Plants are green because photosynthesis is optimized – not to maximize energy, but to minimize stress.
“Photosynthetic light harvesting can achieve a quantum efficiency that approaches 100% (that is, the conversion of 100 photons of light into 100 chemically available electrons), and yet it displays notable robustness in the face of ever-changing external light conditions,” marvels English molecular biologist Christopher D.P. Duffy.
Plants are green because they are throwing away the most energy-intensive bandwidth of sunlight. Instead, plants absorb more reliable wavelength bands. “By absorbing only very specific colors of light, photosynthetic organisms automatically protect themselves against sudden changes – or ‘noise’ – in solar energy,” explains American optical physicist Nathaniel Gabor.
The primary challenge facing early photosynthesizers was the paucity of light in the marine environments where they evolved. During this era of incipiency, the energy of the Sun was 20% weaker than now.
This led to light-harvesting assemblies which employed multiple pigments to capture a broader spectrum. “A mixture of pigments are bound at a high density but with specific relative distances and orientations, resulting in energy transfer that is fast and directional,” furthers Duffy. Photonic energy is converted to electrical storage efficiently.
Efficiency is a double-edged sword. A mismatch between input energy and the ability to usably convert it can cause oxidative damage, called photoinhibition. Although effective repair mechanisms exist, they are slow and costly metabolically. Better to avoid the dilemma altogether. As such, rather than fine-tune light harvesting molecularly, plants employ a suite of regulatory and protective processes to minimize damage.
The primary pigments plants use – chlorophyll a & b – do not absorb at the peak of the spectrum but are instead offset from the peak and each other. This strategy mitigates fluctuations in energy transfer, thereby maximizing robustness by minimizing photoinhibition.
Plant photosynthesis is therefore a brilliant optimization that takes into account the nature of sunlight and the constraints imposed by molecular biology. The verdant picture plants paint with their reflected light is one of radiant intelligence in knowing what to hold on to and what to let go.
References:
Trevor B. Arp et al, “Quieting a noisy antenna reproduces photosynthetic light-harvesting spectra,” Science (26 June 2020).
Christopher D. P. Duffy, “The simplicity of robust light harvesting,” Science (26 June 2020).
“Why are plants green? Research team’s model reproduces photosynthesis,” Phys.org (25 June 2020).