Light and Photosynthesis

Photosynthesis Lab Answer Key

Vocabulary: carbon dioxide, chlorophyll, glucose, limiting factor, nanometer, photosynthesis, wavelength

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get students thinking. Students are not expected to know the answers to the Prior Knowledge Questions.]

• To survive, what gas do we need to breathe in? Oxygen

• Where is this gas produced? In plants

Gizmo Warm-up
During photosynthesis, plants use the energy of light to produce glucose (C6H12O6) from carbon dioxide (CO2), and water (H2O). Glucose is a simple sugar that plants use for energy and as a building block for larger molecules.

A by-product of photosynthesis is oxygen. Plants use some of the oxygen they produce, but most of it is released. In the Photosynthesis Lab Gizmo™, you can monitor the rate of photosynthesis by measuring oxygen production.

1. Observe the left pane closely. What do you think the bubbles are? Oxygen

2. Select the BAR CHART tab. On the graph, notice the Oxygen production bar. Move the Light intensity slider back and forth. How does light intensity affect oxygen production?

Up to 40%, increasing the light intensity increases the oxygen production. Beyond 40% there is no effect.

3. Experiment with the vertical Temperature slider (upper left) and the horizontal CO2 slider.

A. How does temperature affect oxygen production? Oxygen production is maximized around 25° C. Oxygen production goes down when temperature is too hot or too cold.

B. How does CO2 level affect oxygen production? Up to about 3-4%, increasing CO2 production increases oxygen production. Beyond about 4%, there is no change in oxygen production as CO2 level is increased.

C. How does oxygen production relate to the rate of photosynthesis? A greater flow of oxygen corresponds to a higher rate of photosynthesis.

|Activity A: |Get the Gizmo ready: |[pic] |
| |Be sure that the BAR CHART tab is selected. | |
|Ideal conditions |Turn on Show numerical values. | |

Question: In the Gizmo, what are the ideal conditions for photosynthesis?

1. Form hypothesis: During photosynthesis, light energy is used to synthesize carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2). The complex series of chemical reactions is summarized by the following formula:

6CO2 + 6H2O + light energy ( C6H12O6 + 6O2

In the Gizmo, what light intensity and CO2 level do you think will maximize the rate of photosynthesis? Answers will vary.

2. Experiment: Use the Gizmo to find the ideal000000000000000000000000 conditions for photosynthesis. Use any method you like. When you think you have the answer, list the conditions below.

|Temperature |Light intensity |CO2 level |Oxygen production |
|23.0° C to 25.0° C |79% to 90% |5.7% to 10% |51.8 mL/hr |

3. Revise and repeat: One way to test if you’ve found the ideal conditions is to change each variable slightly from the value that you recorded above. If the oxygen production decreases with each change that you make, it is likely you have found the ideal conditions. If a small change causes oxygen production to increase, continue to experiment.

If necessary, revise your numbers in the table above.

4. Think and discuss: Think about the process of finding the ideal conditions.

A. Why would it be hard to find the ideal light intensity if the temperature were very hot or cold?

If the temperature were extremely hot or cold, then oxygen production would be near zero no matter what light intensity value you use.

B. Why would it be hard to find the ideal CO2 level if the light intensity were very low?

If the light intensity were very low, then oxygen production would be near zero no matter what CO2 level you use.

|Activity B: |Get the Gizmo ready: |[pic] |
| |Select the COLOR tab and the BAR CHART tab. | |
|Colored light |Set the Temperature to 24°C, the Light intensity to 90%, and the CO2 level to 10%. | |

Introduction: Plants use a green pigment called chlorophyll to absorb light and convert its energy into a form that the plant can use. Chlorophyll gives plants their green color.

Question: What color of light is the best for photosynthesis?

1. Observe: The color of a light wave is determined by its wavelength. On the COLOR tab, slowly drag the Light wavelength slider back and forth and observe the effect on oxygen production. How does the color of light affect the rate of photosynthesis?

The light color has a major effect on the rate of photosynthesis. Oxygen production is high for orange and blue light, and low with purple, green, and red light.

2. Form hypothesis: Which color of light do you think will maximize the rate of photosynthesis?

Answers will vary.

3. Gather data: Set the Light wavelength to 400 nm. (The symbol “nm” stands for nanometers. A nanometer is a billionth of a meter.) Visible light ranges from 400 to 700 nm.

On the TABLE tab, click Record data. Then set the Light wavelength to 420 nm, and repeat. Continue recording data in the Gizmo every 20 nm until the wavelength is 700 nm.

4. Make a graph: Select the GRAPH tab and select Wavelength. Sketch the graph in the space at right.

A. Which colors were absorbed best by the plant?

Blue (440 nm) and red-orange (660 nm)

B. Which colors were absorbed worst?

Green (560 nm) and yellow (580 nm)

5. Think and discuss: When we look at a leaf, we see the colors of light that are reflected off its surface. How does this explain the relatively low flow of oxygen in green light?

Green light is reflected from leaves, which is why they appear green. Because green light is not absorbed by chlorophyll, photosynthesis does not work well in green light.

|Extension: |Get the Gizmo ready: |[pic] |
| |Select the WHITE tab and the BAR CHART tab. | |
|Limiting factors |Turn on Show numerical values. | |

Introduction: Photosynthesis requires light, water, and CO2 to work. When one of these factors is in short supply, it is called a limiting factor. Temperature can also be a limiting factor when it is too hot or too cold for photosynthesis to work well.

Question: What is the effect of limiting factors on photosynthesis?

1. Observe: Set Temperature to 24°C, Light intensity to 50%, and CO2 level to 2.0%.

A. Move the Temperature slider up and down. Were you able to increase oxygen production? No (Return the slider to 24°C when finished.) B. Move the Light intensity slider back and forth. Were you able to increase oxygen production? No (Return the slider to 50% when finished.) C. Move the CO2 level slider back and forth. Were you able to increase oxygen production? Yes (Return the slider to 2.0% when finished.)

2. Analyze: In this situation, what was the limiting factor? Carbon dioxide

How do you know? If you increase the CO2 level, oxygen production goes up. Therefore, CO2 is the factor that is limiting oxygen production.

3. Challenge: In each of the situations below, use the Gizmo to find the limiting factor.

|Temperature |Light intensity |CO2 level |Limiting factor |
|25°C |60% |7.0% |Light intensity |
|15°C |20% |2.0% |Temperature |
|30°C |50% |4.0% |Temperature |

4. Think and discuss: Suppose you were a farmer trying to grow plants in a greenhouse. Why would it be important to know what the limiting factor is?

The limiting factor limits how much you can grow. If you increase the supply of the limiting factor (or use a more ideal temperature), your plants will grow more quickly. Also, knowing the limiting factor will prevent you from wasting money on supplies that are not limiting.
-----------------------
[pic]

[pic]