Classification Systems
Ordering Life's Diversity📋 Why We Classify Life
Classification (taxonomy) is the science of grouping organisms based on shared characteristics and evolutionary relationships. A good classification system allows scientists worldwide to communicate unambiguously about organisms, reveals evolutionary relationships, and helps predict characteristics of newly discovered species. Modern classification is based on both morphology (structure) and molecular evidence (DNA sequences).
The System
Linnaean Classification — Hierarchy of Life
Carl Linnaeus (1707–1778) devised the system still used today. Seven levels, from broadest to most specific.
DomainEukarya / Bacteria / Archaea
KingdomAnimalia, Plantae, Fungi, Protista, Monera
PhylumChordata, Arthropoda, Ascomycota
ClassMammalia, Insecta, Basidiomycetes
OrderCarnivora, Diptera, Agaricales
FamilyFelidae, Hominidae, Agaricaceae
GenusPanthera, Homo, Agaricus
Speciesleo, sapiens, bisporus
📌 Binomial Nomenclature — The Two-Name System
Every species has a unique two-part Latin name: Genus species. Rules: (1) Always italicised or underlined. (2) Genus starts with a capital letter; species starts lowercase. (3) Example: Homo sapiens (modern human), Panthera leo (lion), Escherichia coli (gut bacterium). The system is universal — scientists in every country use the same names, avoiding confusion from different common names. "Lion" in English = "Leeu" in Afrikaans = "Simba" in Swahili = Panthera leo everywhere.
📌 Memory Aid — King Philip Came Over For Good Spaghetti
Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
Five Kingdoms
Whittaker's Five Kingdom System
The classification system most commonly used in South African Life Sciences curricula.
Animalia
Multicellular, eukaryotic, heterotrophic. No cell wall. Mobile. Examples: insects, fish, mammals.
Plantae
Multicellular, eukaryotic, autotrophic (photosynthesis). Cell walls of cellulose. Examples: mosses, ferns, flowering plants.
Fungi
Multicellular (mostly), eukaryotic, heterotrophic by absorption. Cell walls of chitin. Examples: mushrooms, moulds, yeast.
Protista
Mostly unicellular, eukaryotic. Very diverse — some autotrophic, some heterotrophic. Examples: Amoeba, Paramecium, Euglena, Plasmodium.
Monera
Unicellular, prokaryotic (no nucleus). Includes all bacteria and cyanobacteria. Cell walls of peptidoglycan.
Viruses
Not classified in any kingdom — not considered living by most criteria. Acellular, obligate intracellular parasites.
⚠️ Exam Watch — Prokaryote vs Eukaryote
This is the most fundamental distinction in biology. Prokaryotes (Kingdom Monera — bacteria): no membrane-bound nucleus, no membrane-bound organelles, circular DNA, smaller (1–10 μm), ribosomes are 70S type. Eukaryotes (all other kingdoms): membrane-bound nucleus containing linear DNA, membrane-bound organelles (mitochondria, ER, Golgi), larger (10–100 μm), ribosomes are 80S type. Viruses are neither — they are acellular.
Bacteria (Kingdom Monera)
Earth's Most Successful Life Form🦠 The Prokaryotes
Bacteria are the most abundant organisms on Earth — there are more bacteria in a teaspoon of soil than there are people on the planet. They are prokaryotic (no membrane-bound nucleus), usually unicellular, and extraordinarily metabolically diverse. Some photosynthesize, some fix nitrogen, some live in boiling hot springs, some cause disease — bacteria occupy virtually every ecological niche on Earth.
Structure
Bacterial Cell Structure
Simpler than eukaryotic cells but enormously successful. Know every structure and its function.
| Structure | Present? | Function |
|---|---|---|
| Cell wall | ✅ Yes — peptidoglycan | Maintains shape, prevents osmotic lysis. Gram staining targets peptidoglycan differences. |
| Cell membrane | ✅ Yes | Controls what enters/exits the cell; site of some metabolic reactions |
| Cytoplasm | ✅ Yes | Aqueous matrix where metabolic reactions occur |
| Circular DNA | ✅ Yes — in nucleoid region | Genetic material; NOT enclosed in a membrane-bound nucleus |
| Plasmids | ✅ Sometimes | Small extra circles of DNA; often carry antibiotic resistance genes; used in genetic engineering |
| Ribosomes (70S) | ✅ Yes | Protein synthesis; 70S type — different from eukaryotic 80S (antibiotics exploit this difference) |
| Flagella | ✅ Some species | Locomotion — rotates like a propeller |
| Capsule | ✅ Some species | Slime layer outside cell wall; protects from host immune system; aids attachment |
| Membrane-bound nucleus | ❌ No | Absent — key prokaryotic feature |
| Mitochondria | ❌ No | Absent — respiration occurs at cell membrane |
Identification
Bacterial Shapes & Gram Staining
Three basic shapes. Gram staining divides bacteria into two groups based on cell wall composition — crucial for antibiotic treatment.
