1. Explain the structure and function of stomata.
Answer:
Structure:
- Stomata are tiny pores found primarily on the surface of leaves and stems.
- They are surrounded by two guard cells that control the opening and closing of the stomatal pore.
- Guard cells have a thick inner wall and a thin outer wall.
Function:
- Stomata regulate gas exchange, allowing CO2 to enter for photosynthesis and O2 to exit.
- They also control water loss through transpiration.
2. Describe the role of potassium ions in stomatal opening.
Answer:
- Potassium ions (K⁺) play a key role in stomatal opening by increasing the osmotic potential of guard cells.
- During the daytime, K⁺ ions are actively transported into guard cells.
- This creates a high solute concentration, leading to water influx via osmosis, causing the guard cells to swell and the stomata to open.
3. What triggers stomatal closure?
Answer:
- Stomatal closure is triggered by factors such as water stress, darkness, or high levels of abscisic acid (ABA).
- ABA promotes the efflux of potassium ions and other solutes from guard cells.
- The loss of solutes reduces osmotic pressure, causing water to exit the guard cells, leading to stomatal closure.
4. Explain the role of abscisic acid in stomatal movement.
Answer:
- Abscisic acid (ABA) is a plant hormone that induces stomatal closure during drought conditions.
- ABA activates ion channels, causing the efflux of K⁺, Cl⁻, and malate ions from guard cells.
- This reduces the osmotic potential, leading to water loss from the guard cells and stomatal closure.
5. Discuss the light-dependent mechanism of stomatal opening.
Answer:
- In the presence of light, guard cells activate proton pumps that expel H⁺ ions, creating an electrochemical gradient.
- Potassium ions (K⁺) move into the guard cells through voltage-gated channels.
- This increases the osmotic potential, drawing water into the guard cells, causing them to swell and open the stomata.
6. What is the significance of turgor pressure in guard cells?
Answer:
- Turgor pressure in guard cells is crucial for stomatal movement.
- When guard cells gain water, their turgor pressure increases, leading to stomatal opening.
- Conversely, a loss of water reduces turgor pressure, causing the stomata to close.
7. How does CO2 concentration affect stomatal movement?
Answer:
- Low CO2 levels in the leaf promote stomatal opening to allow more CO2 uptake for photosynthesis.
- High CO2 levels inside the leaf cause stomatal closure as there is no immediate need for additional CO2.
8. Explain the role of guard cell chloroplasts in stomatal regulation.
Answer:
- Guard cell chloroplasts produce ATP and sugars during photosynthesis.
- ATP is used to fuel active transport processes like the movement of K⁺ ions.
- Sugars increase the osmotic potential in guard cells, contributing to water influx and stomatal opening.
9. Describe the role of environmental factors in stomatal movement.
Answer:
Light: Promotes stomatal opening by activating proton pumps.
Temperature: High temperatures increase transpiration, often causing stomatal closure to conserve water.
Humidity: High humidity favors stomatal opening, while low humidity induces closure.
Water Availability: Drought conditions trigger ABA production, leading to stomatal closure.
10. How do malate and chloride ions contribute to stomatal opening?
Answer:
- Malate and chloride ions act as counter-ions to maintain electrical neutrality during K⁺ influx.
- They increase the osmotic potential in guard cells, facilitating water uptake and stomatal opening.
11. Discuss the role of aquaporins in stomatal movement.
Answer:
- Aquaporins are water channel proteins in guard cells.
- They facilitate rapid water movement into and out of guard cells, enabling quick stomatal opening and closure.
12. What is the significance of the stomatal mechanism in plants?
Answer:
- Stomatal movement optimizes gas exchange for photosynthesis while minimizing water loss.
- It plays a critical role in maintaining plant water balance and adapting to environmental conditions.
13. Explain the ionic basis of stomatal movement.
Answer:
- Stomatal movement is regulated by the active transport of ions like K⁺, Cl⁻, and H⁺.
- During opening, K⁺ and Cl⁻ ions accumulate in guard cells, increasing osmotic pressure.
- During closure, these ions exit the guard cells, reducing osmotic pressure and causing water loss.
14. How does temperature influence stomatal behavior?
Answer:
- High temperatures increase the rate of transpiration, potentially leading to stomatal closure to conserve water.
- Moderate temperatures favor photosynthesis and stomatal opening.
15. Describe the role of proton pumps in guard cell function.
Answer:
- Proton pumps expel H⁺ ions from guard cells, creating an electrochemical gradient.
- This gradient facilitates the passive influx of K⁺ ions, which increases osmotic pressure and leads to stomatal opening.
16. How does stomatal closure help plants during drought?
Answer:
- Stomatal closure reduces water loss through transpiration.
- It prevents excessive dehydration, enabling the plant to survive during water scarcity.
17. Discuss the role of circadian rhythms in stomatal movement.
Answer:
- Circadian rhythms influence stomatal opening and closing even in constant light or dark conditions.
- Internal biological clocks ensure stomata open during the day and close at night, optimizing photosynthesis and water use.
18. Explain the differences between stomatal opening in monocots and dicots.
Answer:
- Monocots have dumbbell-shaped guard cells, while dicots have kidney-shaped guard cells.
- The mechanism of stomatal opening and closing is similar but may vary slightly due to structural differences.
19. What happens to stomata under conditions of high salinity?
Answer:
- High salinity can lead to osmotic stress, causing water to move out of guard cells.
- This reduces turgor pressure, resulting in stomatal closure to conserve water.
20. How does stomatal movement affect plant productivity?
Answer:
- Efficient stomatal regulation ensures adequate CO2 uptake for photosynthesis while minimizing water loss.
- Poor stomatal control can lead to reduced photosynthetic efficiency and decreased plant growth.
