I love this lab! It really let's students explore the relationship between surface area, volume, and diffusion time. And quickly allows them to see that a large cell will starve or poison itself since material can't diffuse in or out fast enough. I make bromothymol blue agar (see recipe below), but you just need something safe that will change color as it changes pH. Students cut various sizes of cells and place them in vinegar to watch -- and time -- the movement of color change as the vinegar diffuses in turning the blue to yellow.
The variety of cell designs from my 2007-2008 class
The winning design for 2007. Isn't it amazing how much it models intestinal villi and we didn't even learn about that yet! Congratulations!
The cubes during the diffusion process
Cell design from my 2006-2007 class
Cell designs from my 2006-2007 class
Cell design from my 2006-2007 class
Recipe to make Bromothymol Blue Agar for Cell Races
- Mix 15g agar in 1 liter water.
This is AGAR, not agarose. (Note: 15g is thicker than you would make for growing bacteria, because you want the agar stiffer and tougher for handling.) It is better to use plain agar and not LB agar, so it has less of a chance to grow bacteria.
- Boil slowly in microwave or hot water bath until agar is melted (granules will disappear). Watch for and avoid boil-over.
- Remove from heat. Add 0.1 g powdered bromothymol blue and mix. If the mixture is not dark blue, then add more bromothymol blue. If the mixture is green or yellow, you will need to stir in drops of NaOH (or another base) until it turns blue. Wear safety goggles and gloves when handling NaOH.
- Pour the agar into trays. For the initial lab, I make the agar in rectangular trays, like a wide silverware tray or a square Pyrex dish -- enough to be at least 2cm deep -- and slice chunks for the students to cut from. (I use the tops of my microcentrifuge storage boxes most of the time: See this image, but just tape that little curved indent with masking tape) For the actual contest, I mold them in ice cube trays that I bought at Amazon and use a spatula or butter knife to remove the gelatinous cubes. Make enough for 1-2 ice cubes per student or student group. Let agar harden at room temperature or in refrigerator. Can be made a couple of days in advance. Cover with plastic wrap to keep from drying out and store in refrigerator, otherwise it will get a lot of bacterial growth quickly, especially if you use nutrient agar.
- From the microcentrifuge trays students cut blocks of specific sizes:
1cm x 1cm x 1cm
2cm x 2cm x 2cm
1cm x 1cm x 8cm
- Students have to calculate volume, surface area, and SA:V ratio.
- Students then immerse each block in common household white vinegar either in small beakers or in more tray tops. Agar turns yellow in acid. You can easily see the blue core disappear as diffusion takes place. Students time until blue completely disappears. Helps to put beakers on white paper as a background.
You'll notice that 2x2x2 and 1x1x8 have same volume, but different surface area... so students see the comparative effects. You'll be amazed at how long 2x2x2 takes (45-60 minutes)! I usually have them start the 2x2x2 block first since it takes longest and then run others concurrently. You may need multiple stop watches or good records as to when the later blocks were immersed in their separate beakers.
And now for the Competitive Cell Races!
Then (usually next day) students are given an ice cube block of the agar and they must design their own cell to maximize volume & mass, but minimize diffusion time. This process allows students to confront a lot of misconceptions of cell design.
- No donut-like holes through the agar cell -- cell membranes cannot sustain that shape.
- No poking, prodding, touching beaker containing agar cell in vinegar.
- Teacher determines when 100% diffusion takes place.
- Students mass agar at conclusion of race...cell must not break when handled. Disqualification if cell breaks upon massing (although you can be a bit lenient here).
- Winner = highest ratio of mass divided by time.
We sometimes run a second race after a trial run, so students can improve designs. I make a lot of agar just in case. It's a fun learning day!