Can you help me with the details of an experiment to find the concentration of lead which affects the permeability of onion cell membranes?

I am to determine the lowest concentration of lead ions that causes plant cell membranes to lose their partial permeability, resulting in the cells failing to show plasmolysis. For this investigation, I use the outer leaves of red onions as red pigment allows any plasmolysis to be observed easily and clearly. The problem is that I am not sure of how to measure the degree of plasmolysis, showing the degree of loss of partial permeability. I have thought of using a microscope to count the number of plasmolysed cells and the total number of cells within the observed area to find the percentage of plasmolysed cells as a measurement of plasmolysis. What do you think of this? Is there any other better method of measuring plasmolysis? Also, would 10 minutes be sufficient for lead ions to have any effect on the cells?

Your method of finding the % of plasmolysed cells is exactly the one I have used many times. You need to be careful to take measurements on a range of at least 5 areas, spread at random over the piece of tissue on the slide, so you can produce a fair average count (and perhaps a standard deviation). The problem I have found is that the tissue tends to plasmolyse more at the edges than in the centre of the tissue (even though I allow time for the plamolysis to take place before putting the cover slip in place). You need to explain WHY you have chosen your procedure.

Your estimate of 10 minutes for plasmolysis to take place is quite reasonable, but I think there is an unknown about how long the lead would take to have its effect. I GUESS the lead would have an almost instant effect, but you need to decide whether you will put the lead in with the plasmolysing solution or whether you will use a pre-treatment. Whichever you decide does not really matter - the important thing is that you should be able to justify your choice.

Sometimes, when designing an experiment, it is necessary to carry out some trial experiments first, to determine the effect of some of the variables (eg length of time for lead to have its effect)

John Hewitson.