carbon dioxide wii diffuse into the leaf through the stomata and then dissolve in the water surrounding the spongy mesophyll cells. Because carbon dioxide is being removed by photosynthesis faster than can be supplied by respiration of the leaf cells,a concentration gradient is created. This results in the diffusion of carbon dioxide through the palisade cell into the chloriplasts
The by-product of photosynthesis,oxygen,will diffuse in the opposite direction. That is, from the mesophyll cells out into the air spaces first and then out of the keaf through the stomata