1.6 Mitosis and Cyclins

Serendipity and scientific discoveries—the discovery of cyclins was accidental (1.4)

The NOS  section in the Biology guide tells us that: “Scientists also have to be ready for unplanned, surprising, accidental discoveries.” (p6)  I see this as an opportunity to discuss the human side of the scientific process.  It is also another good time to link science (biology) to TOK. In TOK, we typically think of the natural sciences as being objective, neutral and based predominantly on Reason as a way of knowing. Meticulous observations, carefully planned experiments and independent verification are seen as the hallmarks of science and what gives it a greater sense of certainty than other areas of knowledge such as the human sciences or history. However, while these are important aspects of science, this ignores the fact that science is practiced by humans and as such, is prone to all the messiness that comes from being human.  Further, that like any human endeavour, luck, imagination and creativity are also key parts.

The discovery of cyclins was indeed a serendipitous event. Tim Hunt, who shared the Nobel Prize in Physiology or Medicine in 2001 for this discovery, said in his Nobel lecture…”I did not set out on my scientific career with the intention of studying the cell cycle, and had no idea that the winding road of discovery would lead in that direction.”(p1).  He began his career studying protein synthesis and took to spending summers in  the Marine Biological Laboratory at Woods Hole, Massachusetts, conducting research on mitosis in sea urchins.  It was this work that led to the discovery of the cyclins, proteins that were first identified because they peaked in concentration during interphase and then declined rapidly just before cell division.

While Hunt and his fellow researchers were demonstrating all the traits of the scientific method: – well-designed experiments testing out hypotheses – they had no idea of what they might discover and the joy of the unknown is an integral part of science at all levels.


Hunt, T. 2001. Protein synthesis, proteolysis and cell cycle transitions. Nobel Lecture.  http://www.nobelprize.org/nobel_prizes/medicine/laureates/2001/hunt-lecture.pdf

IB. 2014. Biology Guide: First Exams 2016. Peterson House, Cardiff.

Jackson, P. K. 2008. The Hunt for Cyclin. Cell. 134; 199-202.

Pulverer, B. n.d. Surfing the cyclin wave.  Nature.  http://www.nature.com/celldivision/milestones/full/milestone12.html

Featured Image:

The Cell Cycle. Nobelprize.org. Web Accessed May7, 2019.


1.1 Cells – an Introduction Part 2.

Ethical implications of research—research involving stem cells is growing in importance and raises ethical issues

A prescribed title essay in TOK recently (May 2014) was about the extent that ethics should limit the production of knowledge in the natural sciences. Stem cells was the natural choice of an example for many students who chose this. This topic is the perfect vehicle to link TOK explicitly into the biology classroom and in doing so, allows for some wonderful discussion about this topic.  This year, we used a modified Tug for Truth as a way of structuring the discussion.

Screen Shot 2015-02-04 at 9.22.36 PM

Tug for Truth is part of the Visible Thinking truth routines from Harvard’s Project Zero.  You can download PDFs for all of the different routines from their website.  The lesson plan for the Tug-for-Truth suggests picking a controversial topic and then “tugging” the truth by mentioning either true or false claims.  In our case, one side tugged in favour of fewer restrictions on stem cell research and the other side called for tighter restrictions.  Each side had to justify their claim, thereby moving the “rope” in their direction.  The stronger or better-reasoned the claim, the greater the tug. Focusing on TOK allowed students to use the Ways of Knowing to frame their claims – emotion was one that was used consistently.

For students, there are a range of great websites available for understanding the stem cell debate and what researchers are discovering: try the University of Utah’s excellent Stem Cells information page, this detailed fact-sheet from Euro Stem Cells  and this summary of research advances from the Genetics Policy Institute.

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.

This was based on studies dating back to the 1890s. In particular, the work of Gorter and Grendel was instrumental in establishing that the membrane consisted of a lipid bilayer. The Davson-Danielli 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. One of the key tenets of this model was that the protein layer was embedded within the bilayer, thus preventing the phospholipids from moving around.

This model was adapted by Robertson in 1959 to have the protein layer on top of, but not embedded within, the membrane.  In his model, the internal layer was proposed to be composed of either polysaccharides or polypeptides.

Robertson Model – 1959 (Nuffield Foundation)

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)

We can summarise the development of membrane understanding over time like so, in the format of Scientist(s) – model – evidence – year:

  1. Gorter and Grendel – the phospholipid bilayer – extraction and measurement of cell membrane lipids – 1924
  2. Davson and Danielli – the lipid-protein sandwich – electron microscopy – 1935, revised 1954
  3. Robertson – modified lipid-protein sandwich model – electron microscopy – 1959
  4. Singer and Nicholson – fluid mosaic model – freeze/fracture microscopy – 1972


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;.

“Lesson B: Cell Membranes.” Teaching about Science. The Nuffield Foundation. Web. 26 March, 2019. http://www.nuffieldfoundation.org/teaching-about-science/lesson-b-cell-membranes

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.

1.1 Cells – An Introduction Part 1.

1.1 Looking for trends and discrepancies—although most organisms conform to cell theory, there are exceptions.

A theory is the highest level of certainty in science, built up from a large collection of observations and experiments.  According to the online science dictionary, a theory is:

“A set of statements or principles devised to explain a group of facts or phenomena. Most theories that are accepted by scientists have been repeatedly tested by experiments and can be used to make predictions about natural phenomena. See Note at hypothesis.”

Cell theory states that all living organisms are made of cells, all cells come from pre-existing cells and that cells are the smallest units capable of carrying out life processes. This is based on a huge amount of observation and experimentation involving microscopes. It allows predictions to be made – if a new organism is discovered, say, in a deep ocean vent, we can reasonable expect that it will be made of cells and that these cells are the products of cell division from parental cells and they carry out the functions of life.

What makes biology interesting, of course, is that there are so many exceptions to the rules.  Muscle cells, fungal hyphae and giant algae are all examples of organisms that do not completely follow the above tenets. However, they do not invalidate the theory.  These cells do, to varying degrees, come from pre-existing cells and are part of a living organism; they carry out life processes, have genetic material and a plasma membrane.

The overwhelming evidence to date supports the Cell Theory.


Images courtesy of Grade 11 IB Biology students.

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

Taylor, Stephen. “2.1 Cell Theory.” I-Biology. N.p., n.d. Web. 9 Dec. 2014. <http%3A%2F%2Fi-biology.net%2Fibdpbio%2F02-cells%2Fcell-theory%2F>.