Environmental Issues

Environmental Issues

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental issues in botany cover biotic and abiotic stresses — air/water pollution, climate change, deforestation, ozone depletion, and biodiversity loss — that disrupt plant physiology, community structure, and ecosystem stability.

Must-know anchors:

• Greenhouse effect vs global warming: greenhouse effect is the natural trapping of infrared by CO₂, CH₄, N₂O, CFCs, water vapour; global warming is the anthropogenic rise in mean global temperature from elevated greenhouse gases.
• BOD = DO_initial − DO_final (mg L⁻¹) — measures biodegradable organic load; high BOD → low dissolved O₂ → aquatic plant/animal stress.
• Ozone depletion: CFCs release Cl• radicals that catalytically destroy O₃; increased UV-B damages DNA (thymine dimers), proteins, and PSII.
• Acid rain: SO₂ + NOₓ → H₂SO₄/HNO₃; lowers soil pH, leaches Ca²⁺/Mg²⁺/K⁺, mobilises toxic Al³⁺.
• MDCAT pattern: 2 MCQs from this chapter, usually concept-based (cause-effect matching, AQI components, biomagnification vs bioaccumulation).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Air Pollution and Plant Response

Primary pollutants include SO₂, NO₂, CO, CFCs, particulate matter (PM2.5/PM10), and PAN (peroxyacetyl nitrate). SO₂ enters stomata, dissolves in mesophyll water forming sulphite/bisulphite, and inactivates Rubisco by cleaving its disulphide bridge. PAN (formed from NO₂ + hydrocarbons under sunlight) damages chloroplast membranes and bleaches young leaves. AQI is computed as the maximum sub-index among PM2.5, PM10, SO₂, NO₂, O₃, and CO — values 0–50 = good, 301–500 = hazardous.

Greenhouse Effect and Global Warming

Short-wave solar radiation passes through the atmosphere; Earth re-emits long-wave IR, which is absorbed by greenhouse gases. The net balance keeps Earth 33 °C warmer than bare-rock average (−18 °C). Anthropogenic eCO₂ (currently ~424 ppm, up from 280 ppm pre-industrial) intensifies this, raising mean surface temperature. Botanical consequences:

• C3 plants (rice, wheat) benefit more than C4 (maize, sugarcane) because photorespiration is suppressed.
• Stomatal conductance often decreases, partially offsetting water-loss gains.
• Shifts in species distribution — temperate species move poleward / to higher altitudes.

Ozone Depletion

Stratospheric O₃ absorbs UV-B (280–315 nm). CFCs (e.g., CCl₂F₂) photolyse to release Cl• radicals, which destroy O₃ via: Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂ (Cl• regenerated — catalytic). One Cl atom can destroy ~10⁵ O₃ molecules. Enhanced UV-B causes thymine dimer formation, reduced PSII efficiency, and stunting in crop plants.

Water Pollution, Eutrophication, and Biomagnification

Nitrate/phosphate runoff drives eutrophication: algal blooms → light blockage → algal death → aerobic decomposers consume O₂ → hypoxia (low DO). BOD quantifies this oxygen demand. Lipophilic pesticides (e.g., DDT) undergo bioaccumulation (rising concentration in a single organism over time) and biomagnification (rising concentration up trophic levels).

Remediation

Bioremediation uses microbes; phytoremediation uses plants — Rhizofiltration (roots absorb metals), Phytoextraction (hyperaccumulators like Brassica juncea), Phytostabilisation.

Exam Pattern (MDCAT)

Expect one question on pollutant-effect matching, one numerical/conceptual on BOD or AQI.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Mechanism Depth

• C3 vs C4 response to eCO₂: At 700 ppm, C3 photosynthesis rises 30–60 %; C4 only 10–15 % because CO₂ is already concentrated in the bundle sheath. Under nutrient or drought stress, CO₂ fertilisation effect collapses — field gains are often 0–10 %.
• Acid rain chemistry: 2 SO₂ + O₂ + 2 H₂O → 2 H₂SO₄ (acidic); soil buffering capacity (carbonate-rich soils) resists pH change; cation exchange capacity (CEC) governs leaching speed of Ca²⁺, Mg²⁺, K⁺. Mobilised Al³⁺ damages root meristems and blocks Ca²⁺ uptake.
• BOD vs COD: BOD₅ (5-day, 20 °C) measures biodegradable organics; COD measures total oxidisable matter via K₂Cr₂O₇. BOD₅/COD ratio indicates biodegradability.
• Indicator species: Lichens → SO₂-sensitive biomonitors; their absence signals acidic air pollution.
• Carbon sequestration: forests store ~45 % of terrestrial carbon; oceans ~38 %. Deforestation (≈10 million ha yr⁻¹) releases ~1.5 PgC annually.

Connection to Other Topics

Links directly to Photosynthesis (CO₂ fixation, photorespiration), Ecosystem dynamics (succession, biodiversity hotspots), and Plant Physiology (stomatal regulation, stress proteins like HSPs).

Common Mistakes

1. Treating greenhouse effect and global warming as synonyms.
2. Believing CFCs destroy O₃ directly — they do so via Cl• catalysis.
3. Confusing bioaccumulation (single organism) with biomagnification (trophic rise).
4. Assuming every plant benefits from eCO₂ — nutrient and water limits dominate in nature.
5. Confusing BOD with COD.

Practice Prompts

1. A river receives untreated sewage. Initial DO = 8 mg L⁻¹, after 5 days DO = 2 mg L⁻¹. Calculate BOD₅ and comment on water quality.
2. Explain why DDT concentration rises from 0.003 ppm in water to 25 ppm in a top-predator bird, naming both processes involved and citing two biological reasons.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.