3.5 Microbiology, Evolution Principles & Ecosystem Ecology

3.5 Microbiology, Evolution Principles & Ecosystem Ecology

Microbiology, evolutionary mechanisms, and ecological stability form an essential component of the FSc Biology syllabus tested on the Pakistan Army Medical Cadet (AMC) Initial Test. Mastery of viral kinetics, bacterial architecture, population genetics, and ecosystem dynamics ensures a well-rounded preparation.

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1. Microbiology: Viruses, Bacteria & Fungi

Acellular Entities: Viruses, Prions & Viroids

Viruses are obligate intracellular parasites lacking cellular organelles and independent metabolic machinery.

• Structure: Core nucleic acid genome (either DNA or RNA, never both) surrounded by a protein coat called a capsid (built of capsomere subunits). Enveloped viruses acquire a lipid bilayer from the host cell membrane during budding.
• Bacteriophage Replication Cycles:
  1. Lytic Cycle: Attachment -> Penetration -> Biosynthesis -> Assembly -> Host Cell Lysis (virion release).
  2. Lysogenic Cycle: Viral DNA integrates into host bacterial chromosome as a prophage. The prophage replicates passively alongside host binary fission until stress triggers induction, entering the lytic cycle.
• Retroviruses (e.g., HIV): Enveloped diploid single-stranded RNA viruses carrying the enzyme Reverse Transcriptase, which synthesizes viral cDNA from RNA templates prior to genomic integration via Integrase.
• Prions: Infectious misfolded proteins ($PrP^{Sc}$) lacking nucleic acids, causing transmissible spongiform encephalopathies.

Lytic Cycle: Attachment -> Penetration -> Synthesis -> Assembly -> LYSIS
Lysogenic Cycle: Attachment -> Penetration -> Prophage Integration -> Binary Fission -> Induction -> Lytic Cycle

Kingdom Monera: Bacterial Structure & Genetics

Bacterial cells are prokaryotes lacking membrane-bound organelles and a true nucleus.

Mechanisms of Bacterial Genetic Recombination

Prokaryotes achieve genetic diversity without sexual reproduction via three lateral gene transfer mechanisms:

1. Transformation: Direct uptake of naked exogenous DNA fragments from the environment by competent bacterial cells (Griffith's experiment).
2. Transduction: Transfer of bacterial host DNA from one cell to another mediated by a bacteriophage vector.
3. Conjugation: Direct transfer of plasmid DNA (e.g., F-plasmid) between donor ($F^+$) and recipient ($F^-$) cells via a specialized sex pilus.

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2. Evolution Principles & Population Genetics

Evolutionary biology explains the historical diversification of living organisms through changes in genetic makeup over generations.

Theories of Evolutionary Change

• Lamarckism (Inheritance of Acquired Characteristics): Proposed that organs develop through "use" and atrophy through "disuse," with acquired traits passed to offspring (e.g., giraffe neck stretching). Disproved by August Weismann's germplasm theory.
• Darwinism (Theory of Natural Selection): Charles Darwin proposed that evolutionary change occurs via differential survival and reproduction of individuals possessing advantageous inherited variations.
• Modern Synthetic Theory (Neo-Darwinism): Integrates Darwinian natural selection with Mendelian population genetics, gene mutations, and chromosomal re-arrangements.

Evidence for Evolution

• Homologous Structures: Anatomical features sharing a common embryological origin but modified for different functions (e.g., forelimbs of human, bat, horse, whale). Demonstrates divergent evolution.
• Analogous Structures: Features performing similar functions but arising from different evolutionary origins (e.g., wings of insects vs. wings of birds). Demonstrates convergent evolution.
• Vestigial Organs: Remnants of structures that were functional in ancestors (e.g., human vermiform appendix, coccyx, nictitating membrane).

