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. Higher Plants Efforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The family _Lemnaceae_ (duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.