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I am to conduct a lab investigating how different wavelengths of light affects photosynthesis in Egeria pondweeds.

The idea is to put color filters on light bulbs and shine them on the pondweed in a sodium hydrogen carbonate solution. Then to measure the oxygen, I have an oxygen sensor which measures dissolved oxygen (mg/L).

Now, what I have a trouble with is if measuring the dissolved oxygen is the right way to measure the rate of photosynthesis depending on color of light? Will it work?

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    $\begingroup$ One experiment I saw online, was that during 5 minutes you measure the oxygen dispersed, and it will show you the mg/L depending on time. $\endgroup$
    – Timothy
    Commented Dec 16, 2021 at 23:07
  • $\begingroup$ That solution might just kill the plants. $\endgroup$
    – Polypipe Wrangler
    Commented Dec 17, 2021 at 5:50
  • $\begingroup$ Why not do it in a standardised pondwater, unless you don't mind the results being for only that sodium bicarb solution? $\endgroup$
    – Jiminy Cricket.
    Commented Dec 18, 2021 at 13:19
  • $\begingroup$ A couple thoughts. The plant is undergoing cellular respiration too, so I am not sure that there will be a net oxygen out. I assume that the sodium hudrogen carbonate is going to provide a source of carbon dioxide to a closed system, but even then the plant will release carbon dioxide from respiration from sugar that it has made and burned. $\endgroup$
    – Joseph Hirsch
    Commented Dec 19, 2021 at 1:07
  • $\begingroup$ Is the atmosphere sealed and airtight? You could possibly have Sodium Hydrogen Carbonate in a water reservoir in the container, but not in the water that the pondweeds are growing in. $\endgroup$
    – Joseph Hirsch
    Commented Dec 19, 2021 at 1:12

2 Answers 2

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Bubble formation might be easier to measure than oxygen concentration. Bubbles can be measured optically Or acoustically.

A sealed container with a little carbon dioxide gas might work.

See below article.

“Acoustic effects during photosynthesis of aquatic plants enable new research opportunities”

Helmut G. Kratochvil and Michael Pollirer http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5349586/

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  • $\begingroup$ You can also work out via displacement - pondweed underneath an inverted container, traps bubbles that escape (gentle agitation might help) and measure volume . $\endgroup$
    – bob1
    Commented Jan 18, 2022 at 1:30
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The experiment means anything only if different "colors" (wavelengths of light) affect the rate of photosynthesis. A good experiment requires that you "keep everything same/identical*, to the extent possible, save the color of the light the plants are exposed to. If you detect any changes in the concentration of oxygen, you can plot the color of light to the oxygen concentration.

Photosynthesis is a chemical reaction: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

At the very least, the concentration of $CO_2$ and amount of $H_2O$ must be constant for different colors, as they can alter the rate of photosynthesis, and are therefore confounding factors.

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