Scalar Levels in Space and Time

Starting the hierarchical levels of the parts of plants above at the bottom of the scale with molecules was prudent but not quite right. When we look at photosynthesis we realize that we must extend the scale to a still very much finer level, that is, that of the elementary particles of physics, the photons and electrons. When the appropriate photons of the blue and the red part of the solar spectrum of wavelengths are absorbed by the molecules of chlorophyll, electrons are excited and moved along an electron transport chain in the internal membranes, the thylakoid membranes, of the chloroplasts to generate the reduction equivalents and chemical energy required for the assimilation of CO2. At the other end of the scale, we move from leaves to canopies entire ecosystems, biomes, and even the whole planet. Thus, the spatial scale taken in one dimension (meters) from molecules to the entire globe covers about 15-16 orders of magnitude. For the timescale, we note that the half-life of excited states of chlorophyll are 10_1 to 10~13 s for the so-called second singlet state attained after the absorption of a blue photon and 10~n to 10~9 s for the first singlet state after absorption of a red photon. Time constants for electron transport in the membranes of chloroplasts are up to the range of seconds. Activation of the CO2-reducing Calvin cycle of photosynthesis takes minutes. Stomatal responses in photosynthetic gas exchange may reach several tens of minutes. Time constants of photosynthetic productivity in whole plants, trees, forests, ecosystems, and biomes are weeks, months, and years, decades, and hundreds of years, and the evolution of life on Earth took about (3-4) x 109 years. Thus, the temporal scale (seconds) covers at least 32 orders of magnitude (Figure 4). Another couple of numbers illustrating the vast range of scales covered is that the surface of the globe receives a solar irradiance energy of about 7 x 1016Js~\ while the energy content of one exciton stable for 10_1 to 10~9 s after the absorp tion of a photon by a chlorophyll molecule is (30-45)

io-20J.

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