Lab 5: Photosynthesis and Cell Respiration

Photosynthesis Lab:

Laboratory 4, Biology 2011

Spurthi Tarugu, Kavinmozhi Caldwell, Claudia Osorio

Abstract
Photosynthesis is the process by which plants convert light energy into chemical energy. Plants need sunlight, CO2 and H2O to make sugar, which is the site of energy storage. This process takes place in the chloroplasts. The by-product of photosynthesis is oxygen. In aquatic plants, the photosynthetic rate, which is the rate of photosynthesis, can be measured by bubble production.

Introduction
Observing the bubble production of the leaves of Elodea, an aquatic plant, is a simple way to measure the photosynthetic rate of a plant; that is the purpose of this experiement. We will vary the light intensity by observing the plants at different distances from a constant light source. Then, since CO2 is necessary for photosynthesis, we will be varying the amounts of CO2 in the solution by adding sodium bicarbonate to see the effect of this gas on the photosynthetic rate. We hypothesize that if we add sodium bicarbonate and maximize light intensity, the photosynthetic rate will be higher than low light intensity and no sodium bicarbonate.

Methods

By observing bubble production of Elodea, we are able to measure the photosynthetic rate of the plant. First we need to make sure we have four test tubes, a test tube rack, and tap water, sprigs of Elodea, scissors, Bromothymol, beaker, light source, and measuring tool like a meter stick. Set up the four test tubes in the test tube rack and fill them with tap water and label each of the test tube with a number (1, 2, 3 and 4).  Next take four sprigs of Elodea and cut equal-sized pieces (a few cm long) from the growing tips of each. Then place each of the four pieces in each of the four test tubes with the cut end up. Next fill up the beaker with cool tap water and place it right in front of the light source (preferably a photo flood lamp).Then measure 25 cm from the light and place the test tube rack there. The beaker will insulate the tubes from excess heat.

Leave the test tube rack at its position for five minutes. After five minutes, watch for bubbles at the cut end of the Elodea pieces especially at the bottom of the leaves. Then at 5-minute interval count the number of bubbles that formed in each test tube. As you tally the bubbles that form, record your data. After the counting interval, move the test tube rack to a 50 cm spot from the light source and repeat the same steps as you did after you placed the test tube 25 cm from the light source. After you are finished repeating the process, repeat it once more except this time move the test tubes 75 cm from the light source.

After you record your data for the bubble formation of test tubes 75 cm away from the light source, replace the water in test tubes 1 and 2 with fresh water.  Clean out the water in test tubes 3 and 4 and replace it with 0.5% of sodium bicarbonate.  Either in test tube 1 or 2, place one drop of Bromothymol to test for carbonic acid. If that solution in that test tube turned yellow/green we know it’s positive. After putting these four test tubes back into the test tube rack, place the rack 25 cm from the light source once again. Let the rack sit in its position for 5 minutes before you start counting the bubbles that occur at a 5 minute interval time. After you record your results for bubble count, clean up and dispose the Elodea pieces.

Results

Distance from light source in cm

(intensity in foot-candles if known)

Tube #

25 cm

50 cm

75 cm

1

4

2

0

2

6

4

1

3

3

1

0

4

7

6

4

SD

1.82574

2.21736

1.89297

Distance from light source is 25 cm

(intensity in foot-candles if known)

Freshwater

Sodium Bicarbonate

Tube #

# of Bubbles

Tube #

# of Bubbles

1

3

3

6

2

3

4

6

Analysis
There were several observations that we analyzed in this lab. First and foremost, we insulated the tubes from the heat of the light source to control the temperatures of the plants. As we moved the plants farther from the light source, the bubble formation rate decreases because light intensity decreased. During the second part of the lab with sodium bicarbonate, we saw that adding sodium bicarbonate increased the mean bubble formation rate. Also, because bubble formation was maximized when the light intensity was high, we realized that light reactions are directly involved in oxygen evolution.

Conclusion
By studying the concepts of photosynthesis and cellular respiration in this lab, we are able to clearly see how the light source affects the formation of bubbles from the leaves of the plant. The amount of bubbles released from each sprig of Elodea is indirectly related to the distance from the light source. As the distance from the light source increased, the formation of bubbles decreased. We have proved our hypothesis correct. When we added sodium bicarbonate and maximized light intensity, the photosynthetic rate was higher than when we observed a low light and no sodium bicarbonate.

Literature Cited
“LabBench.” Prentice Hall Bridge Page. Web. 3 Nov. 2011. <http://www.phschool.com/science/biology_place/labbench/lab5/intro.html&gt;.