# How does light intensity affect the rate of photosynthesis?

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jerryyu    10
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Samm    2

Light intensity has a very large effect on the rate of photosynthesis. In most cases, the more light the faster photosynthesis occurs. However, at some point there's a limit that the rate of photosynthesis cannot increase above. This is because it's limited by other factors, such as the level of carbon dioxide in the air.

It's interesting to note that the wavelength of the light also influences the rate of photosynthesis. Here is the absorption levels of two of the different forms of chlorophyll, chlorophyll a and chlorophyll b in a solvent:

The significance of this is that, among other things, chlorophyll's main purpose is to absorb the light energy necessary for photosynthesis.

In short, the intensity and the wavelength of light both affect the rate of photosynthesis.

Sources:

http://en.wikipedia....iki/Chlorophyll

http://en.wikipedia..../Photosynthesis

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Yeah NASA did a experiment a couple years back to see whether different coloured light affected the absorption of photosynthesis. It was basically to testing to maximise growing crops in space, to minimise the space taken up by crops but to maximise the yield.

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Samm    2

Yeah NASA did a experiment a couple years back to see whether different coloured light affected the absorption of photosynthesis. It was basically to testing to maximise growing crops in space, to minimise the space taken up by crops but to maximise the yield.

The article is here: http://www.digikey.c...tests_LEDs.html

That's actually quite interesting. It's odd that different wavelengths of light make the plants grow in different ways.

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lemur    155

Yeah NASA did a experiment a couple years back to see whether different coloured light affected the absorption of photosynthesis. It was basically to testing to maximise growing crops in space, to minimise the space taken up by crops but to maximise the yield.

The article is here: http://www.digikey.c...tests_LEDs.html

This is really interesting considering how much energy traditional grow lamps demand. It's too bad they didn't publish the LED wattage used and crop yield in mass/weight of edible produce.

edit: I don't get the point of adding green LED except to make the plants appear green while they're growing. If they reflect it, then they're not absorbing it, right?

edit2: I also wonder if plants don't get any benefit from infrared and/or UV in addition to visible colors.

Edited by lemur

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Chlorophyll A displays two absorbance "shoulders" and two "peaks" at wavelengths under 400 nm. So, in short chlorophyll absorbs somewhat strongly in the near and middle UV.

It is worth mention that the strong ligand to metal charge transfer, or metal to ligand charge transfer (can't remember for which one for chlorophyll, probably ligand to metal) bands caused by strong $\pi$ back donation to and from the metal center drown out all other signals in the visible region. The peak 670 nm is the $\pi$-metal LMCT.

I tried to post a spectrum to show this but the forum software keeps deleting my image when I preview the post.

Edited by mississippichem

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Samm    2

This website explains it a bit more, the part you are looking for is around the middle.

http://www2.estrella.../BioBookPS.html

That looks like a really good website actually.

edit: I don't get the point of adding green LED except to make the plants appear green while they're growing. If they reflect it, then they're not absorbing it, right?

Well, I don't think they're absorbing it as much as the blue and red light. But it appears as if they're still absorbing it. I believe this website (http://www.digikey.c...tests_LEDs.html) actually said this:

However, the addition of green light may promote increased plant growth since green light penetrates the plant canopy better than red or blue light. Leaves in the lower canopy could use the transmitted green light in photosynthesis

edit2: I also wonder if plants don't get any benefit from infrared and/or UV in addition to visible colors.

Plants don't really get any benefit from infrared light. According the same site:

LEDs can illuminate near the peak light absorption regions of chlorophyll while producing virtually no near-infrared radiation (which is not [used] in photosynthesis),"

Emphasis mine.

Edited by Samm

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lemur    155

Plants don't really get any benefit from infrared light. According the same site:

Maybe photosynthesis doesn't benefit but the plant could benefit for some other reason. It could help with water-transport, for example. I wonder about the UV, though, since energy-density increases rapidly with frequency-increase. Does plant growth consume a lot of energy or not much?

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Samm    2

Maybe photosynthesis doesn't benefit but the plant could benefit for some other reason. It could help with water-transport, for example. I wonder about the UV, though, since energy-density increases rapidly with frequency-increase. Does plant growth consume a lot of energy or not much?

Well, I'd imagine it consumes a fair bit of energy. It's performing an endothermic reaction on quite a large scale in a way that produces very specific products. It's going to take a lot of controlling through various other chemicals, and the process itself is very complex, so I'd imagine it'd eat up a decent amount of energy.

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lemur    155

Well, I'd imagine it consumes a fair bit of energy. It's performing an endothermic reaction on quite a large scale in a way that produces very specific products. It's going to take a lot of controlling through various other chemicals, and the process itself is very complex, so I'd imagine it'd eat up a decent amount of energy.

Enough to necessitate the energy-density of UV frequency? Or could sufficient energy be supplied by the visible spectrum?

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Samm    2

Enough to necessitate the energy-density of UV frequency? Or could sufficient energy be supplied by the visible spectrum?

I wouldn't really know. At the moment, I'd just be saying, the more energy absorbed, the better.

Edited by Samm

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Greippi    33

Light intensity has an effect on the gene expression levels of various components of the photosynthetic machinery, too.

Maybe it's already been said, or maybe it's obvious - but too high light intensities have a damaging effect - some are perfectly happy to hang around in the gloom under the sea.

And, of course, different photosynthetic organisms thrive on different wavelengths of light.