Population Explosion (Yeast Lab) (Day #1 of 3)

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Students will plan and conduct a laboratory investigation that tests the effect of an abiotic or biotic factor on the size of a population of yeast cells.

Big Idea

Populations are able to produce populations of infinite size, but the environment (and its resources) is finite.

Learner Goals

Note: I recommend that you first check out this resource in order to get the most out of this lesson!

In high school I took several drafting classes and, for a while, I had hoped to become an architect. With respect to planning instruction and teaching, I feel that I can still live out the detailed approach to building something intricate and complex even though the product is a lesson rather than a certain "built environment."

This lesson-planning template is a comprehensive overview of how I approach lesson planning. It includes the "Big Three" aspects of the NGSS standards: Disciplinary Core Ideas, Crosscutting Concepts, and Science Practices. Of course, there are many other worthy learning goals, skills, instructional strategies, and assessments that can be integrated into a class session. I don't feel compelled to check every box but, rather, use it as a guide to consider various options and tailor the lesson in light of these.

With regard to this particular lesson, students will investigate the effect of a chosen environmental factor (e.g. temperature, percent glucose solution, etc.) on the population size of yeast. In doing so, students will begin to understand the relationship between organisms and their environment and ways to create a simple model of a complex system and devise a method for measuring population changes therein.

I hope you get some value from my work!

Anticipatory Set ("Hook")

10 minutes

 This is the hook activity for this ecology unit. As such, I expect that students have little to no understanding of the topic. I often employ a KWL strategy to determine student baseline understanding.

To start, have students brainstorm by using a KWL chart. Have them fill in at least four things that they Know about population growth and yeast (K). Next, have them list at least five questions they Would like to know more about the topic (W).

Have them leave the (L) column blank until later; perhaps during the lab or after it has finished, as they will add what they have Learned from the lesson. 

Instructional Input/Student Activities

40 minutes

Teacher note: Use this PPT as a guideline to lead students through these phases of the lab: background context, experimental design, and execution. As you discuss each slide of the PPT, have the students complete their lab handout.

1. DAY #1 Activities: (see Slide 2) The study of ecology typically involves populations, communities & ecosystems, and some form of human impact/restoration. As a start to this unit therefore, I want students to again investigate a process in a hands-on way, practice numeracy, uncover patterns of population growth, and make arguments with evidence.

If done well, this will set them up to broaden their understanding to later ecology topics. In particular, looking at abiotic and biotic factors that influence population growth in areas like Isle Royale, a nature preserve in the Great Lakes area and the interplay among wolf populations, moose populations, and their mutual interactions which will follow this activity. 

 2. DAY #1 Activities: (see Slide 4) The challenge of counting yeast, a microscopic organism, is two-fold: obviously they are incredibly small and their population levels can change rapidly. Therefore keeping track of them can be quite difficult. If grown in a test tube, one might reasonably take a diluted sample, prepare a wet-mount slide of it, and count them using a grid system like the one shown on the PPT slide#3. Of course, this involves estimation due to the fact that a slide represents multiple fields-of-view and the sample is a dilute sample. Students find this to become tedious quite quickly and, unless there are multiple counts taken by group members to verify population numbers, errors can easily mount. The basic nuts and bolts of experimental design are discussed with students with special attention paid to the question, their lab team's choice of manipulated variable and three (or more) controlled variables.

3. DAY #1 Activities: (see Slide 5) In order to activate yeast from powdered form, refer to this video:

This modified apparatus takes a simpler approach to counting yeast cells. A key assumption is that, since yeast perform cellular respiration which in turn creates carbon dioxide bubbles, more yeast will make more bubbles. Conversely, fewer yeast cells will make fewer bubbles. Of course, this is a simplified set-up but it does a reasonable job.

Special note: I use 15 x 150 mm test tubes and 1 mL plastic pipettes (with tips cut down to remain fully immersed in the water bath). The metal nut can be substituted with anything that keeps the pipette at the bottom of the pipette due to buoyancy that is created by the gas.

4. DAY #1 Activities: (Slide 6) Since this lab happens later in the school year, the necessary scaffolding (Lab Writeup Guidelines and the details involved in the process) has happened and students ought to be able to complete the remaining design on their own. That being said, I use a four member small group structure with roles chosen by students. The specific roles are outlined but may need to be specified in particular situations. Slides 2-6 ought to take 55 minutes. Set-up on day#2 should take 20 minutes and each day of counting over the next several days ought to take only 10 minutes.

Closure: What did we learn? Where do we go from here?

5 minutes

After all lab teams have made progress in designing their lab, determine a specific end point that teams ought to have completed by the start of the next class period. I require that the following sections of the student lab template to be completed: I-VII, where VII requires an appropriate data table (as yet blank of course). Note that these section numbers correspond specifically to the Lab Writeup Guidelines handout and generally do so with the student lab assignment.

That being said, I also expect that there will be some stragglers that are not to the designated goal and helping them to get completed must occur in class the next day.

Click here for a sample student's work!