Photosynthesis as a Chloroplast Function

Abstract

The sun is the universal source of energy in the biosphere. During the nuclear-fusion processes occurring in the sun, matter is changed into energy (e.g. 4 protons → helium nucleus + 2 positrons + 4.5 · 10⁻¹² J) which is emitted in the form of electromagnetic radiation (h v) into space. The energy emission of solar radiation corresponds, as a first approximation, to the continuous emission spectrum of a black body at 5800 K. Because of scattering and selective absorption of quanta in the earth’s atmosphere the spectrum of the sun is modified (Fig. 12.1) so that the energy flux is reduced from 1.4 (the solar constant) to ≤0.9 kW · m⁻² (sea level). Approximately half this energy is within the 300–800 nm spectral band (“optical window” of the atmosphere; Fig. 12.1) which is in the centre of the photochemically active radiation band (approximately 100–1000 nm).