Living Organisms Characteristics and Classification

Overview

This lecture summarizes the key points of Topic 1: Characteristics and Classification of Living Organisms from the Cambridge IGCSE Biology syllabus.

Living things, or organisms, share seven characteristics: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.
Movement: Organisms change position or place.
Respiration: Chemical breakdown of nutrients in cells releases energy.
Sensitivity: Ability to detect/respond to environmental changes.
Growth: Permanent increase in size and dry mass.
Reproduction: Production of similar offspring.
Excretion: Removal of metabolic waste and excess substances.
Nutrition: Intake of materials for energy, growth, and development.

A species is a group of organisms able to reproduce fertile offspring.
Classification groups organisms by shared features, reflecting evolutionary relationships.
The binomial system gives every organism a two-part scientific name (Genus capitalized, species lowercase, both in italics, e.g., Homo sapiens).
Dichotomous keys identify organisms by following paired feature-based questions.

Five kingdoms: Animals, Plants, Fungi, Protocists (Protists), Prokaryotes (bacteria).
Animals: Multicellular, nucleus, no cell walls/chloroplasts, feed by ingestion.
Plants: Multicellular, nucleus, chloroplasts, cellulose cell wall, make food via photosynthesis.
Fungi: Usually multicellular, nuclei, non-cellulose cell wall, feed saprophytically or parasitically.
Protists: Mostly unicellular, nucleus, may have cell walls/chloroplasts, can photosynthesize or ingest.
Prokaryotes: Unicellular, cell wall (not cellulose), no nucleus/mitochondria, DNA in plasmids.

Animals split into vertebrates (with backbone) and invertebrates (without backbone).
Vertebrates: Five classes—Mammals (hair, milk, 4-chamber heart), Birds (feathers, eggs, beak, wings), Reptiles (scaly skin, rubbery eggs), Amphibians (moist skin, eggs in water, gills as larvae, lungs as adults), Fish (scales, gills, fins).
Invertebrates: Focus on arthropods—myriapods (segmented, many legs), insects (3 body parts, 6 legs, wings), arachnids (8 legs, book lungs), crustaceans (more than 8 legs, gills).

Plants divided into ferns (fronds, reproduce by spores) and flowering plants (flowers/seeds).
Flowering plants: Monocotyledons (parallel leaf veins, petals in 3s, branching roots), Dicotyledons (branching veins, petals in 4s/5s, taproot).

Viruses are not classified as living; they need host cells to reproduce and are made of genetic material in a protein coat.

Organism — A living thing.
Species — Group of organisms producing fertile offspring.
Binomial system — Two-part scientific naming method (Genus species).
Dichotomous key — Identification tool using paired choices about features.
Kingdom — The highest taxonomic group (Animal, Plant, Fungi, Protist, Prokaryote).
Vertebrate — Animal with a backbone.
Invertebrate — Animal without a backbone.
Arthropod — Invertebrate with jointed legs and segmented body.
Monocotyledon — Flowering plant with one seed leaf and parallel veins.
Dicotyledon — Flowering plant with two seed leaves and branching veins.