The * diffusion
of gases into and but of the cells of an organism. For exchange to occur, the gases usually have to pass through a liquid boundary layer. The rate of diffusion of gases dissolved in water is much slower than diffusion through air, an important consideration in calculations of gaseous exchange. During photosynthesis carbon dioxide is taken in and oxygen given off; the reverse occurs in respiration. The passage of water vapour is usually in one direction only - out of the plant.