IBDP HL Biology Practical 7: It's Potometer Time!
I can't believe that we have reached the end of my series on the seven required lab practicals for IBDP Biology...this post is all about Practical 7, the only HL-only practical in the May 2016 syllabus. It has always been my very last lab practical, as I tend to do the botany lessons of Topic 9 in Year 2. The description of this last lab practical can be found in the IBDP Guide on page 92, the IBO describes it as "Skill: Measurement of transpiration rates using potometers (Practical 7)". This practical requires knowledge from Subtopic 9.1 Transport in the xylem of plants as well as Subtopic 2.2 Water.
In case you weren't aware there are six required practicals for SL students and a seventh for HL students, these practicals can, and do show up on the IB exams throughout all three papers, but especially in Paper 3 Section A. I recommend doing some past paper questions about the practicals in your unit assessments. I have written separate blog posts on the other six practicals if you would like to read them, here are the links:
Unfortunately, this practical is not part of the new guide, but the concept of transportation of water through the xylem of plants is now part of the SL & HL content, so this lab could still be used as part of your Practical Scheme of Work for the new syllabus, you can find my blog post all about this here. I may or may not use it myself, depending on if I can figure out how to use it to investigate water potential. That being said, there are several possible connections in the new syllabus including: B3.1.9 Transpiration as a consequence of gas exchange in a leaf (students should be aware of the factors affecting the rate of transpiration), B3.2.7 Transport of water from roots to leaves during transpiration and D2.3.10 Contributions of solute potential and pressure potential to the water potential of cells with walls (including xylem vessels).
So, how do I run this lab as part of the old syllabus? I don't use a formal lab sheet or procedure, but talk through it with the class using the slides pictured below and let them figure it out. Since this is a pretty low stakes lab figuring it out is an excellent thinking task for my Grade 12 HL biology students. I provide them with the materials and some images to help them and then probe with guiding questions throughout. We also tend to do some simpler labs related to transport in the xylem of plants in the lessons before this practical, such as the classic celery & food colouring lab pictured below and examining some different methodologies to measure transpiration rates (also pictured below). We talk about the various sources of error and evaluate the effectiveness of each method to measure transpiration rates, after which students understand the value of potometers for this kind of research.
Here are some tips for making sure your potometer functions well:
Get a fairly large stem cutting with a sturdy stem, several leaves means more transpiration and therefore more water movement, we have used roses & hibiscus in my classes.
Use two half pieces of cork to hold onto the plant stem in the clamp so that you don't accidentally squish the stem or the tubing
Use Parafilm to attach the pipette to the tubing and then immerse in a basin full of water, keep one end above the water so that the water fills the tube and pipette
You want to have a single bubble of air in the entire water column, this will take several tries, but can be done & students need to keep working at it
Leave your potometers set up in their locations for at least 12 hours to see a significant difference
Don't worry too much if it doesn't work well, students need to understand how the lab is supposed to work and what the data means for the IB exams, nothing more
Remember to have fun with this practical, I recommend you do it later in the course and then check out some past paper questions about potometers, so students can understand what they need to know. Some good ones can be found in HL Paper 3 of the May 2017, 2018 and 2019 exams. I hope you found this blog post helpful and that you and your students enjoy this last lab practical!
Thanks for reading teachers, travelers & curious souls of all kinds.
The Roaming Scientist
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