Purpose. Procedure. Photosynthesis OVERVIEW:

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1 Photosynthesis OVERVIEW: During lab this week you will perform experiments to help you understand two of the basic processes that convert energy from one form to another in biological systems. Your investigations will start with experiments on photosynthesis. This is the process that plants use to convert the kinetic energy of light into the stored chemical energy of sugars. It can be summarized with an equation that looks like this: 6 CO2 + 6 H2O > C6H12O6 + 6 O2 Light In this lab we will investigate the role of light in controlling the amount of photosynthesis that occurs. As in any scientific experiment we will try to isolate one variable, in this case the wavelength of light, and see what effect it has on the amount of photosynthesis that a leaf undergoes. After examining the role of light in photosynthesis this week we will investigate the process of cellular respiration. This process converts the stored chemical energy of sugar molecules (created directly or indirectly by photosynthesis) into a short term cellular storage molecule known as ATP, which is used to fuel a huge number of chemical reactions throughout the cell. This is a complicated process that in a simplified form looks like this: C6H12O6 + 6O2 > 6CO2 + 6 H2O ADP + Pi ATP As you can see from comparing these two summary equations there are some obvious similarities between the end products of photosynthesis and the reactants (the starting chemicals) of cellular respiration. You can also see that the end products of cellular respiration are the reactants of photosynthesis. Remember that the summary reactions above simplify a complex series of reactions (both biosynthetic and degradative) so these reactions are not the exact opposite of each other but the carbon atoms that are a part of all living organisms are constantly being cycled through an ecosystem by the action of these two processes. Purpose In this lab, we will investigate the effect of wavelength of light on the rate of photosynthesis. We will estimate the rate of photosynthesis by observing oxygen production in English Ivy (Hedera helix) leaves exposed to different wavelengths of visible light. Procedure Leaves tend to float in aqueous (water- based) solutions because leaves contain air spaces, which make the leaves less dense than the solution. In this lab, you will be forcing NaHCO 3 solution into and O 2 out of the air spaces in leaf disks. This will cause the disks to become more dense than the solution they are in, and the disks will sink to the bottom of the solution.

2 In order for photosynthesis to occur, water, CO 2, and light need to be present. In this experiment, the NaHCO 3 solution serves as the source of CO 2 and water, and the lamp serves as the light source. The rate of photosynthesis can be determined by measuring the amount of CO 2 used or the amount of O 2 produced over time. In this experiment, we will indirectly measure the amount of O 2 produced over time As the leaf disks photosynthesize, oxygen gas produced during photosynthesis will be released into the air spaces of the disks. As the disks fill up with oxygen, they will become less dense than the NaHCO 3 solution and rise to the surface. The time it takes for the leaf disks to float will be inversely related to the rate of photosynthesis. *Keep in mind that we will be measuring the net amount of O 2 produced (the amount of O 2 produced in photosynthesis minus the amount of O 2 used in cellular respiration) and estimating the net rate of photosynthesis (the rate of photosynthesis that has not been adjusted to account for cellular respiration). Research question How do the wavelengths/colors of visible light received by a plant affect the rate of photosynthesis? Hypothesis Plants receiving wavelengths/colors of visible light that are well absorbed by chlorophyll a and b will have a higher rate of photosynthesis than plants receiving wavelengths/color of visible light that are poorly absorbed by chlorophyll a and b.

3 Materials labeling tape scissors transparent tape 1 beaker (500 ml) red, green, blue, and clear cellophane aluminum foil 8 clear plastic cups (266 ml) NaHCO 3 electronic balance spatula tap water plastic cups for measuring 100 ml graduated cylinder dish detergent 1 plastic pipet (1 ml) stirring rod plastic straw or hole puncher 1 plastic syringe (10 ml) adjustable lamp with 150 watt bulb timer metric ruler (bring your own!) Procedure 1. Label the 500 ml beaker: 0.2M NaHCO 3 solution Label 3 of the clear plastic cups: Heat absorber 2. Create light filters for the remaining 5 clear plastic cups. Cover 1 cup with red cellophane, 1 cup with blue cellophane, 1 cup with green cellophane, 1 cup with colorless cellophane, and 1 cup with aluminum foil. The cellophane/foil that covers the opening of the cup should be removable. Perform steps 3-13 for each cup/filter: 3. Prepare a 0.2M NaHCO 3 (sodium bicarbonate) solution: Add 200 ml of tap water to the 500 ml beaker. Add 3.2 g of NaHCO 3 and stir to dissolve.

