The primary function of the palisade layer of a leaf is to host most of the chloroplasts in the plant, which is where photosynthesis occurs. The palisade layer is a collection of cylindrical-shaped cells just beneath the leaf's surface.
Photosynthesis is the process that plants use to convert energy from sunlight into usable energy for its cells. The sunlight is combined with water and carbon dioxide in chloroplasts, which then produces glucose and oxygen. The oxygen is expelled into the surrounding air, but the glucose is further transformed into adenosine tri-phosphate (ATP), which can be used by all cells for energy.