The Stomata: Gatekeepers of the Plant World
The Anatomy of a Stoma: More Than Just a Hole
If you look at the underside of a leaf under a microscope, you will see thousands of tiny mouth-like structures. These are called stomata (singular: stoma). The word "stoma" comes from the Greek word for "mouth." But a stoma is not just an open pore; it is a sophisticated structure made up of two specialized kidney-shaped cells called guard cells.
The behavior of these guard cells is a masterpiece of cellular engineering, driven by chemistry and physics. Their primary job is to control the opening and closing of the stoma. The walls of these cells are not uniformly thick. The inner wall (the side bordering the pore) is much thicker and more rigid than the outer wall. This asymmetrical design is the secret to their operation.
The Chemistry of Opening and Closing
The opening and closing mechanism is a dance of water and ions, all held together by the power of chemical bonds.
- Sunlight triggers photosynthesis in the guard cells.
- Energy from this process pumps potassium ions (K$^+$) into the guard cells. Ions are atoms or molecules that have an electrical charge because they have gained or lost electrons, often forming ionic bonds.
- The increase in K$^+$ ions lowers the water potential inside the guard cells.
- Water from neighboring cells then moves into the guard cells by osmosis, a process where water moves across a membrane to balance solute concentrations.
- The guard cells swell with water. The thinner outer walls stretch more easily than the thick inner walls, causing the cells to bend and the pore between them to open.
To close the stoma, the process reverses. The plant hormone abscisic acid (ABA)[1] signals the guard cells to pump the potassium ions back out. Water follows by osmosis, the guard cells become flaccid (limp), and the elastic inner walls pull the pore closed. This entire process is a brilliant example of how chemistry dictates form and function in biology.
The Spongy Mesophyll: The Loosely Packed Gas Exchange Hub
Once carbon dioxide (CO$_2$) enters through the open stoma, it doesn't just magically appear where it's needed. It must diffuse through the internal air spaces of the leaf to reach the cells that perform photosynthesis. This is where the spongy mesophyll layer comes in.
Located in the lower part of the leaf, just above the lower epidermis where most stomata are found, the spongy mesophyll is a tissue layer composed of irregularly shaped cells. Unlike the tightly packed, column-shaped cells of the palisade mesophyll layer above it, the cells of the spongy mesophyll are, as the topic states, loosely packed.
The large air spaces between these cells are critical. They create a vast internal surface area and allow for the rapid diffusion of gases. CO$_2$ dissolves in the thin layer of water coating the cells and then diffuses into them. Conversely, oxygen (O$_2$), a waste product of photosynthesis, diffuses out of the cells into the air spaces and eventually exits the leaf through the stomata.
| Layer | Cell Arrangement | Primary Function |
|---|---|---|
| Upper/Lower Epidermis | Tightly packed, flat | Protection; houses stomata |
| Palisade Mesophyll | Tightly packed, column-shaped | Main site of photosynthesis |
| Spongy Mesophyll | Loosely packed, irregular-shaped | Gas exchange and circulation |
A Delicate Balance: Transpiration and Plant Survival
There is a trade-off for this open exchange of gases. While the stomata are open to let CO$_2$ in, water vapor escapes from the moist interior of the leaf into the drier outside air. This process is called transpiration.
Transpiration is not all bad; it actually creates a "transpirational pull" that helps draw water and minerals up from the roots, through the stem, and to the leaves. However, if a plant loses too much water, it will wilt and could die. Therefore, plants are constantly making a calculated decision: open stomata to make food but risk drying out, or close stomata to save water but halt food production.
This is why you might see a plant wilt on a hot, sunny afternoon. It is closing its stomata to conserve water, even though it means temporarily stopping photosynthesis. It will reopen them when the temperature cools down in the evening.
Observing Stomata in Action: A Simple Experiment
You can see evidence of stomata with a simple experiment. Carefully paint a thin layer of clear nail polish on the underside of a leaf (from a tree like a maple or oak works well). Let it dry completely. Then, gently place a piece of clear tape over the dried polish, press down, and peel the tape off. The polish will come with it, creating an impression or a "fingerprint" of the leaf surface. Place the tape on a microscope slide and observe under a microscope. You will be able to see the impressions of the stomata and their guard cells!
Common Mistakes and Important Questions
A: While the vast majority are on leaves, stomata can also be found on green stems and other plant organs. In some plants, like cacti, stomata are on the stem instead of the leaves.
A: Plants perform cellular respiration[2] 24/7, which requires oxygen. At night, when photosynthesis stops, stomata often open to allow oxygen in and carbon dioxide out for respiration. However, this depends on the plant's water status.
A: It refers to the guard cells. The phrase is a bit poetic. The "chemical bond" describes the ionic bonds and osmotic processes that cause the guard cells to swell and create the pore (stoma). The "layer of loosely packed cells" is the spongy mesophyll, which is the internal destination for the gases that enter through the stoma.
Footnote
[1]ABA (Abscisic Acid): A plant hormone that functions in many developmental processes, including bud dormancy and the response to stress, particularly water stress by triggering stomatal closure.
[2]Cellular Respiration: The process by which cells break down sugar (glucose) in the presence of oxygen to produce carbon dioxide, water, and energy (ATP). It is the process all living cells use to generate energy.