Mesophyll is the specialized internal tissue of a plant leaf, sandwiched between the upper and lower layers of the epidermis. This chlorophyll-containing tissue is the central location where photosynthesis occurs. Its cellular architecture is adapted to capture sunlight and atmospheric gases, converting light and simple inorganic molecules into the chemical energy necessary for plant growth.
Location and Structure of the Leaf Engine
The mesophyll is precisely positioned beneath the upper epidermis, an arrangement that maximizes its exposure to incoming solar radiation. This tissue is typically divided into two distinct layers, each possessing a unique cell shape and density that optimizes its specific role within the leaf. The physical placement of these layers establishes a highly efficient internal environment for light harvesting and gas circulation.
The upper portion is the palisade mesophyll, characterized by tightly packed, elongated cells positioned perpendicular to the leaf surface. This dense organization minimizes air spaces, ensuring sunlight is intercepted directly by a high concentration of chloroplasts. The palisade layer is the primary site of light energy absorption for the leaf.
Beneath the palisade layer lies the spongy mesophyll, featuring irregularly shaped, loosely arranged cells. This layer is defined by large, interconnected air spaces that occupy a significant volume. While spongy cells contribute to photosynthesis, their primary purpose is to create a vast internal surface area for the circulation and exchange of gases.
The Primary Function of Photosynthesis
The mesophyll’s defining purpose is to host photosynthesis, the biochemical process that converts light energy into chemical energy in the form of sugars. This conversion takes place inside organelles called chloroplasts, which are highly concentrated within the mesophyll cells, particularly in the palisade layer. The process uses carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) to synthesize glucose, releasing oxygen ($\text{O}_2$) as a byproduct.
Photosynthesis is initiated with the light-dependent reactions, which occur on the thylakoid membranes within the chloroplasts. Here, specialized pigment molecules, like chlorophyll, absorb photons of light energy. This captured energy is then used to split water molecules, a reaction that generates oxygen gas and produces two high-energy molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
The energy carriers ATP and NADPH then fuel the second stage of the process, known as the light-independent reactions or the Calvin cycle, which takes place in the stroma of the chloroplast. During this cycle, atmospheric $\text{CO}_2$ is fixed, or incorporated, into an organic molecule. This carbon fixation step is catalyzed by the enzyme RuBisCO, which attaches the $\text{CO}_2$ molecule to a five-carbon sugar.
Through a series of enzymatic steps powered by the stored energy from the light reactions, the fixed carbon is reduced to form three-carbon sugars. These intermediate sugars are then used to synthesize the final product, glucose, along with other carbohydrates like sucrose and starch.
Facilitating Gas Exchange and Water Management
The porous architecture of the spongy mesophyll supports the constant movement of gases required for photosynthesis. Carbon dioxide, the raw material for sugar production, must travel from the atmosphere into the mesophyll cells. The large air spaces within the spongy layer allow $\text{CO}_2$ to rapidly diffuse to the surfaces of the moist mesophyll cells.
This gaseous exchange begins when atmospheric $\text{CO}_2$ enters the leaf through microscopic pores on the epidermis called stomata. Once inside, the $\text{CO}_2$ molecules quickly circulate through the spongy mesophyll air spaces, eventually dissolving into the film of water covering the cell walls before diffusing into the chloroplasts. This efficient transport system ensures a continuous supply of carbon for the Calvin cycle.
The spongy mesophyll’s air spaces collect gaseous byproducts of photosynthesis and respiration, including oxygen and water vapor. Oxygen, released from the light-dependent reactions, diffuses out of the cells and exits the leaf through the stomata. This exchange also means that evaporating water is released as water vapor, a process known as transpiration.
The regulation of water vapor loss is intricately linked to the activity of the mesophyll, as the opening and closing of the stomata directly controls the rate of transpiration. When the leaf is actively photosynthesizing, the stomata open to allow $\text{CO}_2$ entry, but this inevitably results in water loss.