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Chloroplasts dividing (dumbbell figures) within an enlarging cell of a young spinach leaf, resulting in about 200 chloroplasts per cell at leaf maturity. (Micrograph courtesy John Possingham: Nomarski optics)

In thermodynamic terms, O₂-generating photosynthesis in vascular plants is an improbable process! Improbable, because a weak oxidant (CO₂) must oxidise a weak reductant (H₂O), thereby producing a strong oxidant (O₂) and a strong reductant (carbohydrate). To achieve this ‘uphill’ reaction, a massive and continuous input of chemical energy is required. However, in nature, only radiant energy is available on that scale. How then can green plants achieve this conversion? Chloroplasts are responsible, and in the most significant process in our biosphere, photosynthetically active quanta are trapped and converted into chemically usable forms. This captured energy sustains plant growth and provides a renewable resource base for life on earth.

Thanks to the pioneering work of Calvin and Benson at Berkeley on ¹⁴CO₂ fixation products by Chlorella which began in the 1950s, biochemical aspects of photosynthetic carbon reduction (Calvin cycle) are now comprehensively understood. The transduction of light energy into chemical potential energy is not so well understood, while events surrounding photosynthetic electron flow are defined in some detail and are described here, biophysical processes within the water-splitting apparatus of chloroplasts, and indeed the manner in which photons are captured and their quantum energy harnessed for photolysis, remain something of an enigma and fall outside the scope of our present account.