Introduction
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.
FAQs on Exchange of Gases
1. What is the fundamental process of gaseous exchange?
Gaseous exchange is the biological process where different gases move in opposite directions across a specialized respiratory surface. In living organisms, this typically involves taking in oxygen from the environment and releasing carbon dioxide as a waste product. This movement occurs passively through diffusion, driven by differences in the partial pressures of the gases.
2. Where does the exchange of gases occur in plants?
In plants, the primary sites for gas exchange are tiny pores called stomata, which are mostly found on the surface of leaves. These pores allow carbon dioxide to enter for photosynthesis and oxygen (a byproduct) to be released. Some gas exchange also occurs through lenticels on stems and through the general surface of the roots.
3. What is the main site for the exchange of gases in the human body?
The main site for gas exchange in humans is the alveoli, which are millions of tiny, thin-walled air sacs in the lungs. Each alveolus is surrounded by a dense network of blood capillaries. It is across this extremely thin alveolar-capillary membrane that oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled. You can learn more about the complete human respiratory system.
4. What is the difference between breathing and respiration?
While often used interchangeably, breathing and respiration are distinct processes.
- Breathing is a physical, mechanical process. It involves inhaling air into the lungs (inspiration) and exhaling air out of the lungs (expiration). It is simply the act of moving air.
- Respiration is a biochemical process. It refers to cellular respiration, where cells use the oxygen obtained from breathing to break down glucose and produce energy (ATP). This process releases carbon dioxide as a waste product, which is then removed from the body by breathing. For a detailed comparison, you can explore the difference between breathing and respiration.
5. Why is a partial pressure gradient essential for the exchange of gases?
A partial pressure gradient is the driving force behind gas exchange. Gases naturally move from an area where their partial pressure is high to an area where it is low, without the need for energy. For instance, the partial pressure of oxygen (pO₂) in the alveoli is high, while in the deoxygenated blood arriving at the lungs, it is low. This gradient forces oxygen to diffuse from the alveoli into the blood. Similarly, a high pCO₂ in the blood and low pCO₂ in the alveoli cause carbon dioxide to move out of the blood.
6. How are oxygen and carbon dioxide transported in the blood?
Oxygen and carbon dioxide are transported differently in the blood.
- Oxygen Transport: About 97% of oxygen binds to haemoglobin in red blood cells to form oxyhaemoglobin. The remaining 3% is dissolved directly in the blood plasma.
- Carbon Dioxide Transport: Carbon dioxide is transported in three ways: about 70% is converted to bicarbonate ions (HCO₃⁻) in the plasma, 20-25% binds to haemoglobin (as carbamino-haemoglobin), and about 7% is dissolved in plasma.
7. Why do mountaineers often need supplementary oxygen at very high altitudes?
At very high altitudes, the total atmospheric pressure is significantly lower. While the percentage of oxygen in the air remains about 21%, the lower overall pressure means the partial pressure of oxygen (pO₂) is also much lower. This reduces the pressure gradient between the air in the alveoli and the blood in the capillaries. As a result, not enough oxygen can diffuse into the blood to meet the body's metabolic demands, leading to a condition called hypoxia. Supplementary oxygen increases the pO₂ in the lungs, restoring the necessary gradient for efficient gas exchange.
8. How do the Bohr and Haldane effects facilitate efficient gas exchange?
The Bohr and Haldane effects are complementary physiological phenomena that optimize the transport of O₂ and CO₂.
- The Bohr effect states that haemoglobin's oxygen-binding affinity is inversely related to acidity and CO₂ concentration. In active tissues with high CO₂ levels, the blood becomes more acidic, causing haemoglobin to release oxygen more readily.
- The Haldane effect states that deoxygenated haemoglobin has a greater affinity for CO₂ than oxygenated haemoglobin. In the tissues, as haemoglobin releases O₂, its ability to pick up CO₂ increases. Conversely, in the lungs, as haemoglobin binds to O₂, its affinity for CO₂ decreases, promoting the release of CO₂ into the alveoli. These two effects work together to ensure efficient gas delivery and removal. You can read more about the Bohr and Haldane effects here.
