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Monday, 21 January 2019

January 21, 2019

Gaseous Exchange in Amphibians




 In Amphibians like frog and toad, gaseous exchange occurs through the skin, lining of the buccal cavity and lungs. Only gaseous exchange through the buccal cavity and lungs involves ventilation movements.

 Buccal gaseous exchange

 The buccal cavity in a toad is large and lined by a thin membrane that is richly supplied with blood capillaries. To draw air into the buccal cavity , the mouth is closed, the nostrils are opened , and the floor of the buccal cavity is lowered. This creates a low air pressure in the buccal cavity, causing air from outside to flow through the nostrils. Then, the nostrils and glottis are closed. Gaseous exchange occurs in the buccal cavity between the blood in the capillaries and the inhaled air. To expel air, the floor of the buccal cavity is raised, increasing the air pressure within the cavity. this forces the valves closing the nostrils to open, allowing air from the buccal cavity to flow out. thus the muscular floor of the buccal cavity acts like a pump: it causes air to be sucked in and pumped out alternatively.

Gaseous exchange through the lungs

 Under normal circumstances, the toad gets sufficient oxygen for its needs by gaseous exchange through it's skin and buccal cavity. To draw air into the lungs, air is first sucked into the buccal cavity as just described. Then, the nostrils are closed, the glottis is opened, and the floor of the buccal cavity is raised. This increases the air pressure in the buccal cavity, forcing air into the lungs though the open glottis. The lungs become inflated as air fills the alveoli. Gaseous exchange occurs between the inhaled air in the alveoli and the blood in the capillaries lining the alveoli.

      To expel air from the lungs into the mouth, the floor of the buccal cavity is lowered to create a region of low pressure there. When air fills the buccal cavity, the glottis closes and the floor of the buccal cavity is raised. This causes the valves of the nostrils to open , allowing air flow out of the body.  
January 21, 2019

Gaseous Exchange in Plants



 Plants carryout gaseous exchange for

 • Photosynthesis, and

•Cellular respiration

     Photosynthesis is carried out by chloroplast containing cells in the presence of sunlight. Cellular respiration is carried out by all plant cells all the time. Since plants are inactive, their energy requirements are low, so cellular respiration proceeds slowly. Photosynthesis is a very vigorous process, especially in bright sunlight. Thus, actively photosynthesizing plants take in the absence of photosynthesis, plants take in oxygen and give out carbon dioxide like animals. Besides the gases involved in these two processes, water vapour also escaped from plants during transpiration. 
 
      Plants do not have special gaseous exchange structures like complex animals. Instead, gases enter and leave the plant body through

 • Microscopic openings called stomata on the surfaces of green aerials parts of plants;

 • tiny openings called lenticels on old stems and roots; and •root hairs in young roots.
Stomata

 Stomata are found in the epidermal layer in green aerial parts of plants, especially leaves. They occur mainly on the Lowe surface e of dicotyledonous leaves, although on monocotyledonous leaves they are found on both surfaces.

     Intercellular air spaces found throughout the lead are linked to stomata as a leaf is a very thin flattened organ, individual leaf cells are either in direct contact with intercellular air spaces or very close to such air spaces. This ensures that the rate of gaseous exchange meets the high metabolic demands of an actively photosynthesizing leaf.

 Note : As a leaf is led than 1 mm thick, its cells are less than 0.5mm from intercellular air spaces. This presents no problem for diffusion.

      The gases that enter the intercellular spaces of the lead dissolve in the moisture on the walls of the cells lining these spaces. These walls are kept moist by a continuous stream of water that reaches the leaves from the root.

 Opening and closing of stomata

    The opening and closing of stomata control the flow of gases in and out of the leaves. this control is necessary to prevent excessive loss of water as vapour from the plant body via transpiration. Usually stomata are open during the day and closed at night.

       Each stomata or stomatal pore is flanked by two bean shaped guard cells, the only epidermal cells with chloroplast s. The walls of the guard cells next to the pore are thicker than those adjacent to the epidermal cells the thicker walls cannot stretch as much as the thinner walls.

