Measuring the Effect of Light
Intensity on Photosynthesis
Photosynthesis captures energy from sunlight. Plants, algae, and some bacteria use the energy captured during photosynthesis for their metabolic reactions. During photosynthesis in plants, chlorophyll and enzymes in leaves convert certain wavelengths of light into chemical energy. A simple equation can be used to represent photosynthesis. Enzymes and Chlorophyll
light energy CO2 H2O carbohydrate O2
In this investigation you will examine the relationship between the amount of light energy available and the rate of use of carbon dioxide by a plant in the process of photosynthesis.
What is the relationship between light intensity and the rate of photosynthesis?
Read the entire investigation. Then, work with a partner to answer the following questions.
1. What are the variables in this experiment? Identify the manipulated and responding variables and two controlled variables.
The manipulated variable is light intensity. The responding variable is pH. Controlled variables include the initial pH, the size of the plant sprigs, and time.
2. How will you provide carbon dioxide to the evergreen sprigs? Carbon dioxide is provided by blowing into the BTB solution. 3. How will you know whether the carbon dioxide has disappeared from the solutions in the test tubes?
When the carbon dioxide disappears, the BTB solution changes from yellow to blue. 4. If the carbon dioxide does disappear from the solutions in the test tubes, how will you know whether it was consumed by
photosynthesis or simply evaporated into the air?
Greater loss of carbon dioxide from test tubes closer to the light and containing sprigs supports the idea that photosynthesis is removing the carbon dioxide.
5. How do you expect the intensity of light to affect the color of the BTB indicator? Predict the result you expect for this experiment. In test tubes that contain sprigs, the color should change to blue most quickly closer to the light. Biology Laboratory Manual A/Chapter 8 91
Name______________________________ Class __________________ Date ______________ © Prentice-Hall, Inc.
You may want to refer students to Section
8–3 in the textbook for a discussion of the
relationship between light intensity and
Time required: 40 minutesMaterials (per group)
8 large test tubes dropper
4 test-tube racks straw
250-mL beaker 4 sprigs of an evergreen (such as yew)
distilled water paper towel
bromthymol scalpel or single-edged razor blade
blue indicator solution meter stick
or carbon dioxide probe floodlight
Use caution when working with chemicals, as they may irritate the skin or stain skin or clothing. Put on a laboratory apron and plastic gloves and wear safety goggles. Be careful to avoid breakage when working with glassware. Be careful when handling sharp instruments. Return or dispose of all materials according to the instructions of your teacher. Note all safety symbols next to the steps in the Procedure and review the meaning of each symbol by referring to Safety Symbols on page 8. Procedure
1. Work in groups of two or four students. CAUTION: Wear safety goggles, gloves, and a lab apron. Obtain eight large test tubes and four test-tube racks. Place two test tubes in each test-tube rack. 2. Using a beaker, fill each of the eight test tubes with distilled water to about 4 cm from the top.
3. If you are using a carbon dioxide probe, see your teacher for instructions. Bromthymol blue indicator solution (BTB) indicates the presence of carbon dioxide by turning yellow. Add BTB to one test tube, one drop at a time, stirring with a straw. Count the drops you add. Stop when the water changes color to a pale blue. Then add the same amount of BTB to each of the other seven test tubes. Stir each test tube with a straw.
4. Using a straw, blow gently into each test tube of BTB-water solution to add carbon dioxide to the...
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