This investigation examines the phenomenon of phototaxis, movement away from or toward light. Students may be familiar with time-lapse photography of plants growing toward light, a phenomenon known as phototropism. For this lab, students will gather observations of algae in full light, change habitat conditions by reducing light, and apply their observations to support or refute a claim concerning algae survival with regard to light.
PE 3-LS4-3. Construct an argument with evidence that in a particular habitat some organisms can survive well, some survive less well, and some cannot survive at all.
Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as safety glasses or goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. Ensure that students wash their hands before and after conducting the lab exercises.
In most municipalities, the algae may be washed down the sink after students have completed the initial investigation. The culture tubes can be washed in hot water without detergent if they will be used again for cultures. Local regulations may require treating cultures with a 1:10 bleach solution before disposal. Check your school and local regulations.
The day before the lab activity, prepare a culture tube of algae per group or pair of students. To prepare the tubes:
Label the cup with your name or group number.
Use the magnifying glass to look at algae. Do not shake up the tube.
Draw your observations. Pay attention to algae color and position in the tube.
In 2 or 3 sentences, describe what is in the tube.
Wrap the tube in aluminum foil. The top of the foil should be slightly below the top of the water. The bottom of the tube should be covered.
Recap the tube.
Place the tube in the cup. Put the cup in a window or under a light source for 30 to 45 minutes. Do not shake the tube.
Answer the following questions.
a. Name the organisms and their important traits.
Chlamydomonas, Volvox, Spirulina. All live in water, and all make their own food by photosynthesis. Chlamydomonas and Volvox can swim. Spirulina can’t swim.
b. Identify the habitat and the important characteristics of the habitat.
The habitat is everything in the test tube. It is water at room temperature. Some light can get through the tube.
c. In this experiment, what is the system being investigated?
The system is the light, habitat, and organisms.
d. What do algae need to survive?
Light, carbon dioxide, some other nutrients, water, and a fairly warm temperature.
Get your cup and tube from the window or light bank. Be careful not to shake the tube.
Use the magnifying glass and look at the tube of algae carefully. Do not shake it.
Draw your observations. Pay attention to the color and position of the algae in the tube
In 2 or 3 sentences, describe the algae in the tube in detail. Include any changes in location or depth of color of the algae.
Clean Up: Tighten the cap on the tube of algae and place it in the cup. Return the cup and tube to the location your teacher identified. Throw away the aluminum foil.
Explain how the habitat changed.
The habitat changed when the aluminum foil was placed around the tube. The foil is blocking out the light from the sides and bottom of the algae tube. The only place light can enter is the top of the tube.
How did the change affect the algae? Use your observations
Lack of light made Chlamydomonas and Volvox swim to an area where they could get enough light to photosynthesize. Chlamydomonas and Volvox swim toward the light. The bright yellow-green band of Chlamydomonas got wider, and I could see the Volvox swimming toward the top of the tube.
Which algae adapted to the habitat change the best? What evidence supports your claim.
Chlamydomonas and Volvox adapted to the habitat change the best. Within 30 to 45 minutes they were both swimming toward the light so they could continue photosynthesis and live.
Which algae was least adaptable? Support your claim with observations.
Spirulina did not seem to react to the lack of light at all. It did not change its position from the bottom of the tube.
Use your observations to make an argument for or against this claim: For algae to survive well, they must be able to move up and down through the water column to absorb light for photosynthesis. Algae that cannot move through the water column easily are less likely to survive.
Points students should make in their argument:
a. Algae must have light to photosynthesize.
b. Without photosynthesis, algae have no food.
c. As the amount of light in the habitat changes, algae must move within the water column so they have enough light to photosynthesize.
d. Algae that cannot move or move very quickly will not survive well because they will not be able to make enough food to survive.
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