Stomata: The Plant's Breath
The Structure of a Stoma
Imagine a tiny mouth on the surface of a leaf that can open and close. That is essentially what a stoma (plural: stomata) is. The word "stoma" comes from the Greek word for "mouth." However, this "mouth" is not for eating in the traditional sense; it is for breathing and transpiring.
Each stoma is a complex structure made up of two specialized kidney-shaped or bean-shaped cells called guard cells. These are the only cells in the epidermis of a leaf that contain chloroplasts[1], the organelles where photosynthesis takes place. The guard cells surround the pore, the actual opening. The changing shape of these guard cells controls the size of the pore.
- When the guard cells are turgid (swollen with water), they bend and pull apart, creating an opening between them. The stoma is open.
- When the guard cells are flaccid (limp from water loss), they relax and collapse together, closing the gap between them. The stoma is closed.
This opening and closing mechanism is a brilliant example of osmosis[2] at work. The guard cells have thicker inner walls and thinner, more elastic outer walls. When water flows into the guard cells, the thinner walls stretch more easily, causing the cells to bulge outward and the pore to open.
The Primary Function: Gas Exchange
The main job of stomata is to facilitate the exchange of gases between the plant's internal tissues and the external atmosphere. This is a two-way process critical for two of the plant's most important functions: photosynthesis and respiration.
1. Intake of Carbon Dioxide ($CO_2$): Plants are autotrophs[3], meaning they make their own food. They do this through photosynthesis, a chemical reaction that uses light energy to convert carbon dioxide and water into glucose (sugar) and oxygen. The chemical equation for photosynthesis is:
$6CO_2 + 6H_2O + light \ energy \rightarrow C_6H_{12}O_6 + 6O_2$
For this reaction to happen, the plant needs a constant supply of carbon dioxide. Stomata are the entry points for this essential gas. The $CO_2$ diffuses through the open stomatal pore, into the air spaces within the leaf, and finally into the cells where photosynthesis occurs.
2. Release of Oxygen ($O_2$): Oxygen is a waste product of photosynthesis. If it built up inside the leaf, it would become toxic and halt the food-making process. Therefore, the oxygen produced during photosynthesis diffuses out of the leaf through the very same stomatal pores.
3. Release of Water Vapor (Transpiration): As the stomata are open for gas exchange, water inevitably evaporates from the moist internal surfaces of the leaf and escapes into the atmosphere. This process is called transpiration. While it might seem like a wasteful side effect, transpiration is crucial for pulling water and dissolved nutrients up from the roots through the stem and to the leaves—a process known as the transpirational pull.
| Gas | Direction | Process | Purpose |
|---|---|---|---|
| Carbon Dioxide ($CO_2$) | Into the leaf | Photosynthesis | Raw material for making food (glucose) |
| Oxygen ($O_2$) | Out of the leaf | Photosynthesis (byproduct) & Respiration | Remove waste; supply air for plant cell respiration |
| Water Vapor ($H_2O$) | Out of the leaf | Transpiration | Cool the plant; create transpirational pull for water uptake |
How Stomata Open and Close: The Guard Cell Dance
The opening and closing of stomata is not random; it is a highly regulated process responding to the plant's needs and environmental conditions. The dance of the guard cells is controlled by several key factors:
1. Light: Most plants open their stomata at dawn. Light stimulates photosynthesis in the guard cells' chloroplasts, producing ATP[4] (energy). This energy powers pumps in the guard cell membranes that actively transport potassium ions ($K^+$) into the cells from the surrounding epidermis cells.
2. Water Pressure (Turgor): The influx of $K^+$ ions lowers the water potential inside the guard cells. Following the rules of osmosis, water then flows into the guard cells from neighboring cells. As the guard cells swell with water, their unique structure causes them to bend, and the stoma opens.
3. Carbon Dioxide Concentration: Low levels of $CO_2$ inside the leaf, which occur when photosynthesis is active, signal the stomata to open to let more $CO_2$ in. Conversely, high internal $CO_2$ levels (e.g., when photosynthesis stops at night) cause stomata to close.
4. Other Factors: Wind, temperature, and humidity also play roles. For example, in hot, dry, and windy conditions, plants will often close their stomata to prevent excessive water loss, even if it means temporarily slowing down photosynthesis.
Stomata in Action: A Day in the Life of a Leaf
Let's follow a sunflower leaf over a sunny day to see stomatal regulation in a practical application.
Sunrise: As the first rays of sun hit the leaf, the guard cells sense the light. They begin photosynthesis, producing energy. This energy powers ion pumps, potassium ions flood in, water follows by osmosis, and the stomata begin to open.
Midday: The sun is bright, and photosynthesis is running at full speed, rapidly using up $CO_2$ inside the leaf. This low $CO_2$ level is a strong signal for the stomata to remain wide open. Carbon dioxide streams in, and oxygen and water vapor flow out. The plant is actively making food, but it is also losing a lot of water through transpiration.
Afternoon: It has been a hot, rainless day. The soil is starting to dry out, and the plant's roots are struggling to absorb enough water. Internal water levels drop. The guard cells lose their turgor pressure, and even though it's still light out, the stomata may start to partially close to conserve precious water. The plant is making a trade-off: less food production for better survival.
Sunset: The light fades, photosynthesis stops, and $CO_2$ begins to build up inside the leaf. Without light, the ion pumps in the guard cells stop working. Potassium ions leak out, water follows, and the guard cells become flaccid. The stomata close for the night, sealing the leaf and preventing any further water loss until morning.
Common Mistakes and Important Questions
Footnote
[1]Chloroplasts: Organelles found in plant cells where photosynthesis occurs. They contain chlorophyll, a green pigment that captures light energy.
[2]Osmosis: The movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.
[3]Autotrophs: Organisms that produce their own food from inorganic substances using light or chemical energy. Plants are photoautotrophs.
[4]ATP (Adenosine Triphosphate): The primary energy-carrying molecule found in the cells of all living things. It provides energy for cellular processes.