IB DP Biology: Practical 2 Osmolarity Lab Tips & Tricks
There are six required practicals for Standard Level (SL) IB Diploma Biology course as well as one additional required practical for Higher Level (HL) Biology students. This is my second blog post about each of these required practicals, you can find the Practical 1 or Microscopy practical here. This post is all about Practical 2, or as I call it the Potato Core practical. This lab is an excellent one for students who are learning about osmolarity and collecting reliable and valid data. Like all seven of the practicals they can, and do show up in the IBDP Biology exams; I have seen questions about the skills in this practical in Papers 1, 2 and 3 over the years. It is worth including some of those past paper questions on your unit tests and mock exams to help prepare students for the variety of ways in which the required practicals can be assessed.
Here is the description of Practical 2 which you will find on page 33 of the IBDP Biology Subject Guide: Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and
hypertonic solutions. (Practical 2). After many years of teaching this practical I have broken it down into the following skills:
Calculating the molarity of the different solutions
Creating consistently sized pieces of potato using cork borers
Using a mass balance correctly (tare/zero button)
Correctly identifying and taking into account specific constants & a control group
The difference between distilled water & tap water
Calculating percent change
Making qualitative observations using touch and sight
When it comes to the required lab skills I tend to approach them in a few different ways depending on how well I know the students (did I teach most of them the previous year), how large the class is (bigger classes make demos rather than individual support more feasible) as well as if there is any new equipment or tools that require an introduction. I often require that students read over the lab procedure (seen below) for homework and plan their methodology. In this lab I have them write their research question, hypothesis and do the required math to figure out how to make their required solutions of salt or sugar.
The slide below is what I share with my students as they prepare for the Practical, it goes with the lab sheet you will find below. I find that many students need a few reminders of what would happen if the solution they put their potato core into is isotonic vs. hypotonic vs. hypertonic. I use a few different diagrams and we talk about the use of isotonic solutions for storage of donated organs (Subtopic 1.4) as another example. The goal of this lab is to find the isotonic point for the potato (or other cells you are using) and so students may want to begin more broadly and then narrow down the concentrations as they go.
On lab day I make sure I have a lot of potatoes that are fresh and have been stored in a cool, dry place to prevent water loss as well as lots of distilled water. If you haven't yet spoken to your students about why distilled water is better for making specific solutions this is a great opportunity, the use of distilled water is an important way of making sure that the data is valid & the only thing that is affecting the osmolarity is the salt/sugar no ions or minerals in the water. It is important to have enough time to soak the potato cores so that they will react appropriately to the solution in which they are stored. I usually encourage my students to leave their potato cores for 24 hours, so don't do this lab on a Friday!
This is a very enjoyable lab and I highly recommend going through the results with the class to make sure that everyone understands what happened and why. An examination of sample graphs as well as a discussion of the methodology is a good idea. I hope you find this post helpful and enjoy this lab with your students!
Thanks for reading teachers, travelers & curious souls of all kinds.
The Roaming Scientist
