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Respiration In Plants And Animals


Respiration is a vital process of all living organisms where the chemical energy is made available for the life activities by the oxidation of food. When the food gets oxidised carbon dioxide and water are formed.

Types of Respiration

Living organisms exhibit two types of respiration, namely aerobic and anaerobic.

  • Aerobic respiration is a type of respiration in which food substances are completely oxidised to carbon dioxide and water with the release of energy. For this process atmospheric oxygen is required. All higher organisms respire aerobically.
  • Anaerobic respiration is a type of respiration in which the food gets oxidised and energy is released in absence of oxygen. The lower forms of life like certain bacteria and yeast exhibit anaerobic respiration. In this process, ethyl alcohol and CO2 are formed.

Respiration in Plants


Plants do respire. They give out carbon dioxide and absorb oxygen from the air.

All parts of the plant respire—the leaves, the stem, the roots and even the flowers. The parts above the soil get their oxygen directly from the air through tiny openings in the leaves called stomata and also through the pores in the stems or branches of trees called lenticels. But these pores do not open and close like stomata.

The main respiratory organ of a plant is the leaf. In the presence of sunlight, the green parts of the plants carry out photosynthesis as well as respiration. Respiration occurs through stomata present on the lower surface of the leaves. Stomata are pores perforating the epidermis of the leaves and herbaceous stem. Numerous pores are present in the lower epidermis of leaf and are generally fewer in the upper epidermis and the stem. Their functions are as follows:

  • To allow exchange of carbon dioxide and oxygen between the inside of leaf and the surrounding atmosphere
  • To permit the escape of water vapour from the leaf into the environment

The roots of a plant also need oxygen, which they obtain from the air spaces in the soil.

Respiration in Animals


In animals, lower forms of animals like Amoeba respire by diffusion across the membrane. In earthworm respiration is by diffusion through the moist skin. Among arthropods, terrestrial insects have a system of air tubes called tracheae, arachnids such as spiders and scorpions have book-lungs, and aquatic arthropods such as crabs have gills for respiration. The land animals consisting of amphibians, reptiles, aves and mammals have lungs as the chief respiratory organs. Three types of respiration namely pulmonarycutaneous and buccal respiration are found in frogs. Hibernating frogs show only cutaneous respiration.

Human Respiratory System
There are two major components in the human respiratory system. They are as follows:

  1. The upper respiratory tract is composed of nostrils, nasal cavities, pharynx and larynx.
  2. The lower respiratory tract is composed of trachea, bronchi, bronchioles and alveoli.

shows the human respiratory system.


Human Respiratory System

  • The external nares or nostrils are the openings of the nose. Spiracles of insects are analogous to the nostrils of higher animals. Air enters into the nasal cavity through the nostrils. Nasal cavities open into pharynx.
  • The larynx is more commonly known as voice box or glottis which is responsible for producing sound. The larynx extends from the pharynx to the trachea.
  • The pharynx leads into trachea or windpipe through a slit called glottis. While swallowing food, glottis gets covered by a small cartilaginous flap of skin called epiglottis. The projecting part of the larynx is called Adam’s apple.
  • The lungs are located in the thoracic cavity, which is separated from the abdominal cavity by a muscular partition called diaphragm.
  • Within lungs, bronchi branch into numerous narrow tubes called brochioles. Each bronchiole terminates in a sac called alveoli giving the appearance of a bunch of grapes. Walls of alveoli are very thin and covered by blood capillaries. Exchange of gases takes place here. Alveoli increase the surface area for diffusion of gases. Lungs are covered by thin protective membrane called pleura.

Mechanism of Respiration Following are the important steps of pulmonary respiration.

  1. Breathing or pulmonary ventilation involves the physical movement of air between the atmosphere and the alveoli.
  2. External respiration involves exchange of gases between the alveolar air and the lung capillaries by diffusion.
  3. Internal respiration involves exchange of gases between the blood and the cells of the body tissue which takes place through diffusion.
  • Breathing is effected by the expansion and contraction of lungs. The two processes by which the lungs are expanded or contracted are as follows:
  1. The downward and upward movements of the diaphragm, which lengthens and shortens the chest cavity
  2. The elevation and depression of the ribs, which increase or decrease the diameter of the chest cavity
  • Process of taking in air into the respiratory system is called inspiration. During inspiration intercostal muscles contract, diaphragm flattens out and alveoli are pulled towards the rib cage resulting in increase in volume of thoracic cavity.
  • Expiration is the process of expelling air out of the respiratory system. During expiration intercostal muscles relax, diaphragm regains its dome shape and alveoli compress.
  • Volume of air inspired or expired with each normal breathing is called tidal volume, which is approximately 500–700 mL in man.
  • The maximum possible volume of air, which can be inspired, is called vital lung capacity. After complete exhalation, the lungs of a healthy man contain a litre of gas known as residual volume.
  • Atmospheric air is a mixture of 21% oxygen, 78% nitrogen and approximately 1% of trace gases. Although alveolar air contains 14% oxygen, it gets mixed with rather fresh air in our trachea. This means that the exhaled air may contain 16% of oxygen.
  1. Movement of oxygen: There is more oxygen in the air in the alveoli than in the blood of the capillaries, so oxygen moves from the air to the blood. It then combines with the red-coloured pigment in the blood called haemoglobin to form oxyhaemoglobin. The amount of oxygen normally carried by 100 mL of pure blood is 20 mL.
  2. Movement of carbon dioxide: Carbon dioxide is a waste product of cellular respiration and has to be eliminated from the body. It is absorbed by the blood in the capillaries surrounding the tissues. When this blood reaches the lungs, it flows in the capillaries surrounding the alveoli. Since there is more carbon dioxide in the blood than in the air, carbon dioxide diffuses from the blood of the air in the alveoli. It is sent out of the body when the air is exhaled. Most of the carbon dioxide (70–75%) in the blood is carried in the form of bicarbonates. About 23% of carbon dioxide entering into the RBCs combines with the globin part of haemoglobin to form carbamino-haemoglobin, which is transported to the lungs.

Respiratory Problems

  • Haemoglobin has approximately 200–250 times more affinity to carbon monoxide than to oxygen. Carbon monoxide binds with haemoglobin irreversibly to form carboxyhaemoglobin. It is relatively a stable compound and cannot bind any oxygen. So, the amount of haemoglobin available for oxygen transport is reduced. The resulting deficiency of oxygen causes headache, dizziness, nausea and even death.
  • Chloride shift is connected with the transport of carbon dioxide.
  • A child respires around 26 times per minute. Normal man respires around 14–18 times per minute. If the carbon dioxide concentration in the blood increases, the rate of breathing also increases. When carbon dioxide is dissolved in blood, then blood becomes slightly acidic. The brain stem picks up this change and adjusts the breathing to get rid of CO2 from the blood.
  • Emphysema is a lung disease in which the air sacs are enlarged and damaged resulting in difficulty in breathing. It is caused by smoking.
  • Asthma is a disorder in which breathlessness and wheezing are aggravated by certain stimuli, which cause the bronchi to become constricted. Bronchial asthma may be stimulated by wide range of conditions and substances. It may be an allergic reaction.
  • Mountain sickness is due to the shortage of oxygen at high altitude. In order to overcome this, generally the RBCs increase in number in the people living at high altitude.
  • Pleurisy is due to the inflammation of pleura. It is frequently associated with a pleural effusion (the accumulation of extra fluid in the space between the two layers of pleura).

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