Cell Theory Investigation
Evaluating the evidence for cell theory
The focus of this investigation of cell theory is to see how microscope images provide evidence to support the theory and how some notable exceptions challenge it. Cell theory was an important step in our understanding of living things. It paved the way for Robert Koch and Louis Pasteur to establish the germ theory of disease two hundred years after the establishment of cell theory. This is the basis of much of modern medicine today.
Three activities guide students through the evidence for cell theory, observation of cells, and a consideration of some of the exceptions to the cell theory.
What evidence was there to prove the existence of cells in the 17th century?
What were the technical challenges in getting the evidence?
Are there any organisms that are not made of cells?
Activity 1 Evidence for cell theory
There is a small museum in Delft in the Netherlands where Anton ven Leeuwenhoek lived the whole of his live which houses a small rectangular plate with two screw mechanisms and a minute hole. At first glance it is not obvious how it works. At the time when he used it van Leeuwenhoek was reputed to be one of the finest lens makes in the world. With his simple microscopes he could magnify x250. His friend Reinier de Graaf introduced him to Robert Hook.
Read the Cell theory timeline website, about Robert Hooke and the others involved in the development of the cell theory. Also take a look at Robert Hooke's Micrographia (It's the original script, published in 1665 available free on Project Guthenburg).
A nice concise alternative timeline here
List the evidence of cells and explain how each piece of evidence supports cell theory.
Hints for students:
Activity 2 Observing cells and estimating their size
One of the best things to do with a microscope is to collect a wide range of specimens, mount them on slides and look at them. In Oxford in the 1630s this is exactly what Robert Hook did. Schleiden and Schwann were also observing plant and animal cells in Germany to gather supporting evidence for their new "Cell Theory". In the Netherlands van Leeuwenhoek was making hundreds of his own microscopes and using them to do all sorts of observations. He even calculated the size of a typical cell to be "rather less than" 8.5 µm, a marvellously accurate result given his tools.
Use the Student observing cells worksheet below to observe cells and estimate their size, just as this international group of early microbiologists did almost 400 years ago.
Activity 3 Exceptions to cell theory
Consider the evidence supporting cell theory which has been studied in activities 1 and 2. Answer the questions below.
- How does the multinucleated cytoplasm of fungal hyphae and skeletal muscles bring the cell theory into question?
- How does Acetabularium, the giant algal cell that can grow up to 10 cm in length and has three distinct body parts, disturb the cell theory?
- "Omnis cellula e cellula" (Cells originate from cells) is what R Virchow stated. But where did the first cells come from?
Click the eye icon to reveal model answers for activity 3.
The last question is tricky and could lead to a discussion of the limits of scientific knowledge, useful for TOK.
Robert Hooke's Micrographia - a copy of his original script, published in 1665 has intriguing English spelling and grammar as well as odd syntax from over 300 years ago! An interesting link to language and TOK.
This website (vanleeuwenhoek.com) includes some excellent extra information about the work of Anton van Leeuwenhoek
This is a great microscope simulation from the University of Delaware. It takes students through all the settings that can be controlled on a microscope and has short guided videos for each one. There is also a nice set of puzzles - slides to fix light level, or focusing.
The sheet below instructions about how to set up microscopes and calculate cells size estimates may be a useful alternative / complement to the activity.