The Leaf's Secret Breath: Understanding the Spongy Mesophyll
The Anatomy of a Leaf: Where is the Spongy Mesophyll?
To understand the spongy mesophyll, we must first take a journey inside a typical plant leaf. If you were to slice a leaf incredibly thinly and look at it under a microscope, you would see a beautifully organized structure, much like a multi-layered sandwich.
The top and bottom of the leaf are covered by a thin, transparent layer called the epidermis, which acts like a protective skin. On the underside of the leaf, the epidermis is dotted with tiny pores called stomata (singular: stoma), which are like the leaf's nostrils. Each stoma is flanked by two guard cells that control its opening and closing.
Inside the leaf, between the upper and lower epidermis, lies the mesophyll tissue, which is the main site of photosynthesis. This tissue is divided into two distinct layers:
- Palisade Mesophyll: Located just below the upper epidermis, this layer consists of tightly packed, tall, column-shaped cells that are filled with chloroplasts[1]. They are positioned to absorb maximum sunlight, making them the primary powerhouses for photosynthesis.
- Spongy Mesophyll: Situated below the palisade layer and above the lower epidermis, this is our layer of interest. Its cells are more rounded and arranged very loosely, creating a network of large air spaces. These cells also contain chloroplasts but fewer than the palisade cells.
The contrast between these two layers is a perfect example of how structure relates to function. The palisade layer is designed for light absorption, while the spongy layer is engineered for gas circulation.
The Science of Gas Exchange: How the Spongy Mesophyll Works
Gas exchange is the process of swapping gases between the plant and the atmosphere. For plants, this primarily means taking in carbon dioxide ($CO_2$) and releasing oxygen ($O_2$) and water vapor ($H_2O$). The spongy mesophyll is the central hub for this activity.
Here is the step-by-step process:
- Entry: When the guard cells around a stoma are full of water and turgid, the stoma opens. Carbon dioxide ($CO_2$) from the outside air diffuses through the open pore.
- Circulation: The $CO_2$ enters the vast network of air spaces within the spongy mesophyll. These interconnected channels allow the gas to circulate freely and reach all the mesophyll cells deep inside the leaf.
- Uptake: The $CO_2$ gas dissolves in a thin layer of moisture that coats the surfaces of the spongy mesophyll cells. It then diffuses into these cells.
- Photosynthesis: Inside the chloroplasts of both spongy and palisade cells, the $CO_2$ is used in the photosynthetic reaction. The simplified chemical equation for this process is:
6CO2 + 6H2O ⟶ C6H12O6 + 6O2
This reads: Six molecules of carbon dioxide and six molecules of water, using light energy, are converted into one molecule of glucose (sugar) and six molecules of oxygen. - Release: The oxygen ($O_2$) produced as a waste product of photosynthesis diffuses out of the cells, into the air spaces of the spongy mesophyll, and finally exits the leaf through the open stomata.
This entire process is driven by diffusion[2] – the movement of molecules from an area of high concentration to an area of low concentration. The air spaces are crucial because they ensure the concentration of $CO_2$ inside the leaf is always lower than outside, so the gas keeps flowing in. Similarly, the concentration of $O_2$ is higher inside, so it flows out.
Feature | Palisade Mesophyll | Spongy Mesophyll |
---|---|---|
Location in Leaf | Just below the upper epidermis | Above the lower epidermis, below palisade layer |
Cell Shape | Tall, column-like, tightly packed | Irregular, roundish, very loosely packed |
Air Spaces | Very few, small | Extensive, large network |
Primary Function | Light absorption for photosynthesis | Gas exchange ($CO_2$ in, $O_2$ and $H_2O$ out) |
Chloroplast Count | Very high | Moderate |
A Delicate Balance: Transpiration and the Spongy Mesophyll
The spongy mesophyll is also intimately involved in transpiration, the process where plants lose water vapor through their leaves. The same air spaces that allow $CO_2$ to flow in also allow water vapor to flow out.
This creates a dilemma for the plant: it needs to open its stomata to let $CO_2$ in, but in doing so, it risks losing too much water. This is why plants in hot, dry environments (like cacti) have adapted in incredible ways. Many have very thick waxy coatings on their leaves (cuticles) to prevent water loss, or they have fewer stomata. Some even have their spongy mesophyll located deep inside the plant body, as in the case of cacti where the stem is the main photosynthetic organ and is often folded to create a shaded, humid internal microclimate.
Plants must constantly balance these competing needs – the need for food ($CO_2$) and the need to conserve water. The structure of the spongy mesophyll and the behavior of the stomata are key to managing this balance.
Observing the Spongy Mesophyll in Action
We can see the results of the spongy mesophyll's work all around us. A simple experiment to demonstrate transpiration (and by extension, the pathway through the spongy layer) is the plastic bag experiment.
What you need: A small, healthy potted plant (like a basil plant), a clear plastic bag, and a rubber band.
What to do: Place the plastic bag over a few leaves and the pot, and seal the opening around the pot's stem with the rubber band. Leave the plant in a sunny spot for a few hours.
What you see: Droplets of water will appear on the inside of the plastic bag. This water vapor traveled from the spongy mesophyll's air spaces, out through the open stomata, and condensed on the cooler surface of the plastic bag. This is direct evidence of the water vapor that is a constant passenger in the gas exchange process.
Another practical application of this knowledge is in gardening and agriculture. Understanding that gas exchange happens on the underside of leaves explains why spraying the underside of leaves with insecticide or fungicide is often more effective than just spraying the top surface – you are targeting the entry points (stomata) and the primary tissue (spongy mesophyll) where many pests and fungi operate.
Common Mistakes and Important Questions
A: This is a common misconception. The cells are definitely not just empty space! They are living, functioning cells packed with organelles. While they do have large air gaps between them, the cells themselves are crucial. They contain chloroplasts and contribute to photosynthesis, and their irregular shape is specifically designed to maximize the surface area for gas exchange while creating the necessary air channels.
A: Yes, but it's a different process. During the day, photosynthesis produces more than enough oxygen for the plant's needs. At night, when photosynthesis stops, plants perform cellular respiration[3] just like animals. They take in oxygen ($O_2$) from the air through the stomata and spongy mesophyll and use it to break down sugars for energy, releasing carbon dioxide ($CO_2$) as a byproduct. So, the spongy mesophyll facilitates gas exchange for both processes, just the flow of gases reverses between day and night.
A: Most broad-leaved plants do. However, plants adapted to dry climates (xerophytes[4]) often have a reduced spongy layer or it may be absent, replaced by other water-storing tissues. Conversely, plants that live in water (hydrophytes[5]) often have a huge, exaggerated spongy mesophyll with enormous air spaces (aerenchyma) that helps them float and transport oxygen down to their roots.
Footnote
[1]Chloroplasts: Organelles found in plant cells that contain chlorophyll and are the site of photosynthesis.
[2]Diffusion: The passive movement of molecules or particles from a region of higher concentration to a region of lower concentration.
[3]Cellular Respiration: The process by which cells break down sugar to produce energy, consuming oxygen and releasing carbon dioxide.
[4]Xerophytes: Plants adapted to survive in environments with very little water, such as deserts.
[5]Hydrophytes: Plants adapted to grow in water or in soil that is permanently saturated with water.