Tuesday, November 13, 2012

End of cell biology unit - beginning of Evolution and Genetics


Last week, on Monday, students presented their cell models (photos posted below). They were all very good! I noticed a few misconceptions during the presentations, so we spent a few minutes reviewing some organelles.

Since we had the field trip to McLaughlin the Friday before, I asked students to write two paragraphs about the field trip. The Knoxville area, where we planted the seedlings, has an interested history so I asked the kids to use this restoration project to illustrate John Muir’s quote: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.”

On Wednesday, I had three “stations” for them. One was focused on microscopy, and the other two were worksheets – origin of life on Earth and development. I was at the microscope station talking to them about the parts of a microscope, showing them how to prepare a wet mount, and helping them prepare a slide with a piece of elodea, and a slide with their cheek cells (picture below). While one student was with me, the others were working on their two worksheets in their classroom, with Chris. This was the last “official” class covering cells. My goal was to have them think about how the first cell evolved, and to get an initial appreciation of development. This was the last “official” class because on Wednesday we will discuss the worksheets (recall that they are due on Wednesday!).

We have covered all concepts of the 7th grade standards concerning cells:

Cell Biology
1.      All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:
a.       Students know cells function similarly in all living organisms. b. Students know the characteristics that distinguish plant cells from animal cells, including chloroplasts and cell walls.
c.       Students know the nucleus is the repository for genetic information in plant and animal cells.
d.      Students know that mitochondria liberate energy for the work that cells do and that chloroplasts capture sunlight energy for photosynthesis.
e.       Students know cells divide to increase their numbers through a process of mitosis, which results in two daughter cells with identical sets of chromosomes.
f.          Students know that as multicellular organisms develop, their cells differentiate.


Today we started the evolution/genetics unit. I started by having them tell me what they knew about evolution. They mentioned adaptation, human evolution, and inheritance. We had a good discussion about how these terms are related to evolution, and then I provided a definition, and an example of evolution by natural selection. The example was about the peppered moth during the industrial revolution. We had talked about this example before, and I was surprised to hear how many details they remembered! I then talked to them about Darwin - his voyage, and observations about the finches. I briefly mentioned that Darwin wasn’t the only person working on the idea of evolution by natural selection  (talked about Wallace), and we played a natural selection game at the end of class. The purpose of the game was to illustrate evolution by natural selection at two different levels – predator and prey. Each student had a tool (2 had spoons, 2 had forks, and 2 had plastic tweezers); we walked to a patch of grass by the parking lot where I had previously thrown 100 colored toothpicks (25 of each color - green, yellow, blue and orange), and they had 5 minutes to find, and get toothpicks using their tools. Before leaving the classroom, I asked them to predict the results.  As predicted, colors that could camouflage with the background (yellow toothpicks were blending in with fallen leaves, and green toothpicks are hard to see in the grass) were the most successful at surviving since only 8/50 were found. Orange and blue toothpicks were more easy to spot – 28/50 were found.We spent the last few minutes of class discussing how this game is relevant to evolution by natural selection. 

This is an aggregation of cheek cells. You can see the nucleus in most of the cells.







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