9.1 Transport in the Xylem of Plants

Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.

This is another NOS that provides many links to the prescribed syllabus in Topic 9.1. In fact, you could teach many of the content using the potometer as a demonstration model, and then have students individually use their own and design their own independent variables etc.

  • Prescribed Practical 7: Measurement of transpiration rates using potometers
  • Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing;
  • Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

Potometers can be as simple or technical as you like them to be.  A standard set-up might look something like this:

Basic Potometer (Pearson)

The general plant requirements are to use a woody stemmed-branch and that the leaves have a thin waxy cuticle.

The key to success is ensuring that there are no air bubbles in the tubing, as air bubbles will prevent the transpiration stream from working effectively.

If you have access to Vernier data loggers (or something similar) you can use a gas pressure sensor to record the rate of transpiration.  This can provide a more reliable quantitative measurement of the rate of transpiration and could be a good option for an Individual Investigation (IA).

Here are some pictures of the ones we set-up:

In the bottom right-hand corner you can see that the pressure in the tube is decreasing, indicating that transpiration is taking place.  We did some simple independent variables – removing leaves and changing the light intensity.  You could use a fan to try and stimulate a windier environment and small plastic bags on the leaves to increase humidity.  The advantage with a data logger is the ease of collecting the data and then analysing it, allowing for quick calculations of rate and other statistics.


“LabBench Activity.” Design of the Experiment – Potometer, Pearson Education Inc., http://www.phschool.com/science/biology_place/labbench/lab9/design.html. Web. Accessed Feb 12, 2018.

“Measuring Rate of Water Uptake by a Plant Shoot Using a Potometer.” Practical Biology, Nuffield Foundation, http://www.nuffieldfoundation.org/practical-biology/measuring-rate-water-uptake-plant-shoot-using-potometer.Web. Accessed Feb 12, 2018.


6.1 Digestion and Absorption

Use models as representations of the real world—dialysis tubing can be used to model absorption in the intestine. (1.10)

Models are an essential part of the scientific method and have been discussed in our biology syllabus in topics ranging from Neural Development, the human brain, DNA structure and membranes.

Models allow scientists to represent an idea that is difficult, or impossible to experience directly. By their nature they are simplifications of the real world processes they describe, but they are still extremely useful as a means of explaining processes, making predictions, testing hypotheses and analysing data. Think of the importance of models of climate change or fisheries populations – these models have direct impacts on economics, politics and society.

In 6.1 we are discussing the use of models in the digestive system.  A classic middle-school demonstration is to use visking, or dialysis, tubing to demonstrate the workings of the digestive system.  An example experiment using starch, iodine and Benedict’s solution shows how this might work. The inside of the model gut originally contains both starch and glucose. However, testing over time shows that after 15 minutes, the gut still contains starch, however the liquid outside the “gut” tests positive for glucose.  The movement of glucose out of the gut and into the surrounding fluid, while the larger starch molecules stay behind, helps demonstrate the need to reduce the size of polysaccharides before they can be absorbed and used by the body.

It would be useful to review the different NOS statements on models and develop some common ideas about the use of models in biology.  Turn each statement into a question using an appropriate command term and try to develop some short-answers. For example:

  1. NOS statement for 6.1 – Use models as representations of the real world—dialysis tubing can be used to model absorption in the intestine.
  2. This becomes: Outline how dialysis tubing can be used to model absorption in the intestine (3).


“Evaluating Visking Tubing As A Model For A Gut | Nuffield Foundation”. Nuffieldfoundation.org. N. p., 2016. Web. 11 Dec. 2016.

“Scientific Modelling “. Sciencelearning Hub. The University of Waikato. 2016. Web. 9 Dec. 2016.

A.2 The human brain

Use models as representations of the real world—the sensory homunculus and motor homunculus are models of the relative space human body parts occupy on the somatosensory cortex and the motor cortex.

This follows on from A.1 Neural Development as another example of models in science – perhaps this tells us something about neurobiology?  These models are used to represent the relative amount of cerebral cortex that either receive sensory input or send motor signals out.  What is particularly interesting is that some parts of the body have a much greater amount of cortex dedicated to their sensory or motor information.  These differences have been expressed in the form of a “homunculus” – Latin for “Little man”.


Screen Shot 2016-02-07 at 7.40.38 PM
The sensory and motor and homunculi, Penfield and Rasmussen (1950) in Schott (1993).

As you can see in the image above, The largest sections of both sensory and motor cortexes are concerned with the hands, lips and other parts of the face. This can be transferred to a model as in the image below:


You can check out more interesting information on the homunculus in the eponymous Crash Course Psychology Episode 6

This can manifest itself in some interesting complications.  For instance, you will notice that in the sensory cortex, the region receiving information from the face is directly under the face.  The neuroscientist V.S. Ramachandran, investigating the phenomenon of phantom limbs in patients with amputations, found that when he stroked the cheek of a patient, the patient felt the stroking happening in his phantom hand. The sensory neurons from the facial region had moved into the adjacent area for the hand, which was no longer receiving any input.  This, and other investigations into phantom limbs, helped establish the plasticity of the brain neurons (Topic A.1) and developed treatments for amputees suffering from phantom limb syndrome.

Points that yielded referred sensations in the phantom hand – both on the face and the upper arm. The numbers refer to the digits on the hand.  When these points are touched, it is felt in the phantom hand as well. (Ramachandran and Blakeslee, p30)


Andrews, M. “How To Refresh Your Inner Gollum: Health And The Homunculus”. N. p., 2014. Web. 7 Feb. 2016.

Ramachandran, V. S, and Sandra Blakeslee. Phantoms In The Brain. New York: William Morrow, 1998. Print.

