The mesophyll is the middle layer of cells in a leaf, located between the upper and lower epidermis. It contains many chloroplasts, which are the organelles that perform photosynthesis in plants. The mesophyll is divided into two types of cells: palisade parenchyma and spongy parenchyma. Palisade parenchyma cells are columnar and tightly packed, and they absorb most of the sunlight that reaches the leaf. Spongy parenchyma cells are irregular and loosely arranged, and they allow gas exchange between the leaf and the atmosphere through the stomata. Both types of cells contain chloroplasts, but palisade parenchyma cells have more than spongy parenchyma cells.