Photosynthesis Lab (adapted from: http://media.collegeboard.com/digitalservices/pdf/ap/bio-manual/bio_lab5- Photosynthesis.pdf ) Background and PreLab: Photosynthesis fuels ecosystems and replenishes the Earth's atmosphere with oxygen. Like all enzyme-driven reactions, the rate of photosynthesis can be measured by either the disappearance of substrate, or the accumulation of products. The equation for photosynthesis is: 6CO 2 + 6H 2 O ------light--------> C 6 H 12 O 6 + 6O 2 What could you measure to determine the rate of photosynthesis? 1) The production of oxygen, which is released as photosynthesis occurs 2) The consumption of carbon dioxide Leaf Structure and Function In this investigation, you will use a system that measures the accumulation of oxygen in the leaf. Consider the anatomy of the leaf as shown below. The leaf is composed of layers of cells. The spongy mesophyll layer is normally infused with gases, oxygen and carbon dioxide. Leaves (or disks cut from leaves) will normally float in water because of these gases. If you draw the gases out from the spaces, then the leaves will sink because they become more dense than water. If this leaf disk is
placed in a solution with an alternate source of carbon dioxide in the form of bicarbonate ions, then photosynthesis can occur in a sunken leaf disk. As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy mesophyll and the leaf becomes buoyant and floats. While this is going on, the leaf is also carrying out cellular respiration. This respiration will consume the oxygen that has accumulated and possibly cause the plant disks to sink. The measurement tool that can be used to observe these counteracting processes is the floating (or sinking) of the plant disks. In other words, the bouyancy of the leaf disks is actually an indirect measurement of the net rate of photosynthesis occurring in the leaf tissue. Pre Lab Questions - these should be completed BEFORE the scheduled lab 1. How can the rate of photosynthesis be measured? 2. Where in the cells of the leaf do you find air spaces? 3. What is the function of the stoma? 4. What will happen if you remove the air from these spaces? 5. How will air return to these spaces? 6. Instead of carbon dioxide, what will be used as the reactant in this lab? 7. List any factors that you think may affect the rate of photosynthesis. Consider environmental factors that you could manipulate during the lab. 8. Watch the video that shows the set-up for the floating leaf disk lab at Bozeman Science. (Search for "bozeman leaf disk lab") a) What is the ratio of water to baking soda you will need for your solution? b) What is the purpose of the syringe? c) Why did Mr. Anderson put a watch glass with water on top of the beaker? d) How will you know when photosynthesis is occurring in your leaf disks?
Part 1: Basic Procedure for Measuring the Rate of Photosynthesis Materials: baking soda, liquid soap, plastic syringes, leaves (spinach or ivy), hole punch, cups or beakers, timer, light source Safety: you will need to wear goggles and an apron for this lab 1) Collect at least 20 leaf disks by punching holes in the leaf (try to get them between the veins) 2) Make a solution of sodium bicarbonate by mixing 300 ml of water to a teaspoon of baking soda (or 100 ml to 1g.) Stir until dissolved. 3) Make a diluted solution of liquid detergent (3 drops of soap to 70 ml of water.) 4) Add one drop of this dilute soap solution to your bicarbonate solution - do not generate suds. 5) Remove the plunger from a large clean syringe. Place 20 leaf disks into the body of the syringe. Be sure the leaf disks are near the tip of the syringe as you reinsert the plunger so as not to damage the disks. 6) Insert the tip of the syringe into the beaker of bicarbonate and soap and fill the syringe 1/3 full of solution. The leaf disks should be floating at this time. 7) Hold the syringe tip upward and expel the air by depressing the plunger carefully. Stop before the solution comes out the tip. 8) Seal the tip of the syringe using the index finger of your left hand and hold tightly. Pull back on the plunger creating a partial vacuum within the syringe. If you have a good seal it should be hard to pull on the plunger and you should see bubbles coming from the edge of the leaf disks. Hold for a count of 10. 9) Simultaneously, release your index finger and the plunger. Some of the leaf disks should start to sink. Tap the side of the syringe or shake gently to break any bubbles on the edges of the disks. 10) Repeat steps 8 and 9 until all the disks sink. Do not overdo theses stips. You have been successful if the disks sink to the bottom. Don t repeat just to be sure as it is possible to damage the cells of the leaves. 11) Remove the plunger from the syringe and pour the solution containing the disks into 2 beakers and add the remainder of your solution equally to both beakers. There should be 10 disks per beaker. Make sure they sink to the bottom. 12) Cover one of the beakers to block the light from the leaf disks. Place the second beaker under a light source, approximately 6-8 inches below the light. The lights are best help held by clamping onto a ring stand. Begin timing as soon as the light is turned on. Record your observations in the data table. 13) Notice what is happening to the leaf disks as photosynthesis proceeds. Continue to record your observations in the data table. After each time check, tap the sides of the beaker to make sure the disks are not sticking to the beaker walls. Note: Check the covered beaker quickly to avoid light exposure.
Data table: Time (min) # of floating disks # of floating disks (control) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Analyzing Data 1) Graph your data for the experimental group.
2) What is the variable in the experiment? 3) What questions did you answer in this experiment? 4) Explain why it was important to keep one beaker covered during the experiment. 5) Describe and explain the relationship between the number of disks floating and time, as shown on the graph. 6) Did any leaf disks float in the dark treatment? If so, what may explain this result? 7) What process cannot occur in the dark treatment? 8) Explain the changes that occurred within the leaf tissue that allowed the leaf disks to rise to the surface. Part 2: Design and Conduct Your Own Investigation Now that you have mastered the floating disk technique, you will design an experiment to test another variable that may affect the rate of photosynthesis. You will collect data, analyze data and present your findings in the form of a LAB REPORT. Here are some factors that you might investigate (though you are not limited to these) light intensity, light color, concentration of sodium bicarbonate, different types of leaves (avoid leaves with hairy surfaces), temperature, fertilizer (nitrates), size of leaf Discuss with your group what you will be investigating and how you will design your experiment. Lab Report Guidelines 1) Introduction - stating the problem or question you will be investigating, your predicted outcomes (hypothesis) and relevant information on photosynthesis 2) Procedure - describe how your experiment was set up, include materials (summarize, do not copy procedures from lab guide) 3) Observations including data tables and graphs - present your data in an easy-to read format, include graphs. 4) Analysis and Conclusions - did your experiment answer your question, what did you learn, how could your experiment be improved upon?