Diagram of Stomata

Any of the minute openings or pores in the epidermis of leaves and young stems, often termed stomate, stoma, plural stomata, or stomas. On the underside of leaves, stomata are found extensively. They allow gases to flow between the leaf’s branching system of interconnecting air canals and the outside air.

A stomate opens and closes due to the internal pressure of two sausage-shaped guard cells that surround it. The inner wall of a guard cell is thicker than the outside wall. When the guard cell is filled with water and becomes turgid, the outer wall expands, pulling the inner wall with it and expanding the stomate.

Guard cells help to prevent excessive water loss by closing on hot, dry, or windy days and opening when the weather is more suitable for gas exchange. For most plants, dawn causes a rapid rise in stomatal opening, which peaks about midday and then gradually decreases due to water loss. As darkness approaches, recovery and reopening are followed by another decline.

Types of Stomata

1. Ranunculaceous or Anomocytic: The stoma is surrounded by a small number of subsidiary cells that are identical to the other epidermal cells in this type. 
2. Cruciferous or Anisocytic: The stoma is surrounded by three accessory or subsidiary cells, one of which is smaller than the other two.
3. Rubiaceous or Paracytic: The stoma is encircled by two subsidiary cells that run parallel to the pore’s long axis and guard cells in this type.
4. Caryophyllaceous or Diacytic: The stoma is still surrounded by a pair of subsidiary or accessory cells whose common wall is at right angles to the guard cells in this kind.
5. Gramineous: Guard cells in the gramineous stoma have middle portions that are substantially smaller than the ends, giving the cells a dump-bell appearance in surface view. They are often seen in monocotyledons of the Gramineae and Cyperaceae families.
6. Coniferous Stomata: They have a sunken appearance and appear to be suspended from the secondary cells that arch over them. The guard cells are oval in shape and feature narrow lumina in their median parts. They are triangular in form and feature broader lumina at their ends. The walls of these guard cells, as well as those of the subsidiary cells, are partially lignified and partly non-lignified, which is a distinctive feature.

Functions of Stomata

Stomata have two key functions: they enable carbon dioxide to be taken in and they minimise water loss due to evaporation. However, it also performs additional activities, such as allowing gaseous exchange (CO2 and O2) between the intercellular gaps of plant cells and the surrounding air during photosynthesis. It enables carbon dioxide to enter and oxygen to leave. These gas molecules are the true source of carbon atoms that plants need to make sugars, proteins, and other life-sustaining substances.

In plants, stomata help in the release of surplus water from the surface of the leaves in the form of water vapour. The process is referred to as transpiration.

Carbon dioxide and oxygen are exchanged through diffusion through the stomata of plants. The transfer of a substance from a location of greater concentration to a region of lower concentration is referred to as diffusion.

To maintain the moisture balance, it closes or opens its pores depending on the environmental factors.

Structure of Stomata

Epidermal Cell

It is a plant’s outermost layer, made up of specialised cells that originate in the dermal tissues. Epidermal cells have an irregular structure and are responsible for providing mechanical support to the plant.

Guard Cells

The chloroplast, which is shaped like a kidney or a dumbbell, is found in these cells. They have chlorophyll and can absorb light. The basic role of guard cells is to ensure that the stoma’s opening and closing mechanisms work correctly.

Stomatal Pore

 The apertures discovered beneath the leaf’s structure are referred to as minute ports. These pores are crucial for the exchange of gases such as oxygen and carbon dioxide.

Subsidiary Cells

 It is found in the stroma of a leaf, near to guard cells. Subsidiary cells give support, which helps guard cells in their moment of need. Subsidiary cells are formed in close proximity to mother cells in most cases. They can, however, be seen developing on their own.

MECHANISM OF STOMATA

The turgidity of guard cells determines the process of stomatal opening and closing. When turgidity rises, the pore opens, but when turgidity falls, the pore closes.

Water enters by osmotic pressure, which causes the guard cell to elongate due to the radial orientation of cellulose fibrils.

Turgidity rises when water enters the guard cells, pulling the cells together and causing stomata to open.

Stomata close in the opposite direction of how they open.

Water penetrates the cells during the day because the water potential is lower, resulting in a greater concentration of solutes.

Proton pumps are activated by blue light, which is detected by photoreceptor cells. Protons enter the guard cells from the cytoplasm when proton pumps are activated.

The proton motive force is formed when protons enter the cell, which opens voltage-operated channels in the membrane and allows positive potassium ions to enter the cell.

Chloride ions enter the cell via the chloride-protons symport process when the cell’s potassium content rises, balancing the cell’s internal charges.

Conclusion

On the epidermal layer of leaves, stomata are microscopic pores. Gaseous exchange and transpiration take place mostly at these areas. Guard cells surround the pore and have the ability to close and open it. Some plants have well-developed stomata, whereas others lack them altogether.

Environmental and internal variables influence the opening and closing of these doors. To summarise, stomata play a crucial function in plant physiology by helping plants in removing excess water from their bodies through the process of transpiration.