LAB: OBSERVING THE RELATIONSHIP BETWEEN PHOTOSYNTHESIS & RESPIRATION

Pre-lab discussion:

Green plants bask in sunlight. They use the energy in the sun's rays to make food. The production of food also requires raw materials such as carbon dioxide and water. When plants synthesize food, more precisely carbohydrates, with the aid of the energy in light, this is known as photosynthesis. Plants and animals use the carbohydrates plants make as a source of cellular energy. To release the energy contained in the bonds of carbohydrate molecules, the chemical reactions of photosynthesis must be reversed. The process in which energy is released from food is called respiration. Respiration also produces waste products, carbon dioxide and water, which are the same substances that served as raw materials for photosynthesis.

In water, carbon dioxide dissolves to form a weak acid. As a result, an acid-based indicator such as bromthymol blue can be used to indicate the presence of carbon dioxide. Without carbon dioxide, the indicator remains blue; with the addition of carbon dioxide, it looks yellow-green. In this laboratory investigation, you will use bromthymol blue to investigate whether photosynthesis is occurring.

Problem:

Under what conditions does photosynthesis occur?

Materials:

2 small test tubes

2 #0 rubber stoppers

250 mL flask

bromthymol blue solution

2 pieces of Ascaris

light source

drinking straw

Procedure:

1. Using a graduated cylinder, measure out 50mL of bromthymol blue solution and pour into a beaker. CAUTION: bromthymol blue is a dye and can stain hands and clothing

2. Insert one end of a drinking straw into the bromthymol blue. Gently blow through the straw and keep blowing until there is a change in the appearance of the bromthymol blue solution. Record your visual observations in the data table on back.

3. Place a sprig of elodea into each test tube (approximately 3 cm long).

4. Pour your liquid indicator solution into each of the test tubes until each is full. Stopper and label the test tubes.

5. Place one test tube in the dark for 24 hours. Place the other test tube on a sunny windowsill for the same amount of time. Artificial light may be used to supplement the sunlight.

LIGHT DARK

6. After 24 hours, examine each test tube. Note any change in the appearance of the bromthymol blue solution and record your observations in the data table below.

Observations: Data Table

Bromthymol Blue Color
after you exhaled into both test tubes:
after 24 hours in the LIGHT:
after 24 hours in the DARK:

Analysis & Conclusions:

1.What substance was released into the bromthymol blue solution when you exhaled into it?

2.Explain why the color of the bromthymol blue changed after you exhaled into it.

3.Why was Elodea placed in both flasks?

4.Which flask is the control?

5.Describe an additional control you might use if you were to do this experiment again.

6.Describe the results for the two test tubes and explain why they are different.