A new spin on the agar cell size lab

Carving up blocks of bromthymol blue agar “cells” is one of those go-to biology labs that everybody does, but apparently very few people have a clear idea of what the lab is supposed to show. I had my classes do this lab last year, with lackluster results. My 9th graders aren’t very precise about measuring and cutting the agar blocks, so their results were all over the place, and they couldn’t describe what they were supposed to be learning from this activity. I wasn’t satisfied with the lab, so I went back to square one (hahahaha) to see if there way a way to reconstruct the lab.

The first starting point was to refer to the NGSS – what was the learning goal? The disciplinary core idea of HS-LS-1-4 (From Molecules to Organisms) is that “cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism.” When I boil this down to its core idea for my students, I tell them that a cell functions most efficiently at the sweet spot – when the surface area of the cell membrane is large enough that materials can enter and exit at a rate that serves the volume of the cell – either in receiving the nutrients and materials it needs, or getting rid of accumulated waste products. And how we determine that sweet spot involves looking at the ratio of surface area to volume.

Last year I did the lab where I gave students chunks of BTB agar and had them cut different size cubes plus a long rectangular piece. Students dropped the cubes in vinegar, and then measured (at one-minute intervals) the distance the vinegar had diffused into the cubes. Then students calculated surface area and volume, graphing the change in surface area over time. At the end of the lab, I didn’t feel like students understood any of the concepts about cell size and efficiency. They didn’t really see how increasing the cell’s size affected its ability to move materials in and out of the cell, so I set about rethinking this lab to see if there was a way to make a direct connection between these two concepts.

The first lightbulb went on when I saw ice cube trays that create spheres instead of cubes. Was there a way to make agar spheres of varying sizes? The answer is . . . YES! I poured two different sizes of agar spheres – small ones with a diameter of about 2.5 cm, and larger ones with a diameter of about 5 cm. (The larger ones came from a 6-sphere tray, so I decided to use them as a demonstration; there were plenty of small spheres for students to use.)

Ice sphere trays

Molding and unmolding the spheres is a little tricky. The molds have two pieces – a lower tray with a deep lip, and a top tray with holes in the top of each sphere. You fill the lower tray, then press the top tray down into the lower tray. Any extra liquid oozes out of the holes on top of each sphere. When I poured them, the top tray tended to float on top of the agar, so I weighted down the trays after filling them. I would say out of 40 spheres (two small sphere trays’ worth), about half were usable. Some of them had a dimple in the top due to shrinkage or not having enough agar. The large spheres were less successful – that tray was flexible silicone. When I removed the top part of the mold, I moved too quickly and sheared one sphere in half. I slowed down and took advantage of the flexibility to pull the top tray off the rest of the way and had better success. However, I learned the hard way that trying to pop the spheres out of the lower part of the tray also sheared off parts of the sphere. If you try this, unmold these spheres veeeeerrrrrryyyyy slowly!

The big kahuna! You can see the oops in the lower left corner – take your time when unmolding these.
Baby BTB agar spheres!

The rest of the lab was drama-free. I doled out small spheres to the student groups, warning them to wait for my signal to drop the spheres in vinegar. I took a large sphere and counted down, and we all dropped our spheres in the vinegar at the same time. We waited 15 minutes, removed the spheres, and carefully cut them in half. I had students lay a ruler along the diameter and quickly take pictures (the vinegar continues diffusing into the agar). I shared my picture of the large sphere with them.

Student sample – small sphere
Teacher sample – large sphere

Once they have these pictures, students are calculating surface area and volume of each sphere, measuring the distance the vinegar diffused into the sphere, and calculating the rate of diffusion. After that, they calculate how long it would take for the vinegar to diffuse into the center of the sphere. The final analysis is to infer how increasing the size of a cell affects its ability to efficiently move materials into and out of the cell.