The Mesophyll: The Leaf's Gas Exchange Powerhouse
The Anatomy of a Leaf: A Cross-Sectional View
To understand the mesophyll, we must first look at the leaf's overall structure. Imagine slicing a leaf extremely thinly and looking at the edge under a microscope. You would see a layered structure, much like a multi-layered cake, with each layer having a specific job.
The top and bottom of the leaf are covered by the epidermis, a thin, transparent layer of cells that acts like a protective skin. The epidermis is coated with a waxy, waterproof substance called the cuticle, which prevents the leaf from drying out. Scattered on the epidermis, primarily on the underside of the leaf, are tiny pores called stomata (singular: stoma). Each stoma is flanked by two guard cells that can open and close the pore.
Sandwiched between the upper and lower epidermis is the mesophyll. This is the green, photosynthetic tissue and the main subject of our article. It is not a uniform layer; it is divided into two distinct regions based on the shape and arrangement of its cells:
Feature | Palisade Mesophyll | Spongy Mesophyll |
---|---|---|
Location | Just below the upper epidermis | Between palisade layer and lower epidermis |
Cell Shape | Tall, column-shaped, tightly packed | Irregular, roundish, loosely packed |
Cell Arrangement | Dense, arranged vertically | Loose, with many air spaces |
Primary Function | Absorbing light for photosynthesis | Facilitating gas exchange ($CO_2$ in, $O_2$ out) |
Chloroplast Count | Very high (maximizes light capture) | Fewer than palisade layer |
The Spongy Mesophyll: Master of Gas Diffusion
The spongy mesophyll layer is the star of gas exchange. Its cells are irregularly shaped and, most importantly, loosely arranged. This creates a vast network of interconnected air spaces. Think of it like a kitchen sponge: it's full of holes that can hold air. These air spaces are critical because they form a reservoir for gases.
Here is the step-by-step process of gas exchange:
1. Entry: The stomata on the lower epidermis open. Carbon dioxide (CO2) from the outside air diffuses through the open pore into the air spaces of the spongy mesophyll.
2. Distribution: The CO2 gas fills the air spaces and then dissolves in the thin layer of moisture that coats the surfaces of all the mesophyll cells.
3. Uptake: The dissolved CO2 diffuses into the mesophyll cells, both spongy and palisade, where it is used in the chloroplasts for photosynthesis. The chemical equation for photosynthesis is:
$6CO_2 + 6H_2O + light \ energy \rightarrow C_6H_{12}O_6 + 6O_2$
4. Exit: A waste product of photosynthesis is oxygen gas (O2). It diffuses out of the cells, into the air spaces, and finally out through the stomata. Water vapor from transpiration also exits this way.
The loose packing of the spongy mesophyll maximizes the surface area of the cells that is exposed to the air inside the leaf. More surface area means a faster and more efficient exchange of gases, just like how a crumpled paper towel absorbs water faster than a flat sheet.
A Delicate Balance: Transpiration and Water Conservation
Gas exchange is a double-edged sword. While the plant needs to open its stomata to let CO2 in, it also loses precious water through these same openings in a process called transpiration. The spongy mesophyll's air spaces become saturated with water vapor, which then diffuses out of the leaf.
Plants must constantly balance their need for food (photosynthesis) with their need for water. This is why you'll find more stomata on the shaded underside of leaves—it helps reduce water loss from direct sunlight. Plants in dry climates, like cacti, have evolved to have stomata that open only at night and a mesophyll tissue that is adapted to store water.
Observing Mesophyll in Action: A Simple Experiment
You can see the effect of the mesophyll's gas exchange with a simple experiment. Place a healthy, potted plant (like a geranium) inside a clear plastic bag. Seal the bag tightly around the pot, leaving the plant inside. Place it in a sunny spot for a few hours.
You will notice droplets of water forming on the inside of the bag. This water vapor originated in the mesophyll's air spaces and exited the leaf through the stomata. If you were to test the air inside the bag at the start and end of the experiment, you would also find less carbon dioxide and more oxygen, direct evidence of the leaf's respiratory activities.
Common Mistakes and Important Questions
A: No, this is a common misconception. They are the same type of cell—parenchyma cells2—and both contain chloroplasts. The difference is in their shape and arrangement. The palisade cells are elongated and packed tightly to catch light, while the spongy cells are irregular and loosely packed to trap air.
A: Plants do not breathe in the same way animals do; they have no lungs or circulatory system to transport gases. Instead, they rely on diffusion. Gas exchange is the passive movement of gases from an area of high concentration to an area of low concentration. CO2 diffuses in because it's being used up inside the leaf, making its concentration low. O2 diffuses out because it's being produced, making its concentration high.
A: Not all. This structure is typical of dicot plants (like beans, roses, and oak trees) that have broad leaves. Monocot plants (like grasses and corn) often have leaves that are more uniform inside. Furthermore, plants that live in water (aquatic plants) may have a very large spongy mesophyll with enormous air spaces to help them float, and their palisade layer might be on top to capture light from above.
Footnote
1Stomata (singular: Stoma): Tiny pores primarily on the underside of leaves, bounded by two guard cells, that allow for the exchange of gases ($CO_2$ and $O_2$) and water vapor between the leaf's interior and the external environment.
2Parenchyma Cells: A type of simple plant cell that has thin walls and is responsible for functions including photosynthesis, storage, and secretion. The cells of the mesophyll are a classic example of parenchyma cells.