Basic Components of Life & Biodiversity

Biomolecules — the chemistry of life

Every living cell is built from four classes of macromolecules, all polymers of smaller monomers.

• Carbohydrates: general formula (CH₂O)ₙ. Glucose (C₆H₁₂O₆) is the primary respiratory substrate. Cellulose (β-1,4 glucose) is the most abundant organic polymer on Earth and forms the plant cell wall; starch (α-glucose) is the storage form.
• Proteins: 20 standard amino acids joined by peptide bonds. Four levels of structure — primary, secondary (α-helix, β-pleated sheet), tertiary, quaternary. Enzymes are globular proteins that lower activation energy.
• Lipids: glycerol esterified to fatty acids; phospholipids form the amphipathic bilayer of membranes.
• Nucleic acids: DNA is double-stranded (deoxyribose, A-T-G-C); RNA is usually single-stranded (ribose, A-U-G-C).

High-yield: Water is the universal solvent and the medium of all metabolism; its high specific heat, cohesion, and surface tension drive processes like the transpiration pull.

The cell as the unit of life

The cell theory (Schleiden, Schwann, Virchow) states: all organisms are made of cells, the cell is the structural and functional unit of life, and all cells arise from pre-existing cells (Omnis cellula-e-cellula). Prokaryotes (bacteria, cyanobacteria) lack a true nucleus and membrane-bound organelles; eukaryotes possess both.

Five-kingdom classification

R. H. Whittaker (1969) proposed five kingdoms based on cell type, body organisation, mode of nutrition and phylogeny.

• Two-kingdom system (Linnaeus) split life into Plantae and Animalia only; it failed to separate prokaryotes from eukaryotes.
• Binomial nomenclature (Linnaeus): every species gets a two-word Latin name — genus (capitalised) + species (lower case), italicised, e.g. Oryza sativa (rice).

Plant diversity — from algae to angiosperms

Kingdom Plantae is arranged in ascending complexity:

• Algae (Thallophyta): simple, autotrophic, mostly aquatic; no true roots/stems/leaves. Grouped by pigment — green (Chlorophyceae), brown (Phaeophyceae), red (Rhodophyceae). Reserve food and cell-wall material differ per group.
• Bryophyta (mosses, liverworts): "amphibians of the plant kingdom". Dominant gametophyte (haploid); lack vascular tissue; need water for fertilisation. Example: Funaria, Marchantia.
• Pteridophyta (ferns): first true vascular plants with xylem and phloem. Dominant sporophyte (diploid); reproduce by spores. Example: Dryopteris, Selaginella.
• Gymnosperms: naked seeds (no ovary/fruit); mostly evergreen; well-developed vascular tissue. Example: Pinus, Cycas, Ginkgo.
• Angiosperms: flowering plants with seeds enclosed in fruit; show double fertilisation and vascular tissue with vessels. Divided into dicots and monocots.

Alternation of generations

Land plants alternate between a haploid gametophyte (produces gametes by mitosis) and a diploid sporophyte (produces spores by meiosis). The trend across evolution is progressive reduction of the gametophyte and dominance of the sporophyte.

Dicots vs monocots

Biodiversity concepts

• Biodiversity = variety of life at three levels: genetic, species, and ecosystem diversity.
• Nepal, despite its small area, holds exceptional biodiversity because of its steep altitudinal gradient (60 m to 8,848 m), spanning tropical to alpine zones.
• Hotspots are regions rich in endemic species yet under threat; the Himalaya is a recognised global biodiversity hotspot.
• Conservation is in-situ (national parks, biosphere reserves, sacred groves) or ex-situ (botanical gardens, seed banks, tissue culture).

Exam tip: Remember the diagnostic "firsts" — bryophytes = first land plants, pteridophytes = first vascular plants, gymnosperms = first seed plants, angiosperms = first flowering plants with double fertilisation.

Quick revision

• Cellulose (β-1,4 glucose) is the most abundant organic polymer and forms the plant cell wall.
• Whittaker's five kingdoms (1969): Monera, Protista, Fungi, Plantae, Animalia.
• Binomial nomenclature by Linnaeus — genus + species, italicised.
• Bryophytes are "amphibians of the plant kingdom"; gametophyte dominant.
• Pteridophytes = first vascular plants; gymnosperms bear naked seeds.
• Angiosperms show double fertilisation and enclosed seeds (fruit).
• Dicot: reticulate venation, tap root, tetra/pentamerous flowers; monocot is the reverse.
• Biodiversity operates at genetic, species and ecosystem levels.