell membrane and the nuclear envelope of cells from your mouth in order to extract the DNA contained in the nucleus of these cells.
Isopropyl alcohol (e.g. disinfectant, rum, vodka, etc.), refrigerated
A strainer or colander
- Spit into the glass.
- Add a pinch of salt to it.
- Add some liquid dish soap, juice from a grapefruit, and some drops of alcohol.
- Stir the mixture. The white mucous filaments you observe on top of the mixture is your DNA.
Record your observations here. Include pictures of your extraction.
Use the document, DNA Extraction Chemistry to answer the questions below:
- Why is salt added to your saliva?
- Why is dish shop added? How does it work?
- Why is isopropyl alcohol added? How does it work?
- Predictions: DNA can be extracted from fruit this way. Do you think the DNA will look the same? Why? (This is your hypothesis).
- Try it! Follow the Protocol to Extract DNA from Strawberries. Include pictures and a description of your results. Then explain whether or not you have evidence to support your hypothesis. How can you explain your results?
Part 3: Identifying Lipids
Background: Lipids, or fatty acids, are hydrophobic molecules. This means that because they are uncharged organic molecules, they do not interact with the slight charge that a water molecule. When molecules carry a charge, we describe them as being “polar.” Molecules that are uncharged are considered “nonpolar.” Water molecules are polar. Lipid molecules are nonpolar. This is why you cannot dissolve oil (a lipid) into water.
Water molecule Phospholipid (found in cell membranes)
Objective: In this exercise, you will use a simple test for lipids by observing their ability to produce translucent grease-marks on paper.
Brown paper bag
- Use an eyedropper to add a drop of oil near a corner of a piece of the brown paper bag.
- Add the drop of water near the opposite corner of the paper.
- Let the fluids evaporate
- Hold the paper up to the light and compare the translucency of the two corners.
- Test other foods such as chicken broth, avocado, chips, salad dressing, milk, fruits. (You can crush/smash solid foods onto the brown paper). Propose a hypothesis for each variable tested.
Record your results of the foods you tested here. Include pictures of your results.
- What was your control in this experiment?
- What were your variables?
- Did you find evidence to support any of your hypotheses? Which ones, and what was your evidence?
Do a little research and use your textbook to answer the following questions. Include your sources:
- Why do lipids produce grease spots on paper?
- Where do we find lipids in the human body?
Part 4: Identifying Proteins
Helmenstine, Anne Marie, Ph.D. “How to Test for Protein in Food.” ThoughtCo, Aug. 26, 2020, thoughtco.com/test-for-protein-in-food-607464.
Background: Protein is an essential nutrient that builds structural tissue and enzymes in the body. It’s easy to test for!
Objective: In this experiment you will view a demonstration of the chemical reaction involved in protein metabolism and a test for the products to provide evidence that protein is found in the variable.
Proteins are made of building blocks called amino acids.
Amino acid molecular structure
Proteins are made by our cells according to instructions contained in our DNA to build tissues including muscle, skin, hair, fingernails, and bone. Proteins are also used as enzymes, molecules that facilitate chemical reactions. Individual amino acid building blocks are required to build the proteins these structural tissues and enzymes. We get these from the food we eat. Proteins we consume are broken down into amino acid subunits during protein catabolism. Sometimes our bodies use protein as an energy source as well. Proteins must be converted into carbohydrates in order to metabolize them, and liberate the energy from their chemical bonds to generate ATP.
Ammonia (NH3) forms with the oxidation of amino groups (-NH2), which are removed from the proteins when they convert into carbohydrates. It is a very toxic substance to tissues and extremely soluble in water. We process ammonia in the body by converting it into urea, which we concentrate and remove from our bloodstream in the kidneys and release as urine. All animals must eliminate ammonia. A lot of water is needed for the excretion of ammonia, about 0.5 L of water is needed per 1 g of nitrogen to maintain ammonia levels in the excretory fluid below the level in body fluids to prevent toxicity. Some marine organisms, like fish and crustaceans, excrete ammonia directly into the water.
(Metabolic Wastes https://en.wikipedia.org/wiki/Metabolic_waste#:~:text=Ammonia%20(NH3)%20forms%20with,atom%20is%20removed%20with%20it. Accessed January 10, 2021).
Materials – Do Not Buy These Items, This Lab is Completed by Watching a Video Demonstration on Protein Metabolism
Calcium oxide (sold as quicklime in building supply stores)
Red litmus paper (can be found at Walmart or CVS)
Pot filled with water on the stove or another heat source
Milk or other foods to test
Because milk contains casein and other proteins, it’s a good food to start your testing with. Once you understand what to expect from testing milk, you can examine other foods.
- Add a small amount of calcium oxide and five drops of milk to a test tube.
- Add three drops of water.
- Dampen the litmus paper with water. Water has a neutral pH, so it should not change the color of the paper. If the paper does change color, start again using distilled water rather than tap water.
- Carefully heat the test tube in a pot of water on the stove or other heat source. The increase in temperature helps to facilitate the reaction. Hold the damp litmus paper over the mouth of the test tube and observe any color change.
- If protein is present in a food, the litmus paper will change color from red to blue. Also, smell the test tube: If protein is present, you should be able to detect the odor of ammonia. Both of these indicate a positive test for protein. If protein is not present in the test sample (or is in insufficient concentration to produce adequate ammonia during testing), the litmus paper will not turn blue, resulting in a negative test for protein.
Calcium oxide reacts with protein to break it down into ammonia. The ammonia changes the acidity of the sample, causing a pH change. If your food is already very alkaline, you won’t be able to use this test to detect protein. Test the pH of each food sample with the litmus paper to see if it changes the litmus paper prior to performing the protein test.
Milk is an easy food to test because it’s a liquid. To test solids, such as meat, cheese, or vegetables, you must first grind the food by hand or by using a blender. You may need to mix the food with some water to make a sample you can test.
The test registers a change in pH, which is the concentration of hydrogen ions in an aqueous or water-based solution:
Most foods contain water, so they work fine for the test. However, oily foods may not work as well. You can’t test pure vegetable oil, for example, because it doesn’t contain any water. If you test greasy foods, such as french fries or potato chips, you’ll need to mash them up and mix them with a bit of water first.
View the Demonstration on Protein Metabolism and record the results presented in the video here. Include pictures of the results of the tests by taking screen shots.
- What chemical byproduct are we using to test for the metabolism of protein?
- Do a little research and report how reptiles, birds, and insects eliminate ammonia. Include your references.
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