Peter Nelson    Biology I    1st Period   50 minutes

Objective: Given instruction on the concept of “spontaneous generation”, the student will investigate (blooms level: synthesis) the history of the cell (Biology 2.a)

Materials: Pencil, paper, 3 beakers for visual presentation

Bell Work: Quiz on previous lesson

Set: Have students stand up and look out the windows at the squirrels and lounging stray dogs (they will always be outside your window). Ask them where they come from. What they are made of. What about the worms, roaches, and ants we see outside (and sometimes in the classroom) where do they come from? What are they made of? Give students some time to think and jot down ideas in their notes. Answer questions. Many will be unsure about some of the examples. These are the same problems people struggled with WAY back in the day. Before science and the cell were a part of common knowledge. Today we will...state objectives.

Procedure:

1.    Background — Spontaneous Generation Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know. For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories — in other words, they didn’t use the scientific method (covered previously) to arrive at answers to their questions. Rather, their conclusions were based on untested observations. USE OF PROPER GRAMMAR SHOULD ALWAYS BE EMPLOYED. (Guideline, 10)

2.    Give students examples of “spontaneous generation and allow them to think about the observations before giving them the people’s conclusion.

3.    Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times.

    Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs.

    Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs (like     Shakespeare’s house). As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around.

Conclusion: It was obvious to them that the mice came from the moldy grain.

Observation: In the cities, there were no sewers, no garbage trucks, no electricity, and no refrigeration. Sewage flowed in the gutters along the streets, and the sidewalks were raised above the streets to give people a place to walk. In the intersections, raised stepping stones were strategically placed to allow pedestrians to cross the intersection, yet were spaced such that carriage wheels could pass between them. In the morning, the contents of the chamber pots were tossed out the nearest window. Food was purchased and prepared on a daily basis, and when people were done eating a meal, the bones and left-overs were tossed out the window, too. A chivalrous gentleman always walked closest to the street when escorting a woman, so if a horse and carriage came by and splashed up the filth flowing in the gutters, it would land on him, and not the lady’s expensive silk gown (many of these gowns were so ornately embroidered that they were not easily washable, and neither washing machines nor dry cleaners existed). Many cities also had major rat problems. People back then may or may not have not connected the presence of rats with the spread of Bubonic Plague (Black Death, a dreaded and fatal disease), but they were probably bothered by the rats chewing on things and by the rat fleas biting them (just as cat/dog owners, even now, are bitten by the offspring of their pet’s fleas).

    Conclusion: Obviously, all the sewage and garbage turned into the rats.

Observation: Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in s   summer, meant battling the flies around the carcasses. Typically, carcasses were “hung by their heels,” and customers selected which chunk the butcher would carve off for them.

Conclusion: Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies.

4.   Among these ideas, for centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms (have students put in notes). It was common “knowledge” that simple organisms like worms, beetles, frogs, amd salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life.

5.  Put students into groups of four and ask students how they would develop a way to prove spontaneous generation wrong. Give them about 15-20 minutes to think of an experiment. Have them write down their idea in the format of the scientific method – for credit.

6. When the time is up, listen to their examples and explain how it was eventually proved wrong. 

In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat. This was a true scientific experiment — many people say this was the first real experiment — containing the following elements:

   1. Observation: There are flies around meat carcasses at the butcher shop.

   2. Question: Where do the flies come from? Does rotting meat turn into or produce the flies?

   3. Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies.

   4. Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots.

   5. Testing: Wide-mouth jars each containing a piece of meat were subjected to several variations of “openness” while all other variables were kept the same.

      control group — These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop.

      experimental group(s) — One group of jars were sealed with lids, and another group of jars had gauze placed over them.

      replication — Several jars were included in each group.

   6. Data: Presence or absence of flies and maggots observed in each jar was recorded. In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat. In the sealed jars, no maggots or flies were ever seen on the meat.

   7. Conclusion(s): Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms.

7. Walk through the example using beakers of your own. Explain how this experiment helped fuel the movement that forced us to look closer and eventually discover cells and the miracle of reproduction.

8. Verbal review

Closure: 

Reference to the animals and other living things that we see. “These all come from others of their kind, not dirt, garbage, or dead animals” Review definition of spontaneous generation. How does this relate to the study of the cell?? Wait for responses. Great. Tomorrow we will highlight some of the important historical figures in reference to the discovery of the cell.

Assessment:

Objective: Given instruction on the concept of “spontaneous generation”, the student will investigate the history of the cell.

Formal: The student will complete a group activity (M) on the inquiry based process that disproved spontaneous generation and led to the discovery of the cell (C). Group conclusions will be collected and recorded in the grade book (D)

Informal: The teacher will monitor student progress (M) on the inquiry based process that led disproved spontaneous generation and led to the discovery of the cell (C).

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