2. How big are cells?
It is very difficult for us to get a real idea of very small and very large sizes. So, when we are thinking about things like molecules, cells or the solar system it can be helpful to compare their size with things we are familiar with. In Case study 2, the teacher was fortunate enough to have a good, working microscope and was able to give concrete experience of one of the measurements on the worksheet. When the students do the calculations in Activity 2 they will consider the dimensions of a cell in a number of ways. The activity will help them to develop an understanding of cells, as the building blocks of living things, rather than as diagrams in a book. It will also give them practice of numeracy skills in science and give you an understanding of their ability in maths. This may affect your planning when teaching other science topics with a mathematical content.
Case study 2: Looking at onion cells
Mr Baguma had one microscope to use with his class. He also had 40 glass microscope slides. He did not have cover slips for the slides, but he used a second slide instead of a cover slip when preparing slides with his class. He divided the class into groups of four. Mr Baguma showed the microscope to the whole class and pointed out the main parts and what they do. He demonstrated how to prepare a slide of onion cells to view using the microscope and explained how to use a ruler with the microscope to estimate the size of the cells (Resource 3). He then asked each group to make a slide of onion cells. The groups took it in turns to come up to the front bench to look at their slide using the microscope. While they were waiting to use the microscope, Mr Baguma set some questions and calculations for the class to work on to help them appreciate just how small cells really are (Resource 4). He realised that some of the students were finding the questions difficult, which was a problem as he needed to help with the microscope. So he encouraged the students to help each other. The rule was that they could only write down the answer if they understood where it had come from. Jophus is very good at maths and really enjoyed helping his friends. After each pair had measured their onion cells, they were allowed to write the measurements in a table Mr Baguma had drawn on the board. At the end of the lesson, they could see that there is variation in cell size, but that the variation falls within certain limits.
Activity 2:Thinking about the size of cells
Remind students that you can only see cells with a microscope. Discuss why this is so. Probe their understanding of magnification and use analogies such as buildings made of stones or bricks. If you are far away you can only see the building, but as you get closer you see the bricks or stones. Compare cells to atoms and molecules which are much too small even to see under a normal microscope. Ask pupils to guess how big cells really are. Explain that most cells are between 0.01 mm and 0.1 mm in size. Do they know anything else that is so small? Can they imagine this size? Ask them to carry out all or some of the calculations in Resource 4 . If there are students who find maths difficult, you could ask them to work in pairs. When you check the answers, discuss the extent to which these exercises helped their understanding and ask them to write their own questions.