We all think of the colour green when we think of the environment, due to the fact that plant leaves are almost all green. The reason for this? Much more complex, and perhaps slightly a mystery in this age of science. The GCSE Biology answer may consist of the fact that leaves contain chlorophyll, which is a green pigment. Digging a bit deeper into the subject at A-level, we will find that the photopigments that are found in the stacked thylakoids, i.e. grana, which absorb photons for light energy for photosynthesis, absorb light at different wavelengths, and all have low levels of absorption at 520-570 nm. This means that the green wavelengths in sunlight that shine on leaves are reflected, hence why we view the leaf as green. However, here the fact of why plants are in fact green, and why the photopigments that they contain do not absorb green light, comes into question. It is known that most of the energy that the sun radiates is in the green part of the spectrum, suggesting that plants are not effective in capturing sunlight. It would make more sense for plants to be black, and absorb all wavelengths of light so that they can harness as much light energy as possible for photosynthesis.
Scientists have not been able to give a direct answer to this conundrum until recently, where they suggested that perhaps being efficient at absorbing green wavelengths of light would mean that they would overheat, causing harm to the structural and chemical mechanisms of the plant, where the green light is too strong. Furthermore, when thinking of this problem, I thought that this could possibly be due to a slight evolutionary strategy, where the colour green rather than black would lead to insects that plants rely on to be more attracted to them, similar to the fact that they are more attracted to plants with big brightly coloured petals. An example of this is the honeybee, which pollinates the high majority of insect-pollinated plants - the main colours that it sees are green, yellow, and blue, thus making evolutionary sense for the leaf to be one of these colours. Furthermore, being green would mean that they would be able to camouflage certain organisms from predators, which would increase populations of these organisms. This would then mean that greater populations of these animals would thus die in greater numbers, transforming to a greater volume of detritus which provides nutrients for plants to grow. However, these evolutionary advantages are so slight and perhaps non-existent, so it is evident that there is another factor for the reason behind their colours, that scientists have not spotted until recently.
The answer relies upon the plant not working the most efficiently that it can, however in the most stable way possible. The first step of photosynthesis is nearly perfectly efficient, where the 'antenna' that the pigments are embedded in absorbs light, transferring the energy to areas where the production of chemical energy for the cell’s use is initiated; almost all of the light absorbed is transferred into electrons. However, quick changes in the intensity of light that falls upon plants, bring inefficiency into the process of absorbing light. For such a cell which is harnessing energy, steady inputs/outputs of energy are much more important than the efficiency of absorbing light: too few/too many electrons can either cause an energy failure or lead to free radicals, damaging plant tissue.
Nathaniel Gabor proposed this idea when creating a model for the light-harvesting systems of plants and applying it to the solar spectrum measured below a canopy of leaves. This model showed that although just absorbing green light would be highly efficient, as this is the highest energy wavelength of light, the chance of sunlight flickering at this high light energy would be detrimental to the plant leaf, where 'the noise from the input signal would fluctuate too wildly for the complex to regulate the energy flow'. To combat this challenge and have a safe output, the wavelengths that the photopigments absorb need to both be at similar wavelengths to reduce internal noise, as well as being at different rates to buffer against the external noise caused by swings in light intensity, i.e. the sun going behind/in front of clouds. To match both of these conditions, the safest wavelength for the photopigments to absorb is around the red and blue sections of the spectrum, as these are in the steepest parts of the intensity curve of the solar spectrum. An image of this is below, where the actual light absorption by green plants is charted against the predicted peaks that the model would show (in the blue and red wavelength sections). A weak positive correlation is shown between the two, suggesting that this model is accurate.
Thus it is shown that the machinery of the leaf in undergoing photosynthesis is not based on maximum efficiency, but around a stable input/output of energy, which may explain the constant presence of plants in nature for hundreds of millions of years. When this model was applied to the varying other photosynthetic forms of organisms (e.g. green sulfur bacteria) and the sunlight levels that are found in their respective environments, the predicted peaks of the model were once again in line with the light absorption of the organism.
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