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AP BIO Lab 4: Floating Leaf Disk Assay for Exploring Photosynthesis Instructions: This lab will be conducted in two parts. The first part will introduce you to the technique of the floating leaf disk assay. The second part is a test to see how well you can design and carry out a scientific investigation. Your task for this part is to plan and design an experiment on your own. You have one class period to complete your experiment. Your teacher will review your design and check for safety issues before you begin. Background: Life needs energy. For much of the earth’s living systems that energy is supplied by photosynthesis. The fact that photosynthesis can convert light energy into food energy makes photosynthesis in plants one of the most important of life’s processes to understand. Plants use the energy from light to build food molecules of sugar that are rich sources of energy. The chemical equation that summarizes the process of photosynthesis is: 6 CO2 + 6 H2O + light energy C6H12O6 +6 O2 Part I: The biology behind the floating leaf disk assay: Leaf disks float, normally. When the air spaces are infiltrated with solution the overall density of the leaf disk increases and the disk sinks. The infiltration solution includes a small amount of Sodium bicarbonate. Bicarbonate ion serves as the carbon source for photosynthesis. As photosynthesis proceeds oxygen is released into the interior of the leaf which changes the buoyancy--causing the disks to rise. Since cellular respiration is taking place at the same time, consuming oxygen, the rate that the disks rise is an indirect measurement of the net rate of photosynthesis. Materials: Sodium bicarbonate (Baking soda) Liquid Soap Plastic syringe (10 cc or larger) Leaf material Straw Plastic cups Timer Light source Procedure: 1. Make 300ml bicarbonate solution. This consists of 300ml water + 1/8 tsp. Baking Soda + 3 drops liquid soap. 2. Cut 10 or more uniform leaf disks for each trial. a. Use straw or whole punch to make disks. b. Avoid major vein and dead parts of the leaves c. Take care to not damage the disks. 3. Infiltrate the leaf disks with sodium bicarbonate solution. a. Remove the piston or plunger and place the leaf disks into the syringe barrel. Replace the plunger being careful not to crush the leaf disks. Push on the plunger until only a small volume of air and leaf disk remain in the barrel (< 10%). b. Pull a small volume of sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf disks in the solution. c. Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Let off the vacuum. The bicarbonate solution will infiltrate the air spaces in the leaf causing the disks to sink. d. You will probably have to repeat this procedure 2-3 times in order to get the disks to sink. e. If you have difficulty getting your disks to sink after about 5 evacuations, it is usually because there is not enough soap in the solution. Add a few more drops of soap to your bicarb solution, add fresh leaves to the syringe, and try again. 4. Pour the disks and solution into a clear plastic cup. Add bicarbonate solution to a depth of about 3 centimeters. Use the same depth for each trial. Shallower depths work just as well. 5. For a control infiltrate leaf disks with a solution of only water with a drop of soap--no bicarbonate. 6. Place under the light source and start the timer. At the end of each minute, record the number of floating disks. Then swirl the disks to dislodge any that are stuck against the sides of the cups. Continue until all of the disks are floating. Data Collection and Analysis Rate of photosynthesis of Minutes Disks Spinach leaves 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Part II: Student Designed Photosynthesis Experiment using the Floating Leaf Disk Assay Your Problem to Investigate: Using the floating disk assay, your problem is to design and carry out an experiment to test the effect of a variable on the rate of photosynthesis. You chose which variable you will investigate. Possible Variables to Investigate: Below is a table of possible variables that provides a starting point for developing questions about photosynthesis to investigate. Of course, you may think of other variables to investigate. Look these variables over. Think about how each of these variables might affect the rate of photosynthesis. Environmental Variables Plant or Leaf Variables Method Variables Light intensity (brightness) Stomata distribution Light color Light starved leaves vs leaves kept in bright light Temperature Type of plant Bicarbonate concentration (CO2 Size of leaf disk source) Methods of cutting disks Direction of incoming light Soap amount pH of solution How many times can the procedure be repeated with the same disks? Leaf color (chlorophyll amount) Stomata density Leaf size Questions: Asking good questions is an important skill. It is important to ask the right question in the right way. A good question for scientific inquiry is asked in a way that actually suggests how the question can be answered. A good question should be focused on how one variable affects another. The resources that are available will also limit your question. Also, for this lab test, remember that you only have one day to do the actual lab work. Make sure your question is answerable in one hour of work. If you need more time you can come in after school. Hypothesis: Your research question can now aid you in developing a good hypothesis to guide your research or experimental design. A good, working hypothesis helps the investigator limit his or her investigation to the effect of one variable at a time. This allows the results to be clearly interpreted. To develop a working hypothesis, you need to establish the variables that you are studying and make a prediction on how those variables interact. Forming a hypothesis is a two- step process. 1. Define your variables. Determine which variable will change as you manipulate another. Consider the following question. "Does the temperature of germinating seeds affect the rate of cellular respiration?" The temperature of the germinating seeds is the manipulated variable (independent variable) and the rate of respiration is the experimental, or changing, variable (dependent variable). 2. State the relationship between the two variables in an "If... then" format. If the manipulated variable effects the experimental variable in such and such a way, then the experimental variable should change in such and such a manner when the manipulated variable is changed. Minimal Materials: Plant leaves Sodium bicarbonate (Baking soda) Liquid Soap Plastic syringe Leaf material Straw or whole punch Plastic cups Timer Light source PRE-LAB Design: (Written in NB) *Using the floating disk assay, your problem is to design and carry out an experiment to test the effect of a variable on the rate of photosynthesis. You chose which variable you will investigate. a) What is the question you will be investigating? b) State your hypothesis. c) Describe the independent and dependent variables you are investigating. State your control group and your controlled variables. d) Generate a materials list that includes the special items you would need for your investigation. e) Describe the procedure you will use to investigate your hypothesis. List the steps you will use. Describe the type of data that you will collect. You may want include diagrams or illustrations. Be sure to include any safety procedures you will need to follow. f) Construct a data table that you will use to record your observations. Be sure to correctly label this table and include any important instructions. Submit your lab test to your teacher. Do not proceed until you have your teacher’s approval.
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