2.4-2.5 Respiratory System Adaptations and Gaseous Exchange

Respiratory structures ensure efficient gaseous exchange with the surroundings.
- Thin respiratory structures (one cell thick).
- Large surface area.
- Moist surface.

Fish use gills for gaseous exchange in water.
Gills consist of filaments with lamellae to increase surface area.
Gills are surrounded by water, allowing gases to dissolve and diffuse easily.
Mudskippers are amphibious fish that breathe through gills and moist outer skin.

Insects have a trachea system for respiration.
Air enters/leaves through spiracles controlled by valves.
Trachea divides into tracheoles with thin, moist walls for efficient gaseous exchange.
Large number of tracheoles provides a large surface area for diffusion into cells.
Some insects have air sacs to increase the rate of gas exchange during energetic activities.

Plants exchange gases with their surroundings through leaves, stems, and roots.
These parts provide a large surface area for gaseous exchange.
During the day, plants carry out photosynthesis by taking in carbon dioxide and giving out oxygen.
During the night, plants take in oxygen and give out carbon dioxide.

Equation: Photosynthesis
$6CO2 + 6H2O o C6H{12}O6 + 6O2$

Equation: Respiration
$C6H{12}O6 + 6O2 o 6CO2 + 6H2O + Energy$

Carbon dioxide diffuses through the stoma according to the concentration difference during photosynthesis.
When carbon dioxide is used in photosynthesis, its concentration in cells becomes lower compared to the air space between the cells.
This concentration difference causes dissolved carbon dioxide to diffuse from the air space into the cells.
The concentration of carbon dioxide in the air space becomes lower compared to the air outside the stoma, causing carbon dioxide to diffuse from the atmosphere into the air space through the stoma.

Stoma is made up of a stomatal pore bounded by a pair of guard cells.
Guard cells contain chloroplasts to carry out photosynthesis.
Stomata 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.

Osmosis is the movement of water molecules from a region of high concentration of water molecules to a region of low concentration of water molecules 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.

During the day, water diffuses into the guard cells through osmosis, causing both the guard cells to curve and open the stoma.
At night or on a hot day, water diffuses out of the guard cells through osmosis, causing both the guard cells to become straight and close the stoma.
The environment, especially unpolluted air, is very important to ensure the growth and survival of plants.

If the surrounding is hazy and dusty, the polluted air will be harmful to the growth and survival of plants.
Haze and dust that settle on stomata prevent gaseous exchange between plants and their surrounding.
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.

Preventive Measures: