Green plants contain chlorophyll which captures light energy
thermodynamically required to convert carbon dioxide and water into
carbohydrate which can subsequently be transformed into other foods such
as protein and fat. During this process, carbon dioxide is consumed, and
an approximately equal amount of oxygen gas is liberated. As a first
approximation, photosynthesis is the reverse of the oxidative metabolism
of animal life:
Oxidation
C₆H₁₂O₆ + 6O₂ ———————> 6CO₂ + 6H₂O + heat
Photosynthesis
6CO₂ + 6H₂O + light ———————> C₆H₁₂O₆ + 6O₂
The photosynthetic process in plants and respiration during
photosynthesis have been studied intensively, and several metabolic
pathways have been elucidated. Mechanisms are being studied to explain
the inhibitory effect of strong visible light on this process. This
program may lead to the use of chloroplasts or chlorophyll without cells
in future photosynthetic bioregenerative systems for long-term space
travel.
One of the prime considerations of a closed ecological system is that
the environmental gases shall remain physiologically tolerable to all of
the ecologic components. Ideally, a photosynthetic gas exchange organism
should possess a high ratio of gas exchange to total mass (considering
all equipment and material incidental to growth, harvesting, processing,
and utilization); and a controllable assimilation rate to maintain
steady-state gas composition. It should also be (1) amenable to
confining quarters which may be imposed by inflexibility of rocket or
space station design; (2) genetically and physiologically stable and
highly resistant to anticipated stresses; (3) edible and capable of
supplying most or all human nutritional requirements; (4) capable of
utilizing raw or appropriately treated organic wastes; and (5) amenable
to water recycling as demanded by other components of the ecosystem.