Part 1
In front of students, place a moist food item such as bread or cheese into a sealed container. Ask, “What will happen if we leave this container sealed for a month on the shelf?” Students will probably respond with ideas of it rotting, mold growing, or bacteria growing. Ask, “Would you be surprised if a chicken grew out of the food?” Probe as to why the chicken growth would surprise them. Ask, “If we wanted a chicken to grow out of the container what would we need to do?” A chicken egg would need to be placed into the container.
Show students the How Do These Organisms Begin? Chart on a projector or whiteboard, revealing only the names of the organisms (S-7-4-1_How Organisms Begin Chart.docx). Have students discuss in pairs what each of the organisms start with. After students have talked with their partners, reveal the answers on the chart. Try to elicit specific examples of the following (maybe students have eaten caviar or found turtle eggs) while students share.
Ask students, “Are there any living things that come from nothing/empty space?” (no) To the bottom of the list, add the words and a brief definition: “Spontaneous generation―From nothing.” Explain the hypothesis of spontaneous generation, which stated that living things could arise from nonliving things. For example, insects could appear on a dead, rotting log or tadpoles could arise from muddy soil.
Explain that several scientists tried to disprove spontaneous generation, but many people still accepted the hypothesis in the 1800s. Louis Pasteur conducted an experiment in 1859 in which he disproved spontaneous generation. Show students the Spontaneous Generation Diagram and explain the experiment (S-7-4-1_Spontaneous Generation Diagram.docx). He boiled meat and put it into a sealed container with a small tube-like opening that curved downward. Nothing grew inside the container. He also boiled meat and cut the tube so that microorganisms could enter the container. Over time, organisms grew in that container.
It may also be helpful to demonstrate Pasteur’s experiment for the class (for instructions on how to do so, see http://biology.clc.uc.edu/courses/bio114/spontgen.htm).
Put a line striking through spontaneous generation on the board and the definition. Ask,
- “Why didn’t anything grow in his container?” (The heat destroyed the eggs/spores.)
- “What do we call it when we heat foods to kill spores/eggs/bacteria?” (pasteurizing, named after Louis Pasteur)
- “Why do you think most adults believed in spontaneous generation all the way until 1859?” (Bacteria, spores, and some eggs require extremely high-powered microscopes to see.)
Explain that today the class will be looking at tiny Daphnia, also called “water fleas,” to see if they are pregnant and learn about how they reproduce. Tell students that every spring the local ponds and streams will become populated with these creatures and that without microscopes it may seem like they are spontaneously generated.
Part 2
Live daphnia must be obtained for this lab along with the materials listed on the student handout. They can be ordered from most businesses that supply live organisms. Get enough so that there are at least five per microscope to increase chances of finding pregnant ones. Better-fed daphnia are more likely to be pregnant. They are relatively easy to care for; see the Related Resources section for information about the care of daphnia. Daphnia provides a more exciting and worthwhile introduction to the microscope than a newspaper letter or hair lab, which is often used to introduce microscopes. Daphnia from this lab can easily be reused for other labs if properly cared for. Their heart rate is often studied in response to various chemicals. A simple online search will pull up countless examples of other labs that can be conducted.
Hand out the Daphnia Lab (S-7-4-1_Daphnia Lab and KEY.docx). Have students read the introduction paragraph and answer the question at the end. Go through the biological levels of organization with students and have them give definitions or provide them to students depending on their level of prior knowledge. Have students read through the paragraph and fill in the proper words from the list they are given. Spend this time to make any final preparations for the lab.
Go through the purpose, materials, and procedure with students. If this is their first time using microscopes, also go through basic microscope directions. Additional pointers that are worth mentioning to students:
- Don’t crush the daphnia with the microscope objective (depression slides help reduce the likelihood of this problem).
- Getting a daphnia off of a slide is easily done by dipping the slide into the water-filled daphnia container.
- Hot microscope lights quickly dry out the daphnia, so re-wet them and turn off the microscope light when it is not in use.
Refer to Daphnia Examples (S-7-4-1_Daphnia Examples.docx). These may be useful to print off and use to point out different structures to students while they are working with the microscopes. Have students complete the lab activity and the analysis questions in the Levels of Biological Organization section of the lab worksheet (S-7-4-1_Daphnia Lab and KEY.docx).
Part 3
Brace yourself, and then ask students, “How do you imagine a daphnia gets pregnant?” (Answers will vary.) Tell students that the daphnia they have been looking at can do some strange reproduction that is very unlike human reproduction. Distribute the Basic Life Cycles concept map (S-7-4-1_Basic Life Cycles.docx). Explain that the concept map outlines the human and Daphnia life cycles. Tell students they should read through the concept map and use the information on it to answer the reproduction and life cycles questions at the end of their lab. If students are unfamiliar with concept maps, be sure to walk through the human life cycle with them as an example. Note: The directions in which the arrows point do matter. A factual connection between the two bubbles should be described if a bubble is read then an arrow going away from it, followed by the next bubble. Have a quick discussion about students’ answers to questions 8 and 9 since they tie directly into the upcoming lesson. If time permits, discuss the other questions as well.
On a piece of paper have students write down two ways that sexual reproduction is different from asexual reproduction. Prompt students by writing the following on the board: Sexual reproduction…. Collect the paper as students leave. Answers include: Sexual reproduction requires males and females, sexual reproduction involves sperm and egg; sexual reproduction mixes instructions from parents; sexual reproduction is the only one needing males (since asexual organisms can’t be males only due to a lack of required egg-developing organs).
Extension:
- Students going beyond the standards can research the amount of time it takes to pass through different stages of both the human and Daphnia life cycles. A reasonable time can be found for the transitions both to and from an offspring for both cycles. Students can write the time necessary for those four transitions directly onto the map. (human sexual to offspring = 9 months, human offspring to adult = 12–17 years (varies), Daphnia sexual to offspring = 5–10 days, Daphnia offspring to adult = 2 weeks)
- For students requiring extra practice in locating daphnia under the microscope and/or having difficulty in determining if daphnia are pregnant, supplement the “live” trials with photographs of daphnia and ask students to determine if any of the daphnia in the photos are pregnant (S-7-4-1_Daphnia-Pregnant or Not and KEY.docx).
