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When reading about how green / leafy plants work, I saw that they have chlorophyll A & B, which allow the plant to use the energy from light by capturing and transforming.

When reading about Chlorophyll, I saw some charts showing "peak points" for each type for light wavelengths. Apparently, each chlorophyll type has both peak points.

Since from further reading I learn that plants can use all the light in the visible spectrum for photosynthesis, I'm wondering (my questions >>):

  • What do "peak points" of wavelength correspond to / stand for?
  • How does light peaking in red spectrum (say around 630nm) differ to (or affect plants) compared to light in red spectrum peaking at 680nm (which I gather is Chlorophyll A peak-ish)?
  • When same amount of radiant flux is projected to leaves on different wavelengths, one far from a chlorophyll peak the other near, which one gives more energy to plant and why?
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First thing to note is that the absorption spectrum of chlorophyll A and chlorophyll B is similar but not the same. The reason for the appearance of absorption spectra is because the light is composed of photons of light. Different photon has different oscillation frequency and energy. The essence of Chlorophyll absorption is absorbing photons. After chlorophyll absorption of a photon, the electron transition from the ground state to the excited state, and get water electronic from the water cause decomposition. Only phonton that has resonant vibration frequency with electrons, and can meet the energy requirements of electronic transitions in order to be absorbed by chlorophyll. The Photon energy of different wavelengths of light is different. This is also the reason of different wavelengths of light affect chlorophyll light absorption.

Simply explain the absorption spectrum:

  • 280 ~ 315nm has small effect of plant morphology and physiological processes
  • 315 ~ 400nnm absorption rate is small, impact the photoperiod, prevent stem elongation
  • 400 ~ 520nm (blue) Absorption ratio of of chlorophyll and carotenoid in this light is maximum, it has the greatest impact on photosynthesis
  • 520 ~ 610nm absorption rate is not high
  • 610 ~ 720nm (red) low absorption rate. It has a significant effect on photosynthesis and light cycle effect of chlorophyll.
  • 720 ~ 1000nm Low absorption rate, stimulate cell elongation, impact of flowering and seed germination
  • 1000nm converted into heat

p.s. white light is a mixture of light

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  • $\begingroup$ Hi @meateatingbunny, +1 on rephrasing and simplifying please. Also, can you please make 3 points to answer three parts of my question? Would greatly appreciate it! Thanks. $\endgroup$ – Phil Dec 2 '14 at 10:18
  • $\begingroup$ @meateatingbunny - why do you say that 610 ~ 720nm absorption rate low when all charts show differently? What do you mean, can you please explain? $\endgroup$ – Phil Dec 2 '14 at 18:50
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Lets start with the absorption spectrum first (image from the Wikipedia page in chlorophyll):

enter image description here

What you see in the figure is the absorption of light thoughout the visible spectrum by chlorophyll a (blue) and b (red). The higher the peaks get, the higher the absorption is. What we can see, is that chlorophyll absorbs light roughly until 500 nanometers (nm) and from 620 nm. The light between this is not absorbed, but reflected. This is the green light and the reason why leafs are green. Above 700 nm the light is reflecxcted again, this is the reason why green leafs are visible white in infrared photography.

The peaking of the both chlorophylls ensures that light of the wavelength can be used efficiently for photosynthesis. Since leafs contain both chlorophylls, this gives a wider range for energy absorption than with only one. Additionally both absorption maxima are far enough from each other to prevent one chlorophyll from "stealing" light from the other.

The light closer located to the peak transmits more energy because it will absorbed better. The closer you get to the maxima of the absorption curves, the higher the absorption and the energy transfer will be.

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What do "peak points" of wavelength correspond to / stand for?

The peak points of chlorophyll and their correspondence to the action spectrum indicate that the Chlorophylls play a major role in photosynthesis, it is also clear that chlorophyll isn't the only chemical involved in light harvesting and that other antenna pigments are utilized.

As mentioned in Chris' answer chlorophylls (plural) absorb and reflect light to a various degree at different frequencies, as explained on webpage in the link that was provided (in the section photosynthesis).

From Wikipedia's Chlorophyll webpage, "Measurement of chlorophyll content" section:

"In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; chlorophyll c has a few isomers, peaks for c$_1$ are 442 nm and 630 nm; peaks for chlorophyll c$_2$ are 444 nm and 630 nm; variety c$_3$ is recently discovered; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm;" chlorophyll f (also a newer discovery than information offered on Wikipedia) absorbs at 720nm - making it the most red-shifted chlorophyll to date.

Action Spectrum

Notice that chlorophyll A's Soret peak provides the greatest rate of growth. Blue light also affects phototropism more than red. Different wavelengths affect the plant differently, some cause growth while other wavelengths regulate bending, even seed development.

Here's an image from cell.us's webpage: "Action Spectrum Green Plants":

UV and deep blue

Note that plants reflect infrared light beyond 700nm, that is why they appear white (instead of black) in the right side infrared photograph:

RGB vs. Infrared Click to zoom

How does light peaking in red spectrum (say around 630nm) differ to (or affect plants) compared to light in red spectrum peaking at 680nm (which I gather is Chlorophyll A peak-ish)?

Chlorophyll b simply expands the range of usable light available to the plant.

When same amount of radiant flux is projected to leaves on different wavelengths, one far from a chlorophyll peak the other near, which one gives more energy to plant and why?

Light frequencies closest to one of the peaks has the most affect on growth, as you can see from the action spectrum blue light is more energetic than red and even mid-band (green) light has a small but significant effect on growth. Because green light is not absorbed some is reflected and some passes through the upper leaves to the lower shaded leaves.

Plants use light inefficiently because it isn't a limiting factor in its growth, instead CO$_2$ is.

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