In a simplistic, textbook interpretation of plant cell biology, the organelles of cells and different tissues have specific functions, and house metabolic machinery to catalyse independent metabolic pathways. Thus, chloroplasts are responsible for photosynthesis, mitochondria for respiration, etc. With respect to photosynthesis, there is a combination of biochemical and environmental factors that belie this simple theory and that have affected the course of evolution. The most apparent is that Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase), a key enzyme in the pathway for photosynthesis, functions both in carbon assimilation and in a photorespiratory process, with the magnitude of each being dependent upon the relative concentrations of carbon dioxide and oxygen available to the chloroplast. CO2 and O2 each serve as substrates and react with RuBP (ribulose 1,5-bisphosphate), and each is a competitive inhibitor with respect to the other. In C3 plants, where the supply of CO2 from the atmosphere to the chloroplast is dependent on simple diffusion, the ratio of carboxylase to oxygenase activity is about 2.5:1 under current atmospheric conditions (Sharkey, 1988). Whether or not photorespiration has ever been of benefit to plants, it is known that photosynthesis in C3 plants can be increased by increasing CO2 in the atmosphere, and that some plants, called C4 plants, have evolved a mechanism to concentrate CO2 around Rubisco and minimise photorespiration. This evolutionary development in C4 plants resulted in an obvious benefit in carbon assimilation under environmental conditions where CO2 is most limiting.