16.7 OZONE DEPLETION IN THE STRATOSPHERE 

  • You have earlier studied in the Chemistry textbook of Class XI about ‘bad’ ozone, formed in the lower atmosphere (troposphere) that harms plants and animals. 
  • There is ‘good’ ozone also; this ozone is found in the upper part of the atmosphere called the stratosphere, and it acts as a shield absorbing ultraviolet radiation from the sun. 
  • UV rays are highly injurious to living organisms since DNA and proteins of living organisms preferentially absorb UV rays, and its high energy breaks the chemical bonds within these molecules. 
  • The thickness of the ozone (300 DU) in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU). 
  • Ozone gas is continuously formed by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in the stratosphere. 
  • There should be a balance between production and degradation of ozone in the stratosphere. 
  • Of late, the balance has been disrupted due to enhancement of ozone degradation by chlorofluorocarbons (CFCs). 
  • CFCs find wide use as refrigerants. CFCs discharged in the lower part of atmosphere move upward and reach stratosphere. In stratosphere, UV rays act on them releasing Cl atoms. 
  • Cl degrades ozone releasing molecular oxygen, with these atoms acting merely as catalysts; Cl atoms are not consumed in the reaction. 
  • Hence, whatever CFCs are added to the stratosphere, they have permanent and continuing affects on Ozone levels. 
  • Although ozone depletion is occurring widely in the stratosphere, the depletion is particularly marked over the Antarctic region. 
  • This has resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole (Figure 16.8). 
NCERT Class XII Biology Chapter 16 : Environmental Issues ...
  • UV radiation of wavelengths shorter than UV-B, are almost completely absorbed by Earth’s atmosphere, given that the ozone layer is intact. 
  • But, UV-B damages DNA and mutation may occur. 
  • It causes aging of skin, damage to skin cells and various types of skin cancers. In human eye, cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of cornea, called snow-blindness, cataract, etc. Such exposure may permanently damage the cornea. 
  • Recognising the deleterious affects of ozone depletion, an international treaty, known as the Montreal Protocol, was signed at Montreal (Canada) in 1987 (effective in 1989) to control the emission of ozone depleting substances. 
  • Subsequently many more efforts have been made and protocols have laid down definite roadmaps, separately for developed and developing countries, for reducing the emission of CFCs and other ozone depleting chemicals.

Comments

Post a Comment

Popular posts from this blog

6.10 DNA FINGERPRINTING

Image
  6.10 DNA FINGERPRINTING   DNA fingerprinting is a very quick way to compare the DNA sequences of any two individuals. DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA, because in these sequences, a small stretch of DNA (TANDEM REPEATS) is repeated many times.  Satellite DNA Repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation.  The bulk DNA forms a major peak and the other small peaks are referred to as satellite DNA. Depending on base composition (A : T rich or G:C rich), length of segment, and number of repetitive units, the satellite DNA is classified into many categories, such as micro-satellites, mini-satellites etc.                                 POLYMORPHISM  &  POLYMORPHIC DNA  Polymorphism is a variation at genetic...
Read more

DIGESTION AND ABSORPTION

Image
DIGESTION AND ABSORPTION Food is one of the basic requirements of all living organisms.  The major components of our food are carbohydrates, proteins and fats.  Vitamins and minerals are also required in small quantities.  Food provides energy and organic materials for growth and repair of tissues.  The water we take in, plays an important role in metabolic processes and also prevents dehydration of the body.  Biomacromolecules in food cannot be utilised by our body in their original form. They have to be broken down and converted into simple substances in the digestive system.  This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods.  General organisation of the human digestive system is shown in Figure 16.1. 16.1 DIGESTIVE SYSTEM  The human digestive system consists of the alimentary canal and the a...
Read more

Regulation of Breathing

Image
  Regulation of Breathing The regulation of breathing is based in the body's acid/base balance. The Central Chemoreceptors (CCR), primarily responsible for the breathing stimulation, are affected by the PaCO 2.  The responsiveness of the peripheral receptors is tied to the level of pH and PaCO 2 . Together these provide the ultimate in servo-control - sensors provide feedback that increase or decrease breathing. Medullary Respiratory Center The rhythmic cycle of breathing originates in the medulla. Higher brain centers (voluntary control), systemic receptors, and reflexes modify the medulla's output. However, no truly separate inspiratory and expiratory centers have been identified. The medulla does contain several widely dispersed groups of respiratory-related neurons that form dorsal and ventral respiratory groups. Fig-Schematic illustration of the respiratory components of the lower brainstem (pons and medulla oblongata). PNC = pneumotaxic center; APC = apneustic center; DR...
Read more