36 Environmental Biotechnology
development and so is discussed in some detail alongside the related subject of
photosynthesis.
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