What is a gaseous exchange?
Exchange of oxygen and carbon dioxide between the environment and living organisms is known as a gaseous exchange. It is a very important process that supplies the required gases to the cells for metabolic processes and also removes the waste of the gases produced by the metabolic processes during respiration.
Conditions of gaseous exchange
Efficient gaseous exchange depends upon three conditions:
- Diffusion gradient.
- Large respiratory surface area.
- Moist surface membrane.
Under these conditions, gases can go into solution before they pass across.
How does a gaseous exchange take place?
Gaseous exchange in stem and roots
The gaseous exchange takes place in stem and large roots, only through a very small opening called “ lenticels” oxygen also enters and is dissolved in water from the soil through root hairs. Except for the root, the whole body of the plant is covered by a cuticle wall which is waterproof. This cuticle prevents the excessive evaporation of water and gaseous exchange In plants.
Gaseous exchange in leaves
Leaves are the main places of gaseous exchange because of their large numbers and having a large surface areas.
In the epidermis of leaves, there are tiny pores that are responsible for the exchange of gases in plants. In dicotyledonous plants, the lower epidermis of the leaves have numerous stomata. While in the grasses and other monocotyledons stomata are present in both upper and lower epidermis.
STRUCTURE OF STOMATA
Each pore is covered by a pair of modified epidermal cells, these modified cells are known as “guard cells”. They are bean-shaped and contain chloroplasts and a large sap filled vacuole. The inner wall of the pore is thicker and less elastic than the outer wall.
MECHANISM OF STOMATA
The function of stomata is to allow gaseous exchange. The opening and closing of stomata depend on the changes in fluid pressure of the walls of stomata. When water enters the guard cells, they expand open the pore, when water leaves the guard cells, they straighten up to close the pore.
FACTOR AFFECTING THE OPENING OF STOMATA
Light is the main factor that initiates the opening of stomata. So this is the reason that under natural conditions stomata open at day time and close at night.
DIFFUSION OF GASES INSIDE THE STOMATA
When the gas passes through the sub-stomatal chambers into the intercellular spaces of the mesophyll, it diffusion into the cells. It dissolves in the sap and goes into the solution for photosynthesis and respiration.
Respiration is the process in which organic materials or food is oxidized to produce energy, carbon dioxide and water.
Breathing is concerned with the exchange of gases and outside the body i.e. taking oxygen inside the body or inhale and expelling out carbon dioxide or exhale.
STEPS OF RESPIRATION
Respiration is a complex process and consists of the following two steps:
- Cellular respiration
The breathing of process involves the exchange of gases.
- CELLULAR RESPIRATION
it is defined as the process in which the oxidation of food takes place within the cell with the help of oxygen and enzymes, resulting in the release of energy.
IMPORTANCE OF RESPIRATION
Respiration is the most important process for all living things. Life without respiration is impossible. All the body functions are performed by utilizing the energy obtaining by the process of respiration.
Movement of air in and out is known as breathing or ventilation. It takes place by the movement of the chest. Ventilation can be divided into two steps:
- Inspiration or inhalation.
- Expiration or exhalation
It involves the following steps:
- The muscles of the diaphragm contract.
- This pulls the diaphragm dawn ward making it flat.
- The external costal muscles contract at the same time.
- This pulls the rib cage upwards and dawns ward.
- This movement together, increase the volume of the thorax.
- The pressure inside the lungs decreases.
- The air rushes in to fill and expands the lungs.
Expiration or exhalation includes the following steps:
- The muscles of the diaphragm relax.
- It springs back the lungs into its dome shape.
- The external intracostal muscles relax, at the same time.
- The rib cage drops down again to its normal position.
- The volume of the thorax decreases.
- The pressure inside the lungs increased.
GASEOUS EXCHANGE IN ANIMALS
All animals exchange gases for respiration like plants. They take in oxygen from the environment, air or water and expel CO2 into the environment.
Gaseous exchange in aquatic animals
There is a large difference in the oxygen content of air water. A unit volume of air contains more oxygen in it than an equal volume of water. So aquatic organisms pass a greater volume of water over their surface of a gaseous exchange than terrestrial vertebrates which pass air in order to absorb a sufficient amount of oxygen as required for their metabolic processes.
COMPARISION OF A SAMPLE OF FRESHWATER AND AIR
- Gaseous exchange in unicellular organisms
In unicellular organisms like amoebae, there is a large surface area as compared to their volume, that why diffusion of gases takes place over the whole surface through the plasma membrane.
- gaseous exchange in poriferan and cnidarians
In these groups of animals, all the cells of their body are in contact with water so they can exchange gases through all the cells of their body.
- Gaseous exchange in Platyhelminthes
Platyhelminthes are free-living invertebrates animals obtain oxygen with the help of diffusion through their body surface. The flat bodies of Platyhelminthes help them to exchange gaseous because the flatness of the body increases the surface areas as compared is the volume.
- Gaseous exchange in annelids
There is a blood vascular system in annelids that possess the respiratory compound “hemoglobin”. The oxygen diffuses through the body surface in the blood which circulates all over the body. From the blood, oxygen is combined with hemoglobin and is carried to all the parts of the body. Similarly, the carbon dioxide is brought back to the blood and diffuses out.
- Gaseous exchange in invertebrates with developing respiratory organs
Besides vertebrates, some invertebrates like mollusks, arthropods, echinoderms have developed respiratory organs. These organs are responsible for the exchange of gases in these animals.
Respiratory organs of vertebrates and some invertebrates
Invertebrates and some invertebrates, there are developed respiratory organs for gaseous exchange. These organs are:
- Tracheal tubes
- Book lungs