In most plants, the leaves contain the most chloroplasts and are the major sites of photosynthesis. Within a leaf, the chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue (Figure 8-2).Figure 8-2Photosynthesis takes place in cellular organelles called chloroplasts. In this sunflower, the greatest numbers of chloroplasts are located in the leaves.ithin a leaf, the chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue (Figure 8-2). Figure 8-2Photosynthesis takes place in cellular organelles called chloroplasts.