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Linking Conclusions to Hypotheses

Making the jump from problem to conclusion when carrying out an experiment can be a quantum leap for some students. The lack of focus and the tendency to get things turned around often results in writing a conclusion that causes teachers to cry. What can we do to ensure that the time and effort that goes into an experimental lab bears the fruit of conceptual understanding? Formalized hypotheses are the link to logical conclusions. Let's see how and why this works.

I will use an example of one of the sample labs I have included, "Turgor Pressure and Rigidity in Herbaceous Tissue." The problem for this lab addresses how herbaceous plants derive support since there is no woody tissue and the cells are not lignified as in the cells of woody plants? The observation of wilted flowers and limp salad provide us with a possible clue. Most of us have noticed that if you don't water your yard the herbs wilt and if you soak cut carrots and celery before they are served, they are firm and crisp. From these observations we can develop the following hypothesis:

If rigidity in herbs is related to turgor pressure (water content of cells), then increasing (or decreasing) the water content will increase (or decrease) the rigidity of herbaceous tissue.

In this lab, we use pieces of potato cut to uniform size which are measured for mass and length before and after treatment. To induce the movement of water into or out of the cells, the potato pieces are placed into hypertonic or hypotonic solutions of sucrose.

Students gather data, plot graphs on the gain and loss of length and mass and subjectively rank the relative stiffness of the potato tissue compared to fresh cut samples. The graph shows typical sample data.

So what can a student conclude from this experiment? There are several possibilities. She/he cold say that the potatoes shrink when placed in high sugar concentration and loose mass. Or, the potatoes grow in pure water and gain mass. But these are just observations and do not address the original problem or the hypothesis. Furthermore, the graph alone is not the only data. The students also have data that ranks the relative stiffness of the potatoes. In this case the stiffest piece was in pure water and the least rigid piece was in 18% sucrose solution. By prompting the student to return to the hypothesis and state whether or not the data supports the predicted relationship, i.e., is rigidity related to the contents of the cell? You could get a yes or no answer which is not sufficient for a conclusion. You must also prompt the student to restate what is the relationship, describe it, and tell how she/knows it is so. For example, Rigidity is relatedto turgor pressure, more turgor press ure,more rigidity. Potatoes in pure water gained mass implying that water was gained thus fillingthe cells, creatingmore pressure and swelling their size.

The key to good conclusions is to establish standards for what is expected and then model it. A solid conclusion should always relate back to the original hypothesis, describe how the relationship works, and relate how the data supports this conclusion.

Conclusions are often inferences drawn from what observations and data imply. In biology, conclusions often must describe the limits of the relationship, for example, in hatching brine shrimp eggs, brine shrimp may hatch in distilled water, but they die in short order. Hatching success increases with salinity up to a point in which salinity begins to decrease the hatching response. Enzymes are another good example where a range in the relationship occurs. This means that relations are not always explicit and clear cut. The physical sciences are not plagued by this problem as much as the biological sciences. No wonder that biology is such a challenging science!

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