IBDP Biology: Practical 1 Microscopy Tips & Tricks
There are six required practical for Standard Level (SL) IB Diploma Biology course as well as one additional required practical for Higher Level (HL) Biology students. I will plan to write a blog post about each of these required practicals, starting with Practical 1, also known as the microscopy practical. This is the only required practical to require both drawing and calculation skills. 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 1 which you will find on page 30 of the IBDP Biology Subject Guide: Skill: Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells. Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in drawings or micrographs. (Practical 1) . After many years of teaching this practical I have broken it down into the following skills:
Making accurate & detailed biological drawings from a microscope
Identifying cells, both within tissues of multicellular organisms & in unicellular prokaryotes
Identifying organelles (or ultrastructures) within these cells
Correctly interpreting electron micrograph images as well as specimens on prepared slides
Using a scale bar &/or microscope to determine the magnification of a specimen/image
Converting between centimeters, millimeters, micrometers & nanometers
Calculating the actual size using image size and magnification
While most students enter the IB Diploma with some experience with microscopes, very few have the skills to calculate actual size and this is something that will have to be taught. I like to do Practical 1 after having completed Subtopic 1.1 Introduction to Cells, but before completing 1.2 Ultrastructures of Cells. I am sure many teachers prefer to complete Practical 1 after students have learned about organelles in Subtopic 1.2, but I have found that students are then overly focused on the organelles and do not work enough on their drawings and calculations for Practical 1. Here are some of the slides I use to help teach these skills to my students.
One of the more complex skills for many students is the ability to determine actual size using magnification and image size. For this reason I have students watch a video about how to do this for homework the day before the lab and then we go through the steps and two practice questions as a class first. You can see in the slides above that I use a formula triangle to help my students understand that image size is actual size multiplied by magnification. I also spend some time reviewing metric unit conversions as student must make sure that the image size and actual size are in the same units and often one needs to be converted. After some whole-class instruction we do two calculations together on the board, the first of a tardigrade with clear ultrastructures, and the second of a virus protein coat with no discernible ultrastructures. I then distribute the following lab sheet, the front page includes two electron micrograph images which the students can use as practice since only the images and their drawing on the back page will be assessed (see the rubrics below).
On lab day I put out a few trays of prepared slides which my students will use for their drawing and calculations. I always include some stem and root cross sections (these are particularly good for HL students who will need to study them later in Topic 9 anyway) as well as some interesting slides of bacteria, algae, human tissue and whatever else I have around. I encourage my students to check out a few different slides before they make their final decision to draw a specific slide. The kind of detailed drawing they need to do takes time and is best done over a long lab period. While students are drawing I circulate to make sure that they are following the instructions, drawing in pencil, using a circle to represent the field of view and trying to be accurate rather than artistic.
When it comes to doing the calculations for their drawing they need only use the magnification at which they drew the specimen and its image size (measured with a ruler) to calculate the actual size. While this does introduce some inaccuracy as the image size is usually significantly larger than what they saw through the microscope it teaches them how to use the formula. If you have more time &/or this level of accuracy is important to you you can certainly use a stage micrometer to measure the size of the field of view at the various magnifications to determine the image size more accurately. This is not explicitly required by the IBO and I have found that it takes more time than it is worth.
The last tip I have is to encourage students to take photos with their cell phones for reference, but not to allow them to draw from the photographs. Drawing from a photograph is a completely different skill than drawing from a microscope and students will need time and practice to master this unique biological skill. I hope this blog post helps with Practical 1, don't forget to enjoy this lab, I never tire of examining the world through the microscope!
Thanks for reading teachers, travelers & curious souls of all kinds.
The Roaming Scientist
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