Carbon Dioxide, Photosynthesis, and Cellular Respiration

Introduction

Before beginning this exercise, it is necessary to understand that photosynthesis uses light energy to synthesize carbohydrate from carbon dioxide. The equation is below.

6CO₂ + 6H₂O + Energy ® C₆H₁₂O₆ + 6O₂

This process requires light for some of the reactions.

It is also necessary to understand that the plant is constantly undergoing cellular respiration according to the equation below.

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + Energy

Notice that these two equations appear to be opposites. 

When plants are exposed to light, photosynthesis and cellular respiration both occur. In the dark, only cellular respiration occurs. 

We will study photosynthesis in an aquatic plant (Elodea) We can measure the rate of photosynthesis and cellular respiration by measuring the amount of CO₂ given off or taken up by the plant. 

Carbon dioxide combines with water to form carbonic acid (H₂CO₃) which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The pH drops due to the presence of hydrogen ions.

          CO₂ + H₂O  H₂CO₃ H⁺ + HCO₃^- 2H⁺ + CO₃^-2 

Respiring organisms release CO₂ into the water, causing the pH to decline. During photosynthesis, Elodea takes up CO₂ and the pH increases.

Using the pH Probe

Set up the pH probe by following the instructions given in the link below.

Instructions for setting up and using the pH probe.

Procedure for Observing Photosynthesis and Cellular Respiration

After the pH probe is set up (see the step above), obtain two large test tubes and a 250 ml beaker for this experiment. One test tube will be used to hold Elodea, a common aquarium plant. The second test tube will be used to hold bean or corn seeds. 

A beaker or a test tube rack can be used to hold the two test tubes as shown in the photograph below.

Rinse the tubes and the two stoppers thoroughly with tap water to remove any traces of contaminants that might affect pH. A final thorough rinse can be obtained by putting some tap water in each tube, pushing the stoppers onto the top, and shaking the tubes vigorously. Discard the rinse water.

Fill each tube with tap water to 3 cm from the top.

Cut a a sprig of Elodea that is long enough to fill the entire length of a tube but not protrude from the water. The length of the stem can be adjusted, if necessary, by cutting a piece from the base of the stem. Put the plant in one of the tubes. 

Add 30 germinated bean or corn seeds to the second tube. 

Cap both tubes with a rubber stopper and place them in the test tube rack. 

Fill a one- or two-liter beaker to near the top with water and place it between the lamp and the tube containing Elodea. The water will serve as a heat sink to prevent the Elodea from getting too hot. Adjust the placement of the tube containing elodea so that it will receive as much light as possible. It is not necessary to illuminate the tube containing the seeds.

Click on the photograph to view an enlargement.

Mix the water in the Elodea tube by inverting the tube several times using a stopper to prevent water from spilling out. 

Rinse the pH probe thoroughly and then measure the pH of the water in the Elodea tube. Wait until the reading stabilizes before recording your results. Record the pH to the nearest 0.01 units in the space provided on the answer sheet.

Repeat this procedure for the tube with the germinated seeds.

Turn on the lamp and begin timing the experiment.

Measure the pH of the water in each tube every 10 minutes for one hour.

The probe should be thoroughly rinsed each time it is moved from one solution to another.

When finished, answer the questions on the answer sheet.