Oct
23
2010

Playing around with the Floating Disk Assay—Light Response Curves

Over the years I’ve made the claim that the floating leaf disk assay is quite possibly the best way for students to explore how the process of photosynthesis. The method is inexpensive, accurate, reliably replicable and most importantly accessible for all levels of students from 5th grade to university. However, I’ve got to say that even I was surprised at some data I collected, yesterday. Recently, while working on new AP Biology Labs, I revisited the original (and still the best) paper that first discussed this technique. (or at least the earliest I can find.)

Wickliff, J. L., and R. M. Chasson. 1964. Measurement of photosynthesis in plant tissues using bicarbonate solutions. BioScience 14, no. 3: 32–33.

In this article I saw this graph of a photosynthesis light response curve that got me to thinking:

Last year, the UKanTeach program where I teach acquired a couple of PAR (photosynthetically active radiation) meters to measure photon flux. PAR meters are typically on the expensive side but this model from Apogee runs about $300. I hadn’t taken time to try them out and decided that now was the time.

Yesterday, I went out the north side of Haworth Hall and picked an ivy (Hedera helix) leaf that was growing in deep shade under a shrub.

English Ivy leaf, shade adapted

I picked a shade adapted leaf figuring that a leaf adapted to shade would likely reach photosaturation earlier than a sun adapted leaf. I wasn’t sure whether or not my light source was bright enough to induce photosaturation.

My light source is a clamp shop light with an 8 inch reflector and an 100 watt equivalent compact fluorescent bulb. Actually I found that if I put my meter within a couple of inches of the bulb I can get a flux reading equivalent to a summer’s day. I was sure my light was bright enough for the leaf I had picked.

I modified the technique that I presented here by placing the infiltrated disks in shallow petri dishes instead of plastic cups. I also modified the data collection procedure. Instead of counting disks floating at the end of each minute, I actually attempted to time each disk–a bit of a challenge that I wasn’t quite up to the first time. I should have used a video camera or at least used a computer timer program capable of timing 10 or more “laps” or intervals.

Modified technique

It is real easy to record the first movements of the disks with this technique.

In low light conditions, I started by carefully cutting about 80 disks from one leaf. I then infiltrated ten disks at a time with a dilute bicarbonate solution with a vacuum created with a 10 ml syringe. I placed the 10 sunken disks in separate petri dishes with a total of 30 mls of bicarbonate solution. The dishes with the disks were then placed under a box lid to exclude any light. I then tested 6 of the sets of 10 disks under different light intensities. The data from the highest light intensity are not included because I neglected to use a water heat sink filter to keep the infiltration solution temperature constant. The higher temperatures on this replication affected the outcome. It was only when the light was very close to the petri dish that this was a problem but I need to account for this next time.

Here’s the results:

Note that I’ve plotted plus or minus two estimated Standard Errors for each mean. I was impressed. This is a classic response curve and the parameters of this curve are consistent with data reported in the literature for shade grown English Ivy. I’m more convinced than ever that the floating leaf disk assay is a very valuable tool for a biology teaching laboratory. With this technique students can start their exploration of photosynthesis but the same technique is powerful enough to explore more sophisticated concepts.

Oct
03
2009

Nano Technology in Education

Opportunity for teachers to participate in a project and get summer pay!  Whoo hoo!

Read the following post:

Dear Teacher:

Please join us in supporting the National Science Foundation in facilitating the integration of nanoscience and technology into education!

NanoTeach is a National Science Foundation (NSF) funded professional development project that utilizes the Designing Effective Science Instruction (DESI) framework to integrate nanoscience and technology content into existing science curricula. It is a collaboration between Mid-continent Research for Education and Learning (McREL), the Stanford Nanofabrication Facility (SNF), the Georgia Institute of Technology, the National Nanotechnology Infrastructure Network (NNIN), and ASPEN Associates.

We are seeking 30 public high school science teachers to participate in our year-long, nationwide pilot test of NanoTeach beginning summer 2010. Teachers who complete all requirements will receive a stipend of $3,000 (15 days at $200/day) for the out-of-classroom time required for participation.

The application deadline is January 8, 2010. A special NanoTeach Question-and-Answer webinar is scheduled for November 17 at 5 p.m. EST. For more information, go to:  http://www.mcrel.org/NanoTeach/Recruiting <http://www.mcrel.org/NanoTeach/Recruiting>

Sincerely,

Elisabeth Palmer, Ph.D.

Director of Research

ASPEN Associates, Inc.

John Ristvey

Principal Investigator

NanoTeach Project

Mid-continent Research for Education and Learning (McREL)

Oct
03
2009

Survey on Stem-cell Education

Are you a 7th though H.S. Science teacher? Do you know any 7th though H.S Science teachers?  We are getting a request from the Director of Life  Science Outreach and Project BioEYES, the Institute for Regenerative Medicine  & The Netter Center for Community Partnerships to participate in a survey.  Please help this organization gather data.  After taking the survey, please reply to this post?  Was this beneficial?  Did it help me to participate and make me more aware?  Don’t forget that at NABT conference next month there will be a summit on stem-cell education.  Come and have your questions asked, understood and answered.

