I’d like to report on another great idea from a recent issue of The American Biology Teacher. This time it’s from the October 2015 issue. Dawn A. Tamarkin from Springfield Technical Community College wrote a wonderful article called “Exploring Carbohydrates with Bananas.” I haven’t tried this in my class yet, but my student worker did the activity. It was quick, easy, and informative.
The basic idea is to compare the amount of starch in cells from a green banana to the amount in cells from a ripe banana. The procedure couldn’t be simpler: Just smear a bit of banana on a microscope slide, flood with iodine (we used an I2KI solution left over from a previous lab), and add a cover slip. Any starch-rich plastids in the cells immediately turn purple and are easily visible with a compound microscope.
The article has some nice black-and-white photos of cells from unripe and ripe bananas. Here I’ve included a color photo (taken at 400x) of cells from a ripe banana. The iodine stained the starch-containing plastids an easy-to-see purple/black. Sadly, we did not think to take photos of the unripe banana cells, but the difference was striking. The green banana’s cells were chock full of distinctive purple plastids.
Sometimes it’s not possible to get both green and ripe bananas at the same store; it would be nice to stock up as needed, but bananas are perishable. My student worker and I therefore tested whether frozen banana tissue would work as well as fresh tissue. The answer is no, as you can see from the photos I have included below this post. Freezing the cells made the tissue much harder to interpret; the plastids were present, but they seemed to explode into the cells and were not the dark, distinct organelles that you can see in the fresh (unfrozen) ripe banana photo. We won’t be using that strategy again. Too bad!
This activity is a keeper. I plan to integrate it into our digestion lab, in which we are already using iodine to demonstrate the digestion of starch in a test tube with pancreatic extract. The problem is that students don’t think of digestion as something that plants do. Adding the banana activity, along with a supplemental exercise that will help students see the connections between “test tube” enzymes and the disappearance of starch from a ripening banana, should help us correct that stubborn misconception.
I should add that the original article, though brief, has a really well-developed exploration of banana flesh. It begins by having students taste each type of banana (or “reflect on their prior experiences”), which should lead to the idea that a ripe banana is much sweeter than a green one. The article then explains how to direct students to explore the bananas at the tissue level, cellular level, organelle level, organ level, and organ systems level. This would therefore make a great exercise for a microscopy lab as well. That’s a pretty huge benefit for the price of a couple of bananas and a bit of iodine.