Homologous Structures -> Common Ancestry -> Divergent Evolution (e.g., Pentadactyl limb)
Analogous Structures -> Different Ancestry -> Convergent Evolution (e.g., Bird vs Butterfly wing)

Hardy-Weinberg Equilibrium of Population Genetics

In a non-evolving, ideal population, allele and genotype frequencies remain constant across generations. Represented by the mathematical equations:

$p + q = 1$

$p^2 + 2pq + q^2 = 1$

where $p$ is the frequency of dominant allele $A$, $q$ is the frequency of recessive allele $a$, $p^2$ is homozygous dominant ($AA$), $2pq$ is heterozygous ($Aa$), and $q^2$ is homozygous recessive ($aa$).

Five Conditions Required for Hardy-Weinberg Equilibrium:

1. Infinitely large population size (no genetic drift).
2. Complete random mating (no sexual selection).
3. Absence of mutations.
4. No gene flow (no migration into or out of population).
5. No natural selection (equal reproductive success for all genotypes).

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3. Ecosystem Ecology & Environmental Biology

An ecosystem comprises biotic biological communities interacting with abiotic physical environments to process energy and cycle nutrients.

Trophic Dynamics & Energy Flow

• Energy Flow: Unidirectional movement of solar energy through trophic levels (Producers -> Primary Consumers -> Secondary Consumers -> Tertiary Consumers).
• Lindeman's 10% Law: Only approximately 10% of energy stored as organic biomass at one trophic level is transferred to the next higher level. The remaining 90% is dissipated as metabolic heat through respiration or lost to decomposers.
• Ecological Pyramids:
  • Pyramid of Energy: Always upright (cannot be inverted).
  • Pyramid of Biomass: Usually upright; can be inverted in open ocean aquatic ecosystems where rapid phytoplankton turnover supports a larger standing crop of zooplankton.

[ Solar Energy ] ---> [ Primary Producers (Plants): 10,000 J ]
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                            v (10% Energy Transfer)
                      [ Primary Consumers (Herbivores): 1,000 J ]
                            |
                            v (10% Energy Transfer)
                      [ Secondary Consumers (Carnivores): 100 J ]
                            |
                            v (10% Energy Transfer)
                      [ Tertiary Consumers (Apex Predators): 10 J ]

Nitrogen Cycle & Bacterial Intermediaries

Nitrogen ($N_2$) makes up 78% of Earth's atmosphere but cannot be directly assimilated by plants. It undergoes biochemical transformations mediated by specific soil bacteria:

Atmospheric N2 Gas
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       v [ Nitrogen Fixation: Rhizobium (Legume root nodules), Azotobacter, Anabaena ]
Ammonia (NH3) / Ammonium (NH4+)
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       v [ Nitrification Step 1: Nitrosomonas bacteria ]
Nitrite Ions (NO2-)
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       v [ Nitrification Step 2: Nitrobacter bacteria ]
Nitrate Ions (NO3-)  <--- Preferred Form for Plant Assimilation
       |
       v [ Denitrification: Pseudomonas denitrificans under anaerobic conditions ]
Atmospheric N2 Gas

Environmental Pollution & Eutrophication

• Eutrophication: Runoff containing excess agricultural fertilizers (nitrates and phosphates) enters freshwater bodies:
  1. Triggers rapid algal bloom covering water surface.
  2. Algal die-off stimulates massive proliferation of aerobic decomposers.
  3. Aerobic bacteria consume dissolved oxygen, spiking Biochemical Oxygen Demand (BOD).
  4. Hypoxia leads to widespread aquatic animal mortality.
• Biomagnification: Progressive accumulation and concentration of non-biodegradable, fat-soluble toxins (e.g., DDT, heavy metals like methylmercury) at successively higher trophic levels in a food chain.

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Five-Kingdom Classification & Biodiversity (FSc Screening Hits)

AMC academic papers still pull occasional MCQs from the classical five-kingdom framework (Whittaker): Monera (prokaryotes), Protista, Fungi, Plantae, and Animalia. Know diagnostic traits, not encyclopedic species lists:

Viruses remain acellular and are usually excluded from the five kingdoms—flag any option that places viruses inside Monera without caveat.