incorporated inquiry, as students conducted the investigation and experienced
the changes that the potatoes underwent before understanding the reasons for
these changes. After they completed their investigation, we worked as a class
to deduce the reasons for the changes that the students observed
Topic: Diffusion, osmosis,
. It is important for the discipline of science for students to understand the variety of complex processes occurring within all cells at all times as they move towards a deeper understanding of cellular biology.
. It is important for society to have students capable of comprehending how the minute scientific details can relate to the larger picture.
. It is
important for students to understand the nature of diffusion and osmosis
before exploring how they actually
function within living organisms.
Prior Knowledge: This
lesson assumes prior knowledge of selectively permeable membranes and some
experience with recording laboratory observations neatly.
NJ Core Curriculum
Standard 3.3 (Speaking) All students will speak in clear, concise, organized language that varies in content and form for different audiences and purposes.
A2)Present ideas and
opinions spontaneously in response to a topic or other speakers.
Standard 5.1 (Scientific Processes) All students will develop problem-solving, decisionmaking and inquiry skills, reflected by formulating usable questions and hypotheses, planning experiments, conducting systematic observations, interpreting and analyzing data, drawing conclusions, and interpreting results.
B3) Collect, organize, and interpret the data that
result from experiences.
Standard 5.5 (Characteristics of Life) All students will gain an understanding of the structure, characteristics, and basic needs of organisms and will investigate the diversity oflife.
A2) Recognize that complex multicellular organisms, including humans, are composed of and defined by interactions of.. .cells...
In leading off the lesson, I will ask a few students
to share with the class their homework assignment from yesterday. This
assignment asked them to define diffusion in their own words using examples from
the variety of activities that we did in class. After some people have shared
their definitions of diffusion, I will then write osmosis on the board and
explain that osmosis is a specific kind of diffusion in that it is the diffusion
of water molecules. I will then explain that while yesterday we worked with
models of living things, today we are going to be working with slices of potato.
I will remind them that potatoes come from plants, and ask them what Schleiden
told us about plants. When they explain that he explained that all plants are
made up of cells, I will ask them what this must mean about the potatoes that we
will be using. I will then show them the slices of potato that we are using and
ask them whether they think that they are one cell, or more than one cell. If
confusion ensues, I will remind them of how many cells are in just 1 sq. cm of
their own skin, and this should clarify any confusion about whether or not it is
multiple cells. Lastly, before beginning the lab I will ask students whether or
not they think it is important for water to be able to move into and out of
plant cells, and why.
After briefly introducing the lab, I will ask students to look at their lab report sheet and read over the problem. I will again ask them what a hypothesis is, and ask them to work in their lab groups to generate hypotheses for what will happen to the potato in the fresh and salt water. I will remind them that osmosis (like all diffusion) is very dependent on concentration, and ask them to think: about concentration of water molecules when they are making their educated guesses. After they have had a couple of minutes to make their hypotheses, I will ask some students to volunteer their thoughts, but will remind students not to change their hypotheses, as all that is important at this stage is that they make an educated guess.
After students have made and shared their hypotheses, I will generate a list on the board of some of the adjectives that they used to describe how they think: that the potato slices will look and feel. I will explain to them that it is important that they use descriptive terms such as these in their laboratory reports in order to be able to fully understand what their results mean. At this point, we will go over the procedure together. I will ask students why it is important to cut a fresh slice of potato and place it in a dish with no water. We will briefly discuss the importance of control, and I will ask them what they would compare the potato slice in fresh and salt water to if there were not a regular slice of potato to feel.
At this point, I will give students time to feel the potato slices in the three different situations and fill in their laboratory reports. I will remind them that I want them to be as neat and thorough as possible when recording their observations. As students are making observations, I will wander about the room observing their responses and asking some of them why they think that they are finding the results that they are finding.
After they have had time to complete the lab activity, I will ask students to turn to the back of their laboratory report and complete the sections on whether or not their hypotheses were correct or incorrect. Again, I will emphasize the fact that it is not important whether or not their hypothesis was correct or incorrect, but that they are able to take the experimental data and compare it to their hypothesis in order to draw conclusions. As a class, we will then discuss the two scenarios. I will tell students to take as many notes on this as possible in order to be able to fully complete the final section of the laboratory. I will draw a slice of potato in fresh water on the board, and ask them which part has a higher concentration of water. We will figure out that the water does, and thus it will move to a lower concentration of H2O, thereby entering the potato. On the board I will draw a bunch of water molecules in the petri dish and fewer in the potato, and draw arrows showing water molecules moving into the potato. I will ask the students why this would make the potato in the ftesh water become rigid and hard. Then we will do the same procedure for the potato slice in salt water, and determine that the concentration of H2O is higher in the potato slice than in the salt water. Again, I will draw the potato slice and the petri dish and I will draw more H2O molecules in the potato than in the salt water. We will discuss that the salt was taking up room in the petri dish so there were fewer H2O molecules than in the fresh water. I will ask a student volunteer to draw arrows showing which direction the water will move and then I will ask them where the water goes, and why this made the potato slice get all limp and soggy. Again, we will discuss how H2O flowed from the cell, the vacuole emptied and the potato slice went limp.
After we have analyzed why the potato slices changed in the ways that they did, I will ask students if all of the molecules up and decided to leave, or if they can explain, using the examples we went over yesterday, why they left. I will also ask students whether there was a barrier that they had to cross in order to move, similar to the dialysis tubing laboratory that we completed yesterday.
If time remains, I will ask students whether or not
any of them have ever heard of putting salt on a slug to kill it. If they say
yes, I will ask them to explain to me how this is similar to the potato
experiment we just completed, and why the slug would perish in the situation. In
addition, if time remains, I will tie it to an ecological issue of road salt. I
will draw a plants roots and how plants against the road get a lot of salt in
the soil, especially this time of year. We will diagram where H2O concentration
is highest and determine it is higher in the roots than the soil, and then I
will ask them which direction it will flow in. When they realize it will flow
out of the roots, they will be able to explain why many plants perish in
extremely salty soils.
Assessment will be embedded in the lesson based on
participation in group work and the way in which the students conduct themselves
in the laboratory environment. In addition, the lab report on the potato lab
will be graded to assess understanding of the concepts.
Incorporating lab work, visual demonstration, verbal explanation, and kinesthetic activities into the lesson will allow for different styles of learners to understand the material. In addition, the group-work nature of the laboratory will require students to work cooperatively with their peers.