Characteristics and Classification of Living Organisms
Summary: All living organisms share seven key characteristics (MRS GREN). Organisms are classified using the binomial naming system into a hierarchy of groups: Kingdom, Phylum, Class, Order, Family, Genus, Species. The five kingdoms are Animals, Plants, Fungi, Prokaryotes, and Protoctists. Viruses are not considered living organisms. Tags: igcse biology characteristics-of-life classification taxonomy Created: 2026-07-16 Last Updated: 2026-07-16
1. Characteristics of Living Organisms — MRS GREN
All living organisms share seven key characteristics. The acronym MRS GREN helps to remember them:
| Letter | Characteristic | Meaning |
|---|---|---|
| M | Movement | An action by an organism or part of an organism causing a change of position or place |
| R | Respiration | The chemical reactions in cells that break down nutrient molecules to release energy for metabolism |
| S | Sensitivity | The ability to detect and respond to changes in the internal or external environment (stimuli) |
| G | Growth | A permanent increase in size and dry mass by an increase in cell number or cell size |
| R | Reproduction | The processes that make more of the same kind of organism |
| E | Excretion | The removal of the waste products of metabolism (e.g. carbon dioxide, urea) and substances in excess of requirements |
| N | Nutrition | The taking in of materials for energy, growth, and development |
Important distinction — Excretion vs Egestion:
- Excretion = removal of metabolic waste (CO2 from respiration, urea from breakdown of amino acids)
- Egestion = removal of undigested food (faeces) — this is NOT a characteristic of life
2. Classification Hierarchy
Organisms are grouped into a hierarchical system of classification:
Kingdom -> Phylum -> Class -> Order -> Family -> Genus -> Species
A useful mnemonic: King Philip Came Over For Good Soup
- Species: A group of organisms that can reproduce to produce fertile offspring
- As you go down the hierarchy, organisms become more similar
- As you go up the hierarchy, groups become larger and more diverse
3. The Binomial Naming System
Developed by Carl Linnaeus. Every species is given a two-part scientific name:
- Genus (capitalised first letter) + species (all lowercase)
- Written in italics when typed, or underlined when handwritten
- Same worldwide — avoids confusion from different common names in different languages
Examples:
- Homo sapiens — Modern humans
- Panthera leo — Lion
- Escherichia coli — A bacterium found in the gut
- Canis lupus — Grey wolf
4. The Five Kingdoms
| Kingdom | Key Features | Examples |
|---|---|---|
| Animals | Multicellular, no cell wall, no chloroplasts, heterotrophic (feed on other organisms), usually able to move | Humans, insects, fish, birds |
| Plants | Multicellular, have cell walls (cellulose), contain chloroplasts, autotrophic (photosynthesis), store carbohydrates as starch | Flowering plants, ferns, mosses |
| Fungi | Usually multicellular (except yeast — unicellular), cell walls made of chitin, no chloroplasts, saprophytic or parasitic nutrition (feed by secreting enzymes onto food and absorbing the digested products), store carbohydrates as glycogen, body usually organised into a mycelium made of thread-like hyphae | Mushrooms, mould, yeast |
| Prokaryotes (Bacteria) | Unicellular, no true nucleus (DNA is a single circular chromosome in the cytoplasm), no membrane-bound organelles, have a cell wall (not made of cellulose), some can photosynthesise, reproduce by binary fission | E. coli, Salmonella, Lactobacillus |
| Protoctists | Mostly unicellular (some multicellular), have a true nucleus (eukaryotic), may have chloroplasts (e.g. algae) or not (e.g. amoeba), some are pathogenic | Amoeba, Paramecium, Chlorella, Plasmodium (causes malaria) |
5. Vertebrate Classes
All vertebrates have a backbone (vertebral column). There are five main classes:
| Class | Body Covering | Breathing | Reproduction | Thermoregulation | Examples |
|---|---|---|---|---|---|
| Mammals | Hair/fur | Lungs | Internal fertilisation, live birth, mammary glands produce milk for young | Endothermic (warm-blooded) | Humans, whales, bats |
| Birds | Feathers | Lungs | Internal fertilisation, lay hard-shelled eggs | Endothermic (warm-blooded) | Eagle, penguin, robin |
| Reptiles | Dry, scaly skin | Lungs | Internal fertilisation, lay soft-shelled eggs on land | Ectothermic (cold-blooded) | Snake, lizard, crocodile |
| Amphibians | Moist, smooth skin (no scales) | Gills (larvae), lungs and skin (adults) | External fertilisation, lay soft jelly-coated eggs in water | Ectothermic (cold-blooded) | Frog, toad, salamander |
| Fish | Scales (with mucus coating) | Gills | External fertilisation, lay eggs in water | Ectothermic (cold-blooded) | Salmon, shark, goldfish |
Key distinction:
- Endothermic (warm-blooded): Body temperature is maintained internally, independent of the environment
- Ectothermic (cold-blooded): Body temperature varies with the environment — rely on external heat sources
6. Arthropods
Arthropods are invertebrates with jointed legs and an exoskeleton (external skeleton made of chitin).
