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Rate of Photosynthesis Lab Introduction: This lab will measure the rate of photosynthesis in discs cut from spinach leaves and investigate how a variable might affect that rate. The intercellular spaces in the leaf discs will be filled with a Sodium bicarbonate (NaHCO3, AKA baking soda) solution by decreasing the pressure inside a syringe. This should cause the discs to lose the air in the intercellular spaces and sink in the solution. As photosynthesis occurs, oxygen is produced which fills the intercellular spaces and causes the leaf discs to float. The Sodium bicarbonate solution adds Carbon dioxide to the solution to stimulate photosynthesis. The concept "ET50" will be used in this lab. "ET50 is the Exposure Time required for a defined effect to be observed among 50% of a population when that population is treated with a known amount or concentration of a substance." In other words, given a certain treatment, ET50 is the time it takes for half of the group to change. In this lab, ET50 will be the time it takes for 50% of the leaf discs to float. The reciprocal of the ET50, or 1/ET50, can be used as a simple measure of the rate of photosynthesis per second (rates are always a measurement per unit of time). After mastering the basic technique of sinking spinach discs and making them float again in the Control Group Lab, each pair of students will design and carry out an Experimental Group Lab with one variable changed that might affect photosynthesis. Here are suggestions of different variables that can be tested with the equipment on hand: concentration of Sodium bicarbonate solution, pH of the solution, temperature of the solution, intensity of light, and color (wavelength) of light. Other variables may be used as well; check with your teacher. Make sure that the procedure is a simple departure from the Control Group Lab and that the amount of the variable is specific. Write a hypothesis, write a procedure, and prepare the materials needed for the variable before conducting the Experimental Group Lab. Materials: Control Group Lab Drinking straw Spinach leaves on ice 10mL syringe 50mL 0.2% Sodium bicarbonate 75 to100 Watt light source Ruler Heat-sink beaker or jar Thermometer Possible materials for Experimental Group Lab Sodium bicarbonate solutions of differing concentrations Colored plastic wrap light filters Light bulbs with different wattages pH paper HCl (Hydrochloric acid) in dropper bottle NaOH (Sodium hydroxide) in dropper bottle

Procedure: Control Group Lab 1. Make a table for the data of both the Control and Experimental Group Labs. 2. Using a new, sharp drinking straw, cut ten leaf discs from young spinach leaves by supporting the leaf with a finger while pressing and using a twisting motion of the straw. Avoid cutting through areas with veins. Keep extra leaves on ice and in the dark while not in use. (See Figure 1: A) 3. Remove the plunger from a clean 10 ml syringe. Gently blow the ten discs into the body of the syringe. In order to not damage the discs, be sure the leaf discs are near the tip of the syringe. Now, re-insert and depress the plunger to expel most of the air. (See Figure 1: B) 1

4. Insert the tip of the syringe into a beaker of 0.2% ml Figure 1: Summary of Rate of Photosynthesis Lab Sodium bicarbonate solution and draw about 8 ml into the syringe. The leaf discs should be floating. Hold the syringe upwards and expel any extra air. (See Figure 1: C) 5. Seal the tip of the syringe using an index finger. Pull back on the plunger, creating a partial vacuum in the syringe. If there is a good seal, it should be difficult to pull on the plunger and bubbles should be seen coming from the edge of the leaf discs. Simultaneously release the finger and the plunger. Some of the leaf discs may start to sink. Tap the side of the syringe to dislodge bubbles on the edges of the discs and allow them to sink. (See Figure 1: D) 6. Repeat Step 5 above until all discs sink. However, do not overdo that step as it is possible to damage the cells of the leaves. If after a few tries some discs will not sink, discard them and make other discs to take their place. (See Figure 1: E) 7. Once the syringe is prepared, keep it in a dark place until the experiment begins. To eliminate as much ambient light as possible during this lab, the room lights will be kept off and the curtains will be closed. 8. Fill the syringe up to 10 ml if any solution has been lost. Take and record the temperature of the solution. 9. Set the syringe, standing tip up, 15 cm from the light source and begin timing. Place a beaker full of water between the light and the syringe to act as a heat sink. 10. As the leaf discs photosynthesize they should float. At 1-minute intervals, gently shake the syringe to agitate the leaf discs and immediately return the syringe to the position in front of the light. Count the number of discs that are floating after each 1-minute interval until all discs are floating. Enter data in the table as the lab proceeds. (See Figure 1: F) 11. Complete a "best fit line" graph of the data on graph paper: time in minutes vs. number of floating discs. Using the graph, determine the time necessary for 50% of the discs to float (the ET50) as described. Find the ET50 by drawing a line on the graph perpendicular to the "floating disc axis," starting at the "five discs" location, and ending at its intersection with the graphed line. Then draw a line perpendicular to the "time axis" through the point just found on the graphed line. This time shown on the "time axis" is the ET50 in minutes. Next to the graph, show the calculations for the rate of photosynthesis: Photosynthetic rate = 1/ET50. Label the calculations and include the units (rate/minute). 12. Clean up. 2

Experimental Group Lab 1. Before the Experimental Group Lab day, write a hypothesis about the affect of the chosen variable on the rate of photosynthesis. Write the hypothesis in this form: If ... [this change is made to the control group set up] ...then... [this change in the rate will take place] ... because ... [this principle about how photosynthesis works]. 2. Before the Experimental Group Lab day, write a procedure for the Experimental Group Lab. Remember to test only ONE variable at a time. 3. Before the Experimental Group Lab day, prepare the necessary materials for the Experimental Group Lab. 4. On the lab day, follow the Experimental Group Lab procedure. Enter the data in the table as the lab proceeds. Make as many trials of the procedure as possible in the class period. 5. Graph the results of the Experimental Group Lab in a different color on the same graph as the Control Group Lab. Determine the ET50 and the photosynthetic rate (1/ET50) as in the Control Group Lab. 6. Clean up. Lab Write-up Make a formal write-up of the Rate of Photosynthesis Lab. Follow the guidelines! Discussion Question: 1. Summarize the Experimental Group Labs of others in the class (give the variable and the results for each) and explain if the results of each would be what one would expect from what is known about photosynthesis.



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Part II Measuring Photosynthetic Rate in Spinach Leaf Disks
Microsoft Word - Spinach photo lab lah.doc