      Changes in the solute concentration of the guard cells cause water to flow in and out of them by osmosis. When the solute concentration of the guards cells is high, water flows into them from the surrounding epidermal cells. As a result, the volume and turgidity of the guards cells increase. The thin walls strectch more than the thicker walls, causing the guard cells increase. The thin walls stretch more than the thicker walls causing the guards cells to become more curved, and so open the stoma. When the solute Concentration of the guard cells is low, hence their volume, decrease, I.e. the guard cells becomes flaccid. As the walls of the guard cells are elastic, they return to their original position. This causes the guard cells to straighten up and close the stoma.

      Recent studies have shown that guard cells can actively pump in ions, especially potassium, from the surrounding cells, thereby increasing their solute concentration. this active transport mechanism needs energy (ATP) which is probably supplied by photosynthesis in the guards cells. When active transport of Ione into the guard cells stops, the ions in them diffuse out, causing water to flow out also. The guard cells thus become flaccid.

 Lenticels

 These are the air pores found in the bark of stem and roots. They appear as scars or small protrusions on the surface of stems and roots.

    Lenticels are formed when stems and roots undergo secondary thickening. Usually a lenticels develops below a stoma, where the cork cambium, instead of forming compact rows of cork cells, produces irregularly shaped cork cells which are loosely arranged with a lot of intercellular spaces. As these cork cells increase in number and size, the epidermis ruptures to form an opening or lenticels thorough which air can diffuse in and out of the plant. The intercellular spaces in the cork cambium and cortex ensure that the living plant cells in the stem and root are in contact with or close to the air from outside.

, Note : Most of the cells in a woody stem, however, are composer of dead cells.

Root hairs

 These provide a large surface area for the absorption of water, mineral salts and oxygen. Oxygen , present in the soil air, dissolves in the soil moisture and diffuses into the root hairs. From here, it diffuses into the other root cells. The carbon dioxide produced by the root cells diffuses out of the root into the soil via the root hairs.  

Thursday, 17 January 2019

January 17, 2019

Gills and Tracheal system



 • Bony fishes have the most complex gills, composed of gills filament which are made up of richly vascularized transverse plates.

• A continuous one way circulation of water from outside, through the mouth and over the the gill filaments ventilates the gills. Gaseous exchange is enhanced by blood flowing in the opposite direction to water flow.

•  The tracheal system in arthropods consists of a branching system of air tubes (tracheae and tracheoles) which open to the exterior through spiracles.

 • Air flow in the tracheal system is regulated by opening and closing the spiracles. In large and active insects, air is pumped in and out of the tracheal system by alternately flattening and relaxing the body.  
January 17, 2019

Gaseous Exchange in Lower Animals



 Sponges and coelenterates All cells in the bodies of sponges and coelenterates are in contact with the water in which they live. Oxygen diffuses into each cell and carbon dioxide diffuses out simultaneously. Soon, however, the water bathing these cells would become deficient in oxygen and saturated with carbon dioxide, bringing gaseous diffusion to a stand still.

      To maintain a high diffusion rate, flagellated cells in the body walls of these animals beat rhythmically to create water currents. The water bathing the cells is continuously replaced by a fresh supply of water rich in oxygen and poor in carbon dioxide. As a result , the diffusion gradient for the gases remains high, enabling gaseous exchange to proceed at a sufficiently high rate.

 Annelids

 In the terrestrial earthworms, gaseous exchange occurs by diffusion through their moist skins. There is no special mechanism to circulate air over their skins. Instead , a rich blood supply to the skin rapidly removes the oxygen that diffuses into the epidermal cells, thus maintaining a sufficiently high diffusion gradient. Simultaneously, the carbon dioxide in the blood capillaries diffuses out of them, to enter the epidermal cells. From here, the gas diffuses out of the body into the external environment.

