Exchange of Gases

During the process of respiration in plants, the sugar that is produced during photosynthesis along with oxygen is used to produce energy for the growth of the plant. In many ways, respiration and photosynthesis are two different processes.

As we already know, plants produce their food to survive. They use the carbon dioxide (CO2) from the environment to produce sugars and oxygen (O2), which acts as a source of energy for them. While photosynthesis occurs in the leaves and stems only, respiration takes place in the leaves, stems, and roots of the plant. Respiration occurs in the mitochondria of the cell in the presence of oxygen, which is known as aerobic respiration. In plants, there are two types of respiration: dark respiration and photorespiration. Dark respiration can take place in the presence or absence of light, while photorespiration takes place only in the presence of light.

Respiration is a continuous process that takes place in the plants throughout the day and night. Photosynthesis gas exchange occurs only in the daytime in the presence of sunlight and oxygen. Photosynthesis requires the plants to have a proper supply of carbon dioxide to produce oxygen, water, and energy that is required in the plants. Just like the process of exchange of gases takes place through the lungs in the animals, the plants have stomata to take out this process. The gas exchange in plants takes place through stomata, and the process is called diffusion.

The stomata are found in all the aerial plants and not in the roots. Epidermal cells present in the stomata surrounding the guard cells of the stoma are called subsidiary cells. The guard cells are living cells and contain chloroplasts. These cells contain more protoplasm than the other cells. The stomata cells are found on the dicotyledonous leaves, and they are arranged parallel in the case of monocotyledons. A higher concentration of stomata cells is found towards the lower surface of the leaf.

How does Exchange of Gases Take Place in Plants?

In plants, the exchange of gases takes place through stomata. Each of the stomata is surrounded by two guard cells, and these cells contain chloroplasts. A respiratory opening is found under each stoma, and the process of opening and closing of stomata depends on the presence of sugar and starch in the guard cells. In the daylight, the guard cells of the stomata contain sugar synthesized by the chloroplasts present in them. The sugar is soluble, and it increases the concentration of the sap present in the guard cells. As the concentration becomes higher, the water from the neighbouring cells comes into the guard cell by osmosis, and they become turgid. Because of this, the stomata remain open.

In the absence of light, the sugar present in the guard cells converts into starch. The starch is insoluble, and thus the sap of the guard cells remains of much lower concentration than the neighbouring cells, and these cells take out the water from the guard cells by osmosis, making the stomata stay closed.

Factors Affecting the Stomata Movement

• Light: The stomata generally control the presence of the light and close in the dark. Some plants require proper sunlight to keep the stomata open, but other plants may keep the stomata open even in the moonlight. 

• Temperature: Generally, the stomata open up as the temperature rises, provided that the water does not become a limiting factor. In some plant species, the stomata remain closed under continuous light at 0-degree temperature.

• Water Availability: In case the rate of transpiration on the plants is more, the availability of the water becomes less, and they go under water stress. Such plants are called water deficit plants. The majority of the plants close their stomata under such conditions to protect them from the damage that may result due to extreme water shortage. The stomata reopen once the water potential of the plants is restored. This type of control of the movement of stomata is called hydro-passive control. When the plants go under water stress, the accumulation of the phytohormone abscisic acid (ABA) in the guard cells takes place. When the water potential is stored, the stomata reopen, and the phytohormone abscisic acid gradually disappears from the guard cells. 

• Carbon Dioxide Concentration: The reduction of CO2 concentration is a favourable condition for the stomatal opening and the increase in its concentration results in the closing of the stomatal closing. This generally happens in the daylight. These kinds of stomata usually open in the dark. This condition happens when the CO2 trapped inside the leaf is consumed in photosynthesis during the process of photosynthesis. This shows that the internal leaf CO2 concentration is responsible for the stomatal opening rather than the atmospheric CO2. 

The opening and closing of the stomata are a function of the guard cells. The guard cells swell when the water flows into them, which results in the opening of the stomata cells. Similarly, the stomatal pores close when the water moves out, and the guard cells shrink, resulting in the closing of the stomata. 

Stomata

Different parts of plants play different roles in carrying out the processes which are essential for the survival of plants. Stomata is one such part that is involved in the exchange of gases by plants. There are thousands of stomata located on the lower surface of the leaves.

Stomata are kind of tiny openings present on the epidermis of the leaves. Stomata can be seen under a light microscope. In some plants, the stomata are located on the stem and other parts of plants.  Stomata play a huge role in gaseous exchange and photosynthesis. The opening and closing of stomata control the transpiration rate in plants.

The Mechanism of Stomatal Opening and Closure

The mechanism of the opening and closing of the stomata depends on the presence of sugar and starch in the guard cells. When light is present during daytime, the guard cells of the stomata contain sugar which is synthesized by their chloroplasts. The sugar is soluble and increases the concentration of the guard cells. Due to the higher concentration of the cytoplasm of guard cells, the water enters into these cells from the neighbouring cells by the process of osmosis. And the stomata remain open. In the absence of light, the sugar present in guard cells gets converted into starch. The starch is insoluble, and therefore, the guard cells remain in lower concentration than the neighbouring cells, and the neighbouring cells take out the water from the guard cells by osmosis which leads to the closing of stomata.