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.

Lesson Description

Guiding Question

What evidence was there to prove the existence of cells in the 17th century?

Are any organisms not made of cells?

Explain the discovery of cells as a paradigm shift in the scientific world.

Activity 1 Evidence for cell theory

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).

List the evidence of cells and explain how each piece of evidence supports cell theory.

Hints for students:

Include the contributions of:

  • Hooke -1st to describe cells
  • van Leeuwenhoek - 1st to view living cells, called "animalcules"
  • Schwann - all animals are made up of cells
  • Schleiden - all plants are made up of cells
  • Virchow - cells come from pre-existing cells

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 marvelously 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?
    • The cell theory states that cells are the smallest unit of life and that organisms are made of cells. Both skeletal muscle cells and fungal hyphae are made from elongated cytoplasm with more than one nucleus. These structures are a problem for cell theory because they are not strictly made of cells.

  • 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?
    • Cell theory states that organisms are made of cells. Acetabularium is a whole organism and it is larger than a cell. This disturbs the cell theory because it seems to break the rule that all organisms are made of cells by not itself being made of cells.

  • "Omnis cellula e cellula" (Cells originate from cells) is what R Virchow stated. But where did the first cells come from?
    • There is some evidence that organic molecules could form in the 'primordial soup' of the early Earth's atmosphere. Some of these molecules, like RNA or enzymes are thought to be capable of reproducing themselves. Other molecules have properties similar to parts of a membrane. They could form the main parts of a cell.
      This is a question which cannot be answered fully by science. It is not possible to observe the process happening, nor to recreate accurately the conditions of the early earth's atmosphere.

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.

Teachers notes

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.

This Magnification worksheet can be found in the lesson activities Calculating Magnification and Size is a step through worksheet to learn the process of calculating the magnification of images.

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