On Photosynthetic Entropy

I remember when I first read What Is Life a while back and immediately became obsessed with biological thermodynamic/informatical principles. To me it was much more enchanting than anything quantum mechanical. That’s why I’m spending so much time on it in the next book.

Plants, through photosynthesis, serve as an exemplary system for examining entropy fluxes in conjunction with low-entropy reservoirs and biological processes. The process supports the second law of thermodynamics, which dictates an overall increase in entropy, even as the initial photochemical stages generate debate. Photosynthesis begins with low-entropy radiant energy from the sun. This energy is partially transformed into chemical energy stored in biomolecules, which have lower entropy compared to the heat energy generated during energy dissipation.

Photosynthesis captures a portion of the entropy potential, sometimes referred to as negentropy, and incorporates it into the free energy of synthesized compounds. These compounds later release energy and entropy during respiratory processes in plants and non-photosynthetic organisms. The entropy associated with the radiant energy can be estimated based on its energy-to-entropy ratio, which reflects the energy's quality. Radiant energy, with a high associated temperature equivalent, undergoes conversion to heat energy at ambient temperatures, leading to a significant increase in entropy.

The free energy captured in glucose molecules during photosynthesis, though lower in quality than the original radiant energy, remains far superior to that of thermal energy. While less than 1% of the absorbed radiant energy is retained as biomass, the entropy associated with glucose molecules is much lower than that of the absorbed light. However, the overall entropy of the system increases, primarily due to heat dissipation.

Photosynthesis produces a substantial entropy increase during the photophysical stages, where photon absorption and charge separation occur. Subsequent biochemical reactions, including proton pumping and enzyme-catalyzed pathways, contribute minimally to the total entropy production. Excess light energy not utilized in photosynthesis increases entropy further through mechanisms such as non-photochemical quenching. The entropy production during photosynthesis, even when accounting for light and heat dissipation, aligns with established thermodynamic principles, highlighting the efficiency and limitations of energy capture in biological systems.

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