In This Laboratory Exploration, You Will

Investigating Respiration and Photosynthesis in Plants

OBJECTIVES

In this laboratory exploration, you will

Use probes to measure the oxygen and carbon dioxide concentrationsin a chamber.
Use the inferences about the amount of oxygenand carbon dioxide in the chamber to make conclusions about whether the plant is respiring or photosynthesizing more in the light.
Reinforce concepts about respiration and photosynthesis.

introduction

♦Recall that plant cells, like animal cells, use their mitochondria to produce energy in the form of ATP. This process is known as cellular respiration. We can measure rates of respiration in several ways, all of which come from the basic equation of cellular respiration:

C6H12O6 + 6O2  6 H2O + 6 CO2 + energy

(glucose) (oxygen) (water) (carbon dioxide)

Thus, if we wanted to know how much respiration was occurring in an organism, we could measure any of the following:

Rates of the disappearance (consumption) of glucose
Rates of the disappearance (consumption) of oxygen
Rates of the production of water
Rates of the production of carbon dioxide
Rates of the production of energy

Because we are measuring respiration in living organisms, it is not easy to measure the consumption of glucose or the production of water molecules or energy. Also, remember that some of the energy produced is captured as ATP (and some is lost as heat). If we want to measure respiration, the easiest things to measure, is either the consumption of oxygen or the production of carbon dioxide. In this laboratory exploration, we will concentrate on the consumption or production of these two gases.

♦As animals, we acquire the sugars we use in our mitochondria by ingestion (eating!). Most plants, however, produce their own sugars using carbon dioxide and sunlight in a process known as photosynthesis. We can measure rates of photosynthesis in several ways, all of which come from the basic equation of photosynthesis:

6 H2O + 6 CO2 + sunlight energy  C6H12O6 + 6O2

(water) (carbon dioxide) (glucose) (oxygen)

Thus, if we wanted to know how much photosynthesis was occurring in an organism, we could measure any of the following:

Rates of the appearance (production) of glucose
Rates of the appearance (production) of oxygen
Rates of the disappearance (consumption) of water
Rates of the disappearance (consumption) of carbon dioxide
Rates of the consumption of light energy

Of these possibilities, again the easiest to measure is the appearance of oxygen or the disappearance of carbon dioxide. For this lab exploration, we will measure the concentration of both these gases to estimate the rate of photosynthesis and respiration. Remember, however, that plants are made of cells and so they must also undergo cellular respiration. Therefore, we will be measuring both respiration and photosynthesis by measuring changes in oxygen levels under light conditions. 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 you expect to happen to oxygen levels in light compared to dark conditions? These are questions you will address as part of this exploration.

♦Prelab: Complete the pre-lab worksheet at the end of this document. Like all prelabs, this is due at the start of class on your lab day.

MATERIALS (per group)

LabQuest / Vernier O2 Gas Sensor / Vernier CO2 Gas Sensor
BioChamber 2000
Calibration Stick / Spinach / Ring Light Apparatus

PROCEDURE

1. Work in groups of 4. To work efficiently, split up the work!

2. Connect the white connectors from the CO2 and O2 sensors to the top of the LabQuest. Make sure the switch on the CO2 Gas Sensor is set to low (0-10,000 ppm) setting. The O2 sensor must remain upright during the entire experiment!

3. Change the units to ppt by tapping Sensors at the top of the screen, thenChange Units -> CO2Gas Sensor -> ppt. Repeat this process to select ppt as the units for the O2 Gas Sensor.

4. Only the O2 Sensor will need to be calibrated for this experiment. To do this tap Sensors -> Calibrate -> Oxygen Gas -> Calibrate Now. Have your partner use the Calibration Stick to hold down the CAL button on the O2 Sensor. While this is happening enter 0.0 for the Reading 1 Known Value and tap Keep. Your partner can release the CAL button the probe. Enter 20.8 for the Reading 2 Known Value and tap Keep then OK. Your O2probe is now calibrated.