4 4. Add 1 drop of dish detergent to the NaHCO 3 solution and stir to dissolve (the dish detergent will help the NaHCO 3 enter the leaf.) If suds develop, dilute the solution by making more 0.2M NaHCO 3 solution and adding it to the beaker. 5. Remove 1 or more ivy leaves from the plant and punch out 10 uniform leaf disks with a hole puncher or straw. Avoid the major leaf veins. 6. Remove the plunger of the syringe and place the leaf disks inside the barrel. Replace the plunger, being careful not to crush the leaf disks. Depress the plunger until only the leaf disks and a small volume of air remain in the barrel. 7. Draw 7 ml of NaHCO 3 solution into the barrel, and tap the barrel to suspend the leaf disks in the solution. 8. Hold the syringe upright (open end up), and gently depress the plunger to remove any air left in the barrel. 9. Hold a finger firmly over the opening of the syringe, pull back on the plunger, and wait for about 10 seconds. This will create a vacuum inside the barrel and draw NaHCO 3 into the air spaces inside the leaf disks. It will also force O 2 out of the air spaces in the leaf disks. Once the NaHCO 3 has entered the leaf disks, the disks will sink to the bottom of the barrel. You may have to create the vacuum several times before all the leaf disks sink. Shake the syringe if the leaf disks stick to the barrel or each other. 10. Pour the contents of the barrel into one of the plastic cups covered with a light filter. Add 150 ml of NaHCO 3 solution. All of the leaf disks should settle at the bottom of the cup. 11. Place the cup under a lamp with a 150 watt bulb. Adjust the lamp so that the bulb is 10 cm away from the bottom of the cup. 12. Add 150 ml of tap water to each of the clear plastic cups labeled heat absorber. Surround the cup containing the leaf disks with the heat absorbers. 13. Turn on the lamp, start the timer, and record the number of disks that are floating at minute 0 (the beginning of data collection). 14. When 1 minute has elapsed, record the number of disks that are floating and then swirl the cup to dislodge any disks that are stuck to the side of the cup or each other. Use a clean pipet to dislodge the disks if necessary. 15. Repeat step 14 until all of the disks are floating. 16. Repeat steps 3-15 for the remaining cups.

5 Pre- Lab Assignment #2 Table 1. Colors and wavelengths of visible light Color of visible light Wavelength of visible light / nm violet blue green yellow orange red Figure 1. Absorption spectra for chlorophyll a and b

6 1. Study Table 1 and Figure 1 above. a. Rank the colors of visible light absorbed by chlorophyll a from most absorbed to least absorbed. b. Rank the colors of visible light absorbed by chlorophyll b from most absorbed to least absorbed. 2. The wavelengths/colors of visible light received by a plant affect the rate of photosynthesis. In our experiment, we will cover the leaf disks with light filters to limit the wavelengths/colors of light they are exposed to. Identify the colors of light each group of leaf disks will receive. Light filter material covering the leaf disks Colors of visible light the leaf disks will be exposed to Red cellophane Blue cellophane Green cellophane Clear cellophane Aluminum foil

7 3. Our research question is: How do the wavelengths/colors of visible light received by a plant affect the rate of photosynthesis? Our hypothesis is: Plants receiving wavelengths/colors of visible light that are well absorbed by chlorophyll a and b will have a higher rate of photosynthesis than plants receiving wavelengths/color of visible light that are poorly absorbed by chlorophyll a and b. Extend the hypothesis by predicting the specific outcome of our experiment. Rank the groups of leaf disks from highest to lowest expected rate of photosynthesis, and then explain your reasoning. 4. What are the independent and dependent variables in this experiment? 5. Which leaf disks belong to the experimental group and which a leaf disks belong to the control group? 6. Identify as many controlled variables as possible.