| Group | Body Segments | Legs | Other Features | Examples |
|---|---|---|---|---|
| Insects | 3 parts (head, thorax, abdomen) | 3 pairs (6 legs) | Usually 2 pairs of wings, 1 pair of antennae | Butterfly, ant, bee, beetle |
| Arachnids | 2 parts (cephalothorax, abdomen) | 4 pairs (8 legs) | No antennae, simple eyes | Spider, scorpion, tick |
| Crustaceans | 2 parts (cephalothorax, abdomen) | 5+ pairs (10+ legs) | 2 pairs of antennae, mostly aquatic | Crab, lobster, woodlouse |
| Myriapods | Many similar segments | Many pairs of legs (1 or 2 pairs per segment) | 1 pair of antennae | Centipede, millipede |
7. Plant Classification — Ferns and Flowering Plants
Ferns:
- Have true roots, stems, and leaves (fronds)
- Leaves have spores on the underside (reproduce by spores, not seeds)
- Have vascular tissue (xylem and phloem)
Flowering Plants (Angiosperms): Divided into monocotyledons (monocots) and dicotyledons (dicots).
| Feature | Monocotyledons | Dicotyledons |
|---|---|---|
| Number of cotyledons (seed leaves) | 1 | 2 |
| Leaf venation | Parallel veins | Branched/reticulate (net-like) veins |
| Vascular bundles in stem | Scattered | Arranged in a ring |
| Flower parts (petals etc.) | In multiples of 3 | In multiples of 4 or 5 |
| Root system | Fibrous roots | Taproot with lateral roots |
| Examples | Grasses, wheat, maize, lilies | Sunflowers, beans, roses, oak trees |
8. Viruses — Not Living Organisms
Viruses are not classified as living because:
- They have no cellular structure (no nucleus, no cytoplasm, no cell membrane, no organelles)
- They cannot reproduce on their own — they must infect a host cell and use the host’s machinery to replicate
- They do not carry out any of the MRS GREN life processes independently
- They do not respire, grow, or excrete
Structure of a virus:
- A protein coat (capsid) surrounding genetic material (DNA or RNA)
- Some have an additional lipid envelope (e.g. influenza virus, HIV)
Examples:
- Tobacco mosaic virus (TMV) — infects plants, causes discolouration of leaves
- Influenza virus — causes flu in humans
- HIV — attacks the immune system
Sources
- BBC Bitesize GCSE Biology — Classification of living organisms, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 1 The Study of Life, Ch. 20 Phylogenies and the History of Life, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 1: Characteristics and classification of living organisms, Cambridge Assessment International Education
- CK-12 Biology for High School — Classification of Living Organisms, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Cell Structure and Organisation — How cells form the basis of all living organisms
- Inheritance — DNA, genes, and how species pass on characteristics
- Variation and Selection — Natural selection and how classification reflects evolutionary relationships
- Organisms and their Environment — How different organisms interact in ecosystems
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Excretion is the same as egestion” | Excretion removes metabolic waste (CO2, urea); egestion removes undigested food (faeces) |
| “All bacteria are harmful” | Many bacteria are useful — decomposers, nitrogen-fixing bacteria, gut flora, used in food production (yogurt, cheese) |
| “Fungi are plants” | Fungi belong to their own kingdom — they have cell walls of chitin (not cellulose) and cannot photosynthesise |
| ”All protoctists are unicellular” | Some protoctists (e.g. certain algae, seaweeds) are multicellular |
| ”Viruses are the smallest living organism” | Viruses are NOT living organisms — they have no cellular structure and cannot carry out life processes independently |
| ”Fish are endothermic” | Fish are ectothermic — their body temperature changes with the surrounding water temperature |
| ”All arachnids are spiders” | Arachnids also include scorpions, ticks, and mites — all have 8 legs and 2 body parts |