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.