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Q11(c):
Discuss briefly the following:
(c) Ultraviolet B
Solution :
1. Physical Definition and Electromagnetic Properties
Ultraviolet B (UV-B) is a specific band of electromagnetic radiation emitted by the Sun, occupying a position within the ultraviolet spectrum. The ultraviolet spectrum is broadly categorized into three distinct regions based on their wavelength ($\lambda$):
- UV-C: $100 \text{ nm} - 280 \text{ nm}$ (Highest energy, completely absorbed by the atmosphere)
- UV-B: $280 \text{ nm} - 320 \text{ nm}$ (Moderate energy, biologically highly active)
- UV-A: $320 \text{ nm} - 400 \text{ nm}$ (Lowest energy, largely penetrates the atmosphere)
Because the energy of a photon is inversely proportional to its wavelength [per the Planck-Einstein relation, $E = \frac{hc}{\lambda}$], UV-B carries sufficient energy to break chemical bonds, making it highly reactive and biologically hazardous upon contact with living tissues.
2. Interaction with the Stratospheric Ozone Layer
Under undisturbed atmospheric conditions, the stratospheric ozone layer ($O_3$) acts as a critical protective shield, absorbing the vast majority of incoming UV-B radiation. The absorption mechanism relies on the continuous cycle of ozone creation and destruction (the Chapman Cycle). However, anthropogenic ozone depletion—driven primarily by the release of chlorofluorocarbons (CFCs) and the subsequent catalytic destruction of ozone by chlorine free radicals ($Cl^\bullet$)—diminishes this shield. A thinning ozone layer directly correlates with a quantifiable increase in the flux of UV-B radiation reaching the Earth's surface.
3. Mutagenic and Biological Pathogenesis
The penetration of UV-B into biological systems triggers highly localized molecular degradation. Its principal target is Deoxyribonucleic Acid (DNA). The pathological steps are outlined below:
- DNA Damage (Pyramidine Dimers): UV-B radiation is intensely absorbed by cellular DNA. The energy induces adjacent pyrimidine bases (predominantly Thymine) on the same DNA strand to form abnormal covalent bonds, known as thymine dimers. This structural distortion disrupts DNA replication and transcription.
- Mutagenesis and Skin Cancer: While cellular repair mechanisms (like nucleotide excision repair) attempt to correct these dimers, overwhelming UV-B exposure leads to uncorrected errors (mutations). Accumulation of these mutations in skin cells can lead to malignant transformations, yielding various skin cancers, including basal cell carcinoma, squamous cell carcinoma, and the highly lethal melanoma.
- Ocular Damage (Photokeratitis and Cataracts): The human cornea and lens are highly efficient absorbers of UV-B. High-dose exposure causes inflammation of the cornea, clinically termed snow-blindness (photokeratitis). Chronic, long-term exposure induces protein cross-linking and aggregation in the lens, resulting in cataracts and eventual vision impairment.
- Immunosuppression: UV-B radiation alters the function of Langerhans cells in the epidermis, suppressing the skin's local immune response and compromising the body's defense against pathogens and tumor antigens.
4. Ecological Consequences
The implications of enhanced UV-B flux extend far beyond human health, fundamentally disrupting trophic structures within ecosystems:
| Ecosystem Type | Impact of Elevated UV-B Exposure |
|---|---|
| Marine Ecosystems | Inhibits photosynthesis in phytoplankton (the foundational producers of aquatic food webs). A decrease in phytoplankton productivity severely limits biomass for higher trophic levels and diminishes the ocean's capacity to sequester atmospheric $CO_2$. |
| Terrestrial Botany | Stunts plant growth, reduces leaf surface area, and alters flowering times. It degrades essential plant hormones and secondary metabolites, ultimately reducing agricultural crop yields. |
Final Solution: Ultraviolet B (UV-B) is high-energy electromagnetic radiation with a wavelength ranging from $280 \text{ nm}$ to $320 \text{ nm}$. Primarily regulated by the stratospheric ozone layer, any elevation in its surface flux—resulting from ozone depletion—acts as a potent mutagen. It causes thymine dimerization in DNA, culminating in heightened incidences of skin cancers, immunosuppression, and ocular pathologies (such as snow-blindness and cataracts), whilst simultaneously triggering systemic damage to terrestrial flora and marine phytoplankton productivity.
More Questions from Class 12 Biology Environmental Issues EXERCISES
- Q1: What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.
- Q1: What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.
- Q2: List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? Which would be difficult or rather impossible to reduce?
- Q2: List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? Which would be difficult or rather impossible to reduce?
- Q3: Discuss the causes and effects of global warming. What measures need to be taken to control global warming?
- Q3: Discuss the causes and effects of global warming. What measures need to be taken to control global warming?
- Q4: Match the items given in column A and B:
- Q4: Match the items given in column A and B: Column A Column B (a) Catalytic converter (i) Particulate matter (b) Electrostatic precipitator (ii) Carbon monoxide and nitrogen oxides (c) Earmuffs (iii) High noise level (d) Landfills (iv) Solid wastes
- Q5(a): Write critical notes on the following: (a) Eutrophication
- Q5(a): Write critical notes on the following: (a) Eutrophication
- Q5(b): Write critical notes on the following: (b) Biological magnification
- Q5(b): Write critical notes on the following: (b) Biological magnification
- Q5(c): Write critical notes on the following: (c) Groundwater depletion and ways for its replenishment
- Q5(c): Write critical notes on the following: (c) Groundwater depletion and ways for its replenishment
- Q6: Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
- Q6: Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
- Q7: Discuss the role of women and communities in protection and conservation of forests.
- Q7: Discuss the role of women and communities in protection and conservation of forests.
- Q8: What measures, as an individual, would you take to reduce environmental pollution?
- Q8: What measures, as an individual, would you take to reduce environmental pollution?
- Q9(a): Discuss briefly the following: (a) Radioactive wastes
- Q9(a): Discuss briefly the following: (a) Radioactive wastes
- Q9(b): Discuss briefly the following: (b) Defunct ships and e-wastes
- Q9(b): Discuss briefly the following: (b) Defunct ships and e-wastes
- Q9(c): Discuss briefly the following: (c) Municipal solid wastes
- Q9(c): Discuss briefly the following: (c) Municipal solid wastes
- Q10: What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?
- Q10: What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?
- Q11(a): Discuss briefly the following: (a) Greenhouse gases
- Q11(a): Discuss briefly the following: (a) Greenhouse gases
- Q11(b): Discuss briefly the following: (b) Catalytic converter
- Q11(b): Discuss briefly the following: (b) Catalytic converter
- Q11(c): Discuss briefly the following: (c) Ultraviolet B
CBSE Solutions for Class 12 Biology Environmental Issues
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
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