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Why doesn't the rate of photosynthesis increase with increasing light energy (frequency)? I know that green light is not absorbed by chlorophyll, yet I am confused as to why this certain "frequency" is not absorbed. Intuitively, it makes sense that increasing frequency would make the rate faster.

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To understand why the wavelength doesn't change the rate of photosynthis you have to look at the chemical processes. I won't name the molecules and all the stuff but I'll try to paraphrase it in generic terms.

Photosynthesis means that in the end a carbohydrate has been assembled where you had only water and carbon dioxide when you started. The assembly of carbohydrates can be broken down in very clear steps. The power needed for this assembly comes in well defined quantities. These quantities are fixed by the amount of energy needed to activate a certain center (in fact two different ones) inside the chlorophyll. It is clear, that any surplus energy doesn't change the modification of the molecule (electron transport), because when the needed energy is delivered, the reaction takes place. The surplus energy is lost and dissipates in different ways (photons, phonons etc.), because there is no other place where it can foster a reaction.

The other fact related is: Light also comes in portions and not as a continuum. Single light quantums accidentally hit the centers where they can deliver the energy for a single reaction. So the energy from a light quantum can start the reaction if it contains enough energy but only one. Neither it can't be spread over more reaction centres nor can the surplus energy transferred as "change" to another reaction. Even if possible, the arrival of the surplus energy of photon A used in reaction 1 must be rather synchronous to the arrival of photon B at reaction 2. This is very unlikely.

That's why lamp manufacturers declare the usable power of their grow lights with μmol/s and not in Watts or lumens. Only the number of photons counts. More energetic photons (having a shorter wavelength) can't start more than one reaction per photon.

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There are 2 frequencies/wavelengths absorbed and used by green chlorophyll ( forgot the numbers). To increase photosynthesis rate you need to increase the amount of light ( lumens) at those frequencies. Not provide different frequencies .

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    $\begingroup$ side note: It is unlucky to use lumens in this context. Because depending on wavelength there's a very different power and also a very different photon count for one lumen. I recommend referring to the particle flux (μmol/s) when talking about light for photosynthesis. $\endgroup$
    – Ariser
    Nov 24 '20 at 19:52

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