🔵 Three Basic Shapes
- Coccus (cocci) — spherical; can form chains (streptococci), clusters (staphylococci), or pairs (diplococci). Example: Streptococcus pneumoniae
- Bacillus (bacilli) — rod-shaped; can be single or in chains. Example: Bacillus anthracis, Escherichia coli
- Spirillum (spirilla) — spiral/corkscrew shaped. Example: Helicobacter pylori (causes stomach ulcers)
🔴🟣 Gram Staining
- Developed by Hans Christian Gram (1884)
- Gram-positive — thick peptidoglycan wall; stains purple. Examples: Staphylococcus, Streptococcus
- Gram-negative — thin peptidoglycan + outer lipid membrane; stains pink/red. Examples: E. coli, Salmonella
- Critical for choosing correct antibiotic — different drugs target Gram+ vs Gram- differently
Reproduction
Binary Fission & Antibiotic Resistance
Bacteria divide every 20 minutes under ideal conditions. This speed of reproduction drives rapid evolution of resistance.
✂️ Binary Fission
- Asexual reproduction — one cell splits into two identical cells
- Circular DNA replicates → cell elongates → cell wall forms between the two copies → cell divides
- Generation time as fast as 20 minutes under ideal conditions
- One bacterium → 2 → 4 → 8 → 16... exponential growth
- After 24 hours at 20-min generation time: theoretically 4.7 × 10²¹ bacteria
💊 Antibiotic Resistance — Evolution in Action
- Antibiotics kill most bacteria — but random mutations occasionally create resistant individuals
- Resistant bacteria survive and reproduce → resistance allele becomes common (directional selection)
- Resistance genes can be transferred between bacteria via plasmids (horizontal gene transfer)
- Overuse and misuse of antibiotics accelerates this process dramatically
- MRSA (Methicillin-resistant Staphylococcus aureus) is a major hospital crisis
Viruses
At the Edge of Life⚡ Are Viruses Alive?
Viruses occupy a strange category — they are not classified in any of the five kingdoms because they lack the characteristics we use to define life. They have no cells, cannot carry out metabolism independently, cannot reproduce without a host cell, and do not grow. Yet they have genetic material (DNA or RNA), evolve by natural selection, and can replicate. Most biologists consider them acellular, obligate intracellular parasites — not alive, but not simple chemistry either.
Structure
Viral Structure — Minimal but Effective
Just genetic material in a protein coat. Sometimes with an extra lipid envelope. That's all it takes.
🧬 Core Components
- Nucleic acid — either DNA OR RNA (never both); single or double stranded; contains genes for replication and new virus assembly
- Capsid — protein coat surrounding the nucleic acid; made of protein subunits called capsomeres; protects genetic material; gives virus its shape (helical, icosahedral, complex)
- Together = nucleocapsid
🫧 Envelope (Some Viruses)
- Some viruses (HIV, influenza, herpes) have a lipid membrane envelope outside the capsid
- Derived from the host cell's membrane when the virus buds out
- Contains viral glycoproteins (spikes) used for host cell recognition and attachment
- Enveloped viruses are more easily destroyed by alcohol/soap — why handwashing works against COVID-19 and flu
- Non-enveloped (naked) viruses are more resistant to disinfectants
⚠️ Exam Watch — Why Antibiotics Don't Work on Viruses
Antibiotics target bacterial-specific structures — cell walls (peptidoglycan), bacterial ribosomes (70S), or bacterial enzymes. Viruses have NONE of these structures. They have no cell wall, no ribosomes, and no metabolism of their own. Therefore antibiotics are completely ineffective against viral infections (flu, COVID-19, common cold, HIV). Antivirals are used instead — they target virus-specific steps like RNA replication or envelope protein function.
Replication
The Lytic & Lysogenic Cycles
Two strategies for viral replication — lytic destroys the cell immediately; lysogenic hides inside and waits.
