Plants, Animals and Microbiology
Overview
Questions are application-based rather than purely factual—expect queries linking biological concepts to agriculture, health, environment, and recent scientific developments.
The scope covers three interconnected areas: cellular foundations common to all life, the diversity of plant and animal kingdoms, and the world of microorganisms. UPSC frequently tests the practical significance of these concepts—why nitrogen fixation matters for farming, how vaccines work, or why certain plants are medicinally important. A strong grasp here also supports Environment and Ecology questions.
Students must focus on fundamental distinctions (prokaryote vs eukaryote, monocot vs dicot), economically important species, and microorganisms in news (disease outbreaks, probiotics, biofertilisers). Avoid memorising excessive taxonomic details—understand principles and applications instead.
Key Concepts
- Cell Theory: All living organisms are composed of cells; the cell is the basic structural and functional unit of life; all cells arise from pre-existing cells (Schleiden, Schwann, Virchow).
- Prokaryotes vs Eukaryotes: Prokaryotes (bacteria, cyanobacteria) lack a membrane-bound nucleus and organelles; eukaryotes (plants, animals, fungi, protists) have a true nucleus, mitochondria, and other organelles.
- Plant Cell Uniqueness: Plant cells possess a rigid cell wall (cellulose), chloroplasts for photosynthesis, and large central vacuoles—absent in animal cells.
- Photosynthesis Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Occurs in chloroplasts; converts solar energy into chemical energy stored in glucose.
- Respiration vs Fermentation: Aerobic respiration (with oxygen) yields ~36-38 ATP per glucose molecule; anaerobic fermentation yields only 2 ATP. Fermentation produces ethanol (yeast) or lactic acid (bacteria, muscles).
- Five-Kingdom Classification (Whittaker): Monera (bacteria), Protista (unicellular eukaryotes), Fungi, Plantae, Animalia—based on cell structure, mode of nutrition, and body organisation.
- Nitrogen Cycle Role of Microbes: Rhizobium (legume root nodules) and Azotobacter fix atmospheric nitrogen into ammonia; Nitrosomonas and Nitrobacter convert ammonia to nitrates usable by plants.
- Symbiosis Types: Mutualism (both benefit—lichens), Commensalism (one benefits, other unaffected—orchids on trees), Parasitism (one benefits at other's expense—tapeworm).
Formulas / Key Facts
| Fact | Significance |
|---|---|
| Mitochondria = "Powerhouse of cell" | Site of ATP production via cellular respiration |
| Ribosomes = Protein factories | Found in both prokaryotes and eukaryotes |
| Chlorophyll absorbs red and blue light | Reflects green; basis of plant colour |
| Monocots vs Dicots | Monocots: parallel veins, fibrous roots, floral parts in 3s (rice, wheat). Dicots: reticulate veins, taproot, floral parts in 4s/5s (pulses, mustard) |
| Xylem transports water upward | Dead tissue; unidirectional flow |
| Phloem transports food | Living tissue; bidirectional flow |
| Bacteria reproduce by binary fission | Can double every 20 minutes under ideal conditions |
| Viruses are obligate parasites | Cannot reproduce outside a living host cell |
| Fungi are heterotrophs | Decomposers; cell walls of chitin (not cellulose) |
| Lichens = Algae + Fungi | Bioindicators of air pollution (sensitive to SO₂) |
Worked Examples
Example 1: Why do leguminous crops improve soil fertility?
Solution: Legumes (pulses like gram, lentil) have root nodules containing Rhizobium bacteria. These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants absorb as nitrates. When the crop is harvested or residues decompose, this fixed nitrogen enriches the soil, reducing the need for synthetic fertilisers. This is why crop rotation with legumes is recommended.
Example 2: A question states: "Assertion: Cyanobacteria are used as biofertilisers in paddy fields. Reason: They can fix atmospheric nitrogen."
Solution: Both assertion and reason are true, and the reason correctly explains the assertion. Cyanobacteria (blue-green algae like Anabaena, Nostoc) possess specialised cells called heterocysts that fix nitrogen. In waterlogged paddy fields, they thrive and contribute 20-30 kg nitrogen per hectare naturally.
Example 3: Why are antibiotics ineffective against viral infections?
Solution: Antibiotics target bacterial cell structures—cell wall synthesis (penicillin), protein synthesis (streptomycin), or DNA replication. Viruses lack these structures; they use host cell machinery to replicate. Hence, antiviral drugs work differently (block viral enzymes like reverse transcriptase in HIV), and antibiotics are useless against cold, flu, or COVID-19.
Common Mistakes
- Confusing bacteria and viruses → Bacteria are living cells that can reproduce independently; viruses are non-living outside hosts, lack cellular machinery, and are much smaller. Antibiotics work only on bacteria.
- Assuming all bacteria are harmful → Most bacteria are beneficial (gut microbiome, nitrogen fixers, decomposers). Only a small fraction are pathogenic.
- Mixing up xylem and phloem functions → Remember: Xylem = water Xits upward (unidirectional); Phloem = Food flows both ways.
- Believing fungi are plants → Fungi lack chlorophyll, cannot photosynthesise, have chitin cell walls, and are placed in a separate kingdom.
- Forgetting that mitochondria and chloroplasts have their own DNA → This supports the endosymbiotic theory (they were once free-living prokaryotes). Important for questions on cell evolution.
- Ignoring economic importance → UPSC often asks about useful microbes: Lactobacillus (curd), Saccharomyces/yeast (bread, alcohol), Penicillium (antibiotic), Streptomyces (many antibiotics).
Quick Reference
- Cell wall composition: Plants = cellulose; Fungi = chitin; Bacteria = peptidoglycan
- Biofertilisers: Rhizobium (legumes), Azotobacter (free-living), Cyanobacteria (paddy), Mycorrhiza (phosphorus uptake)
- Disease-causing microbes: Bacteria (TB, cholera), Viruses (dengue, COVID), Fungi (ringworm), Protozoa (malaria)
- Lichens = Pollution indicators; absent in highly polluted areas
- Vectors: Mosquito (malaria, dengue), Housefly (typhoid, cholera), Rat flea (plague)
- Probiotics: Live beneficial bacteria (Lactobacillus, Bifidobacterium) that improve gut health