Understanding Guard Cell Function and Stomatal Opening

The question asks what happens to the guard cells when water flows into them. To answer this, we need to understand the structure and function of guard cells and stomatal pores.

What are Guard Cells and Stomatal Pores?

• Guard cells are specialized plant cells found in the epidermis of leaves, stems, and other organs.
• They surround tiny pores called stomatal pores (or stomata, singular stoma).
• These stomatal pores regulate gas exchange (like carbon dioxide uptake for photosynthesis and oxygen release) and transpiration (water vapor release) between the plant and the atmosphere.
• The opening and closing of the stomatal pores are controlled by the guard cells.

Water Movement into Guard Cells

Water moves into cells through a process called osmosis. Guard cells can accumulate solutes, such as potassium ions ($K^+$) or sugars, which lowers their water potential compared to the surrounding cells. This difference in water potential causes water to move from the surrounding epidermal cells into the guard cells.

Effect of Water Influx on Guard Cells

When water flows into the guard cells, they absorb the water and swell. This swelling is due to the increase in turgor pressure inside the cells. The cells become turgid.

How Turgidity Affects Stomatal Pores

The structure of guard cells is unique and allows them to control the stomatal pore size. Guard cells have unevenly thickened cell walls:

• The cell wall area adjacent to the stomatal pore (the inner wall) is thicker and less elastic.
• The cell wall area away from the stomatal pore (the outer wall) is thinner and more elastic.

When water enters the guard cells and they become turgid, the thinner, more elastic outer walls bulge outwards significantly more than the thicker inner walls. Because the guard cells are attached at their ends, this bulging of the outer walls pulls the thicker inner walls apart, causing the stomatal pore to open.

Conversely, when guard cells lose water, they become flaccid, the outer walls relax inwards, and the inner walls move closer together, causing the stomatal pore to close.

Analysis of Options:

Let's evaluate the given options based on our understanding:

• The guard cells close: Guard cells are cells, they don't 'close' in this context. They change shape. This option is incorrect.
• Stomatal pores open: When guard cells become turgid due to water influx, their shape changes and the stomatal pore opens. This aligns with our understanding.
• Stomatal pores shrink: Shrinking implies the pore gets smaller, which is the opposite of opening. This is incorrect.
• Stomatal pores close: Closing means the pore size reduces or is eliminated, which happens when guard cells lose water and become flaccid, not when they gain water and become turgid. This is incorrect.

Therefore, when water flows into the guard cells, the stomatal pores open.

Revision Table: Guard Cells and Stomata

Key points about the relationship between guard cells, water, and stomata:

• Guard cells control stomatal pore size.
• Water potential difference drives water movement into/out of guard cells.
• Water entering guard cells makes them turgid.
• Turgid guard cells, due to uneven wall thickness, open the stomatal pore.
• Water leaving guard cells makes them flaccid.
• Flaccid guard cells close the stomatal pore.

Additional Information: Factors Affecting Stomatal Opening

While water availability is crucial for guard cell turgidity and stomatal opening, other factors also influence stomatal behavior:

• Light: Stomata generally open in the presence of light, which is required for photosynthesis (and thus CO₂ uptake). Light triggers mechanisms that increase solute concentration in guard cells.
• Carbon Dioxide Concentration: Low CO₂ levels inside the leaf often lead to stomatal opening to allow more CO₂ uptake. High CO₂ levels tend to cause closing.
• Temperature: Stomata may close at very high temperatures to reduce water loss through transpiration.
• Humidity: Low humidity increases the rate of transpiration, which can lead to water stress and cause stomata to close.