HelloTeachers,

Together with the Genetics Policy Institute, the University of Pennsylvania is seeking funding to develop a new and innovative Stem Cell education website and live classroom demonstration that will expand on Project BioEYES. For those teachers who are not yet involved with BioEYES, it is a live classroom experiment that uses zebrafish to teach students about cell biology, development, and genetics. It has reached over 18,000 students since 2002 and we hope to continue to offer new and exciting classroom opportunities.

This survey will help us gain insight into your interest and knowledge about how
to best develop online and classroom-based stem cells resources for teachers.

Please complete this survey so that you can have a voice in the project’s
development. We truly appreciate you taking the time to complete this! It will
only take a few minutes.

http://www.surveymonkey.com/s.aspx?sm=zf9lwefON6Im4_2bkWSs1jwQ_3d_3d

Sincerely,
The Project BioEYES team


May
25
2009

Using Spreadsheets to Introduce the Logistic Population Growth Model

This post is a continuation of exploring the use of spreadsheets in high school biology.  I’ve started with a rather obvious topic:  population growth.  What I present is only one possible scenario which is meant only as a starting point.  Two themes I hope are apparent as you read through these posts:  1.  I use questioning techniques to help the students connect to their previous knowledge while they are developing new understandings and 2.  I really work hard to have the patience to allow the students time to work out their own solutions on the spreadsheets with only a little intervention from me.  That’s the beauty of spreadsheets–they can quickly provide feedback to the students as to whether or not they’ve entered their formulas correctly or even if their proposed formulas work the way the student hoped.   In other words, making mistakes and fixing them is a critical part of these exercises.  Don’t cheat the students out of a learning opportunity by providing too much help/guidance.   In these posts I’ve suggested that you work out the spreadsheet yourself before checking out the embedded sheets.   In my experience, my mistakes help to inform my teaching as well.  I doubt that I’ve ever created an original spreadsheet model the first time from scratch that I didn’t subsequently correct or modify–that’s an essential part of the process.

Earlier posts in this series:

Sparrow Lab

Exponential Growth

At the end of the exponential growth post I mentioned that mathematical models can be additive–perhaps I should have said modular.  The exponential equation developed in the earlier sheet now serves as the core for more sophisticated models.

logistic

At this point with my students I enter a conversation that explores what they see in the real world.  Do populations continue to grow exponentially?  Why not?  What factors might limit population size?  Eventually, using guiding questions we follow a path that leads to a new concept:  carrying capacity.   At this point, with student input, I sketch a graph on the board that has the x-axis labeled time and the y-axis labeled population size.  I then draw a horizontal line across the top of the graph that I label carrying capacity.  I ask the student to do the same on a piece of paper and then challenge them to sketch a line that represents a population that grows exponentially at first but as the population size approaches the carrying capacity the population growth slows and the population size levels off.   Eventually, the class agrees that a likely scenario would be an S-shaped line, with an increasing slope early on, with a transition zone where the slope changes to a decreasing slope and an eventual leveling.

With the target in mind, I bring the class back to their earlier spreadsheet model of exponential growth that had two terms:  N and r.   I ask a number of question such as:  “Which of the two terms change as the exponential equation is recalculated”  “Which term is constant?”  “If we wanted to modify the exponential growth curve into the S-shaped curve what has to happen to r?” (no longer constant)   At this point I introduce a new variable to the work:  “K” which represents carrying capacity.  (Naturally, there is further discussion about carrying capacity in the real world and in the model.)

Now, for the hard part—having the students come up with the logistic expression themselves.  First I remind the students about the algebraic form of the exponential equation that they represented their earlier spreadsheet:

Nt = N(t-1) + r*N(t-1)

The discussion has already focused on the “r” term which is in the second expression.  I ask the questions such:  “What part of the graph is population growth maximal?  minimal?”  “How can we change ‘r’ to maximize growth? minimize growth?”  “Now if the spreadsheet has a constant value of ‘r’ how might we change that value during the calculations?”    At this point I will introduce the idea of adding another expression to the equation–the logistic.  “Is there some mathematical expression that we could add to this equation that maximizes ‘r’ early but minimizes ‘r’ in later generations?”  “Can you think of an expression that includes just the N variable and the K variable that can be multiplied times ‘r’ to fill the needs of the model?”  or  “Can you think of an expression that is approximately equal to “1″ when N (the population size) is small but approximately equal to “0″ when N approaches K in size?”  At this point I let the students “discover” this expression themselves.  I ask them to try out the expressions they think will work in their spreadsheet.  To evaluate their proposed expression put it in the spreadsheet and use the graph produced to evaluate whether the expression works as planned.

