Variety of Life
Bio-diversity covers how living things are named, classified and grouped into kingdoms, and the distinguishing features of each group. The PMDC MDCAT 2026 syllabus expects you to use binomial nomenclature and the taxonomic hierarchy, justify the five-kingdom system, and recall the diagnostic characters of bacteria, protists, fungi, plants and animals. Read this alongside Acellular Life, which covers viruses. Expect 2–4 MCQs from this chapter.
Taxonomy and Nomenclature
Carolus Linnaeus introduced binomial nomenclature: every organism receives a two-word Latin name — genus (capital initial) followed by the specific epithet (small initial), both italicised or underlined. Homo sapiens, Pisum sativum, Escherichia coli.
The hierarchy runs from most to least inclusive:
Kingdom → Phylum → Class → Order → Family → Genus → Species
Each step down contains fewer, more closely related organisms sharing more characters. Species is the basic unit — a group of similar organisms able to interbreed and produce fertile offspring.
Five Kingdom System
Robert Whittaker (1969) classified life on three criteria: cell structure (prokaryotic or eukaryotic), level of organisation (unicellular, colonial, multicellular) and mode of nutrition.
| Kingdom | Cell type | Organisation | Nutrition | Cell wall |
|---|---|---|---|---|
| Prokaryotae | Prokaryotic | Unicellular / colonial | Auto- or heterotrophic | Peptidoglycan |
| Protista | Eukaryotic | Mostly unicellular | Auto- or heterotrophic | Variable / absent |
| Fungi | Eukaryotic | Multicellular (mostly) | Absorptive heterotroph | Chitin |
| Plantae | Eukaryotic | Multicellular | Photosynthetic autotroph | Cellulose |
| Animalia | Eukaryotic | Multicellular | Ingestive heterotroph | Absent |
Bacteria
Shapes: coccus (spherical), bacillus (rod), spirillum (rigid spiral), vibrio (short curved comma).
| Feature | Gram positive | Gram negative |
|---|---|---|
| Peptidoglycan | Thick layer | Thin layer |
| Outer membrane | Absent | Present (lipopolysaccharide) |
| Crystal violet retained | Yes — appears purple | No — appears pink |
| Penicillin sensitivity | Generally sensitive | Often resistant |
- Endospore
- A dehydrated resistant body formed inside the cell; survives boiling and drying and germinates when conditions improve.
- Capsule
- A slime layer outside the wall; protects against phagocytosis and aids adhesion.
- Reproduction
- Binary fission — asexual, needs no spindle, can complete in about 20 minutes.
Genetic recombination in bacteria occurs three ways, and telling them apart is a favourite MCQ:
| Process | How DNA moves |
|---|---|
| Transformation | Uptake of free DNA from the surroundings |
| Conjugation | Direct cell-to-cell transfer through a sex pilus |
| Transduction | Carried from one cell to another by a bacteriophage |
Plasmids — small self-replicating DNA circles that move between cells — are the standard vectors of genetic engineering.
Cyanobacteria
Photosynthetic prokaryotes (formerly “blue-green algae”). They have no chloroplasts: pigment sits on infoldings of the plasma membrane. Many possess heterocysts that fix atmospheric nitrogen into ammonia, enriching soil and water. Their oxygen-releasing photosynthesis progressively oxygenated the early atmosphere, making aerobic respiration possible.
Protista
Mostly unicellular eukaryotes, classified by locomotion and nutrition.
| Organism | Locomotion | Note |
|---|---|---|
| Amoeba | Pseudopodia | Entamoeba causes amoebic dysentery |
| Paramecium | Cilia | Oral groove sweeps in food |
| Trypanosoma | Flagellum | Sleeping sickness (tsetse fly) |
| Plasmodium | Non-motile (apicomplexan) | Malaria; vector Anopheles |
Fungi
Walls of chitin; body a mycelium of branching hyphae; nutrition absorptive — enzymes are secreted onto the food and soluble products absorbed. Reserve carbohydrate is glycogen, as in animals.
- Lichen
- Mutualism of a fungus with an alga or cyanobacterium; the partner photosynthesises, the fungus supplies anchorage, water and minerals. Together they colonise bare rock.