Schott, G D. “Penfield’s Homunculus: A Note on Cerebral Cartography.”Journal of Neurology, Neurosurgery, and Psychiatry 56.4 (1993): 329–333. Print.`

The Motor Homunculus. 2014. Web. 7 Feb. 2016.

A.1 Neural Development

Use models as representations of the real world—developmental neuroscience uses a variety of animal models.

Animal models are crucial in developmental biology, as they allow observations and experiments that would not be possible, feasibly or ethically, with humans. One example of a model organism is Xenopus sp. collectively known as the African clawed frogs.  The guide asks you to label developmental images from this animal and it is one of the most important model organisms used in embryological research.

Besides, Xenopus sp. there are a range of other model organisms used in research. Here’s how they compare in terms of their importance to scientists:

Screen Shot 2016-02-01 at 3.59.44 PM

C. elegans is a nematode worm and Drosophila sp. refers to the fruit fly, so important in genetic studies.  Each has its advantages and disadvantages. For instance, C. elegans has a fixed number of cells as an adult (959), which makes it very useful for studying cell differentiation. The zebrafish (Danio rerio) produces almost transparent tissues.

The relative evolutionary relatedness of these organisms to humans can be seen here (and a good chance to review Cladograms!):

Screen Shot 2016-02-01 at 4.03.07 PM

You can see the importance of Xenopus sp. as a model organism – here is the summary of this:

  1. Easy to maintain in the laboratory
  2. They produce eggs throughout the year
  3. The eggs are a reliable and flexible material for research
  4. The eggs themselves are very large, making manipulation and observation much easier
  5. The embryos are a good model for vertebrate development
  6. Genetically similar to humans and so a good model for human disease

Points 4 and 5 are particularly relevant for studying neural development.

The source listed below on Xenbase.org is a wonderful resource for all things African clawed frog – especially developmental imaging.

The use of model organisms, of course, has ethical considerations, as we have discussed previously on this blog.


Cbs.umn.edu,. “What Is C. Elegans? | College Of Biological Sciences”. N. p., 2015. Web. 1 Feb. 2016. http://cbs.umn.edu/cgc/what-c-elegans

Xenbase.org,. ” Introduction To Xenopus – Xenbase | A Xenopus Laevis And Xenopus Tropicalis Resource “. N. p., 2016. Web. 1 Feb. 2016. http://www.xenbase.org/anatomy/intro.do


Content, Practicals and the Nature of Science

As I have mentioned at various times on this blog, I think one of the challenges with the new syllabus is the idea that the NOS represents an “add-on” that will somehow impact teaching and learning.  Some of them certainly are new concepts and content, but some are also linked directly to either content, lab-work or both.  In these cases, making the connections is easy and can help reinforce what the students are already learning.  Some examples that would work here include (IBO,2014).:

1.4 Membrane transport Experimental design—accurate quantitative measurement in osmosis experiments are essential. (3.1)
This links to Practical 2:  Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. 

2.5 Enzymes Experimental design—accurate, quantitative measurements in enzyme experiments require replicates to ensure reliability. (3.2)
This links to Practical 3 – Experimental investigation of a factor affecting enzyme activity.

2.9 Photosynthesis Experimental design—controlling relevant variables in photosynthesis experiments is essential. (3.1)

4.3 Carbon cycling Making accurate, quantitative measurements—it is important to obtain reliable data on the concentration of carbon dioxide and methane in the atmosphere. (3.1)
See my post on Carbon Database Analysis

6.1 Digestion and absorption Use models as representations of the real world—dialysis tubing can be used to model absorption in the intestine. (1.10)

9.1 Transport in the xylem of plants Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.
This links to Practical 7 – Measurement of transpiration rates using potometers.

With a bit of planning, a single lesson can combine content, practical work and the Nature of Science.  Further, linking NOS to an experiment can help reinforce understanding in a much more effective way. As you begin to work towards the IA, these then provide additional inspiration for students to develop their own investigations.

In terms of preparing for examinations, students should draw on their experience with these practical experiments. This looks especially important for the new Paper 3, which includes a Section A with unseen data based on the core/AHL syllabus, but could also be important on the other papers as well.

I could envisage the following sorts of short-answer questions (to be clear, I have made these up myself!):

  • Outline the use of models in:
    • measuring transpiration in plants
    • showing how absorption in the small intestine works
  • Explain the need to control variables when designing experiments to measure photosynthesis
  • Outline the importance of collecting adequate quantitative data when conducting osmosis experiments/measuring the rate of reaction in enzyme experiments.
  • Explain the importance of quantitative data in providing evidence to support climate change

Remember to look back over your experimental notebooks or old lab reports here – this does not require so much in terms of memorisation of facts but rather the process and justification of experimental procedures.

Biology Guide: First Assessment 2016. Cardiff: IBO, 2014. Print.

2.6 Making Models – DNA

Topic 2.6:  Using models as representation of the real world—Crick and Watson used model making to discover the structure of DNA. (IBO, 2014)

This year, while trying to locate our ball-and-stick DNA models, we found an old cardboard-puzzle DNA kit.

Piecing together the puzzle
Piecing together the puzzle

It proved a great (and unplanned) way to introduce the structure of DNA and have the students examine the chemical features to deduce their own answers to the structural significance of DNA.  The advantage of this kit was that it had the chemical structure painted onto the puzzle pieces and the students, much like Watson and Crick in the early 1950s, were able to experiment with no guidance from me and determine which pieces needed to fit where.   There were many “Aha” moments as different students determined out where the different chemical pieces fit best.

The finished product!
The finished product!

We were thus able to figure out the significance of anti-parallel strands, purine + pyramidine pairing, 3′ →5′ linkages and the sugar-phosphate backbone.  Models in action!

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.