The mesophyll cells in a plant leaf play a vital role in photosynthesis by enabling the gas exchange portion of the process, and through the actions of their specialized chlorophyll-containing organelles, called chloroplasts, in which the primary photosynthetic reactions take place. Because they are surrounded by intercellular air pockets, the mesophyll cells are able to perform a gas exchange when the stomatal pores in the leaf's epidermis open. The Greek term "mesophyll" means "middle leaf," and it reflects the mesophyll cells' location between the leaf's thinner and much stiffer upper and lower epiderma. Mesophyll cells are found in the plant's leaves. They are a type of ground tissue that is actually found as two distinct types in the leaves. In a nice organized order we find the palisade parenchyma cells of the mesophyll. These cells have a lot of chloroplasts in them, which is why they're the main sites of photosynthesis in the leaf (since chloroplasts conduct photosynthesis). There's usually only one layer of these particular cells, and they're located near the surface of the leaf just under the epidermis, which is the skin of the plant; it helps protect the rest of the leaf. The following image is an excellent view of the layers of the leaf. Notice the mesophyll layers. The palisade parenchyma cells are on top and the spongy mesophyll is below.
The mesophyll cells make up what is known as a plant's assimilation tissue, which refers to the primary location of photosynthetic reactions. In the majority of flowering plants and in ferns, the mesophyll tissue is comprised of two layers: the palisade layer and the spongy layer. The palisade layer lies directly beneath the upper, or adaxial, epidermal layer and contains vertically elongated mesophyll cells. The cells in this layer contain a greater number of chloroplasts. These mesophyll cells also have air spaces between them that make it possible for the cells to absorb carbon dioxide.
The spongy layer of mesophyll cells lies below the palisade layer and contains larger air spaces that enable carbon dioxide and oxygen to diffuse through the cell walls during photosynthesis and respiration. To enable the exchange of gases in and out of the plant, the stomatal pores in the epidermis lead into substomatal chambers, which connect to the intercellular air pockets between the mesophyll cells in the spongy layer.
Cells of the mesophyll make up the bulk of internal leaf tissue and are the major site of photosynthesis in a plant by virtue of containing large populations of chloroplast organelles. The differentiation of the mesophyll and its coordinated expansion is important to leaf function because light interception by chloroplasts and gas exchange in the internal airspaces of the leaf are crucial to optimise rates of photosynthesis. In dicotyledonous leaves there are two types of mesophyll cell; palisade mesophyll and spongy mesophyll. Palisade mesophyll cells are elongate and form a layer beneath the upper epidermis, whereas spongy mesophyll cells are internal to the lower epidermis. Mesophyll cells in monocotyledonous leaves are often highly lobed. All mesophyll cells contain large populations of chloroplasts, which enable the leaf to carry out photosynthetic carbon assimilation.