Monthly Archives: February 2011

Stations of the Cell

>I read an interesting thread on the NSTA biology listserv that suggested teaching mitosis with DNA structure/replication instead of with meiosis as it is usually paired.  The idea is that mitosis is asexual reproduction and thus conceptually separate from meiosis, which is the basis of sexual reproduction.  I’d chosen cloning as the context for teaching about mitosis, and my students had started a webquest using the Learn.Genetics Cloning module.  I collected them to gauge their progress, and ended up with three groups of those who were almost done, those about halfway, and those who had barely started.  I put a different stamp on the papers of those in each group, and this was how I divided the class.  (Amazingly, this was the first time I had tried grouping the class this way, and it made my giant class of 25–which doesn’t seem that big until you consider I only have 24 seats–a lot more manageable.)

I set up three stations and gave them about 25 minutes at each one.

Station 1 – Finish the cloning webquest.
Station 2Online Onion Root Tips (also available in Spanish).  Students read about the 5 phases of mitosis and sort cells into the right phase to determine how much time is spent in each one.  I gave them an online quiz through Schoology at the end.  Our state assessment is very general about mitosis, but knowing the phases is important for understanding meiosis, which is in turn a conceptual underpinning of sexual reproduction and chromosomal inheritance.
Station 3Build a DNA Molecule on the computer, then using Twizzlers and colored marshmallows.  This was everyone’s favorite, of course, and the takeaway was very simple: A with T, G with C. Make sure students refer to the base-pair letters rather than just the marshmallow colors.

Much cheaper than DNA modeling kits, and every student gets to have one.

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>Egg Diffusion Lab

>So I meant to blog about this weeks ago, but between the great Snowpocalypse closing school for 3 days, parent-teacher conferences, and the general mayhem of life, I never got around to it.

Some of you may not realize this, but teachers are all filthy thieves.  We beg, steal, borrow, and mash up anything and everything to get a decent lesson out because, in all honesty, it is simply too much work to write a new lesson every single solitary day.  Plus there are a plethora of amazing ideas and resources, and a good teacher will pick and choose what best fits the needs of her students without having to reinvent the wheel every time.  I have a “take a penny, leave a penny” philosophy when it comes to sharing lessons, so while I have no idea who reads this thing, I am at least doing my part to share.

To introduce the concept of diffusion, I did a clicker case called The Agony and the Ecstasy from National Center for Case Study Teaching in Science, which is one of my favorite resources ever.  The case deals with the physiological effects of using ecstasy, so I primed the kids with Learn.Genetics Mouse Party animation, and they always get a big kick out of the drugged up mice.  After that we did the classic egg diffusion lab, which you can find many variations of online.  Here’s the gist of it, though…
  1. Place a raw egg in vinegar and leave for 2-3 days so that the shell dissolves.  This is a good lab to start on Friday, let it sit over the weekend, and pick up again on Monday.  Be careful there are no cracks in the shell though, or you will end up with Humpty Dumpty!

  2. Once the shell is gone, the egg will be held together by the outer and inner membranes.  It’s very fragile at this point, so tell your kids to be really careful!  (Every semester I always have at least one group break theirs at this point, so it’s not a bad idea to make a few extra.)  Mass the egg and record.
  3. You can do the next part several ways.  In order to move more quickly, I have each group put their egg in a different solvent and predict what will happen.  You can also put the egg first in distilled water, then in salt water or some other hypertonic solvent.  In either case, mass the egg after letting it sit overnight in the solvent and compare it to the pre-solvent weight.
  4. Discuss the movement of water based on concentration.  I’ve found that this is a really tricky concept for kids to grasp and explain articulately, and I myself had a ton of trouble understanding osmotic pressure until I thought of it in terms of water concentration.  I always tell them that diffusion is the movement from HIGH concentration to low concentration, and then I say that higher concentrated solutions are “HIGH”-pertonic and lower concentrated solutions are “hypo for low”-tonic.  (I’ve also found that I almost never speak in a normal voice during class, but who’s keeping track?)

 
We put this egg in Gatorade and it came out a lovely orange color and did not change much in mass, which suggests that Gatorade is in fact isotonic with our cells.