The first time I tried this, the students took most of an hour and went through quite a bit of frustration.  I’m not really sure why I thought they could “empirically” determine this expression or what I thought they’d get out of it but I realized part of the value of the exercise when all of a sudden, one of the girls jumped up and yelled “Yeaaaah, I’ve got it”.  I decided to not have her share her strategy with the others—but instead prompted them to keep trying.  Eventually the entire class came around to the logistic expression:  (K-N)/K    Definitely a powerful experience.  The students learn that they can solve seemingly impossible problems with hard work but they also learn how to think about mathematical models in of biology.  It’s fairly easy to discuss  now, the limitations and the power of the model.  BTW, that first student is now a professional biologist.

I hope that you try to create this spreadsheet yourself before you ask students to do so.  Here is an example of how the spreadsheet model might be formulated.

Link to the spreadsheet in case the embed feature is not working.

Apr
16
2009

Reframing Biology

biologyIt’s a perennial discussion… in what order do you teach the biology units.

Like many of you, when I started teaching AP Biology years ago I organized it by domains of scale:

  • The Domain of BioMolecules
  • The Domain of Cells
  • The Domain of Organisms
  • The Domain of Populations
  • The Domain of Communities & Ecosystems

I did it that way because I was taught that way and the textbooks were organized that way. But I became disenchanted with it because I felt like I was merely marching through the material instead of making connections between domains. So I started mixing it up — teaching principles and then teaching a unit that highlights a body’s application of that principle (form and function) — like teaching osmosis and then teaching kidney function as an example of osmosis.

But over the last couple of years, I have been brewing on a re-framing of the course that takes this idea further. I have started to view the material as being divided up between (1) large-scale interactions and (2) cellular processes.

Under LARGE-SCALE interactions I place evolution and ecology, because these are built on long term processes or interactions between organisms or groups of organisms. And I start my course with these because (1) evolution is my guiding principle for the rest of the year and (2) interactions between organisms and populations are easier for students to grasp this early in the year of their intellectual development.

SideNote: Many people have asked me how I teach evolution before teaching genetics. That always makes me laugh because if you think about it, Darwin developed the principles of evolution by natural selection without having been taught genetics himself!

I teach evolution before genetics, because you don’t have to know the nitty gritty of genetics to understand evolution. You only have to know that inheritance happens — and every high school kid knows that s/he looks like one or other of their parents.

Specifically for population genetics, you get to introduce/review some concepts and vocab early on in the course this way too, like you can introduce them to allele, heterozygote, homozygote… but each can be explained in one sentence and I consider that an advantage instead of a disadvantage.
I leave evolution by segueing from speciation into phylogenetics/taxonomy (who has evolved on this earth) and then into ecology (how they all interact).

Then I introduce CELLULAR PROCESSES by discussing that organisms are coordinated masses of cells that must perform a set of shared tasks. And I now organize this unit within the framework that cells have 3 main jobs: (1) to make energy, (2) to make more cells, (3) to make proteins. And for me everything else in the course falls under those functions.

First you have to discuss cell structure to lay the foundation — that includes biomolecules & their behavior, cell organelles, cell membrane, and movement across the membrane. Then we discuss making energy and all the animal & plant systems that have evolved to support that in one way or another:

  • MAKING ENERGY
    • Respiration
      • Digestion — taking in fuel
      • Gas exchange — taking in O2 & releasing CO2
      • Circulation — moving raw materials to & wastes from cells
      • Excretion — removing intracellular waste
      • Immune System — protecting an interconnected mass of cells & tissues
      • Motor System — using the energy produced in respiration
      • Nervous & Endocrine Systems — coordinating an interconnected mass of cells & tissues to make it an organism
    • Photosynthesis
      • Gas exchange — taking in CO2 & releasing O2
      • Plant Structure & Growth — highlighting the differences & similarities between plants & animals but how each structure supports making energy or using products

Then we discuss making new cells both for asexual reproduction and for the special case of sexual reproduction & all that extends from those topics:

  • MAKING CELLS
    • Mitosis
      • DNA replication
    • Meiosis
      • Genetics

Then we discuss making proteins & that opens the topics that have come from the new DNA-centric world that we live in:

  • MAKING PROTEINS
    • Protein Synthesis — transcription & translation
      • Gene Regulation
      • Biotechnology

And that’s where I end the course.

I hope this offers you another perspective than the one dictated by your textbook. I strongly believe that students get a more integrated view of the biological world this way. I feel like it tells a story that both holds their attention and makes sense, rather than marching through a mass of vocabulary as if we are teaching a foreign language.

Maybe someday there will be a textbook that breaks the mold of domains of scale.

Kim Foglia

Kim Foglia