Measuring rate of photosynthesis

[ronthepon]

Now, for a science project I have to illuminate leaves with lights of different wavelengths and plot a rate-wavelength graph.

 

Now how on earth do I measure the rate of photosynthesis?

 

I have considered placing the leaves in bags posessing light polarising abilities (called cellofane or something of that sort). Then, I shall measure the oxygen released by studying the time for which an Incense stick will remain burning inside.

Is this a good idea?

 

Or is there a better alternative?

 

(I love this site)

[Tim_Lou]

well, maybe the increase of mass of the plant/leaves? mass can be easily measured.

[ronthepon]

Well, mass is not a good idea, because a plant also has a good deal of other matter coming and going, further the true rate of photosynthesis focusses on conversion of low energy mass to high energy mass...

 

Thanks anyway.

[sebbysteiny]

Yeah, the mass is a great idea.

 

From E = mc^2, the energy absorbed by the plant will increase the mass of the plant. Now all you need is science to invent a super sensitive mass metre and your experiment will work a treat.

 

There is one way to actually measure the concentration. Put a sample of the air into a measuring cylinder. Put the measuring cylinder upsidedown into water. Use a substance that absorbes oxygen into the cylinder and measure the volume of water that has entered the test tube. Use this to find the %age. If you can find a suitable oxygen absorber, it should work without much problems.

[ronthepon]

Okay, here is why mass wont be good even if we get supersensitive mass measurers.

 

1- There is active movement of matter through phloem in leaves.

 

2- Transpiration: water movements are enormous.

 

3- Carbon-di-oxide in and Oxygen out movements need not be exact enough compared to each other.

 

So the fluctuations will be so enormous compared to energy related mass, that its not an excellent idea...

 

As for the oxygen absorber, I could try burning a little peice of charcoal until all oxygen is used up. Then I dissolve the carbon-di-oxide into the water below by pressurising.

[UncleAl]

Google

photosynthesis "action spectrum" 34,000 hits

 

http://www.phschool.com/science/biology_place/biocoach/photosynth/action.html

 

Does Uncle Al get an A in the course for being minimally competent?

[sebbysteiny]

At the risk of pursuing the most silly tangent to a thread ever, with a super sensitive mass meter, you could make the measurements. Just make sure your measuring the WHOLE system, air, water, soil, etc, and the mass of the whole system will increase as the solar energy is absorbed. Just ensure you make your measurements at exactly the same temperature, so you might need a supersensitive thermomitor as well and a super fine thermostat. No probs.

[Tim_Lou]

nevermind

[Tim_Lou]

after some more thinking, the best way to do this seems to be putting leaves underwater, and collect air bubbles. for a give amount of time, measure the air produced.

[sebbysteiny]

correct me if I'm wrong, but doesn't photosynthesis require carbon dioxide gas?

[ronthepon]

Yes. Leaves do need [math]{CO}_2[/math]. How would it be if I put the leaves under jars, with the bottoms of the jars dipped inside water?

 

Then I measure the oxygen evolved somehow.

 

UncleAl, I happen to be rather impatient at the moment, so I cannot try google surfing...

[UncleAl]

I happen to be rather impatient at the moment, so I cannot try google surfing

Six months in the lab will save you an afternoon in the library.

[InfiniteNow]

correct me if I'm wrong, but doesn't photosynthesis require carbon dioxide gas?

I'd like to clarify that we cannot know if plants on other planets (come on, you know there are at least a few...) require CO2 for photosynthesis, nor if EVERY plant on Earth does.

[sebbysteiny]

I did a quick google search and I found the following link that might be useful.

http://waltonfeed.com/self/upack/oxyintro.html

 

The problem with your experiment is that there is very little %age of CO2 in air. Therefore, the photosynthisis will stop reletively quickly. Instead, might I suggest filling the jar with carbon dioxide. This isn't too tricky. Just add a large amount of carbonate to acid in a test tube filtering the contents into water. Obviously ignore the first few bubbles (because it's air) and then use the bubbles to fill up a jar full of water. Then put the plant in the upside down jar making sure the CO2 doesn't escape. But instead of a jar, use a measuring cylinder and also, make sure an oxygen absorber is inside too.

 

From what I've seen, oxygen absorbers only cost about £5 or something, and I'm sure your school will be prepared to refund the costs.

 

For bonus points, do you know which colour light will be the worst for photosynthasis?

[InfiniteNow]

For bonus points, do you know which colour light will be the worst for photosynthasis?

Higher frequency... blues, purples, ultra-purples... causes heating and evaporation and probably some structural damage.

 

 

What do I get? Do I get a cookie? Can these bonus points be turned in for prizes? I want the hand-cuffs! :)

[Mercedes Benzene]

If you are able to choose the type of plant used, I think you could do this:

 

Get some sort of aquatic plant...

Put samples of the plants into jars or test tubes or something, submerged with water.

Put an acid indicator into the test tubes.

Blow Carbon dioxide into the test tubes through a straw (the color should change due to the presence of Carbonic acid in solution)

Put your samples under the different light sources.

Measure the rate of change of CO2 in the jars.

 

Not only will you see oxygen gas being evolved, but the indicator color will also change color!!

Just my idea... i don't know. GOOD LUCK!

[sebbysteiny]

What do I get? Do I get a cookie? Can these bonus points be turned in for prizes? I want the hand-cuffs!

 

Yes, you do get a cookie and some hand-cuffs too :) .......

but only if you were right first time!!

 

I'm sorry to say it but ;) :doh:

 

try again :)