Photosynthesis and the Basis of Phytotechnology
The sun is the biosphere’s ultimate source of energy and photosynthesis is the only means there is on this planet to trap incident sunlight and convert it into chemical energy available to biological processes. Thus, with very rare excep-tions, organisms which do not photosynthesise, which is the majority, are totally dependent on those which do. With this introduction it is hardly surprising to find a description of this process in a book which specifically addresses the capabili-ties of biological organisms and their interplay. Leafy plants obviously feature in this section but so too do photosynthetic eukaryotic micro-organisms and bacte-ria. A knowledge of this vital process is essential to appreciating the role which photosynthesising organisms play in the environment, their limitations and the strengths upon which biotechnology can capitalise.
The energy from this process is used to drive all the biochemical synthesis and degradation reactions occurring in the cell in addition to various other energy-requiring processes such as the movement and transport of molecules across membranes. Energy is finally dissipated as heat, and entropy rises in accordance with the laws of thermodynamics. Any interference with the flow from the sun either by reducing the ability of the energy to penetrate the atmosphere, or by reducing the total photosynthetic capacity of the planet, has dramatic conse-quences to all forms of life. Conversely, too intense a radiation from the sun resulting from thinning of the ozone layer runs the risk of damaging the pho-tosynthetic machinery. This can be compensated for by the organism acquiring pigments to absorb harmful radiation, but this requires time for such an evolu-tionary adjustment to take place.
It is noteworthy that the bulk of photosynthesis is performed by unicellular organisms, such as photosynthetic algae, rather than the macrophytes as might reasonably be supposed. Photosynthesis occurs in two parts; the first is the trap-ping of light with associated reduction of NADP+ and ATP synthesis, and the second is the fixing of carbon dioxide by its incorporation into a carbohydrate molecule. This is most commonly a hexose sugar, and typically glucose, the syn-thesis of which utilises the NADPH and ATP produced in the light-dependent part 1. The processes of carbohydrate synthesis occurring in the second part are described as the dark reactions, so called because they may proceed in the dark after a period of illumination to activate part 1. The sugar produced during these dark reactions will then be utilised by the cell, transferred to another cell or ingested by a larger organism and eventually catabolised to carbon dioxide and water, releasing the energy consumed originally to synthesise the molecule. Here is another example of a natural cycle, where carbon is introduced, as carbon diox-ide, into the synthesis of a sugar which is then interconverted through the various metabolic pathways until finally it is released as carbon dioxide thus completing the cycle. Eukaryotes capable of carrying out photosynthesis include higher green plants, multicellular green, brown and red algae and various unicellular organisms such as the euglenoids and dinoflagellates both of which are commonly found in fresh water environments, and diatoms which are also found in salt water. The diatoms which are unicellular algae, are particularly noteworthy given the current estimates that they are responsible for fixing 20 to 25% of the world’s carbon through photosynthesis (Round, Crawford and Mann 1990). Prokaryotes capable of photosynthesis include blue-green algae, and both the sulphur and nonsulphur purple and green bacteria. The blue-green algae which are oxygenic bacteria and are alternatively named cyanobacteria, operate light reactions very similar to those of eukaryotes. Conversely, the green and the purple nonsulphur bacteria which are both facultative aerobes and the strictly anaerobic green and the purple sulphur bacteria utilise a rather different set of light reactions as a con-sequence of their possessing a ‘simpler’ photosystem. Eukaryotic and bacterial systems are both described in the following sections.