Transmission %/ethanol concentration graph..please help!

[futureless]

Hello, I'm an AS Biology student, I was drawing my graph for my coursework and wanted to know whether the lines of best fit (for the 4 repeats) on the graph should start from a point level with the 100% transmission mark on the axis, even if the actual point to plot for 10% ethanol is lower than that or doesn't it matter. Also, it is a *line* of best fit isn't it? or is it a curve? Furthermore, do the lines of best fit (or curves) have to touch the vertical axis?

I'd be grateful if anyone could help as it's really quite urgent...thanks!

[prion]

lines of best fit are plotted so that they are as close as possible to all the data points, regardless of where they cross the axes. If you knew for sure that 10% ethanol is 100% transmission then presumably you would have a data point there? Whether you draw it across the axes is up to you and depends what you're trying to show or work out.

[Tartaglia]

Absorbance = ln(100/% transmittance) = E*c*l

 

Where E = extinction coefficient, c = conc, l = path length of cell.

 

You should really be fitting a straight line to ln(transmittance) (yaxis) and concentration (x axis)

[futureless]

I am trying to see the effect of increasing concentrations of ethanol on the leakage of pigment in beetroot. I know that increasing concentration DOES break down the cell membrane and so pigment leaks out and hence transmission decreases, but I have to draw a graph of the results to show this. I don't have to work anything out. Really 0% ethanol should be 100% transmission (as it is clear) so what should I do? Should I draw the line starting at that point on the axis, even if I haven't actually got a result like that?

 

Oh and Tartaglia, erm I think that is a bit advanced for me, as I have nooo idea what "ln(transmittance)" is. Thanks for trying anyway!

[Yggdrasil]

In this case, 0% ethanol will not necessarily mean 100% transmission. Although in theory this should be true, there may be other factors which will decrease trasmission at 0% ethanol. If you want, however, you may be able to set the 0% ethanol reading to 100% by adding a constant to all the data points such that the 0% ethanol reading is 100% transmission (however, this is really only a good idea if you expect a linear relationship between conc. ethanol and % transmission, in my opinion it would be best just to plot your raw data).

[Tartaglia]

Futureless - This isn't a dig at you specifically. Its more a reflection of the English educational system. Logs were taught to 12 year olds in the 1970's and so "ln" isn't advanced at all. I'm sure you can see it on your scientific calculator

[futureless]

Yes, it is on my calculator. I'm sure many people would agree that a lot has been removed from the syllabus and moved up to learn at a later date. My chemistry teacher says the very same thing

 

In this case, 0% ethanol will not necessarily mean 100% transmission. Although in theory this should be true, there may be other factors which will decrease trasmission at 0% ethanol. If you want, however, you may be able to set the 0% ethanol reading to 100% by adding a constant to all the data points such that the 0% ethanol reading is 100% transmission (however, this is really only a good idea if you expect a linear relationship between conc. ethanol and % transmission, in my opinion it would be best just to plot your raw data).

 

by linear realtionship you mean if they are proportional right? They are proportional to each other just not directly.

 

Thanks for everyones help, these forums are so friendly!

[Yggdrasil]

Linear and proportional pretty much mean the same thing. Linear means x and y satisfy the relationship y = ax + b (a and b constant), while proportional is the special case of a linear relationship where b = 0.