2.5 | Gaseous Exchange in Plants
Mechanism of Gaseous Exchange in Plants
Most plants carry out the process of gaseous exchange with their surroundings through their leaves, stems and roots.
These three parts provide a large surface area for gaseous exchange.
Diffusion of Carbon Dioxide
The structure in leaves that shows the pathway of gaseous exchange is as shown in diagram below. The diffusion of carbon dioxide occurs through the stoma according to the difference in concentration of carbon dioxide in the cells and in the air spaces between the cells during photosynthesis.
When carbon dioxide is used in photosynthesis, the concentration of carbon dioxide in the cells becomes lower compared to the concentration of carbon dioxide in the air space between the cells.
This difference in concentrations allows the dissolved carbon dioxide in the moist surface of cells to diffuse from the air space between the cells into the cells.
This causes the concentration of carbon dioxide in the air space between the cells to become lower compared to the concentration of carbon dioxide in the air outside the stoma.
This difference in concentrations causes the diffusion of carbon dioxide from the atmosphere into the air space between the cells through the stoma which is open.
Stomatal Pore and Guard Cells
Stoma is made up of a stomatal pore bounded by a pair of guard cells. Guard cells contain chloroplasts to carry out photosynthesis.
Stomata of plants open during photosynthesis when there is light and close when it gets dark or when the plant loses a lot of water on a hot day
Process of Osmosis Affects the Stoma
Concept of Osmosis
Osmosis is the process of movement of water molecules from a region of high concentration of water molecules (solution with a low concentration of solutes) to a region of low concentration of water molecules (solution with a high concentration of solutes) through a semipermeable membrane.
This membrane is permeable to water but not permeable to some solutes such as sucrose molecules.
Process of Osmosis in Guard Cells
When there is light, guard cells carry out photosynthesis to produce glucose.
The concentration of glucose in guard cells increases and causes water from surrounding cells to diffuse into the guard cells through osmosis. Hence, the guard cells become turgid and curved.
Conversely, at night or on a hot day, water diffuses out of the guard cells also through osmosis and causes the guard cells to become flaccid and straight.
Importance of Unpolluted Environment for the Survival of Plants
Effects of Haze and Dust on the Survival of Plants
The reducing sunlight from reaching the plants and reducing the rate of photosynthesis, haze and dust that settle on stomata prevent gaseous exchange between plants and their surroundings.
Effects of Acidic Gases in the Air on the Survival of Plants
Air pollutant gases which are acidic such as sulphur dioxide and nitrogen dioxide dissolve in rainwater to produce acid rain.
Acid rain kills plant cells and causes soil to be acidic and less fertile.
Most plants cannot live in highly acidic soil. This will reduce agricultural produce and cause food shortage.
The preventive measures against the effects of pollution on plants
Ban open burning in Indonesia and Malaysia
Limit the number of motor vehicles on the road in Beijing, China
Encourage the use of alternative energy such as solar energy