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Q6:
Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
Solution :
Step 1: Mechanism of Ozone Hole Formation over Antarctica
The formation of the "ozone hole"—a region of exceptionally depleted ozone in the stratosphere—is primarily triggered by the presence of Anthropogenic Chlorofluorocarbons (CFCs) and specific meteorological conditions unique to the Antarctic region. The chemical cycle is as follows:
- Chemical Activation: CFCs are transported to the stratosphere where UV radiation breaks them down to release chlorine atoms ($Cl\cdot$). These act as catalysts in the destruction of ozone ($O_3$).
- Polar Stratospheric Clouds (PSCs): During the dark Antarctic winter, temperatures drop below $-78^{\circ}C$, allowing the formation of PSCs. These clouds provide a solid surface for heterogeneous chemical reactions that convert inactive chlorine reservoirs ($HCl$ and $ClONO_2$) into active chlorine gases ($Cl_2$).
- Springtime Photolysis: When sunlight returns in the Antarctic spring (September/October), the $Cl_2$ is photolyzed by UV radiation into highly reactive $Cl\cdot$ radicals.
- Catalytic Cycle: Each chlorine atom can destroy thousands of ozone molecules:
$Cl\cdot + O_3 \rightarrow ClO + O_2$
$ClO + O \rightarrow Cl\cdot + O_2$
Net Result: $O_3 + O \rightarrow 2O_2$ [Per the Molina-Rowland Hypothesis].
Step 2: Biological and Environmental Impacts of Enhanced UV Radiation
The depletion of the ozone layer allows higher fluxes of UV-B radiation ($280–315\ nm$) to reach the Earth's surface. The physiological implications are categorized below:
| Target System | Pathophysiological Impact |
|---|---|
| Human Integumentary System | Increased incidence of skin cancers (basal cell and squamous cell carcinoma) and accelerated skin aging [Per Photobiological Damage Principles]. |
| Human Ocular System | High-intensity UV exposure leads to inflammation of the cornea (photokeratitis) and the development of senile cataracts. |
| DNA/Molecular Level | UV-B is absorbed by DNA, causing the formation of pyrimidine dimers (specifically thymine dimers), which lead to mutagenic mutations and inhibited DNA repair mechanisms. |
| Photosynthetic Organisms | Reduction in phytoplankton productivity at the base of marine food webs, leading to ecosystem-wide trophic instability. |
Step 3: Synthesis of Causality
The Antarctic stratosphere acts as a "chemical reactor" due to the Polar Vortex, a massive circular wind pattern that isolates the air mass, preventing the mixing of ozone-rich air from lower latitudes with the depleted polar air. Once the vortex breaks up in late spring, the ozone-depleted air disperses toward mid-latitudes, increasing global UV exposure. The biological impact is not localized; it represents a systemic threat to global biodiversity, specifically affecting the genetic integrity of all organisms exposed to high-energy solar radiation.
Final Solution: The ozone hole forms over Antarctica due to the synergistic effect of CFC-derived chlorine and the formation of Polar Stratospheric Clouds within the thermally isolated Antarctic Polar Vortex. Enhanced UV-B radiation causes severe biological damage, including DNA mutation, increased incidence of skin carcinomas, cataracts, and a decrease in primary marine 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?
- 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
- 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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