As mankind explores the planets, human needs for air, clean water, and food suggest that plants be carried to and exist on his colonies. The complexities of even a simple ecosystem of humans and a single plant crop require a sophisticated understanding of the interactions between atmosphere, nutrients and lifeforms. While many experiments could be done to find the relationships between mass flows and chemical/energy transformations, it would be simpler to develop a generalized model of plant growth, to validate it, and to use it to test the variations possible within a closed environment.Such a model specifically designed for a closed space system should focus on gas mass transfers through the photosynthetic processes, leaf radiation/heat balances, and the production/distribution of carbohydrates. The development of these relationships will lay the groundwork for future, more complex modeling exercises, involving uptake of major nutrients and partitioning of amino acids and carbohydrates into food, plant support, photosynthetic, and root tissues. The photosynthetic processes will be the hub of the model described in this paper and will determine the gross parameters of plant needs and produce for long duration space exploration.