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.

Working with enzymes is something that all biology students get very familiar with over their studies!  This is particular true in the new syllabus as Practical 3 requires an enzyme experiment and they are popular as topics for the IA.

IMG_2768
Testing the effect of substrate concentration on potato catalase.

Although there are several other practical-themed NOS statements, this one makes particular reference to the idea of reliability and replicates.  In terms of assessment this is most likely to appear in the Section A of Paper 3, when students are provided with experimental scenarios and have to apply their knowledge. However, it is also important for the IA.  The chosen investigation must design a methodology that will collect sufficient data, the data must be processed with appropriate awareness of uncertainties, and the reliability of the results reviewed and evaluated in the conclusion and evaluation.

This is thus an important lesson to not just experience the practical side of biology, but to understand the importance of replicates and how this impacts the IA.  So what could this look like? Here are a few ideas:

  • If an enzyme-based experiment, aim for five variations of the independent variable (five different pHs; five different temperatures etc). As enzyme experiments are invariably time-based, this will allow you to plot a graph with more confidence (five data points rather than, say, three).
  • Try to repeat each variation five times.  This will provide enough data to calculate the average and standard deviation. Of course there are more processing options than this (think rate of reaction) but these two are the basics.
  • The conclusion/evaluation needs to then assess how the range of the independent variable, the sample size and the processed data contribute to the reliability of the experiment. The more replicates you have, the more robust this section will be.
  • There are always time constraints on how many replicates you can collect – so factor this into your methods.  If your experiment is only collecting data for three minutes for each run, then you should be able to get more replicates (and you will be expected to collect more data).  If, in contrast, you are collecting data for more than an hour per experiment, then you will need to be aware of this.
  • Finally, remember that design, processing and evaluation are all relative to the specific experiment you carried out – so always think in terms of the context for your investigation and the resources available to you.

 

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1.4 Membrane Transport

Experimental design—accurate quantitative measurement in osmosis experiments are essential. (3.1)

This is one of the NOS that is relatively easy to incorporate into your learning.  Prescribed practical 2 is Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions; when you do this lab, you get a first-hand experience in why these measurements are important.

In my class we use potatoes and sucrose solutions of 0.1-0.5M, plus distilled water. We cut them into approximately equal-sized “chips” and place them in test tubes containing each of the six solutions.  Next class (or within 24h) we then remove them, measure again and investigate the changes.

While the potatoes are bathing in the solutions, we discuss the NOS as a class.  Here are some of our talking points:

  • The movement of water, which will influence the change in size of the potatoes, is likely to be small. Thus accurate measurements are needed to demonstrate that there has indeed been change, rather than just random variation.
  • Following on from this, the more measurements that can contribute to the data, the more accurate picture we might have – thus we need to accurately measure length, height, width and mass.
  • Accurate replicates will enable us to process the data to investigate any changes with confidence.
  • Measuring grams/mm requires careful attention to the uncertainties attached to those measurements.
  • While qualitative data is still important, it is less objective than accurate quantitative measurements.
  • These measurements need to be coupled with carefully controlled variables to allow the most accurate conclusion to be drawn.

 

2.8 Respiration and Ethics

Assessing the ethics of scientific research: the use of invertebrates in respirometers has ethical implications.

The use of animal models in biological experiments has a long history. Indeed, many of our most important discoveries were made possible by using animal test subjects. However, using animals at any time during an experiment has ethical implications that need to be evaluated.

Any scientific research involving animals will have to satisfy an ethics board as to the justification for using and/or experimenting on animals. Two key issues that scientists have to consider might include: what suffering or pain will the animal experience and are there alternatives to using animals?  There is a process in the UK called the 3R’s – replacement, refinement and reduction of the use of animals in research (Festing, S. and Wilkinson, R.).  This process, while acknowledging that animals may be required in certain circumstances, aims to ultimately reduce these to only the most essential experiments.

There is often a difference in concern between invertebrates and vertebrates in terms of what ethical rules apply to them.  Most people probably care less about the fate of cockroaches or crickets in a respirometer experiment than about the use of mammals in medical research.  However, it is still important to evaluate the ethical use of invertebrates in the same way as vertebrates. In addition to the issues of pain/suffering and replacement, we should consider:

  • whether the animals can be released back into their natural habitat
  • whether it is ethical to remove them in the first pace
  • whether we can minimise any pain or suffering that may take place in the experiment.

The IBO has published a document on the use of animals in experiments and it is very clear that any animal (invertebrate or not) must be treated ethically and must not be subject to any suffering or environment outside its normal range. This link from the Nuffield Foundation outlines an experiment based on this; take note of their ethical issues paragraph after the methods.

Sources:

Allott, Andrew, and David Mindorff. Biology. Oxford: Oxford University Press, 2014. Print.

Festing, Simon, and Robin Wilkinson. The Ethics Of Animal Research. Talking Point On The Use Of Animals In Scientific Research. EMBO Reports 8.6. 2007: 526-530. Web. 27 Jan. 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.