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.


A.6 Ethology

Testing a hypothesis—experiments to test hypotheses on the migratory behaviour of blackcaps have been carried out.

Male blackcap warbler (Bird fieldguide)

In the 1950s, blackcap warblers (Sylvia atricapilla), a small European songbird, began to be observed wintering in Great Britain, instead of North Africa. These observations led to the formation of hypotheses regarding blackcap migratory behaviour: were the changes due to inherited (innate) factors, was it a response to the environment or did the birds simply lose the ability to migrate normally?

To test these hypotheses, it was necessary to use experimental methods, as fieldwork would have been unfeasible.  The birds were kept in specially designed cages that could register if birds began to become restless during the migratory season and then what direction they tried to orient towards.

The results showed that the offspring of birds that migrated to Britain oriented consistently in a NW direction, despite being raised in isolation from their parents. Follow-up genetic analysis showed a strong heritability for this trait in both British wintering blackcaps and those from other parts of Europe.  The authors were led to suggest that:

“Under moderate selection intensities and environmental conditions similar to those presented in this study, the southern German blackcap population could evolve into a short-distance migrant in 10-20 generations.” (Berthold and Pulido; p311)

These results represent rapid evolutionary changes in behaviour- it is worth considering what selection pressures are working to promote these changes.  Can you relate this back to the Evolution/Natural Selection topics (5.1/5.2)?


Berthold, P and Pulido, F. Heritability of Migratory Activity in a Natural Bird Population.
Proc. R. Soc. Lond. B. 257. 1994. 311-315. Web. Mar 15, 2016. Full text available: at https://www.researchgate.net/profile/Francisco_Pulido/publication/216768532_Heritability_of_migratory_activity_in_a_natural_bird_population/links/0fcfd50c5c5adc29c9000000.pdf

Berthold, P. et al. Rapid microevolution of migratory behaviour in a wild bird species. Nature. 360.  1992. Web. Mar 15, 2016.

“Identify A Blackcap, Sylvia Atricapilla”. Birdfieldguide.co.uk. N. p., 2016. Web. 15 Mar. 2016.

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.