💥 Lytic Cycle (Active)
- Attachment — virus binds to specific receptor on host cell surface
- Injection/Entry — viral nucleic acid enters host cell
- Replication — host cell's machinery hijacked to replicate viral DNA/RNA and produce viral proteins
- Assembly — new virus particles (virions) assembled inside host cell
- Lysis — host cell bursts, releasing hundreds of new virions to infect more cells
- Cell is destroyed. Causes active infection and disease symptoms.
😴 Lysogenic Cycle (Latent)
- Viral DNA integrates into host cell's chromosome — becomes a prophage
- Virus replicates silently with the host cell — every time the host divides, the viral DNA is copied too
- No immediate symptoms; host cell is not destroyed
- Can remain latent for years (HIV, herpes, chickenpox → shingles)
- Triggered by stress, immune suppression → enters lytic cycle → active disease
- Hard to treat because virus is hidden inside host DNA
Case Study — South Africa
HIV/AIDS
South Africa has the world's largest HIV-positive population. Understanding this virus is a curriculum and civic priority.
🧬 HIV Biology
- HIV = Human Immunodeficiency Virus; a retrovirus — carries RNA, uses reverse transcriptase to make DNA from RNA (reverse of normal flow)
- Enveloped virus; glycoprotein spikes (gp120) bind to CD4 receptors on T-helper lymphocytes
- Destroys T-helper cells over time → immune system collapses
- AIDS = Acquired Immune Deficiency Syndrome — the disease state caused by advanced HIV infection (CD4 count below 200 cells/μL)
- Death is usually from opportunistic infections (TB, pneumonia) the damaged immune system cannot fight
💊 Treatment & Prevention
- ARVs (Antiretrovirals) — suppress virus replication; do not cure but allow normal life expectancy; must be taken lifelong
- U=U: Undetectable = Untransmittable — ARVs reduce viral load to undetectable levels; person cannot transmit HIV
- Transmission: unprotected sex, sharing needles, mother to child (pregnancy/birth/breastfeeding), blood transfusion
- Prevention: condoms, PrEP (pre-exposure prophylaxis), needle exchange programmes, PMTCT (prevention of mother-to-child transmission)
- No vaccine available yet — HIV mutates rapidly
Fungi & Protists
The Overlooked Kingdoms🍄 Two More Microbial Kingdoms
Fungi and Protists are eukaryotic kingdoms containing many microscopic members. Fungi are the great recyclers — their digestive enzymes outside their bodies break down dead organic matter and return nutrients to the soil. Protists are a "catch-all" kingdom of diverse unicellular eukaryotes, including some of the most important disease-causing organisms (Plasmodium, Entamoeba) and some of the most important ecological players (phytoplankton).
Kingdom Fungi
Decomposers, Pathogens & Partners
Not plants. Not animals. Fungi absorb food from outside their bodies and are more closely related to animals than to plants.
🏗️ Key Characteristics
- Eukaryotic; heterotrophic by absorption (secrete digestive enzymes outside body, absorb products)
- Cell walls made of chitin (NOT cellulose like plants)
- Body made of thread-like hyphae; network of hyphae = mycelium
- Mycelium is the main body — mushroom is just the reproductive structure
- Reproduce by spores (sexual and asexual)
- Most are multicellular; yeast is unicellular
🌍 Ecological & Medical Roles
- Decomposers — break down dead wood, leaf litter, animal remains; essential for nutrient cycling
- Mycorrhizae — mutualistic fungi living on plant roots; extend root area, improve mineral/water uptake; ~90% of plant species depend on them
- Pathogens — athlete's foot (Tinea pedis), ringworm, thrush (Candida albicans), aspergillosis in immunocompromised patients
- Food/industry — bread, beer, wine (yeast Saccharomyces cerevisiae); penicillin from Penicillium mould
Kingdom Protista
The Catch-All Kingdom
A hugely diverse group — united mainly by what they are NOT (not animal, plant, or fungus) rather than what they are.
