Why are plants not black?
All kinds of reasons have been put forward for why plants apparently fail to make maximum use of the available light. None of these reasons can explain why after two billion years of evolution they are not black, like industrial photovoltaic solar cells. Are we missing something?
Image: Chloroplasts in plant cells.
In the February edition of Photosynthesis Research, Leiden biophysicists Marcell Marosvölgyi and Hans van Gorkom demonstrate that the colour of solar cells is determined by the costs. It costs energy to make a solar cell, and this process is only cost-effective if the solar cell can extract more energy from the sun. The same applies to chloroplasts, the biological solar cells that plants use to stay alive. The more sunlight they absorb, the better, you might think. And a black system absorbs all the available light.
Plants and other photosynthetic organisms are largely filled with pigment protein complexes that they produce to absorb sunlight. The part of the photosynthesis yield that they invest in this therefore has to be in proportion. The pigment in the lowest layer has to receive enough light to recoup its energy costs, which cannot happen if a black upper layer absorbs all the light. A black system can therefore only be optimal if it does not cost anything.
Image: A solar panel with photovoltaic solar cells.
This explains why the higher the costs are, the more light the system allows to penetrate, since it is not profitable to add more pigments. That part of the light that can be absorbed profitably depends on the amount of light absorbed for each colour. Apparently, plants are able to achieve a greater yield from red light, which is why plants are green, red being the complement to green.
The absorption spectrum is made up of numerous contributions by the individual pigments. Given static production costs, if you can correlate the colour of each pigment with the place in the spectrum where the capacity of the as yet unabsorbed light is greatest, this will give rise to a system that absorbs all the capacity at all frequencies above a given threshold, this threshold being determined by the price per pigment.
Figure 1 is a representation of the light from the solar spectrum that penetrates a plant. This spectrum has many dips. The flow of light is not even and less light penetrates the dips. Given this information, if a plant has to invest a lot of energy in pigments, it will not locate these in the dips. In an eventual absorption spectrum, the dips reccur as an absence of absorption. If the pigments use only a small amount of energy, the plant could produce them in the dips since they require a lower yield in order to be profitable.
The solar spectrum in a mud pool looks very different from in full sunlight. All that is left, in fact, is infrared; the rest is obstructed by the water and whatever else goes to make up the mud. An organism living in the pool therefore has to shift its absorption to infrared. Figure 2 shows that the purple bacterium, that lives in mud pools, has adapted to the mud pool spectrum. In other words, it has adapted to infra red absorption.
If it does not cost any energy to produce the pigments, then black would be the optimal colour for solar cells. The question is whether we should make black solar cells, if the energy costs of the solar cell material are included in optimising the net yield?
(26 January 2010)