- Mycorrhiza
- Fungus–root association; hyphae greatly extend absorption (especially of phosphate) and receive carbohydrate in return.
- Yeast
- Unicellular; anaerobic fermentation yields ethanol and carbon dioxide — the gas raises bread, the alcohol is wanted in brewing.
Plantae
| Group | Vascular tissue | Reproduction | Water for fertilisation |
|---|---|---|---|
| Bryophytes (mosses) | Absent | Spores | Required |
| Pteridophytes (ferns) | Present | Spores | Required |
| Gymnosperms | Present | Naked seeds in cones | Not required |
| Angiosperms | Present | Seeds enclosed in a fruit | Not required |
Vascular tissue was the key innovation: xylem conducts water and gives support while phloem distributes sugars, so plants could grow tall and colonise drier land. In ferns the conspicuous plant is the diploid sporophyte; the gametophyte is a small short-lived prothallus.
Animalia
Multicellular ingestive heterotrophs whose cells lack walls — which is what permits contractile movement. Classification uses symmetry, germ layers, body cavity and embryonic development.
- Coelom
- A true body cavity lined with mesoderm. It separates gut from body wall, so peristalsis and locomotion can proceed independently, and leaves room for organs to develop.
- Protostome vs deuterostome
- In protostomes (annelids, molluscs, arthropods) the blastopore becomes the mouth; in deuterostomes (echinoderms, chordates) it becomes the anus and the mouth forms later.
- Arthropods vs annelids
- Both segmented, but arthropods add a hardened chitinous exoskeleton and jointed appendages — and therefore must moult to grow.
Worked MCQs
Five MCQs covering the high-yield testing patterns for this chapter. Read every explanation carefully.
Q1. Which sequence of taxonomic categories runs from the most inclusive to the least inclusive?
The full hierarchy is kingdom → phylum → class → order → family → genus → species. Two distractors here are the same list reversed — read the direction asked for before choosing.
Q2. The cell wall of a typical fungus is composed largely of:
Fungal walls use chitin, the same nitrogen-containing polysaccharide as the arthropod exoskeleton. Cellulose belongs to plants and peptidoglycan to bacteria — this three-way contrast is worth memorising as a set.
Q3. Transfer of genetic material between bacteria through direct contact by a pilus is called:
Conjugation needs a cytoplasmic bridge formed by a sex pilus. Transformation is uptake of free DNA from the medium; transduction uses a bacteriophage as the carrier; binary fission is not a recombination process at all.
Q4. Bryophytes are restricted to damp habitats mainly because they:
Two limitations act together: without xylem and phloem they cannot conduct water efficiently, and their motile male gametes must swim to the egg through a film of water. Bryophytes do photosynthesise and do not make seeds.
Q5. On what grounds are fungi placed in a kingdom separate from plants?
Fungi were once grouped with plants because they are non-motile and walled, but they cannot photosynthesise, their walls are chitin rather than cellulose, and they digest food externally then absorb it. They are eukaryotic and mostly multicellular, which rules out the other options.
Quick Recap
- Linnaeus = binomial nomenclature; genus capitalised, species epithet small, both italicised.
- Hierarchy: Kingdom, Phylum, Class, Order, Family, Genus, Species.
- Whittaker’s five kingdoms use cell type, organisation and nutrition.
- Cell walls: bacteria = peptidoglycan, fungi = chitin, plants = cellulose, animals = none.
- Gram positive = thick peptidoglycan, purple; Gram negative = thin wall + outer membrane, pink, often penicillin-resistant.
- Bacterial recombination: transformation (free DNA), conjugation (pilus), transduction (phage).
- Cyanobacteria: no chloroplast, heterocysts fix N2, oxygenated the early atmosphere.
- Lichen = fungus + alga; mycorrhiza = fungus + root; yeast ferments to ethanol + CO2.
- Vascular tissue let plants grow tall; gymnosperm seeds naked, angiosperm seeds in a fruit.
- Protostome blastopore → mouth; deuterostome blastopore → anus.