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.


6.6 Hormones, Homeostasis and Reproduction

Developments in scientific research follow improvements in apparatus—William Harvey was hampered in his observational research into reproduction by lack of equipment. The microscope was invented 17 years after his death.

Harvey’s 1651 Publication. (Anatomia Animata)

We encountered Harvey in the NOS for Topic 6.2 on Circulation. In addition to his studies of the circulatory system, Harvey was responsible for a key publication on reproduction -the 75 chapter opus Exercitationes de Generatione Animalium (Experiments in Animal Generation),  published in 1651. This was an important publication, as Harvey made many key observations and overturned existing ideas on reproduction. Some of the highlights include:

  • landmark drawings and observations of the stages of embryonic development in chickens
  • denouncing the theory of spontaneous generation by discussing how maggots emerge from eggs
  • providing evidence, through comparative anatomy and observations, that argued against the idea of preformation – that the sperm or egg contains an already perfectly formed tiny animal (like a series of Russian dolls)
  • overturning the Aristotelian theory that conception resulted from the fusion of menstrual blood with semen.

In addition, his work on the dissection of female deer reproductive systems at different stages of pregnancy provided a new insight into this previously misunderstood aspect of physiology. However, he was limited in what he could observe here.  He was unable to find eggs or embryos until about six or seven weeks into the pregnancy, hampered by a lack of technology that could magnify the tissue. Thus his theory that epigenesis, rather than preformation, was the correct path for development could not be confirmed until the microscope was invented.



“Anatomia Animata: Generation And Reproduction”. Indiana.edu. N. p., 2009. Web. 16 Mar. 2016.

“Embryology – History Of Embryology As A Science”. Science.jrank.org. N. p., 2016. Web. 16 Mar. 2016.

Lopez, Angel, “William Harvey (1578-1657)”. Embryo Project Encyclopedia. Arizona State University. 2010. Web. Mar 16, 2016.

6.5 Neurons and synapses

Cooperation and collaboration between groups of scientists—biologists are contributing to research into memory and learning. (4.3)

As we learn in TOK, the image of the lone scientist coming up with brilliant, individual insights is generally false; science is a discipline where collaboration and cooperation are critical to developing new scientific knowledge. This is particularly true today, as disciplines become ever more specialised.

There are innumerable examples of this taking place in memory studies, as the interaction between neurons, the brain and other aspects of physiology and biochemistry mean that researchers from many different disciplines are needed.

One recent example is a study investigating the role of hormones (specifically estrogen) on learning and cognition (Phan et al.).  Adding the hormone to female mouse brains seemed to boost short-term learning; the team concluded that it helps induce the formation of immature synapses (which occurs during development) and that this provides the foundation for developing and then storing new memories.

A look at the author affiliations shows the importance of collaboration:

  • Department of Psychology, University of Guelph, ON, Canada
  • Neuroscience Program, University of Guelph, Guelph, ON, Canada
  • Department of Biomedical Sciences, University of Guelph, Guelph, ON, Canada
  • Laboratory of Neurobiology and Behavior, The Rockefeller University, New York, NY 10065

Such examples can be found with any search of the current scientific literature, highlighting that scientists must work together not just in studies of learning and memory, but in all fields.


Phan, Anna et al. “Rapid Increases In Immature Synapses Parallel Estrogen-Induced Hippocampal Learning Enhancements”. Proceedings of the National Academy of Sciences (2015): 201522150. Web. 16 Dec. 2015.

University of Guelph. “Tie between estrogen, memory explored by researchers.” ScienceDaily. ScienceDaily, 15 December 2015.

6.4 Gas Exchange

Obtain evidence for theories—epidemiological studies have contributed to our understanding of the causes of lung cancer. (1.8)

In 1964, the US Public Health Service published a report on the links between smoking and health that has since been recognised as a turning point in public health.  The conclusion of the report was clear: “Cigarette smoking is a health hazard of sufficient importance in the United States to warrant appropriate remedial action.” (Dela Cruz, C. S) This was based in large part on two large-scale epidemiological studies completed in 1950.