     When the oxygen rich blood from the skin reaches the various body tissues oxygen diffuses out of the capillaries and enters the individual body cells. At the same time, carbon dioxide waste diffuses out of the body cells and enter the blood capillaries to be transported to the skin , where they can be got rid of to the external environment. 

Note : in annelids, the removal and transport of oxygen from the skin is enhanced by the presence of the oxygen carrying pigment, haemoglobin, in their blood. This type of blood transports oxygen more efficiently than one without such a pigment.

Insects

 Air enters and leaves the body of an insect through its tracheal system. This flow of air is controlled by adjusting the size of the spiracular openings. The spiracles open fully when the carbon dioxide concentration in the body tissues is high. This occurs when the insect is active. When the insects is at rest, the carbon dioxide Concentration in the tissues drops. This triggers the valves guarding the spiracular openings to behave like tiny doors and decreases the size of the openings.

     Usually, oxygen diffuses in and carbon dioxide diffuses out passively through the tracheal system. In a large and active insects, like the locust, air is actively pumped in and out of the tracheae by ventilation movements, also referred to as breathing movements. In this mechanism , dorso ventral muscles (vertical muscles connecting the roof and floor of the body segments) contract the flatten the body. This reduces the volume of the tracheal system and forces air out of the body (expiration). When the muscles relax, the body returns to its normal size. The tracheal system, too, returns to its original (larger) size. This causes air to flow into the body (inspiration). In co ordination with these ventilation movements, the spiracles in the anterior and posterior parts of the body open and close alternately. This causes a one way flow of air through the tracheal system, with air being sucked in through the anterior spiracles and expelled through the posterior ones.

     The oxygen in the air that enters the tracheal system dissolves in the tissue fluid in the fine tracheoles. From here, the oxygen diffuse in the body cells. At the same time, carbon dioxide wastes in the cells diffuse out in to the tissue fluid. In the fine tracheoles, the carbon dioxide in the tissue fluid escapes as a gas that leaves the body through the spiracular openings.  
January 17, 2019

Gaseous Exchange Structures



 • In monerans, protists, fungi, simple multicellular animals and plants, gaseous exchange occurs through the body coverings such as plasma membrane, epidermis and skin

 • Complex animals have specialized respiratory or gaseous exchange structures. These include the gills in aquatic animals, tracheae in terrestrial arthropods and lungs in air breathing vertebrates.

 • Gaseous exchange occurs by diffusion or dissolved gases. To improve the rate of diffusion, respiratory structures must have (i) thin gaseous exchange membranes with large, moist (ii) ventilation mechanisms to maintain steep diffusion gradients, and (iii) a close link with the Organism's transport system.  
January 17, 2019

Respiratory Mechanisms



 In practically all living organisms, cellular respiration uses oxygen and produces carbon dioxide as waste. These gases enter or leave the bodies of all organisms by diffusion at the gaseous exchange surfaces.

      In a simple multicellular animal, lime the hydra, gaseous exchange occurs directly between the external environ and the individual body cells. In a complex animal, however, gaseous exchange occurs by diffusion at two sites:

 • the surfaces in respiratory organs like the lungs and gills , and

• The surfaces of individual body cells.

     In many multicellular organisms, air or water from the external environment is continuously circulated over the gaseous exchange surfaces. This process is known as ventilation. Mechanisms which brings about ventilation differ according to the complexity of the organisms and the medium in which the organism lives.

   I am going to discuss the various mechanisms which brings about ventilation and gaseous exchange in the respiratory systems of animals and plants.

 Note: Cellular respiration is sometimes referred to as tissue or internal respiration, while ventilation and gaseous exchange are referred to as external respiration.  
January 17, 2019

Gaseous Exchange in Plants




 Gases enter and leave the plant through (i) stomata, (ii) lenticels, and (iii) root hairs.

 • Opening and closing of stomata regulate gaseous exchange, especially in leaves. This is brought about by changes in the solute concentration of guard cells, making them turgid (stoma is open) or flaccid (stoma is closed).