| Group | Key Features | Examples | Significance |
|---|---|---|---|
| Animal-like (Protozoa) | Unicellular, heterotrophic, motile — pseudopodia, flagella, or cilia | Amoeba, Paramecium, Plasmodium | Plasmodium causes malaria — kills ~600 000 people/year; Trypanosoma causes sleeping sickness |
| Plant-like (Algae) | Photosynthetic, contain chlorophyll, produce O₂ | Diatoms, dinoflagellates, green algae (Chlamydomonas), kelp | Phytoplankton produce ~50% of Earth's oxygen; base of marine food webs |
| Fungus-like (Slime moulds) | Move like animals when feeding; form spores like fungi | Physarum polycephalum | Used in research on cell signalling and distributed computation |
📌 Malaria — A Protist Disease, Not a Bacterial or Viral One
Malaria is caused by Plasmodium — a protist (protozoan parasite), NOT a bacterium or virus. It is transmitted by the Anopheles mosquito (the vector). The parasite's life cycle alternates between the mosquito (sexual reproduction) and human (asexual reproduction in red blood cells). This is why antibiotics and antivirals don't treat malaria — antimalarial drugs specifically target the protist parasite.
Roles & Impact
Friend, Foe & Everything Between🌍 Micro-organisms Run the World
Most people think of micro-organisms primarily as disease-causing agents. But the vast majority are harmless or essential to life on Earth. They fix nitrogen, drive the carbon cycle, produce the oxygen we breathe, form the base of food chains, ferment our food, produce medicines, and live in mutualistic relationships with every plant and animal on Earth — including us.
| Role | How | Organism Examples | Significance |
|---|---|---|---|
| Nitrogen fixation | Convert atmospheric N₂ to NH₄⁺/NO₃⁻ usable by plants | Rhizobium (root nodules), Azotobacter (free-living) | Essential for all plant protein synthesis — no nitrogen fixation = no agriculture |
| Decomposition | Break down dead organic matter, releasing inorganic nutrients | Soil bacteria, Penicillium, Aspergillus | Drives all nutrient cycles; without decomposers, nutrients locked in dead matter |
| Oxygen production | Photosynthesis by phytoplankton and cyanobacteria | Cyanobacteria, diatoms, dinoflagellates | Produce ~50% of Earth's oxygen; cyanobacteria oxygenated early Earth ~2.4 billion years ago |
| Food production | Fermentation — convert sugars to alcohol, CO₂, lactic acid | Saccharomyces cerevisiae (yeast), Lactobacillus | Bread, beer, wine, cheese, yoghurt, sauerkraut, kimchi, vinegar |
| Medicine production | Produce antibiotics, insulin, vitamins as natural products | Penicillium notatum, Streptomyces, engineered E. coli | Penicillin (1928) transformed medicine; recombinant insulin produced by bacteria saves millions |
| Disease (pathogens) | Invade host, cause infection, damage tissues | M. tuberculosis, HIV, Plasmodium, Vibrio cholerae | TB kills ~1.5M/year; malaria ~600K/year; HIV ~650K/year; still major global health burden |
| Biotechnology | Genetic engineering, bioremediation, biofuels | E. coli, Bacillus thuringiensis | Produce human proteins (insulin, growth hormone); clean up oil spills; produce biofuels |
📌 The Human Microbiome — More Microbial Than Human
The human body contains approximately 38 trillion bacteria — roughly equal to the number of human cells. These microbiome bacteria live primarily in the gut and play essential roles: producing vitamin K and some B vitamins, training the immune system, protecting against pathogens by competitive exclusion, and breaking down dietary fibre. Disruption of the microbiome (e.g. by antibiotics) is linked to a range of health conditions including inflammatory bowel disease, obesity, and allergies.
🎯 Invisible Majority Assessment
Eight questions on classification and micro-organisms.
Question 1 of 8
What is the correct order of taxonomic levels from broadest to most specific?
Question 2 of 8
What is the key structural difference between prokaryotic and eukaryotic cells?
Question 3 of 8
Why are viruses NOT classified in any of the five kingdoms?
Question 4 of 8
Why are antibiotics effective against bacterial infections but NOT effective against viral infections such as influenza?
Question 5 of 8
What is the scientific name for the domestic dog, given that it belongs to the genus Canis and species lupus familiaris? How should this name be written?
Question 6 of 8
Fungi are sometimes mistaken for plants. Give TWO characteristics that distinguish fungi from plants.
Question 7 of 8
Describe the difference between the lytic and lysogenic cycles of a bacteriophage (bacterial virus).
Question 8 of 8
Malaria is caused by Plasmodium, which is transmitted by Anopheles mosquitoes. To which kingdom does Plasmodium belong, and why does this make malaria difficult to treat with standard antibiotics?