Cell Structure and the Cell Theory

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Cell theory states that the cell is the smallest structural and functional unit of life. Eukaryotic cells carry a membrane-bound nucleus and organelles (mitochondria, ER, Golgi); prokaryotic cells (Monera) lack a true nucleus and most organelles. Living things are sorted into the five-kingdom system: Monera, Protista, Fungi, Plantae, Animalia. In any ecosystem, energy flows from producers → primary consumers → secondary consumers → decomposers, with roughly the 10% rule between trophic levels, while matter cycles through biogeochemical loops (carbon, nitrogen, water).

Shannon-Wiener diversity index: H′ = −Σ (nᵢ/N) · ln(nᵢ/N).
Exponential population growth: Nₜ = N₀ · e^(rt).
A fungus is not a plant — it is heterotrophic and digests food externally.

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Cell Structure and the Cell Theory

All organisms are built from cells, which arise only from pre-existing cells (Virchow) and are the basic unit of life (Schleiden, Schwann). Prokaryotes (Bacteria, Archaea) are 1–10 µm, lack a nuclear envelope, and have 70S ribosomes. Eukaryotes (Protista, Fungi, Plantae, Animalia) measure 10–100 µm, house DNA inside a nucleus, and contain 80S ribosomes plus membrane-bound organelles: mitochondria (ATP via Krebs cycle), chloroplasts (photosynthesis), rough ER (protein synthesis), and Golgi apparatus (modification, packaging).

Five-Kingdom Classification

Whittaker’s system separates organisms by cell type, nuclear organisation, and nutrition mode:

Kingdom	Cell type	Nutrition	Example
Monera	Prokaryote	Aut/heter	Escherichia coli, cyanobacteria
Protista	Eukaryote	Aut/heter	Amoeba, Euglena
Fungi	Eukaryote	Heterotroph (absorptive)	Yeast, mushroom
Plantae	Eukaryote	Autotroph	Moss, angiosperm
Animalia	Eukaryote	Heterotroph (ingestive)	Sponge, mammal

Ecology Basics

Population density = individuals ÷ sampled area; distinct from total population size N. Species interact as mutualism (both benefit), commensalism (one benefits, other neutral), parasitism (+,−), competition (−,−), and predation (+,−). An ecological pyramid shows standing biomass or energy at successive trophic levels — energy shrinks by ~10% at each step (Lindeman’s rule), while biomass can be inverted in aquatic systems.

Quantitative Tools

Shannon-Wiener H′ rises with both species richness and evenness; values > 3 indicate high diversity.
Simpson’s D rises as one species dominates (0 = infinite diversity, 1 = monoculture).
Exponential growth Nₜ = N₀ · e^(rt) applies when resources are unlimited; logistic growth adds the carrying capacity K.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mitosis vs Meiosis — the Classic Trap

Examiners frequently test whether you know that mitosis produces 2 diploid daughters with chromosome number preserved, while meiosis produces 4 haploid gametes with chromosome count halved. Meiosis I separates homologous pairs (reductional); meiosis II separates sister chromatids (equational). Crossing-over in prophase I is the source of genetic recombination.

The 10% Rule — Misread Carefully

Lindeman’s ~10% energy transfer is a field average, not a fixed constant. Real transfers range 5–20% depending on tissue digestibility and metabolism of the consumer. Biomass pyramids can therefore invert in marine ecosystems where phytoplankton biomass at any moment is far below that of zooplankton + fish — yet energy flow still upholds the 10% pattern when measured over time.

Worked Example — Shannon-Wiener

A pond sample yields: Species A = 50, B = 30, C = 20 (N = 100). Compute H′:

A: p = 0.50 → −0.50 · ln 0.50 = 0.3466
B: p = 0.30 → −0.30 · ln 0.30 = 0.3612
C: p = 0.20 → −0.20 · ln 0.20 = 0.3219

H′ ≈ 1.03 nats, indicating low-moderate diversity dominated by Species A.

Common Mistakes

Calling decomposers a trophic level — they operate across all levels as a functional group.
Treating Euglena (Protista) as a plant: it has a chloroplast but is heterotrophic in the dark.
Comparing density of two species from unequal sample areas without standardising.

Practice Prompts

Calculate N₅ for a bacterial culture starting at N₀ = 1,000 with r = 0.4 h⁻¹.
A grassland loses a keystone grazer. Predict the cascade using trophic-level logic and the 10% rule.

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Cell Structure and the Cell Theory