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.


9.3 Growth in Plants

Developments in scientific research follow improvements in analysis and deduction—improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression.

In this case, the NOS statement is referring to the use of genomics to analyse gene expression in plants. Some of the earliest experiments into tropisms were conducted by none other than Charles Darwin, whose 1880 book The power of movement in plants represented the first attempt to synthesise available evidence on tropisms and included many of his own experiments in this field.  As developments in technology increased, the role of hormones became increasingly important and better understood.

Hormones influence gene expression; by detecting changes in gene expression, we can determine the role of hormones in this process. DNA sequences have been analysed to determine how these change in response to hormone exposure and mRNA levels (evidence of transcription and hence gene expression) can also be detected, pinpointing the cells that are responding to these hormones. Scientists have detected a range of common short sequences of nucleotides, from four to twelve bases in length. Different combinations of these appear to be linked to specific hormones and allow the genes to be affected by different classes of hormones. Details on some of these nucleotide sequences can be seen in the table below (Plants in Action).  Experiments have revealed that plant hormones can act extremely fast – with mRNA changes detected as quickly as 2-5 minutes from exposure.

Table 9.02.png
Image from Plants in Action 1st Ed.

The advances in microarray analysis of DNA and mRNA was thus critical to our improved understanding of how plant hormones work.


Jennifer J. Holland, Diana Roberts, Emmanuel Liscum; Understanding phototropism: from Darwin to today, Journal of Experimental Botany, Volume 60, Issue 7, 1 May 2009, Pages 1969–1978, https://doi.org/10.1093/jxb/erp113 Web. Accessed 6 Feb, 2018.

“Modified Gene Expression.” Plants in Action, 1st ed., Australian Society of Plant Physiologists, 1998, plantsinaction.science.uq.edu.au/edition1/?q=content/9-2-4-modi-ed-gene-expression. Web. Accessed 6 Feb, 2018.

9.2 Transport in the Phloem

Developments in scientific research follow improvements in apparatus—experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available.

This NOS is another example that fits nicely into the syllabus content.  In 9.2, students are asked to analyse …data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide. We can learn about this process through the NOS to understand how it works and why it enables us to understand the flow of sap through the phloem.  Two for the price of one!

Radioisotopes have been encountered before in your IB Biology studies. They become widely available to researchers after the second world war.  As the molecules that are radioactive can be traced, it became possible to track the flow of these molecules through cells, tissues and organisms over time.

Phloem transport is based on high hydrostatic pressure.  Thus if the phloem can be punctured, the contents should continue to exude out. If the plant is exposed to radioactively labelled carbon dioxide, the sap can be tested for the presence of the isotopes and the rate of translocation can then be estimated.

Collecting Phloem Sap using Aphid Stylets (D. Fischer)
Image from D. Fischer, Plants in Action.

The use of the aphids can be summarised as follows (base on the images above; Plants in Action; image D. Fischer):

Top Image: Aphid feeding, inset is the stylet (St) penetrating to the phloem (p)
Image a: feeding aphid with stylet penetrating the plant
Images b-d: the stylet is removed from the aphid (they would be anaesthetised beforehand – there are some links here to the use of animals in experiments)
Image e: phloem sap starts to accumulate from the stylet.

The droplet of phloem sap can then be analysed for traces of the isotope to determine transport rate.  Aphids can be placed along the length of the plant stem to show transport along various distances.


Fischer, D. “Collection of Sap from Aphid Stylet.” Plants in Action, University of Queensland, 2018, plantsinaction.science.uq.edu.au/content/522-techniques-collect-phloem-sap. Digital image. Feb 1, 2018.

“5.2.2 – Techniques to Collect Phloem Sap.” Plants in Action, Australian Society of Plant Scientists, 2018, plantsinaction.science.uq.edu.au/content/522-techniques-collect-phloem-sap. Online Textbook. Feb 1, 2018.

9.4 Reproduction in plants

Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.

Albert Einstein is claimed to have said: “If the bee disappears from the surface of the earth, man would have no more than four years to live.” (Quote Investigator). While it appears he did not actually ever say this, it is a remarkably prescient observation about the role that pollinators play in continuing the survival of plants.

While wind, water and explosive propulsion do work for some flowering species, the majority rely on animal pollinators. The co-evolution of pollinators with the plants they pollinate means that, in many cases, species may be pollinated by only a select few animals.  Should the animals decline in population, so will the plants.

A recent example in New Zealand reminds us of this (Biello). The endemic flowering shrub Rhabdothamnus solandri, or New Zealand gloxinia, relies primarily on the bellbird (Anothornis melanura) and stitchbird (Notiomystis cincta) to pollinate its flowers.  These birds have long beaks and tongues to access the long, tubular flowers of the shrub.  However, the bellbird and stitchbird have recently become extinct on New Zealand’s North island. To investigate this impact on the flower, researchers conducted a study on three smaller offshore islands, where the birds were still present. The results were alarming – in the absence of the two birds on the North island, just 22% of of flowers produced fruit and had only 37 seeds per flower. This compares to the the islands that still have the birds, where they produced 232 seeds per flower and 58% produced fruit. In order to save this flower, we must also save the birds.


New Zealand Gloxinia – Rhabdothamnus solandri. 


A paradigm shift represents a radical change in thinking based on new evidence.  Understanding that protecting birds, for example, will also protect plants, is an important change in thinking from purely looking at the conservation of single species. Conservation methods that focus on ecosystems as a holistic unit reflect our increased understanding of the way animals and plants in particular are inter-related.


Biello, David. “For Want Of A Pollinator, A Flower May Be Lost–Or A Forest”. Scientific American. N. p., 2016. Web. 13 Oct. 2016.

“If The Bee Disappeared Off The Face Of The Earth, Man Would Only Have Four Years Left To Live | Quote Investigator”. Quoteinvestigator.com. N. p., 2013. Web. 13 Oct. 2016.

“T.E.R:R.A.I.N – Taranaki Educational Resource: Research, Analysis And Information Network – Rhabdothamnus Solandri (Taurepo) “. Terrain.net.nz. N. p., 2016. Web. 14 Oct. 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.