Investigating Respiration and Photosynthesis in Plants
OBJECTIVES:
In this laboratory exploration, you will
· Use a DO meter to measure the dissolved oxygen level of water.
· Use the inferences about the amount of oxygen in the water to make conclusions about whether plants consume or produce carbon dioxide in the light.
· Use the inferences about the amount of carbon dioxide in the water to make conclusions about whether the plant is respiring or photosynthesizing more in the light
introduction
Dissolved Oxygen (DO) is essential for the maintenance of healthy lakes and rivers. The presence of oxygen in water is a positive sign of a healthy body of water butthe absence of oxygen is a signal of severe pollution. Rivers range from high to very low levels of dissolved oxygen - so low, in some cases, that they are practically devoid of aquatic life.
Most aquatic plants and animals need oxygen to survive. Fish and some aquatic insects have gills to extract the necessary oxygen from the water. Some aquatic organism, like pike and trout, require minimum-to-high levels of dissolved oxygen to live. Other animals, like carp and catfish, flourish in waters of low dissolved oxygen. Waters of consistently high dissolved oxygen are usually considered healthy and stable ecosystems capable of supporting many different kinds of aquatic organisms.
Some of the dissolved oxygen in water comes from the atmosphere through diffusion. Other sources of DO include algae and rooted aquatic plants who deliver oxygen to water through photosynthesis:
6 H2O + 6 CO2 (g) + sunlight energy à C6H12O6 + 6O2(g)
Algae and aquatic plants also use up dissolved oxygen through respiration:
C6H12O6 + 6O2 (g) à 6 H2O + 6 CO2 (g) + energy
Measuring Dissolved oxygen in oceans, rivers, wetlands, etc is therefore common practice for scientists attempting to better understand the health of aquatic environments. It is also common practice when trying to gain a better understanding of global biogeochemical cycles such as the carbon cycle. If we can measure DO in an aquatic environment we can then calculate both photosynthetic rates, respiration rates and ultimately have a better understanding of the sources and sinks of CO2.
In this lab, we will begin our exploration by measuring dissolved oxygen levels in the water surrounding our familiar aquatic plant, Elodea. As O2 is both produced in photosynthesis and consumed during cellular respiration we’ll be able to estimate the relative levels of photosynthesis and respiration under varied conditions in the lab.
Based on what you know about photosynthesis and respiration, which would you expect to be more prevalent in a plant under light conditions compared to dark conditions? What would happen to DO levels in light compared to dark conditions? These are questions you will address as part of this exploration.
MATERIALS (per group)
4 beakers / 2+ sprigs of Elodea / scale1 O2 probe / wax pencil / 1 weigh boat
well water
PROCEDURE
1. Work in groups of 4. To work efficiently, split up the work!
2. Obtain and label 4-250 ml beakers. Using a wax pencil, label them “Light-Control”, “Light-Experimental”, “Dark-Control”, and “Dark-Experimental”. Also label all beakers with your group name.
3. Obtain an O2 meter, and turn it on to warm up.
4. Fill each beaker with well water (about ½ full).
5. Obtain 2-4 large sprigs of Elodea (or other aquatic plant). You will eventually divide your sprigs between the “Light-Experimental” and “Dark-Experimental” beakers. Obtain enough plant to fill the water in the test tube. Pat the plants dry with a paper towel, weigh them and record the data in an organized table in your lab notebook.
6. Place 1-2 sprigs in beaker “Light-Experimental”, and the other sprigs in beaker “Dark-Experimental”. The sprigs should be submerged (under water).
7. Follow the directions provided on the O2 meter, placing it into the beaker“Light-Control” and gently swirl briefly to allow water to move past the probe’s tip. When the reading stabilizes, record the O2 value in a table in your lab notebook. Repeat this process for your “Light-Experimental” tube.
8. Place the probe into beaker“Dark-Control” and gently swirl briefly to allow water to move past the probe’s tip. When the reading stabilizes, record the O2 value. Repeat this process for your “Dark-Experimental” beaker.
9. When all readings have been taken, rinse the O2 probe with distilled water, replace the cap, and turn off the meter.
10. Place beakers “Light-Control” and “Light-Experimental” under light conditions in the light rack. Leave the beakers for 40 minutes.
11. Place beakers “Dark-Control” and “Dark-Experimental” under dark conditions in the cabinet at the front of the lab. Leave them for 40 minutes.
12. After 35 minutes, turn on your O2 meter, so that it has time to warm up.
13. After 40 minutes, measure O2 for each of the 4 beakers. Record the data in the table in your lab notebook.
14. Clean up by returning the well water and Elodea to the Elodea container, and putting your beakers, wax pencil, squirt bottle and the O2 meter back at the front of the room. Wipe up any spilled water, and throw away paper towels and the weigh boat.
15. Before leaving the lab, be sure to record both your own data and class data in the table below.
Optional Extension
If time and conditions permit, your instructor may ask you to bring your O2 meters out into the wetland and collect DO data at various sites. Be sure to capture this work in your lab notebook, too!
PROCESSING THE DATA
♦ Calculate the change in O2, D O2, for your tubes. Do this by subtracting the starting O2 from the ending O2. Record your results in your data table.
♦ Calculate the Corrected D O2. You must correct for any changes that occurred to the O2 of the water that were NOT the direct result of photosynthesis or respiration by Elodea. Do this by subtracting the D O2 of the control tube from that of the Elodea tube. Record this “Corrected D O2”.
♦ Calculate the Corrected D O2/g. Do this by dividing the “Corrected D O2” for the Eldoea tube by its weight, in grams.
♦ Report your findings to the class. Report only your Corrected D O2/g on the table on the wipe board at the front of the classroom. Neatly copy all class data from the wipe board to your notebook.
♦ Calculate the “Average Corrected D O2/g” from the class data for both Light and Dark Elodea beakers. For each condition (Light or Dark), find the average by adding all class data (across the table) and then dividing by the number of groups.
POST-LAB: Your Post-lab report this week should include a hypothesis and prediction for the plant in the light condition, and for the plant in the dark condition. In addition, provide a graph of the average class data (Corrected D O2/g), using graph paper or a computer. Recall that in all sciences, we almost always place the independent variable (the one you selected and manipulated) on the horizontal, or X-axis, and the dependent variable (the one that depends on your manipulations) on the vertical or Y-axis. Be sure to correctly label the x- and y-axes. And finally, include a brief paragraph explaining your graph. Do these results support your hypothesis? Why or why not? Are the class averages consistent with the data your group generated? Why or why not?
Lab Notebook Check: Your lab notebook entry this week should contain a title and objective for this lab. As always, you should include detailed procedures. (You may simply tape them in from this handout if you like.) In addition, please prepare an organized data table. The data you’ll collect might be organized like this:
Sample Table: Data from respiration/photosynthesis experiment with Elodeabeaker / treatment / weight (g) / starting O2 / ending O2 / D O2 / Corrected D O2 /
“Light-Control” / control
“Light-Experimental” /
Elodea
“Dark-Control” / control“Dark-Experimental” /
Elodea
Feel free to use this table, or create your own as long as your data is organized and easy to interpret!