5. Tap on the data collection box on the right side of the home screen. Change the rate to 15 and the length to 15 minutes.

6. Weight out 40 grams of spinach and place them into the BioChamber 2000. Insert the CO2 and O2 Gas Sensors into the top of the respiration chamber. Make sure they fit snuggly so no air can escape.

7. Turn the ring light on and place the BioChamber 2000 in the center of the light. The lamp should be on for 5 minutes prior to beginning data collection. After 5 minutes start data collection by pressing the Start button on the LabQuest.

8. After 15 minutes the data collection will end. Turn off the light and sketch copies of the CO2 and O2 vs. time graphs in your lab notebook.

9. Perform a linear regression to calculate the rate of respiration / photosynthesis for the CO2 Gas Sensor.

Choose Curve Fit from the Analyze menu and select CO2 Gas.
Select Linear as the Fit Equation. The linear-regression statistics are displayed to the right of the graph for the equation in the form y = mx + b. X
Record the absolute value of the slope, m, as the rate of respiration / photosynthesis for the CO2 Gas Sensor in an organized table in your lab notebook.
Select OK.

10. Repeat 9a-d for the O2 graph.

11. Place the BioChamber 2000, probes and the LabQuest handheld into a dark cabinet for 5 minutes to equalize. After 5 minutes start data collection by pressing the Start button on the LabQuest.When prompted, select Discard to start a fresh graph.

12. Data will be collected for 15 minutes and stop automatically. Take the apparatus out of the cabinet and sketch both CO2 and O2 graphs in your lab notebook.

13. Repeat steps 9 and 10 to determine the rate (slope) of photosynthesis.

14. Remove the plant leaves from the respiration chamber and return them to the container in the front of the classroom. Clean and dry the BioChamber 2000.

15. Turn off the LabQuest by holding down the power button and tap Discard to completely turn off the machine. Neatly wrap up all cords and return the CO2 probe and the BioChamber to the lab box. Return the O2 probe to the probe rack at the front of the classroom. Plug the LabQuest back into its charging dock and make sure the stylus lanyard is not blocking the power connector.

DATA

Record the rate of O2 production or consumption (ppt/s) and CO2 production or consumption (ppt/s) in the class data sheet at the front of the room. Be sure you include the final class averages in your lab notebook as you’ll need these for your postlab! (Note that the rate of production or consumption is simply the slope of the line from your graphs!)

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 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? What biological processes were responsible for the changes you observed? What types of factors might have influenced these rates? Please remember that college-level post-labs should generally be typed.

Lab Quiz Preparation: For the lab quiz, be able to write clear and effective hypotheses and predictions for these experiments. Be prepared to graph or interpret results similar to those you’ve obtained today.

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:

Name: ______

Biology 211

Pre-Lab Assignment: Investigating Respiration and Photosynthesis in Plants

Pre-Lab Worksheet

Do plants have mitochondria? ______
Why do plants do cellular respiration? To produce ______.
Do plants do cellular respiration in light? ______
Do plants do cellular respiration in the dark? ______
Do plants photosynthesize in the light? ______
Do plants photosynthesize in the dark? ______
Why do plants photosynthesize? To produce ______.
What are two possible ways a plant might use the product you identified in “g”? ______
Given your answer to the previous questions, what would happen to a plant cell that did not photosynthesize and why? ______

j. Consider a plant in optimal light conditions (during daylight hours) and answer the following three questions.

Is it possible for this plant to consistently do more cellular respiration than photosynthesis? Why or why not? ______

Is it possible for this plant to consistently do the same amount of photosynthesis and respiration? (Beware! The sun will set eventually, leaving our plant in the dark!) Why or why not? ______

Is it possible for this plant to do more photosynthesis than respiration under light conditions? Why or why not? ______

k. So, given your answer to the previous questions (j), what do you predict will happen to the oxygen concentration in the light, and why? ______