Which light is least effective in photosynthesis?

Photosynthesis factor light color

The influence of the environmental factor light intensity on the photosynthesis rate of a green plant could still be predicted with common sense: the more light, the higher the photosynthesis rate. What influence the lightcolour on the photosynthesis rate cannot be inferred that simply. You can, however, proceed experimentally by slightly modifying the waterweed experiment that we got to know on the method page. This time we do not irradiate the waterweed plants white Light, but with colored Light of different wavelengths. For this purpose, the teaching material dispatch department offers special color filters that can be held in the beam path.

Method: color filter

Why can't you just use the inkjet printer to print overhead transparencies in different colors and hold them in the beam path?

The reason is very simple: if we want to investigate the photosynthesis factor light color, we have to keep all other photosynthesis factors constant. This also and especially applies to the light intensity.

In the case of self-made foils or filters, however, it may well be that, for example, the blue filter absorbs more light than the red filter. Then it is only logical if the photosynthesis rate is lower in blue light than in red light. But that's not because of the lightcolour, but simply because the blue light is less intense than the red light.

This factor has already been taken into account in the filters that are offered in the teaching material trade. These filters are tuned so that they all allow the same light intensity to pass through, regardless of the color.

ENGELMANN's bacterial experiment

That the photosynthesis rate depends on the light color can be proven with an ingenious experiment:

Method: ENGELMANN bacterial experiment

Take a filamentous green alga, put it under a microscope, put oxygen-loving bacteria in the water droplet and then irradiate the algae thread not with normal light, but with a light spectrum by first passing the light through a prism.

Result of ENGELMANN's bacterial experiment on the dependence of the photosynthesis rate on the light color

You can see that a particularly large number of bacteria collect in the blue and especially in the red area of ​​the spectrum, obviously the algae thread produces a lot of oxygen there.

Although ENGELMANN's attempts (see "external links") were made over 110 years ago, the statements still apply. ENGELMANN's claims could be confirmed with modern research methods.

Oxygen production depending on the light color

If you want to know what light and color are and what is meant by an absorption spectrum and so on, please go to my pages on dye chemistry.

The absorption spectrum of leaf green

Absorption spectrum of chlorophyll

The direct reason for the fact that oxygen is mainly produced in blue and red light is due to the chemical-physical properties of the green leaf pigment chlorophyll. Above you can see the absorption spectrum of the chlorophyll. Here you can clearly see that the green leaf pigment mainly absorbs blue light and partly also red light. Green and yellow light, on the other hand, are as good as not absorbed. Chlorophyll is now the dye responsible for photosynthesis, which converts light energy into chemical energy.

Absorption spectrum method

A solution of leaf green is sprayed with light of different wavelengths but the same intensity. A suitable device can then be used to measure the percentage of the incident light that is absorbed by the liquid. In terms of measurement technology, it works very simply: You compare the irradiated light intensity with the intensity of the light that comes out behind the container with the green leaf solution.

With simple photometers you work with white light that you send through different color filters (see above), and then you write down the respective absorption values.

In modern photometers, the white light is sent through a prism that rotates with a certain angular speed, so that the sample is irradiated one after the other with the different wavelengths. The photometer is then connected to a computer, which records the absorption for each wavelength and then displays it as a graphic - the absorption spectrum is ready.

Very modern photometers somehow irradiate the sample with all wavelengths at the same time or at least so quickly in succession that the absorption spectrum is available on the computer monitor within a few seconds.

Why does photosynthesis work particularly well in blue light?

The "deeper" reason for the fact that photosynthesis works best with blue or red light lies in the composition of the sunlight. Bright sunlight is composed of all visible colors. You can see that immediately when you break the white sunlight down into its spectral components with the help of a prism (which works very well with the unprinted side of a CD, by the way). The blue component in sunlight is particularly high, higher than the red, green or yellow component. You don't see that in the spectrum, you have to find out with sensitive measuring devices. In any case, that's why photosynthesis works particularly well in blue light.

Why does photosynthesis also work in red light?

The question now is, why does photosynthesis work particularly well in red light? The immediate answer would be: Because the chlorophyll also absorbs red light quite strongly. But then the more in-depth question should immediately follow: Why does chlorophyll absorb red light? What sense does that make? What is the advantage of those plants that have chlorophyll molecules in their chloroplasts that are able to absorb not only blue but also red light?

To do this, one must not only look at the spectral composition of sunlight, where the blue component clearly predominates, but one also has to examine how the light is composed in the shade, in which many plants have to live. And here it turns out that shadow light contains a fairly high proportion of red.

Plants with chlorophyll, which absorbs blue AND red and uses them chemically, have a clear survival advantage over plants whose chlorophyll only absorbs blue light. You can do photosynthesis in the sun as well as in the shade. For this reason, over the course of millions of years, such plants have prevailed over plants without red absorption.