MS-EXCEL: GOING FURTHER WITH GRAPHS iI: CALCULATING SLOPES

Using MS Excel to determine rates from a graph

Biological processes are often analysed by comparing their rates. That is the speed at which they progress.

Often the rates of the reactions are not constant. Living organisms need time to acclimatize to the experimental conditions or reactions may slow down as substrates are used up. The fastest rate or the most uniform rate can be determined by inspecting a graph.

The graph below shows the results of an experiment where a water plant was given sodium hydrogen carbonate (a source of dissolved carbon dioxide). The plant was left for 10 minutes in a constant light source in a closed test tube. A pressure sensor measured the build up of oxygen gas as a measure of the rate of photosynthesis.

It can be seen from this graph that the increase in pressure due to the build up of oxygen takes 5 minutes before it starts to give consistent results. So by inspection the slope will be calculated from the readings between 5 and 10 minutes.

The slope of a line can be calculated in Excel using the slope function.

Select a cell and click on the Functions menu. Choose Other functions

From the menu select Slope (Fr: Pente)

A dialogue box will appear into which you must put the series of x and y values that will be used to calculate the slope.

The graph indicates that the slope, from which the rate of photosynthesis can be calculated, should be taken from the period between 5 and 10 minutes. Select these values from the y-axis (changes in pressure).

Then do the same for the x-axis (time).

Click on OK or press Enter

The value for the slope will appear in the cell that was selected.

The number of decimal places may now be adjusted.

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© Paul Billiet 2007