QUESTION: I have been trying to design an experiment by which students can see that leaves kept in light will produce more glucose than leaves kept in dark. The idea is to have them cut equal size pieces into test tubes and use Benedict's solution as an indicator.
I have tried this three times with ivy leaves and each time the two samples turned the Benedict's solution the same color (yellow). I tried leaving the leaves in light/dark for as long as 3 weeks, and they still come out the same.
Any ideas why they are testing the same? Any suggestions for a better way to have students see how light affects sugar production in plants?
ANSWER: Hi Jon
Carbohydrates stored in the leaves are polysaccharides (starches) not monosaccharides. Benedicts tests positive for simple sugars and will not test for glucose.
In my Biology we labs we subject leaves to light and dark and then remove the chlorophyll by boiling the leaf in alcohol and the stain the leaf with Iodine which indicates the presence of starch.
This experiment can only indicate that light is required to make carbohydrates and does not indicate the quantity produced.
I have never tried a quantitative study of photosynthesis.
You might try comparing the amount of starch observed in leaves grown in different time frames. This would depend only on observation and you could not attach any mathematical number.
Another possibility would be to weigh the leaves if you had a sensitive digital scale
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QUESTION: This is very helpful; thank you! :)
May I ask what kind of leaves you use and for how long you leave them in light/dark conditions?
And are you saying that the leaves in light look noticeably darker than the leaves in dark when stained with iodine?
Looks like a great experiment for my classes. Thanks again for volunteering your time this way!
I noticed that in the first sentence of my reply I stated that Benedicts will not test for glucose. That was a mistake It will not test for polysaccharides. I did not realize that you were a teacher. It seems I contributed a lot of information you already knew.
I use Geranium plants. Have students pick a leave and cover a part of the leaf with aluminum foil ( While still on the plant of course) Expose the plant to light for about a week. Remove the leaf and boil it in alcohol to remove the chlorophyll. Place it in a dish and flood with Lugols Iodine. The part exposed to light will test for starch. (very dark blue or brown)
I have never tried a quantitative study. You may want to vary the amount of exposure to light to see if the exposed part appears darker. This would seem to indicate a greater rate of photosynthesis although it could not suggest any mathematical number