Mesophyll cells are the internal tissue of a plant leaf, acting as the primary factory for the plant’s food supply. They are specialized cells located between the protective outer layers of the leaf. This tissue performs the light-harvesting and sugar-producing work that sustains the plant. Understanding these cells reveals the process that converts sunlight and atmospheric carbon dioxide into the energy that fuels nearly all life on Earth.
What Are Mesophyll Cells and Where Are They Found?
The term “mesophyll” translates from Greek to “middle leaf,” describing the tissue’s location within the leaf structure. This tissue is situated entirely between the upper and lower layers of the protective epidermis. The mesophyll is primarily composed of thin-walled parenchyma cells, which are adaptable and involved in various functions like photosynthesis and storage.
These cells are organized into layers fed by the leaf’s vascular bundles (veins), which transport water and nutrients. Gases required for function enter the leaf through tiny pores on the epidermis called stomata. Once inside, these gases move into the intercellular spaces of the tissue, where they are absorbed by the cells. This arrangement places the mesophyll at the intersection of light, water, and atmospheric gases, making it an ideal location for energy conversion.
The Primary Function: Driving Photosynthesis
The primary role of the mesophyll is to perform photosynthesis, the process of converting light energy into chemical energy. Mesophyll cells are equipped for this task because they contain a high concentration of organelles called chloroplasts. Chloroplasts house the green pigment chlorophyll, which absorbs the energy from sunlight, specifically the red and blue wavelengths.
Within the chloroplasts, the absorbed light energy drives reactions that convert carbon dioxide and water into glucose, a sugar used for plant energy, and oxygen. In many plants, up to one-fifth of the mesophyll cells’ volume is occupied by chloroplasts. The mesophyll is the primary site of photosynthetic carbon assimilation, generating the molecules necessary for the plant’s growth. The resulting glucose is then transported out of the leaf through the vascular bundles for use or storage.
Two Distinct Types: Palisade and Spongy Mesophyll
The mesophyll is organized into two layers: the palisade mesophyll and the spongy mesophyll. The palisade layer is found directly beneath the upper epidermis and consists of elongated, tightly packed, column-shaped cells. This arrangement maximizes light absorption, as these cells are the first to intercept incoming sunlight and contain the highest number of chloroplasts per cell. Their vertical orientation optimizes the capture of high-intensity light.
The spongy mesophyll sits below the palisade layer, closer to the lower epidermis. It is made up of irregularly shaped cells that are loosely arranged, creating extensive intercellular air spaces. While spongy cells perform some photosynthesis, their main function is to facilitate the rapid movement and exchange of gases. The air spaces allow carbon dioxide to diffuse quickly from the stomata to the palisade cells, and they also allow oxygen and water vapor to move out of the leaf.