One study, in the US, found that 96.5% of lung cancers in a study group of 605 patients were in men who were moderate to heavy smokers for many years (Wynder and Graham). The other, in the UK, found an association between lung cancer and smoking cigarettes and between cancer development and the amount of smoking (Doll and Hill).

Additional research establishing the nature of the 50 carcinogens and 400-500 major constituents of cigarette smoke provided the supporting data to establish this causation (Dela Cruz, C. S). It is now estimated that a heavy smoker has between a 10-30 fold greater risk of developing lung cancer than a non-smoker (Dela Cruz, C. S).

Now, this is not meant to be a History of Science section, or a study of epidemiology, but an analysis of the Nature of Science.  So what can we determine from this?

Well, Theories are the backbone of science, establishing explanations for observable phenomena that are supported by experimental data and evidence. Until such evidence was established, tobacco companies could brush aside health concerns because of a lack of data – it could be genetic, environmental, the product of other health concerns etc etc. Large scale studies that established causal links between cigarette use and lung cancer determined that smoking was the key cause and thus public health policy and laws were able to move in the direction of reducing smoking.

The adult per capita cigarette consumption in the US, 1900-2006 (Dela Cruz, C.S)

The graph above shows the dramatic effect that the studies outlined above had on smoking, as well as on social and legal factors such as taxes and bans on advertising.  One could extend this aspect into some TOK-territory by examining advertising and cigarettes: Australia has some of the most restrictive laws regarding cigarette packets and warnings, which could lead to some interesting knowledge questions.

Ultimately, it is the collection experimental data that leads to the formations of theories which can then be used to explain observations.  The power of science is that these theories can then be used to support legislation to protect the public against powerful interests.


Dela Cruz, Charles S., Lynn T. Tanoue, and Richard A. Matthay. ‘Lung Cancer: Epidemiology, Etiology, And Prevention’. Clinics in Chest Medicine 32.4 (2011): 605-644. Web. 4 Dec. 2015.

Doll, Richard, and A. Bradford Hill. “Smoking and Carcinoma of the Lung.”British Medical Journal 2.4682 (1950): 739–748. Print.

Wynder EL, Graham EA. Etiologic factors in bronchiogenic carcinoma with special reference to industrial exposures; report of eight hundred fifty-seven proved cases. A M A Arch Ind Hyg Occup Med.1951;4(3):221–235

6.3 – Ethics and Penicillin

6.3 Risks associated with scientific research—Florey and Chain’s tests on the safety of penicillin would not be compliant with current protocol on testing.

Study this NOS in conjunction with the Application from 6.3:
Florey and Chain’s experiments to test penicillin on bacterial infections in mice

While Alexander Fleming gets much of the credit for discovering penicillin, it was the work of Florey and Chain during the Second World War that led to its availability as a breakthrough medicine: the two “…transformed penicillin from an interesting observation into a life saver. ” (Torok).   All three shared the Nobel prize in physiology or medicine in 1945 and Florey was knighted and decorated by the US, UK, France and Australia for his role in influencing the outcome of the war.

The NOS refers to the trials Florey and Chain and their team undertook to get penicillin out to the troops as quickly as possible. Florey and Chain began their research in 1939 and were faced with the challenge of extracting enough useful penicillin to be used.  This was compounded by the outbreak of war and the fact that such a medicine could be crucial to the Allies.


The penicillin mould (Bos)

In 1940, they were ready to test on mice.  Eight were chosen and inoculated with lethal doses of streptococcia.  Four were then given injections of penicillin and four were left as controls. Within 24 hours, the untreated mice were all dead and those that received penicillin survived. This promising result led to an immediate human trial – a policeman suffering from a facial infection caused by a scratch from a rose bush.  The patient responded very favourably to treatment but supplies of penicillin ran out and he relapsed and died.  Successful subsequent human trials on five patients were enough to convince them of its efficacy and production was increased for release to patients and eventually scaled up to provide the Allied troops with the drug.

