Category : 11th Class

Respiration is a process which involves intake of oxygen from environment and to deliver it to the cells. It include stepwise oxidation of food in cells with incoming oxygen, elimination of \[C{{O}_{2}}\] produced in oxidation, release of energy during oxidation and storing it in the form of ATP. It takes place in three basic steps -

(1) Pulmonary ventilation : The first process, pulmonary (pulmo = lung) ventilation, or breathing, is the inspiration (inflow) and expiration (outflow) of air between the atmosphere and the lungs.

(2) External (pulmonary) respiration : This is the exchange of gases between the air spaces of the lungs and blood in pulmonary capillaries. The blood gains \[{{O}_{2}}\] and loses \[C{{O}_{2}}.\]

(3) Internal (tissue) respiration : The exchange of gases between blood in systemic capillaries and tissue cells is known as internal (tissue) respiration. The blood loses \[{{O}_{2}}\] and gains \[C{{O}_{2}}.\] Within cells, the metabolic reactions that consume \[{{O}_{2}}\] and give off \[C{{O}_{2}}\]and give off \[C{{O}_{2}}\]during production of ATP are termed cellular respiration.

Respiratory surface

The surface at which exchange of gases (\[C{{O}_{2}}\]  and \[{{O}_{2}}\]) takes place is called respiratory surface. Respiratory surface must be vascular and have enough area for gas exchange. For example - plasma membrane in protozoa, body wall (skin) in annelids, alveocapillary membrane in men.

Respiratory medium

Oxygen is dissolved in air and water. Thus water an air are source of oxygen for animals and called respiratory medium. Water and air are external respiratory medium. Inside the body an internal respiratory medium is also found. This internal respiratory medium is tissue fluid.

Types of respiration : It is of two types

(1) Aerobic respiration : It occurs in the presence of molecular oxygen. The oxygen completely oxidizes the food to carbon dioxide and water, releasing large amount of energy. The organisms showing aerobic respiration, are called aerobes. It is found in most of animals and plants. Aerobic respiration is of two main types direct and indirect.

\[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+\underset{\text{oxygen}}{\mathop{6{{O}_{2}}}}\,\to \underset{\text{Carbon dioxide}}{\mathop{6C{{O}_{2}}}}\,+\underset{\text{Water}}{\mathop{6{{H}_{2}}O}}\,+\underset{\text{Energy}}{\mathop{2830\,kJ}}\,\]

(i) Direct respiration : It is the exchange of environmental oxygen with the carbon dioxide of the body cells without special respiratory organs and without the aid of blood. It is found in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most arthropods.

(ii) Indirect respiration : It involves special respiratory organs, such as skin, buccopharyngeal lining, gills and lungs, and needs the help of blood. The respiration in the skin, buccopharyngeal lining, gills and lungs is respectively called cutaneous buccopharyngeal, bronchial and pulmonary respiration. Cutaneous respiration takes place in annelids, some crustaceans, eel fish, amphibians and marine snakes. It occurs both in water and in air. Buccopharyngeal respiration is found in certain amphibians such as frog and toad. It occurs in the air. Branchial respiration is found in many annelids, most crustaceans and mollusks, some insect larvae, echinoderms, all fishes and some amphibians. It occurs in water only. Pulmonary respiration is found in snails, pila, some amphibians and in all reptiles, birds and mammals. It takes place in air only.

(2) Anaerobic respiration : It occurs in the absence of molecular oxygen and is also called fermentation. In this, the food is only partially oxidised so only a part of energy (5%) is released and of energy remains trapped in the intermediate compounds. It is found in lower organisms like bacteria and yeast. It is also found in certain parasitic worms (Ascaris, Taenia) which live in deficient medium. The organism showing anaerobic respiration, are called anaerobes. These involve one of following reactions.

\[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,\underset{\text{(Fermentation of sugars)}}{\mathop{\xrightarrow{\text{In yeasts }}}}\,\underset{\text{Ethanol}}{\mathop{2{{C}_{2}}{{H}_{5}}OH}}\,+2C{{O}_{2}}+118\,kJ\]

\[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,\xrightarrow{\text{In intestinal worms}}\underset{\text{Lactic acid}}{\mathop{2C{{H}_{3}}CHOHCOOH}}\,+\text{Energy}\]

Certain body tissues of even aerobes also show anaerobic metabolism e.g., during the vigorous contraction of skeletal muscle fibres. In this, the glucose is metabolised into the lactic acid in anaerobic conditions. The rapid formation and accumulation of lactic acid are responsible for muscle-fatigue. The mammalian RBCs shows anaerobic respiration as these lack the mitochondria. In lens of eye and cornea of eye respiration is anaerobic because these structures are a non vascular. Anaerobic respiration appeared first in primitive organisms because there was absence of \[{{O}_{2}}\] in primitive atmosphere.


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