The word mesophyll literally translates from Greek as “middle leaf,” describing the tissue’s location within a plant’s leaf structure. This specialized tissue forms the bulk of the leaf’s interior, positioned directly between the protective layers of the upper and lower epidermis. The mesophyll is the primary functional tissue of the leaf, acting as the centralized hub for light energy conversion and gas exchange.
What the Mesophyll Is
The mesophyll is composed of a type of ground tissue called parenchyma cells, often referred to as chlorenchyma because of their high chloroplast content. These cells are fundamentally different from the epidermal cells that surround them, as the epidermis lacks these photosynthetic organelles. The tissue is situated in a layer running horizontally across the leaf’s cross-section, filling the space between the top and bottom surfaces.
The high density of chloroplasts makes the mesophyll the exclusive site where light energy is captured and converted into chemical energy. This tissue is also laced with a network of vascular bundles, commonly known as veins, which supply the necessary water and nutrients.
Specialized Structure of the Mesophyll
The mesophyll is structurally divided into two distinct layers, each specialized for a different aspect of the leaf’s overall function.
Palisade Mesophyll
The palisade mesophyll forms the upper layer, located directly beneath the upper epidermis. Its cells are elongated and columnar, tightly packed together and oriented vertically. This compact arrangement maximizes the absorption of incoming sunlight, making the palisade layer responsible for the majority of the leaf’s photosynthesis. The upright positioning ensures that light travels through the length of the cell, increasing the probability of absorption by the numerous chloroplasts concentrated near the cell walls.
Spongy Mesophyll
Beneath the palisade layer is the spongy mesophyll, characterized by its irregularly shaped cells and the presence of large, interconnected air spaces. This layer is situated closer to the lower epidermis, where most of the microscopic pores called stomata are located. The loose packing of cells creates a vast internal surface area for the exchange of gases. These air spaces are directly connected to the outside atmosphere via the stomata, which open and close to regulate the flow of gases.
The Engine of Photosynthesis
The mesophyll’s structure directly enables photosynthesis, which converts carbon dioxide and water into glucose and oxygen using light energy. The high concentration of chloroplasts, particularly in the palisade layer, performs the light-dependent reactions. Light striking the leaf is absorbed by the chlorophyll within these organelles, initiating the energy conversion process.
The spongy layer ensures a continuous supply of carbon dioxide by acting as a diffusion pathway. Carbon dioxide enters through the stomata and quickly disperses into the intercellular air spaces. From there, it dissolves into the thin film of moisture surrounding the mesophyll cells before entering the cell for the carbon-fixing reactions.
Water, the other primary reactant, is delivered directly to the mesophyll cells through the leaf’s vascular bundles that weave throughout the tissue. This integrated system of light capture, gas delivery, and water supply makes the mesophyll an efficient biological reactor. The resulting glucose sugar is then transported out of the mesophyll via the same vein network to fuel the growth and metabolism of the rest of the plant.