Light Absorption for Photosynthesis

Photosynthesis depends upon the absorption of light by pigments in the leaves of plants. The most important of these is chlorophyll-a, but there are several accessory pigments that also contribute.

The measured rate of photosynthesis as a function of absorbed wavelength correlates well with the absorption frequencies of chlorophyll a, but makes it evident that there are some other contributors to the absorption.

The plot of the absorption spectra of the chlorophylls plus beta carotene correlates well with the observed photosynthetic output. The measure of photochemical efficiency is made by meauring the amount of oxygen produced by leaves following exposure to various wavelengths.

It is evident from these absorption and output plots that only the red and blue ends of the visible part of the electromagnetic spectrum are used by plants in photosynthesis. The reflection and transmission of the middle of the spectrum gives the leaves their green visual color.

Why are Leaves not Black?

Light must be absorbed for nutrients to be created by Photosynthesis. So why reflect the green and waste the whole middle part of the spectrum? According to Moore, et al., this is a long story; in fact ancient history!

It looks like chlorophyll takes the part of the spectrum that bacteriorhodopsin doesn't take. Bacteriorhodopsin is a purple pigment that resembles the light-sensitive pigment in our eyes.

Current understanding is that the earliest photosynthetic organisms were aquatic bacteria, some of which are still around today. One of these, halobacterium halobium, grows in extremely salty water. It makes use of the bacteriorhodopsin pigment. The chlorophyll system developed to use the available light, as if it developed in strata below the purple bacteria and had to use what it could get.

But what about the development of land plants? Why did they stay green? The thoughts are that they had plenty of light and were not pressured to develop more efficient light gathering. That is, the light was not the limiting resource in photosynthesis for plants.

That being said, there is some extension toward the middle of the spectrum with the beta carotene and other pigments.

Photosynthetic Efficiency

The energy derived from light absorption is used in particular pathways to achieve the final result of synthesis of sugars. Since the pathways are known, a theoretical maximum efficiency can be calculated. It is known that a total of 8 photons of light must be absorbed to reduce two molecules of NADP⁺. Operating in the Calvin cycle, the resulting two molecules of NADPH can produce one hexose molecule. The photon energy of a median energy photon at 600nm is 2.07 eV, and for 8 moles of such photons the energy absorbed is

It takes 114 Kcal to reduce one mole of CO₂ to hexose, so the theoretical efficiency is 114/381 or 30%. Remarkably, Moore, et al. report that 25% has been achieved under laboratory conditions. The top efficiency they reported under natural growing conditions was the winter-evening primrose growing in Death Valley at 8% (if you can call Death Valley natural conditions!). Sugarcane has registered 7% , which is very important for a food crop. Sugarcane is a C4 plant, and under high sunlight conditions they will usually outperform C3 plants and others.

The intensively cultivated agricultural plants average about 3% in photosynthetic efficiency, and most crops range from 1-4%. This is also typical of algae.