1.5 The Origin of Cells

1.5  Testing the general principles that underlie the natural world—the principle that cells only come from pre-existing cells needs to be verified.

If Cell Theory tells us that all cells come from pre-existing cells, then where did the first cell come from?  What a wonderfully intriguing question!  What is the origin of life?

This is a topic that dovetails nicely with TOK, as it helps establish the process by which the natural sciences develop knowledge.  Although we cannot, of course, travel back to the early years of the earth, we can develop hypotheses and test them experimentally, discarding those for which the evidence does not support.  I think this NOS is also important because it emphasises that biologists can attempt to answer even the most perplexing questions through the scientific process.

Pasteur demonstrated that new cells could not spontaneously arise – they must therefore develop from existing cells.


Urey and Miller demonstrated that inorganic compounds could become organic under the right environmental and atmospheric conditions.


And, my favourite, the endosymbiotic theory – we all have prokaryotes inside us!

Ongoing research, demonstrated in the excellent Exploring Life’s Origins website, provides evidence of how protocells and membranes may have evolved.  It all fits rather nicely with the new Crash Course Big History series, of which episode 5 is on the origin of life.


1.3 Membrane Structure

Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model.

As we saw with Watson and Crick, models play an important role in developing knowledge in biology.  This is particularly important when studying microscopic structures. Davson (a physiologist) and Danielli (a chemist) proposed a model in 1935 that was based on twin layers of protein surrounding the membrane – a protein bilayer.

Diagram of the Davson-Danielli model (Eichman)
Diagram of the Davson-Danielli model (Eichman)

This was based on studies dating back to the 1890s that had established a phospholipid bilayer surrounding cells. Their model appeared to be confirmed by subsequent electron micrographs taken in the 1950s that showed a darker band, thought to be protein, surrounding a lighter  core of phospholipids.

Electron micrograph supporting the Davson-Danielli hypothesis (Childs)
Electron micrograph supporting the Davson-Danielli hypothesis (Childs)

Developments in microscopy techniques, however, soon led to the revision of this theory and its replacement with the Singer-Nicholson model, which is the basis of our understanding today. They based their model on Freeze-Fracture techniques, which involved rapidly freezing cells and then fracturing them. This fracture plane is between the phospholipid bilayer.  Visible in these micrographs were a series of bumps or protrusions – which turned out to be the integral proteins embedded within the membrane.  The structure of the proteins themselves were also able to be studied in more detail and it was revealed that they were globular, rather than fibrous,  and thus unlikely to be find in a structural role. From this developed our modern understanding of a fluid, phospholipid bilayer containing a range of peripheral and integral proteins within it.

Freeze-Fracture Image (Childs)
Freeze-Fracture Image (Childs)

The current TOK Guide tells us that:

The methods of the natural sciences based on observation of the world as a means of testing hypotheses about it are designed to reduce the effects of human desires, expectations and preferences, in other words they are considered objective. (p36).

In what ways does this example demonstrate this? Why is it important to understand theories that are no longer used? If our understanding of science is dependent on technology, and technology is continually advancing, will we ever arrive at a single, final understanding of the natural world?


Historical Development of Membrane Structure:

Childs, G. “Membrane Structure and Function.” Membrane Structure and Function. N.p., 19 July 2001. Web. 27 Jan. 2015.

Eichman, P. “From the Lipid Bilayer to the Fluid Mosaic: A Brief History of Membrane Models.” SHiPS Resource Center || History of Biological Membranes. SHiPS Resource Centre, n.d. Web. 26 Jan. 2015. <http://www1.umn.edu/ships/9-2/membrane.htm&gt;.

Original Published Articles

Danielli, J. F., and H. Davson. “A Contribution to the Theory of Permeability of Thin Films.” Journal of Cellular and Comparative Physiology 5.4 (1935): 495-508. Wiley Online. Web. <http://onlinelibrary.wiley.com/doi/10.1002/jcp.1030050409/abstract&gt;.

Singer, S. J., and G. L. Nicolson. “The Fluid Mosaic Model of the Structure of Cell Membranes.” Science 175.4023 (1972): 720-31. Web. 27 Jan. 2015. <http://life.umd.edu/cbmg/Faculty/song/688D/Paperdiscussion/Singer%20and%20Nicolson%201972.pdf&gt;.


Allott, Andrew, and David Mindorff. Biology: Course Companion. Oxford: Oxford UP, 2014. Print.

Diploma Programme Theory of Knowledge Guide. Publication. Cardiff: IB, 2013. Print.