This season, we focused on water, specifically diffusion, osmosis, and water potential.
Our AP Bio class kicked off the season with the documentary FLOW – For the Love of Water. FLOW pointed out the advantages that companies take of local people and land when these companies privatize water, a natural, essential, and originally free resource, and sell it to the public at a high cost. The movie focused on how Nestle pumps water from Michigan, but also mentions various other places in the States, even Mount Shasta, which is near us! Coca Cola was trashing the villages in India for their bottling plants and gave the villages free “fertilizer” that was filled with toxins. Suez privatized water and sold it to poor communities in Bolivia, where communities only had water contaminated with blood from slaughterhouses.The entire class was shocked at how cruel the companies are to deplete the earth and communities of water and sell it for profit. To anyone who has not watched FLOW, watch it!
The class then broke up into groups of 3 to make large posters that depicted all the processes that occur in a cell membrane. We drew out the phagocytosis, pinocytosis, endocytosis, exocytosis, active transport, simple diffusion, and facilitated diffusion.
We experimented with the agar cubes and vinegar in our first lab. We were given large and small agar cubes dyed with bromethymol blue, a pH indicator that turns yellow when a substance is acidic. The cubes were places in vinegar, an acidic substance. After letting the cubes rest for about 25 minutes, we saw that the outsides of the cubes had turned yellow. However, the small cube had a much thicker border of yellow than the big cube. This is because the surface area to volume ratio was smaller in the smaller cube, so more of the bromethymol blue could diffuse out of the cell. This lab shows that cells are small because waste products need to efficiently leave the cell and materials need to enter through the membrane, which can only expand so much as volume increases.
Our second lab was the sucrose mystery lab. The AP Bio team figured out how to uncover the mystery and the entire process of the lab all together! Ms. Girard even said it was one of the fastest she’s ever seen 🙂 There were six different solutions of sucrose of water, from 0.0 to 1.0 M, each time raising in increments of 0.2 M. We were to determined the concentrations of each, based on our knowledge of osmosis. Each group of two put some of their assigned solution in a dialysis bag, which is made of cellulose, much like a plant cell. The sucrose would not diffuse out, but water would osmose in. The higher the concentration of sucrose, the more water would enter the dialysis tube. We were almost successful in labeling which solution matched which concentration, just the green and red were mixed up.
Our final lab involved various vegetables and the same sucrose solutions. The goal was to determined the concentrations of solute in each vegetable. We focused on the concept of water potential, or the likeliness of water to move out of a system. We soaked red potato, sweet potato, and turnip cores in the 6 different sucrose solutions to see if the cores would gain, lose, or maintain the same amount of water content. The vegetables were hypertonic relative to the solutions would gain water, and the vegetables were hypotonic relative to the solutions would lose water, and if the vegetables were isotonic relative to the solutions, the vegetables would remain the same. In the end, we graphed our data and found the approximate solute concentrations of the vegetables based on where the vegetables’ masses (affected by water gain or loss) remained about the same after the experiment.
Overall, this was a very fun and lab-intense season. I look forward to more!
Photos courtesy of Ms. Girard