Mechanism of Opening and Closing of Stomata | Biology for JAMB PDF Download

Introduction

Stomata are small openings found on the surface of plant leaves and stems, primarily in the epidermis. They play a vital role in regulating the exchange of gases, such as carbon dioxide and oxygen, as well as water vapor. Understanding the mechanism of opening and closing of stomata is essential for comprehending the processes involved in plant physiology. This note aims to outline the mechanism of stomatal opening and closing according to the JAMB Biology exam guidelines.

Stomatal Opening

Stomatal opening is influenced by various environmental factors and plant hormones. The main steps involved in stomatal opening are as follows:

• Light Perception: When light intensity increases, it is perceived by specialized cells called guard cells. This triggers the synthesis of ATP (adenosine triphosphate) through photosynthesis in the guard cells.
• Activation of Proton Pump: The ATP produced is utilized to activate a proton pump present in the plasma membrane of guard cells. This pump actively transports protons (H+) from the cytoplasm of the guard cells to the cell wall.
• Potassium Ion Influx: The transport of protons results in the creation of an electrochemical gradient across the plasma membrane. This gradient promotes the influx of potassium ions (K+) into the guard cells through potassium channels.
• Increased Osmotic Potential: The entry of potassium ions into the guard cells causes an increase in osmotic potential. As a result, water molecules enter the guard cells by osmosis from neighboring cells, leading to their swelling.
• Turgor Pressure: The influx of water and accumulation of potassium ions within the guard cells increase their turgor pressure. Due to their kidney-shaped structure, this increased pressure causes the guard cells to bulge outwards and the stomatal pore to open.

Stomatal Closure

Stomatal closure is a protective mechanism to prevent excessive water loss and maintain plant hydration. The following steps outline the mechanism of stomatal closure:

• Environmental Factors: Various environmental conditions such as high temperature, low humidity, or water scarcity can trigger stomatal closure.
• Abscisic Acid (ABA) Release: In response to adverse environmental conditions, the plant hormone abscisic acid (ABA) is synthesized and released from the cells of the leaves and roots. ABA acts as a signal for stomatal closure.
• ABA Receptors: ABA receptors present on the plasma membrane of guard cells bind with the released ABA molecules.
• Ion Transport Regulation: The binding of ABA to its receptors initiates a signaling cascade that leads to changes in ion transport across the plasma membrane. Potassium channels close, preventing the efflux of potassium ions from the guard cells.
• Loss of Turgor Pressure: The closure of potassium channels decreases the osmotic potential in the guard cells, causing water to move out of the cells. As a result, the guard cells lose turgor pressure and become flaccid.
• Stomatal Pore Closure: When the guard cells become flaccid, they relax and return to their original shape. This causes the stomatal pore to close, restricting the movement of gases and water vapor.

Conclusion

The opening and closing of stomata are crucial processes in plant physiology that enable the exchange of gases and regulate water loss. Stomatal opening is primarily influenced by light perception, ATP synthesis, potassium ion influx, and increased turgor pressure. Stomatal closure, on the other hand, is triggered by adverse environmental conditions and involves ABA release, ion transport regulation, loss of turgor pressure, and stomatal pore closure.