Consider these points as the start for a discussion on the ethical issues and possible risks involved in these experiments:

  1. The original study sample used only eight mice.
  2. The mice were exposed to lethal doses of infection.
  3. Human trials were begun without enough of the drug.
  4. The drug was released after studies on a sample size of five people.
  5. The potential for alleviating human suffering and curing disease was enormous.

On an ethical side note, Florey did not patent the extraction of penicillin, even though he would have been fabulously wealthy as a result.  The UK government advised him that it should be available for all humankind.  US drug companies however did patent it, with the result that the UK had to pay royalties to them.


Abc.net.au,. ‘Howard Florey – Maker Of The Miracle Mould’. N. p., 2015. Web. 18 Nov. 2015.

Bos, Carole “Alexander Fleming and Penicillin – “The Wonder Drug”” AwesomeStories.com. May 25, 2015. Nov 18, 2015.


6.2 Circulation – Galen and Harvey

6.2 Theories are regarded as uncertain—William Harvey overturned theories developed by the ancient Greek philosopher Galen on movement of blood in the body.

This NOS highlights the advancement of knowledge in biology (and the Natural Sciences) – new discoveries, led by advancing technology or new insights, leads to previous theories being overturned as we develop new understandings.

Galen of Pergamon was a scientist and philosopher in the second century AD and provided the first systematic explanation of the circulatory system.
The key points of this theory included (key parts in bold):

  1. Blood is created in the liver from ingested food
  2. Some of this is sent to the lungs via the right side of the heart
  3. Some blood crosses invisible pores from the right to the left side of the heart
  4. Air from the lungs mixes with blood in the left side of the heart
  5. Blood is used by the tissues; any that is not is dissipated away

Galen was highly educated for the time and an accomplished physician and based his conclusions on some experimental work and deductive logic. He was on the right track in some places – structural and blood differences between veins and arteries, the heart is myogenic, arteries contain blood (not air) – but was clearly wayward on many other key points.

His central premise – that blood is continually produced and consumed by the body – was to undermine faulty medical practices for over a thousand years in Europe.

Harvey, a 17th-century physician, identified that if blood were to be consumed, the liver would have to produce many times a person’s body weight in blood each day, something that did not seem possible. He also identified that veins and arteries were connected in a circuit, circulating the same blood between them.

Blood systems

Schematic of the cardiovascular system over time. (Aird, W. C.)

Harvey’s work was based on a range of experiments and observations, including applying ligatures to arms to compare the flow of blood through arteries and veins and to establish the role of valves and some live experimentation on the hearts and vessels of fish and snakes.


Schematic of Harvey’s experiments with ligatures (Aird, W.C)

Harvey’s work is often regarded as the basis of modern medicine, yet it was ridiculed in his own lifetime.  In fact, he became a recluse after publishing his work, not wanting to attract any more attention to himself.  This is often seen when long-standing dogma is threatened by new ideas.


AIRD, W. C. ‘Discovery Of The Cardiovascular System: From Galen To William Harvey’. Journal of Thrombosis and Haemostasis 9 (2011): 118-129. Web. 18 Nov. 2015.

Bbc.co.uk,. ‘BBC – GCSE Bitesize Science – Circulatory Systems And The Cardiac Cycle : Revision, Page 5’. N. p., 2015. Web. 18 Nov. 2015.

Busch, Georg Paul. Portrait of Galen. Photograph of original from The Lancet. Wikimedia Commons. Accessed March 15 2018.

Membercentral.aaas.org,. ‘The Circulatory System, From Galen To Harvey | AAAS Member-central’. N. p., 2015. Web. 18 Nov. 2015.

Mijtens, Daniël. Portrait of William Harvey. National Portrait Gallery, London. Photograph original, 2012. Wikimedia Commons.  Accessed Mar 15 2018.


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.